VOLT, in the manege, a round or circular tread, hence, by the phrase to make volts, is understood a gate of two treads, made by a horse going sidewise round a centre, in such a manner that these two treads make parallel tracks; one larger, made by the fore-feet, and another smaller, made by the hind-feet; the croup approaching towards the centre, and the shoulders bearing out.
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Voltaic Electricity properly designates that branch of electricity to which the name of Galvanism is generally applied. The term voltaic has been given to it in consequence of the science having been founded by M. Volta, professor of natural philosophy at Pavia, although the experiments of Galvani were prior to those of his countryman. At present we propose to comprehend under this general title, the sciences of galvanism, electro-magnetism, magneto-electricity, and thermo-electricity ; and we have adopted this arrangement in order that we might avail ourselves of the various discoveries which might be made up to the close of the work.^[1. The names electro-magnetism and galvanism occur so early in the alphabet, that the arrangement referred to could not with propriety be avoided. ]

PART I.—GALVANISM.

The science of galvanism derives its name from some interesting experiments performed in 1789 by Μ. Galvani, professor of anatomy at Bologna. When one of his pupils was using an electrical machine, a number of frogs were lying skinned on an adjoining table for the purposes of cookery. The machine being in action, the young man happened to touch with a scalpel the nerve of the leg of one of the frogs, when, to his great surprise, the leg was thrown into violent convulsions. Madame Galvani, having observed the fact, communicated it to her husband, who speedily repeated and extended the experiment. He found that the convulsions took place when a spark was drawn from the prime conductor; and when the blade of the knife, or any other good conductor, was brought into contact with the nerve. When a frog formed part of an electric circuit, a very small quantity of electricity, whether common or atmospheric, produced convulsions in the muscles. Having hung a number of frogs by metallic hoops on an iron railing, he observed that the limbs were frequently convulsed when no electricity was indicated in the atmosphere. In studying this experiment, he was led to the conclusion that the convulsions were not produced by extraneous electricity, but that they always took place when the muscle and nerve of a frog were each placed in contact with metallic bodies, themselves connected by a metal. A still more powerful effect was produced when two metals, such as zinc and silver, were employed, the nerve being armed or coated with one of these metals, the muscle brought into contact with the other, and the two metals joined by an electrical conductor.

This experiment is shewn in fig. 1, where Z is the rod of zinc and C the rod of copper. The extremity B, of the zinc is brought into contact with the armed muscle of the suspended limb FD, while the extremity of the copper is brought into contact with the nerve at D. When the two metals are made to touch at A, the limbs are convulsed, and take the dotted position Galvani explained this phenomenon by saying, that the muscle of the frog was a sort of Leyden phial, the nerves representing the interior, and the muscles the exterior coating of the phial, and the discharge or shock taking place by the metal or metals, which form a communication between the two electrified coatings. The fluid which thus passed along the nerves and muscles was called the Galvanic fluid.

The publication of Galvani’s discoveries excited great interest. The subject was prosecuted by Valli, Fowler, Robison, Volta, Wells, Humboldt, Fabbrici, and others; but the labours of Volta were the most successful, and, by his discovery of the Voltaic pile, he may be considered as the great founder of the science. In his earliest inquiries, Volta saw the true cause of the phenomena described by Galvani. He maintained that the exciting fluid was ordinary electricity, produced by the contact of the two metals, and that the convulsions of the frog arose from the electricity thus developed passing along its nerves and muscles. Volta afterwards proved that the force, which gave rise to these phenomena, was generated by the contact of heterogeneous bodies, that it decomposed their natural electricity at the point of junction, continually separating the two fluids, and making the positive electricity pass along the one and the negative along the other.

According to this view of the subject, every two heterogeneous bodies form a galvanic circle or are, as it is sometimes called, in which electricity is generated; and hence Volta was led, in 1800, to invent the Voltaic pile, or compound galvanic circle.

This apparatus, shewn in figs. 2, 3, consists of a number of single galvanic circles, ZC, ZC, &c., each of which is composed of a disc of zinc and another of copper. These single galvanic circles are separated by a disc D, of paper, card, or cloth soaked in water or any other fluid. When thirty or forty pair or couple of zinc and copper discs, four inches square, are combined, as in fig. 2, the electricity developed will be sufficient to make the gilt leaves of an electroscope diverge, the upper or zinc end with positive, and the lower or copper end with negative electricity; and if we touch the upper end with the wetted fingers of one hand, and the lower end with the wetted fingers of the other, a distinct electrical shock will be experienced.

The zinc extremity of the pile is therefore called the positive extremity or pole, and the copper extremity the negative extremity or pole. The wet card, or disc, which separates each pair of metallic discs is called the conductor.
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In order to understand how the electricity of each galvanic pair is accumulated in the pile, let C, fig. 4, be a plate of copper, or a negative element, communicating with the ground by a wire or chain, W. When a plate of zinc Z, or a positive element, of the same size, is placed above it, a decomposition of electricity will take place at the instant of contact; the negative electricity will pass into the copper and through the wire W into the ground; while the positive electricity will enter the zinc, and accumulate, till its tension, or the thickness of the electrical stratum, is a maximum which we shall call 1. If we were to make the zinc plate Z communicate with the ground by another wire, the positive electricity with which it is charged will be carried off like the negative electricity from the copper disc, and the electricity, set free by the contact of the two metals, would be carried off as soon as it is generated. By uniting the extremities of the two wires the electricities would be recomposed, and a continual circulation of electricity would take place. The disc of copper C, communicating with the ground as in the figure, and the zinc disc Z, having an electrical intensity equal to 1, let us place upon Z a wet disc D of card. The positive electricity will pass from Z to D till the tension of the electricity in D is equal to 1, a fresh supply arriving from the contact of the two metals. The same electrical state will continue when a second plate of copper, C 2 , is placed above the disc D. But if we place a second plate of zinc, Z 2 . It will acquire from the copper beneath it and the wet disc an electrical intensity equal to 1, and from its own action on the copper it will acquire another portion of electricity equal to 1, so that its electrical intensity will be 2. While this is going on, the negative electricity developed in the copper will be neutralised by the positive electricity which it possesses, and in the first pair, CZ, there will be a new development, by which the first zinc disc Z will be brought back to an intensity 1, as well as the disc D and the second copper C 2 . Hence the second zinc disc, Z 2 , can be in equilibrium only when it has an intensity double of that possessed by the first. In like manner the third zinc disc will have an intensity 3, the fourth 4, and so on, the fortieth having an intensity 40.

In the voltaic pile which we have now described, the negative, or copper pile, communicates with the earth, and the intensity of the positive electricity increases, at every pair, from 1 to 40. If we take another pile of similar dimensions, in which the zinc or positive pile communicates with the earth, then the intensity of the negative electricity will increase from 1 to 40. Let us now place these two poles together, so that the two poles in communication with the ground are supported by a wet disc of card, we shall have a pile of eighty pair of plates, in the middle of which the electricity is in its natural state. Its intensity being there 0, while at one end there will be a positive pile whose intensity is 40, and at the other end a negative pile whose intensity is also 40.

In a voltaic pile thus insulated we have electricity of opposite kinds accumulated at its two poles, and of any intensity we choose. If we now place a wire in contact with one pole, and another wire in contact with the other, and bring their extremities together, we shall observe an electric spark. By separating them and again bringing them together, another spark will be seen, so that there is a continual current of fire passing from the one extremity or pole of the wire to the other.

If we now unite the two extremities of the two wires, so as to close the circuit, every thing will appear to be at rest, but, notwithstanding this state of apparent repose, the elec trical actions are still going on; the electrical fluid is decomposed in each pair of plates, and again recomposed in the conducting wires. In order to prove this, we have only to interpose a piece of slender wire between the extremities or poles of the conducting wires, when it will either become hot, or red hot, or white hot, or be fused, according as it is longer or shorter, or of a greater or a less diameter. In like manner, water, acids, and other compound substances are all decomposed when placed between the poles and the wires, so as to form part of the galvanic circuit.

CHAP. I.—DESCRIPTION OF GALVANIC APPARATUS.

In performing these experiments, and drawing from this powerful agent all the electrical energy which it is calculated to yield, a great variety of apparatus has been employed. When a voltaic pile consists of many couples, their superincumbent weight presses all the water or other fluid from the discs of card or cloth, and thus injures the action of the pile. In order to avoid this, Volta introduced the “couronne des tasses,” which is represented in fig. 5, where A, B, D, &c., are three or more glass vessels containing acidulated water or diluted sulphuric acid; the plates Z and C of zinc and copper, about two inches square, are soldered to the ends of a bent metallic wire, M, N, &c., and are immersed in the water in the vessels. About thirty of these cups are sufficient to give a shock. When the circuit is closed, by joining the ends of the wires W, W,gas is actively evolved at all the surfaces of the plates, but when the circuit is broken the evolution of gas ceases in the copper plate, and becomes less copious in the zinc.

A valuable modification of the “couronne des tasses,” called the trough battery, was suggested by Dr Wilkinson and Dr Babington. Plates of zinc and copper, about four inches square, are joined together in pairs, by being soldered at one point. The pairs of plates are then attached to a strip of well dried and varnished wood, A B, fig. 6, so that the whole can be placed in a trough, T T, made of earthenware or wood, with as many partitions as there are pairs of plates. When the trough is of earthenware the partitions are of the same substance, but if they are made of wood the partitions are made of glass. When diluted sulphuric acid is poured into the cells of the trough T T, the battery A B, is immersed in it, so that each pair shall be separated from the adjoining one by a partition of the trough. This apparatus has the great advantage of allowing us to clean or repair the plates, without pouring out the fluid, which can also be changed with great facility. The powerful voltaic battery, constructed for the Royal Institution by Mr Eastwick, under the direction of Sir H. Davy and Mr Children, is upon this plan. It consists of 2000 double plates, and its acting surface is 128,000 square inches. When a battery consists of a number of these troughs united[7:21:666] together, the terminal plates of the adjacent troughs are joined by slips of copper, which unite the zinc end of one trough with the copper end of the other.

The galvanic trough, as it is called, is shewn in fig. 7, as constructed by Mr Cruikshanks of Woolwich. Plates of zinc and copper C Z, C Z, &c., figfl, soldered together, are made to constitute the partitions of a trough, T T, of baked wood, by fixing them into grooves formed in its side, all the zinc surfaces being on one side and all the copper ones on the other. The spaces or cells between them are then filled with a solution of salt and water, or with diluted muriatic or sulphuric acid. Troughs of this description are very apt to get out of order from the warping of the wood by the action of the acid solution, from the cracking of the cement, and other causes which affect the condition of the cells. This trough was still farther improved by keeping the plates of zinc and copper separate as at Z, C, but united at their summits S, with a small ribbon of copper. Each of these double plates was then placed upon the earthenware or glass partitions of a trough like that in fig. 6.

Dr Wollaston found that the power of a galvanic battery was greatly increased, when each surface of the zinc plate was opposed to a surface of copper, and in 1815 he constructed on this principle what be calls an elementary galvanic battery. From a series of experiments, made for the purpose of ascertaining the most compendious form of apparatus, by which visible ignition might be shewn, he found that a single plate of zinc, one inch square, when rightly mounted, was more than sufficient to ignite a wire of platina 1 / 3000 dth of an inch in diameter, even when the acid is very diluted (fifty parts of water to one of sulphuric acid). “But for this purpose,” he adds, “each surface of the zinc must have its counterpart of copper, or other meta), opposed to it; for when copper is opposed only to one face, the action on the posterior surface of the zinc is wasted to little or no purpose. The smallest battery that I formed of this construction consisted of a thimble without its top, flattened till its opposite sides were about 2 / 10 ths of an inch asunder. The bottom part was then nearly one inch wide, and the top about 8/ 10 ths, and as its length did not exceed 9 / 10 ths of an inch, the plate of zinc to be inserted was less than ¾ths of an inch square.”^[2. Annals of Philosophy, vol. vi. p. 210. ]

The plan thus suggested by Dr Wollaston was employed by Mr Children in the construction of a magnificent battery, in which each plate presented a surface of thirty-two square feet; the plates being six feet long, and two feet eight inches broad. The plates are attached to a strong wooden frame, suspended by ropes and pullies, which, being counterpoised, is easily lowered and elevated, so as to immerse the plates in, or raise them out, of the acid. The cells of the battery were twenty-one in number and their united capacities 945 gallons.^[3. Phil. Trans. 1815, p. 363. ]

Having seen a new battery of Dr Wollaston’s, constructed on a large scale by Newman, Mr Hart^[4. Edin. Journal of Science, No. vii. p. 19, Jan. 1826. ] of Glasgow conceived the idea of adding sides and bottoms to the double copper plates, so as to make them form cells of themselves, for holding the acidulous liquid. In this way, each galvanic pair became a triad, consisting of two plates of copper inclosing one of zinc. The following is the method of constructing such a battery as given by Mr Hart :—

“The cells are formed by cutting the copper in the form represented by fig. 9, they are then folded up as seen in fig. 10, and the seams grooved. A drop of tin is run into each lower corner, to render the cells perfectly tight, and at the same time to increase the positive state of the copper. Fig. 11. represents the zinc plate cast in the usual manner, and having a piece of screwed brass wire cast into the top of it, in order to suspend it by.

“Fig. 12. is a section of the battery, showing how the copper tail of the first cell is connected with the zinc plate of the second, and so on. This connection is rendered perfect, by joining them with a drop of solder. The zinc plates are kept firm in their place in the cells by three small pieces of wood, in the same manner as in Dr Wollaston’s battery; the whole are then fixed (by means of screw nuts fitted on the brass wires) to a bar of baked wood, previously well varnished. Fig. 13 represents the battery in its complete state.

"When the battery is small, two may be suspended on one frame. When used for shocks, they may be arranged with the positive or negative poles together, and joined with wire to complete the circuit; but when employed for deflagration, the batteries ought to be placed alongside of [7:21:667]each other, with all the positive poles at one end, and the negative at the other, and the poles of the same name joined. This arrangement will increase the surface, while the number is the same.

“When the battery is to be used. It is to be lifted off the frame, and dipped into a wooden trough lined with lead, into which the acid has been poured, or it may be placed in the leaden trough, and the liquid poured into it, till the cells are full. It is then to be placed on the frame, and the rest charged in succession.”

Mr Candie of Glasgow constructed six batteries on this principle, of twenty-five triads each, and they were found superior to a battery on Wollaston’s construction, “with the same number of plates, but the plates of which contained double the surface.” This comparison was made in the manner proposed by M. Μ. Gay Lussac and Thenard. “Two of Dr Wollaston’s batteries, each containing ten triads in porcelain troughs, evolved a certain volume of gas in seventeen minutes, while a battery of the new construction, with the same number of triads, but presenting only one half the surface of the other, yielded the same volume of gas in fourteen minutes.” This effect, as Mr Hart remarks, could arise only from the superior means of insulation possessed by his battery.

Very great improvements have been made on the galvanic battery by Dr Robert Hare of Philadelphia, whose Galvanic deflagrator, as he calls it, is represented in fig. 14, which represents an apparatus consisting of two troughs, each of which is ten feet long. Each trough contains 150 galvanic pairs. The galvanic series, A B, in the upper trough, is shewn, as it appears when the acid is off the plates, CD being the part of the trough containing the acid when it is off the plates. In the lower trough EF, the galvanic series is omitted, in order that the interior may be better understood. The series belonging to this trough is shewn in fig. 15. The pairs are contained in three boxes, each containing fifty pairs. In placing these three boxes in the trough, a little space is left between them and that side of the trough in which the acid enters, so that instead of flowing over them. It may run down outside, and rise up within them.

The pairs of the series consist of copper cases, about seven inches long, three wide, and half an inch thick, each containing a zinc plate equally distant from its sides, and prevented from touching it by grooved stripes of wood. Each zinc plate is soldered to one side of the adjacent case of copper, as shewn in fig. 16, the copper cases being open only at the top and bottom. The copper cases are separated from each other by very thin veneers of wood.

The two troughs, AB, EF, are joined lengthwise, edge to edge, so that when the sides of the one are vertical, those of the other must be horizontal. Hence, by turning the handle H a quarter of a revolution, the two troughs thus united upon pivots which support them at the ends, will be so raised that any fluid in the one trough must flow into the other, and by reversing the action must flow back again. In this way, the galvanic series being placed in one of the troughs, and the acid in the other, the plates may, by means of the handle H, be all simultaneously subjected to the action of the acid, or relieved from it. The pivots are made of iron, coated with brass or copper, and a metallic communication is made between the coating of the pivots and the galvanic series. The lower trough, EF, is connected with the upper one, AB, by metallic rods, mn, op, joining the two handles, H, h. ^[5. Silliman’s Journal, vol. vii p. 347, and vol. v. p 94. ]

In the course of his experimental researches in electricity, Dr Faraday was led to the construction of a voltaic trough, in which the coppers, passing round both surfaces of the zincs, should not he separaten from each other, except by an intervening thickness of paper, or in some other way, so as to prevent metallic contact, and should thus constitute a compact, powerful, and economical instrument. He found, however, that Dr Hare had in the trough, which we have above described, anticipated him in his contrivance. The arrangement of Dr Hare, who separated the copper plates by thin veneers of wood, and poured the acid on and off the plates, by a quarter revolution of an axis, which carries both the troughs with the plates, and another trough to collect and hold the liquid, was applied by Mr Faraday as the most convenient. His zinc plates were cut from rolled metal, and, when soldered to the copper ones, had the form shewn in fig.

17. They were then bent over a guage into the shape fig. 18, and when packed in the wooden trough, were disposed as in fig. 19, small plugs of cork being used to prevent the zinc from touching the copper plates, and a single or double thickness of cartridge paper being interposed between the contiguous surfaces of copper to prevent their contact. A trough of forty pairs of plates could thus be unpacked in five minutes, and repacked again in half an hour; and the whole series occupied only fifteen inches in length. A trough of this kind, with forty pairs of plates three inches square, was compared with one of forty pairs of four inch plates, having double coppers, and used in porcelain troughs, with insulating cells, and having the same strength of acid; and the former was found equal to the latter in the ignition of platina wire, in the discharge between points of charcoal, and in the strength of the shock. The following are the advantages of this form of trough enumerated by Dr Faraday:—1. It is so compact that one hundred pair of plates may go into a trough three feet long. 2. The copper bearings, on which the pivots rest, afford fixed terminations, which Dr Faraday connects with two cups of mercury fastened in front of the stand of the instrument. These fixed terminations give [7:21:668] the great advantage of arranging an apparatus to be used in connection with the battery, before the latter is put in action. 3. The trough is ready for use in a few seconds, a single jug of diluted acid being sufficient to charge one hundred plates. 4. When the trough has performed a quarter of a revolution. It becomes active, and the experiment has the advantage of the first contact of the zinc and acid, which is twice, and sometimes thrice, that which the battery can produce a minute or two after. 5. When the experiment is finished, the liquid can be instantly poured from between the plates, and hence the latter is never needlessly wasted, the acid unnecessarily exhausted, or the zinc uselessly consumed. The charge, too, is mixed and made uniform, and the advantage of a first contact is obtained in the next experiment. 6. The saving of zinc is so very great, that Dr Faraday estimates the zinc to be thrice as effective as that in the ordinary form of battery. 7. The surfaces of the zinc and copper plates may be brought much nearer to each other when the battery is constructed, and remain so till it is worn out. 8. Thinner plates of zinc will do the duty of thicker ones, and rolled zinc, which is the purest,^[6. Dr Faraday found rolled Liege or Moiselman’s zinc the purest. ] may be used. 9. The purity of the diluted acid is proportioned to the quantity of zinc dissolved. 10. The acid is more easily exhausted, so that we need not use an old charge a second time. 11. By using a due mixture of nitric and sulphuric acid for the charge, no gas is evolved from the troughs. Among the defects of this form of the battery, Dr Faraday enumerates the precipitation of copper on the zinc plates, which he considers as arising chiefly from the papers between the coppers retaining acid when the trough is emptied, which acid, acting slowly on the copper, forms a salt, which gradually mingles with the next charge, and is reduced on the zinc plate by the local action, and hence the power of the whole battery is reduced. Dr Faraday proposed to remedy this evil, by using slips of glass to separate the coppers at their edges.^[7. Phil. Trans. 1835, Parc ii. ]

The defect thus pointed out by Mr Faraday, was particularly experienced by Mr James Young of the Andersonian University, Glasgow, who has proposed a form of battery in which these papers are not required, and having constructed several dozens of instruments in the new form, he found that from the same surfaces of zinc, electricity, the same in quantity and tension, is produced in both Dr Hare’s form and his, but that in the new construction, this effect is produced with half the quantity of sheet copper, which arises from both sides of the copper plates being presented to surfaces of zinc. The following is Mr Young’s construction. Supposing the breadth of the required plates to be two inches, the sheet copper and zinc are cut into ribbands, two inches broad and five inches long, and a portion cut out as in fig. 20. The ribbon is thus divided into two squares of two inches, and united at A, and having a piece projecting at B. Fig. 20, representing a single plate, either of zinc or copper, is bent at A, as in fig. 21. A plate of zinc thus bent, is then united to a similar one of copper, by soldering together the projecting parts BB, as in fig. 22, and this is the only metallic communication existing between them. Each pair of plates is constructed as in fig. 22. In arranging a number of pairs to form a battery, they are interlaced so that a copper square comes in between each couple of zinc squares, and vice versa. This arrangement is not easily described. At the positive end of the battery there is a single copper plate, which is soldered at the top to the last double copper plate, as seen in fig. 23, which represents three pairs properly arranged, and also the way in which they should be fitted up, and kept steadily apart in a wooden frame. This frame is made of two solid pieces of wood, into which are dove-tailed two cross bars, ee, e', in front, with two similar ones behind. The grooves in the cross bars for receiving the edges of the plates, are formed by placing the four cross bars together, and sawing a little way into one side of them all every eighth of an inch or so in their length, so as to form a set of parallel grooves. This affords us a better security against metallic contact than the wedges of cork in Dr Hare’s battery, which are apt to slip out.

The frame, fig. 23, with its plates, may be introduced into a porcelain or wooden trough, TT, containing the diluted acid. Mr Young prefers a single trough to Dr Hare’s two connected troughs, and by means of an axis of stout wire, A B, carrying two pullies, P P, the frame and battery can be raised out of the fluid.

Mr Warren de la Rue made an important step, by using a solution of sulphate of copper as the exciting agent in voltaic batteries. Oxygen is thus supplied to the zinc by the oxide of copper; no gas is evolved; and the action being thus rendered continuous, the effect is fully equal to that momentarily produced by immersion in acids. The battery which Μ. de la Rue considers best adapted to the use of sulphate of copper, is shown in fig. 28. Thezinc plateis shown in fig.25. It should be tinned on the top A, previous to the amalgamation of the rest of the plate. The zinc plate is retained in its place by grooves cut out of the two slips of wood BB, to within 3-4ths of an inch of the bottom. The copper plates are formed into cells, painted on the outside (as in fig. 26,) five inches square and one inch wide, E,E, being two ears of copper for suspending the cell in its place, and A, a slip of copper to be soldered to the zinc plate in the adjacent cell. As the zinc plates do not descend lower than within 3-4ths of an inch of the bottom of the cells, the space thus left may contain the deposits arising from decomposition of the sulphate of copper. The cells are supported in a long wooden frame, by means of the ears E,E, by hooks driven through them as shown in fig.

27. In order to receive the charge when the battery is in action, Μ. de la Rue employs the contrivance shown in fig.
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28. A spout L, a quarter of an inch deep, is placed at the top of each cell, and these spouts overhang a wooden gutter extending along the frame. The solution of the sulphate must then be renewed by means of a funnel with a long neck, the long end being made to descend nearly to the bottom of the cell. When the fresh solution is thus poured in, the spent liquor will run out by the spout into the gutter. When a series of experiments is over, the battery must be emptied, and the plates well cleaned by dashing water between the cells.^[8. London and Edinburgh Philosophical Magazine, April 1837, vol. x., p. 244. ] Mr Noad justly recommends this battery as a very valuable one, and as likely to supersede the acid charged battery altogether. Its initial action was equal to one of Wollaston’s batteries of equal surface, and what is of far more importance. Its action was permanent.

Professor DanieD has published in the Philosophical Transactions for 1836, an account of two new voltaic batteries, possessing valuable properties. The first of these he calls the dissected battery, by means of which he says, “many detached facts well-known before, had become clear, and of more importance, from their connection and comparison with each other by its means.” The following is Mr Daniell’s own description of it :—The battery consists of ten glass cells, a section of one of which is represented in the accompanying figures.

"abed, fig. 29, is a foot of solid glass, containing a cavity e f g h, the upper part of which is fitted with a stopper, g h. Through this stopper the stems of the two plates i; k l in n, pass into the lower part of the cavity, which is divided into two cells by the partition o p, and each of which contains mercury, into which the wires respectively dip. This arrangement admits of the plates being changed at pleasure with little difficulty. The plates may be connected together, or with the plates of other cells, by means of wires, ρ q, passing through the lateral holes, t, u, and dipping also into the cups of mercury. To the glass foot thus arranged, a glass shade v w x y z z, is fitted by grinding, and constitutes a cell for the reception of the liquid. A graduated glass jar, A, B, may be suspended over either plate by means of a brass clip, proceeding from a rod placed by the side of the cell in the manner represented by fig. 30, which is a perspective drawing of a circular arrangement of ten such cells.

Fig. 31 represents the section of a cell which is adapted to the same purposes, but is less expensive in construction. It is supported in a perforated table C, D, by its projecting rim v w y z, and the stems of the plates pass through the glass stopper a b c d, into the exterior mercury cups o, p, by means of which all the necessary connexions may be made.

The circular arrangement of the cells of the battery, fig. 30, admits of their being combined together in various ways with the greatest facility, by means of small cups of mercury g, h, i, placed at proper intervals. My next disposition was to connect all the platinum plates together by wires radiating from them to a central cup k, of mercury, and all the zinc plates by wires, dipping into a ring of the same metal, placed in a groove a b c d e f, surrounding the whole arrangement. In this state of things no action was of course manifest, for there was no complete circuit; but upon making a connection by means of a wire, between the centra) cup and the exterior circle of mercury, the current was enabled to circulate, and was manifested by the simultaneous evolution of gas from all the cells. The inequality of action became again apparent, and the differences between the cells was nearly the same, as when they were connected in separate single circuits.”

Notwithstanding the numerous improvements which have been made in the voltaic battery, no successful attempts had been made till the time of Professor Daniell, to discover the causes of the variations and decline of its force, after the first immersion of the plates in the diluted acid. The principal causes of these variations he proved to be the evolution of hydrogen gas from the negative metallic surface, which not only consumes a considerable portion of the generated electricity, but reduces at the conducting plates the oxide of zinc, formed by the action of the battery at the generating plates, and here the conducting plates were ultimately so encrusted with metallic zinc, as to diminish and finally annihilate the circulating force. Hence he was led to the construction of what he calls a constant battery, for producing an invariable current of force, and therefore applicable to many important researches, which cannot be successfully carried on under variation of the voltaic current. But beside the attainment of this great object, Professor Daniell considers it as promising the following advantages:

1. The abolition of all local action, by the facility of applying amalgamated zinc.

2. The trifling expense of replacing the zinc rods when worn out, and the total absence of any wear of the copper.

3. The dispensing with the use of nitric acid, and the substitution of the cheaper materials, sulphate of copper and oil of vitriol, and the absence of any annoying fumes; and,

4. The facility and perfection with which all metallic communications may be made, and different combinations of the plates arranged.

The following is Professor Daniell’s description of this valuable instrument.

Fig. 32 represents a section of one of the cells, ten of which are shown in connexion at fig. 53; a b c d is a cylinder of copper six inches high and three and a half inches wide; it is open at the top a b, but closed at the bottom, except a collar, er, one and a half inch wide, intended for the reception of a cork into which a glass siphon-tube, g h i j k, is fitted. On the top, a b, a copper collar, corresponding with the one at the bottom, rests by two horizontal[7:21:670] arms. Previously to the fixing of the cork siphontube in its place, a membranous tube, formed of a part of the gullet of an ox, is drawn through the lower collar, e f and fastened with twine to the upper, l in n o; and when tightly fixed by the cork below, forming an internal cavity to the cell, communicating with the siphon-tube in such a way as that, when filled with any liquid to the level m o, any addition causes it to flow out at the aperture k. In this state, for any number of drops allowed to fall into the top of the cavity, an equal number are discharged from the bottom, p q, is a rod of cast zinc, amalgamated with mercury, six inches long and half an inch diameter, supported on the rim of the upper collar by a stick of wood, r , s , passing through a hole drilled in its upper extremity; t is a small cup for the reception of mercury, by which, and the cavity a , at the top of the zinc rod, various connexions of the copper and zinc, of the different cells, may be made by means of wires proceeding from one to the other.

In fig. 33 the ten cells are represented as connected in single series, the zinc of one with the copper of the next. They stand upon a small table in a circle, with the apertures of the siphon-tubes turned inwards, surrounding a large funnel, communicating with the basin underneath, for the reception of any liquid which may overflow. A smaller funnel is supported over the internal cavity of each cell by a ring sliding upon rods of brass placed between each pair of cells. One of these only is shown in the drawing to avoid the crowding of the sketch.”

In the preceding construction, Mr Daniell had two main objects in view. 1. To remove out of the circuit the oxide of zinc as soon as its solution is formed; and, 2. To absorb the hydrogen evolved upon the copper without precipitating any substance injurious to the latter. The first of these objects is completely effected by suspending the rod in the membranous cell, into which fresh acidulated water is allowed to drop slowly from the funnel above, whi1st the heavier solution of the oxide is withdrawn from the bottom at an equal rate by the siphon-tube, g h i j k. The second object was attained by charging the space round the membrane with a saturated solution of sulphate of copper instead of dilute acid. When the circuit was completed the current passed freely through this solution, no hydrogen appeared upon the conducting plate, but a beautiful thick coating of pure copper was precipitated upon it, thus perpetually renewing its surface.

Notwithstanding these charges, there was still a gradual, though very slow, decline in the force of the battery, which Mr Daniell traced to the weakening of the saline solution by the precipitation of the copper, and consequent decline of its conducting power. In order to remedy this defect, he suspended some solid sulphate of copper in small muslin bags, which just dipped below the surface of the solution in the cylinder, and kept it in a state of saturation by its gradual dissolution. With this improvement the voltaic current became perfectly steady for six hours together. An improvement upon this arrangement is shewn in fig. 34, where a c f h, is a perforated colander of copper, into which, instead of muslin bags, the sulphate of copper is placed. The central collar, b d e g, rests by a small ledge upon the rim of the cylinder. The membrane is then drawn through the collar, and, after being turned over its edge. It is fastened with twine.

Professor Daniell having found it of great advantage to increase the number of cells, he now places them in two parallel lines, of ten each, upon a long table, the siphon-tubes being disposed opposite each other, and hanging over a small gutter, placed between the rows to carry off the refuse solution when the acid requires to be changed; and as a uniform action may be kept up by occasionally adding a small quantity of fresh liquid, he now dispenses with the dripping funnels.

Professor Daniell considers a battery of twenty cells as amply sufficient for all the purposes of demonstration and investigation. It keeps eight inches of platinum wire, 1 / 100 th of an inch in diameter, permanently red hot in the open air, and it is even an economical source of the purest oxygen for laboratory purposes. For this latter purpose he has fitted up a cell by inclosing a platinum plate, instead of the zinc rod, within the membranous tube, which is closed at the upper end by a glass tube bent in a convenient form to deliver the disengaged gas under a receiver. When this cell is included in the circuit of double cells, the hydrogen is absorbed as formerly by the oxide of copper, but the oxygen is evolved from the platina at the rate of eighty-four cubic inches in the hour.

In a subsequent paper on voltaic combinations^[9. Phil. Trans, vol. xxxvii. p. 119, &c ] Professor Daniell found that the power of the battery was greatly increased by an increase of temperature. Having dissolved the sulphate of copper in standard acid, in place of water, the battery produced thirteen in place of eleven cubic inches of mixed gases every five minutes. On another occasion, he added one part of sulphuric acid to eight parts of the saturated solution of the sulphate, and poured it into the cells, when of the high temperature produced[7:21:671] by the disengagement of heat during the mixture, which was about 110°. The battery afforded at first twenty-two inches of the mixed gases in five minutes.

Wishing now to try the effect of higher temperatures, he replaced the membranous tubes with cylinders of porous earthenware. These cylinders, closed at the lower ends, had their diameter one and a half inches, and the same height as the copper cells. The bottoms of the latter are fitted with sockets in which the tubes are placed, and which confine them in their proper position, the perforated colanders, which hold the sulphate of copper, passing over their upper ends. These porous tubes require to be thoroughly soaked in dilute acid. The increase of temperature was obtained from steam, and the general result of many experiments was, that the working rate of this battery was nearly doubled at a temperature of 212°, provided no secondary action interfered with it.^[10. The experiments of Marianini and Rogers on the influence of heat upon single voltaic circuits will be found in the Annales de Chimie, tom. miii. p. 182, and Sillimans' Journal, vol. xxvii. p. 67, January 1835. In Roger’s experiments the deflection of the galvanometer, rose from 70° to 147° while the temperature rose from 75“ to 210“. ]

In the interesting paper which contains these observations, Professor Daniell has described an improved constant battery of large dimensions, the effects of which exceeded his most sanguine expectation, and which he thinks cannot be farther improved in point of simplicity and cheapness. This battery consists of ten copper cells, twenty inches high, and three and a half inches diameter. The interior partitions are formed by merely tying the open ends of the oxen’s gullets to the rings of the colanders which hold the blue vitriol, and which are made deeper than before, and suspending them in the cells, to the bottoms of which they reach. Each bag contains rather more than a quart of the dilute acid. The zinc rods are five-8ths of an inch in diameter, and well amalgamated, and their connexions the same as formerly. At the temperature of 67° this battery produces, in the voltameter, twelve cubic inches of the mixed gases per minute, or 720 in the hour. It has great power of ignition, and while it will maintain six inches of platinum wire, 1 / 100 of an inch in diameter, red hot. It will still decompose water at the rate of fourteen cubic inches in jive minutes. When the battery is not in use, the zinc rods are taken out and wiped, and the membranous bags carefully lifted out of their cells, emptied of their acid, filled with water, and suspended from a frame placed for their reception. Professor Daniell adds, that there is no reason to think that the limits of efficiency have yet been nearly attained, and the gullets could easily be connected together so as to obtain bags of any required length. Professor Daniell has more recently put in action seventy series of his large constant battery, which, on the 16th February 1839, fused titanium, and heated sixteen feet four inches of No. 20 platinum wire.

Another form of the constant battery we owe to J. W. Mullins, Esq., M.P., who calls it the quantity battery. It consists of an earthenware pot six inches deep and four wide, which is shewn in action in fig. 35, and in perspective in fig. 36, a cylinder of amalgamated zinc, Z, Z, standing on legs half an inch long, and cut out of the cylinder, is placed in the pot; the height of the cylinder, including the legs, is only two inches. Within this cylinder, and at the distance of three-8ths of an inch from it, is placed a copper vessel c c , having round its outer edge a rim a quarter of an inch wide, round which a thin bladder, well cleaned and moistened, is tied. The bottom of the pot rests on a circular piece of baked wood projecting a quarter of an inch beyond the cylinder. The bladder is then drawn all over and fastened round the upper rim by a cord, and it is kept clear of the copper by the circular piece of wood. The copper cylinder c c, which is as deep as the pot, is perforated with six holes equidistant from the top and bottom. These holes form communications with an inner cylinder of copper C, C, three quarters of an inch distant from the outer one. The shelf bottom of the space between the two cylinders, is on a level with the lower edge of the holes, and soldered to the large cylinder. The object of this cylindrical chamber is to hold crystals of sulphate of copper when required, and to contain the solution, which should not rise higher than the upper edge of the holes. A small quantity of sal-ammoniac, (muriate of ammonia,) in the proportion of five parts of the saturated solution to 100 of water, is then poured outside the bladder till it reaches the upper edge of the zinc cylinder Z Z. The solution of sulphate of copper will require a few crystals of the sulphate to be added every four hours, but the ammoniacal solution needs no renewal. The connexions are formed, as in fig. 36, by strips of copper soldered to the zinc cylinder Z Z, and to the inner copper cylinder C C. The wires bend over the edge of the pot and enter two cups holding mercury, from which the wires that transmit the electrical current through any apparatus, may proceed. The action of this battery will continue as long as a particle of metallic salt remains in solution. If six drops of a saturated solution of the sulphate of copper are added to the exhausted and colourless solution, the battery instantly resumes its original power. A constant current, therefore, may be kept up by having a few crystals of the blue vitriol on the shelf, which, by being gradually dissolved, will pass to the external surface of the copper.

Mr Mullins has constructed also a battery for intensity, of the effects of which he has given the following description:^[11. Lend, and Edin. Phil. Mag. 1836, vol. ix. p. 283. ] —“I have put,” he says, “as in the quantity battery, a shallow cylinder of zinc within, and close to the internal surface of the earthenware pot, next the copper cylinder, as before; but, instead of letting the inside of this cylinder go for nothing, the internal surface of the copper is lined with very thin caoutchouc for insulation; then comes another small cylinder of zinc, then a copper one lined as the last, then a zinc, and lastly, a copper cock, copper, of course, enveloped in membrane. In this battery the power is immense in proportion to the quantity of the metals used, which, in my opinion, depends upon a new principle, which is developed in this mode of construction and arrangement, that is, restricting the electric current to gradually diminishing metallic surfaces as it advances, so that, as the quantity accumulates, the conducting surfaces are reduced, and of course a much higher degree of intensity is a necessary consequence. By merely altering the connexions of the plates, which, by the mode I have adopted, can be done with the utmost facility, this battery can be turned into a powerful quantity one, and probably a wine glass full of the solution is amply sufficient.”

Having found that a single piece of zinc, of three square inches, surrounded by a membrane, could be easily fitted up, Mr Shillibear constructed the galvanic apparatus shown [7:21:672]in figs. 37 and 38. It consists of a copper trough, C C, three inches deep, and two and a quarter wide, divided into as many compartments, with copper partitions, as we wish to have zinc plates. A section of a trough, for five plates, is shewn in fig. 38. The zinc plates are soldered firmly to a copper bar, and this copper bar is fastened by a screw to a piece of hard wood which serves as a cover to the battery. The pole director, shewn at the top of fig. 37, for directing the course of the electric current, is constructed thus. In the wood cover, C C, there is cut a groove on each side of the screw B, in connexion with the zinc, and into this groove is fitted a copper slide, which carries two moveable wings, D d , Ee, which may be easily brought into contact with the copper or zinc. When the wing D d is in contact with C, and Ee with B, the current of electricity will go out from the wire in connection with the wing D d and return by the wire connected with the wing Ee, into the zinc plates through B. If we now shift the slide, so that E e is in contact with C, and D d with B, the current will be reversed, going out by the wire at Ee, and returning by the wire at Dd, to the zinc through B. In order to prevent any precipitation of the sulphate of copper upon them, the zinc plates should be lightly covered with bladder. When the trough is to be employed for sustaining a weight, the membranes should be slipped off, and diluted nitrous acid used instead of the sulphate of copper solution.^[12. Sturgeon’s Annals of Electricity, April 1887, p. 224. ]

A galvanic battery of a very simple kind, and easily constructed, has been employed by M. Bachhoffner.^[13. Id. Id. p. 214. ] A piece of thin sheet copper, coiled up in the form of a cylinder, 4 inches by 2½, is kept in that state by a fine copper wire. The cylinder is then placed in a small bladder tied round the copper with pack-thread. The bladder is open at top, and its bottom forms the bottom of the cylinder. A sheet of rolled zinc is coiled up in a similar manner, and a piece of copper wire, previously soldered to each zinc and copper cylinder, forms the connexion between them. This battery may then be placed in a jelly-pot. It is excited by a saturated solution of sulphate of copper, poured into the copper cylinder. Another solution of common salt is poured in on the outside of the copper, and in contact with the zinc. If it be required to keep the battery in action for two or three days, a few crystals of sulphate of copper must be added, to keep up the strength of the solution within the copper cylinder.

Before concluding this part of the subject, we must notice the spiral galvanic batteries. The first battery of this kind on a large scale, was made by Dr Robert Hare, who called it a calorimotor from its remarkable power of producing heat. It consists of sheets of zinc and copper formed into coils, the copper coil encircling the zinc at a distance not exceeding a quarter of an inch. The sheets of zinc were about 9 inches by 6, and the copper 14 by 6, more of the copper being necessary as shown in fig. 39, where ZZ represents the zinc, and C C the copper coil. Each coil was about 2½ inches in diameter, and they were 80 in number. All these coils were let down by means of a lever, into 80 glass jars, 2¾ diameter, and 8 inches large, containing the acid solution for exciting them.^[14. Phil. Transactions, 1823, p. 187. ]

Μ. Pouillet constructed one of these with twelve couples for the Faculty of Sciences at Paris, and found it very powerful in producing great quantities of electricity with low tension.

Mr Pepys constructed a similar instrument on a grand scale, for the London Institution, in 1822, for electro-magnetic purposes. It is represented in fig. 40, where M is the battery, C C the conductors, W the counterpoise weight, and T T the tubs, one for holding the dilute acid, and the other water. The battery M, consists of two plates of copper and zinc each fifty feet long, and two wide, exposing a superficial surface of 400 feet. They are rolled or wrapped round a cylinder of wood, with three ropes of horse hair between each fold to keep them from contact, and these ropes are kept in their position by notched sticks, occasionally introduced in the rolling. Two conductors CC, of copper about three quarters of an inch thick, are soldered to the end of each plate, from which the electric force is obtained when the instrument is in action. The battery is suspended by ropes and pullies, with a counterpoise W, to permit its immersion in a tub of diluted acid, or when not in use in a tub of water. It requires about 55 gallons of fluid, and the solution used contains about 1 / 40 th of strong nitrous acid.^[15. ‘ Trans. American Philosophical Society, vol. v. ]

A very excellent galvanic battery for producing electricity of different intensities, has been described in 1835, by Mr Joseph Henry, of New Jersey college.^[16. Silliman’s Journal, vol. iii. 1821, p. 105. ] The object of the apparatus is to exhibit most of the phenomena of galvanism, and of all those of electro-magnetism on a large [7:21:673]scale, with one battery. It consists of 88 pairs, each of which is composed of a plate of rolled zinc, 9 inches wide and 12 long, inserted in copper cases, open at top and bottom. These elements are suspended in groups of 11 pairs, or 8 sets in all; and each of the 8 troughs which are raised up to the elements, are divided into 11 cells by wooden partitions coated with cement

An electric apparatus, in which the phenomena were supposed to be independent of chemical action, was invented by J. A. de Luc, in 1809. It consists of a great number of alternations of small discs of zinc, and silvered paper, about an inch in diameter. These discs succeed each other in the following order: zinc, silver, paper; zinc, silver, paper. When from 500 to 1000 discs are enclosed in a dry glass tube, and the plates are pressed together by a brass cap and screw at each end, the pile will produce distinct electrical effects. When the columns of 1000 series each, are fitted up as in fig. 41, and placed vertically in a glass receiver, a brass ball, suspended by a thread of raw silk, will, by the action of the two piles, continually strike the two bells placed at the lower end of the piles. Mr B. M. Forster succeeded in making an apparatus analogous to the preceding, which rung continually for five months. Mr Singer made one, which rung continually for 14 months, and De Luc had a pendulum which kept vibrating for more than two years.

Mr Singer found that when the paper was perfectly dry, the pile lost all its power, and that it was deteriorated when too much moisture was present. M. Jaeger, however, observed, that when the paper, after being dried to excess, was heated by exposing the pile to a temperature of from 104° to 140°, the pile began to act as powerfully as before. When the paper is in its driest natural state, the pile is active, and it loses its activity only when the paper is subjected to a degree of heat capableof scorching it. By means of a pile of 20,000 groups of silver, zinc, and double discs of writing paper, a series of distinct sparks were obtained. A jar, having a coated disc of 50 square inches, was charged in 1.0 minutes, and gave a disagreeable shock in the elbows and shoulders. The charge of this jar fused one inch of platina wire, the five-thousandth of an inch in diameter. This pile, though exhibiting such power, did not exercise the slightest chemical action.

The two ends of the electric pile are in opposite electric states, the zinc extremity being positive, and the silver extremity negative, the middle part being in a neutral state.

M. de Luc and Μ. Hausman observed that the rays of the sun increased the power of the column, an effect which they thought was not due to heat. Mr Singer, however, found that his column was always more powerful in summer than in winter, and in a room with a fire than in one without it.

M. De Luc has shown how the dry pile may be used in determining the conducting power of bodies, and also their insulating qualities, and he has likewise employed it as an aerial electroscope for indicating the electrical changes which take place in the atmosphere.^[17. See Nicholson’s Journal, vol. xxvii. p. 81, 161,241, and also vol. xxviii. p. 5 Phil. Mag. 1810, vol. xxxv., vi. and vii. and Singer’s Elements of Electricity. ] '

In 1812, Professor Zamboni of Verona made a considerable improvement on the pile of De Luc. He dispenses entirely with the discs of zinc, and employs only discs of paper, one of whose surfaces is silvered, or rather tinned, and the other covered with a thin film of the peroxide of manganese pulverised in a mixture of milk and flour. The faces of tin are placed in contact with those of manganese, the tin being the positive, and the peroxide the negative element. M. Zamboni has been endeavouring during the last 20 years, to produce by this pile a perpetual or long-continued motion; but the motion, though often long-continued, frequently ceases for a while, and sometimes altogether, when the electric force of the apparatus has been enfeebled.

Chemists and natural philosophers had in vain endeavoured to produce chemical effects by means of the dry pile; but M. Gassiot has very recently^[18. ’ Phil. Trans., 1840, part i. p. 191, note. ] succeeded in exhibiting its chemical power. Having constructed a pile of 10,000 series of discs of laminated zinc, paper, and oxide of manganese, each about one inch in diameter, he divided it into separate piles of 1000 each. With this apparatus he succeeded in obtaining sparks which passed through the space of 1 / 10 th of an inch. When the distance of the points was 1 / 50 th of an inch, the stream of sparks was so powerful as to produce that peculiar phosphorescent odour which is always perceptible in the action of the electrical machine. With one of the piles of 1000 series, a spark passed through a space of the 12 / 5000 dth of an inch; but what was of more interest, M. Gassiot succeeded, after many trials, in obtaining chemical decomposition of a solution of iodide of potassium. He fastened about two inches of platinum wire to each end of the pile of 10,000 series, the two points of his micrometer electrometer being brought parallel to each other, so as to be about a quarter of an inch apart. A piece of bibulous paper saturated with a solution of iodide of potassium was placed on a slip of glass, and then brought into contact with the ends of the wires from each extremity of the pile. The iodine then appeared invariably on the end of the wire attached to the end of the pile, which terminated with the oxide of manganese.

The dry pile has been applied with much success by Μ. Bohnenberg, in constructing an electroscope of great delicacy. Having suspended between the two opposite poles of two piles a single strip of gold leaf, he found that this leaf, however slightly it was electrified, was drawn to one or other of the poles, according to the nature of the electricity with which it was influenced. In this way he obtained an instrument, not only sensible to small electrical influences, but capable of indicating the kind of electricity which was present.^[19. See Annales de Chim. et de Phys. tom. Xvi. p. 91, and Bibl Univ. tom. xv. p. 163 Gilbert’s Annalen der Physik, vol. xlix. ]

Before concluding this part of the subject, we shall describe some series of apparatus, which have been employed in the most recent researches, both in this country and on the continent, in conducting the important researches to which we shall afterwards refer, respecting the reduction of oxides and earths by weak electric currents. Μ. Becquerel employed only a single pair of plates, in connection with his decomposing cell. Having closed a glass tube at one end by a plug of moistened clay, he immersed it in a weak solution of common salt. The solution of the metallic salt to be decomposed, was then placed in this glass tube, and a compound metallic arc, formed of zinc and platinum, was placed in the solution in such a manner, that the platinum leg was immersed in the tube containing the metallic solution (the negative tube of BecquereD, while the zinc dips in the solution of salt. Chemical decomposition then takes place, and in a few hours or more, sometimes a few weeks, the metal appears on the plate of platinum, in a form more or less crystallised.

Dr Golding Bird, in prosecuting similar researches, has contrived a simple form of the battery, which with Becquerel’s cell, enables us to perform this class of experiments with facility and certainty. It consists of a large glass cylinder A, (tig. 42,) 8 inches deep and 2 in diameter. Within[7:21:674] this is fixed, by means of corks, another glass cylinder B, 1½ inch in diameter, and 4 inches long, and closed at one end with a plug or bottom, D of plaster of Paris, 0∙7 of an inch in diameter. A piece of sheet copper C, 6 inches long, and 3 wide, loosely coiled up, and having the conducting copper wire F, soldered to it, is placed, in the cylinder D, while an equal sized piece of zinc E, loosely coiled up, and furnished with a conducting wire G, is placed in the cylinder A. When the cylinder A is nearly filled with weak brine, and the smaller one B, with a saturated solution of sulphate of copper, the apparatus is complete; and if the fluids in the two cylinders are kept at the same level, a continuous current of electricity will be maintained for some weeks. The mode of connecting this battery with the decomposing cell, is shown in fig. 43. This cell is the counter part of the battery, consisting of two glass cylinders, A, B, the latter having a plaster of Paris bottom. The tube B, is about 3 inches long and half an inch wide, and receives the metallic or other solution to be decomposed, the outer tube A being filled with weak brine. Into this brine is plunged a strip of amalgamated zinc C, connected with the wire F, of the battery, while a strip of platinum foil D, is immersed in the metallic solution, and is connected with the wire G of the battery. This apparatus, therefore, consists of an active single battery, of which C, E, is one of the two metallic elements, and C and D the other; and the fluid between C and E, separated by the porous diaphragm D, one fluid element, and the fluid between C and D, separated by a porous diaphragm, another fluid element.^[20. See Phil. Trans. 1837, part i. p. 39—40; end Graham’s Elements of Chemistry, p. 237, 8. ]

Another instrument necessary in voltaic researches is the volta-electrometer, or voltameter, invented by Dr Faraday, for measuring the quantity of voltaic electricity, by means of the quantity of oxygen and hydrogen generated by the battery. It consists of a graduated glass tube, a , fig. 44, closed at the upper end. Platina wires b , b', terminating in two platinum plates within the tube, pass through the tube, and are fused into the glass. The tube is fitted by grinding into one of the necks of a two necked bottle. If the bottle is ½ or 2 / 3 ds full of dilute sulphuric acid. It will by enclosing the tube, flow into the tube, and fill it. When an electric current, therefore, is passed through the instrument between the plates, the evolved gases collect in the upper part of the tube, without being subject to the recombining power, of the platina, the stopper c is taken out.

By receiving the wires connected with δ, δ', replacing the stopper, and refilling the tube with the liquid by inverting the bottle, a second measure of gas may be obtained on replacing the wires at b , and b' .

Dr Faraday has given in fig. 45, another form of the voltameter, which he found very useful in experiments continued for days together, and where large quantities of indicating gas are to be collected. The gases, in place of being measured in the tube, as in fig. 44, are carried by the bent tube δ, into a graduated jar, placed in a small pneumatic trough.

CHAP. II —ON THE GENERAI. PHENOMENA AND EFFECTS OF GALVANISM.

In our article on Electricity, ^[21. Vol. viii p. 574, 5. ] we have already given a brief account of the results obtained by Dr Faraday, which established the identity of all the various kinds of electricity, and the relation by measure of ordinary and voltaic electricity, as obtained by the same distinguished philosopher, and of his new law of electrical conduction.

Notwithstanding the identity of character of common and and voltaic electricity, the effects which they produce are almost infinitely varied, some of these effects being exalted, while others are diminished. All these variations, however, are explicable by the differences in quantity and intensity of these two kinds of electricity.

In the case of ordinary electricity, a piece of glass or sealing-wax, excited by friction and kept near the cap of a gold leaf electrometer, will produce a great and instantaneous divergence of the leaves; but in voltaic electricity the same effect is not produced, even by a battery of 100 pair of plates.

When the extremities or poles of such a battery are examined by the electrometer, they are found to be positive and negative, the gold leaves repelling each other at the same pole, and attracting each other at different poles, even when above half an inch of air intervenes. Hence ordinary differs from voltaic electricity, in having a much higher degree of tension, or intensity, that is, in acting with a greater elastic force in a given direction. From this property it acts so powerfully on the electrometer, and is discharged with such facility through air, whether highly rarefied or heated. On the other hand, voltaic differs from ordinary electricity in the enormous quantity of electricity which it delevopes and puts in motion, and in the continuity or perpetual reproduction of the current.

In order to convey some idea of the immense difference in this respect of the two electricities, Dr Faraday has stated that “the chemical action of a grain of water upon four grains of zinc, can evolve electricity equal in quantity to that of a powerful thunder-storm.” That if a Leyden battery is charged with 30 turns of a large and powerful plate electrical machine in full action. It would require 800,000 such charges to supply electricity to decompose a single grain of water, or to equal the quantity which is naturally associated with the elements of that grain of water, endowing them with their mutual chemical affinity.^[22. Faraday’s Exp. Researches, p 263, 258, and 861, 873. ] Or to put the comparison differently, the quantity of electricity in 25¼ grains of water is equal to above 24 millions of charges of the Leyden battery above mentioned, or would keep any length of platina wire 1 / 104 th of an inch in diameter, red hot for an hour and a half.
[7:21:675]

We have already seen that voltaic electricity at rest, like ordinary electricity, produces attractions and repulsions. We shall therefore proceed to give an account of the effects of voltaic electricity in motion, or of voltaic currents.

Sect. I.— On the conducting Power of Solids and Fluids for Voltaic Electricity.

When a voltaic battery is in a state of activity, and when wires of different metals are placed between the poles of the battery, so as to complete the circuit, the current of electricity passes through them with different degrees of facility, that is, the different metals transmit the electricity with different degrees of resistance.

The following table contains the results obtained by Davy, Becquerel, Pouillet, &c., respecting the conducting power of different metals, for different kinds of electricity.
[table]

These results are, generally speaking, greatly at variance, the only ones that admit of comparison being those of Becquerel and Mr Snow Harris. Much depends on the purity of the metals; and Mr Harris has ascertained that the conducting power of alloys is very different from that of their component metals. This appears very distinctly from the number for brass, which bears no relation in Μ. Pouillet’s column, to the measures for copper and zinc, and also from his measure for gold of 18 karats, which we find to be only the 7th of copper, and the 6th of fine gold. The conducting power in the same metal increases directly as the area of the section of the wires, and inversely as the length of the wire.

Μ. Pouillet has found that the same law holds in liquids included in cylindrical tubes. By comparing, in this way, the conducting power of different saline solutions, the conducting wires being formed of the metal whose oxide was in solution, he found, as Fechner had done, that the intensity was rigorously in the direct ratio of the section and the inverse ratio of the conductibility. In this way he found that 433 feet of platinum wire 0·006 inch in diameter, had the same conducting power as a column of saturated solution of sulphate of copper 3¼ feet in length, and 0∙8 inch in diameter, from which it follows that the conducting power of the platinum is two million and a half times greater than that of the solution.

The following table shows the results of his observations, the conducting power of the copper solution at 59° Fahr., being taken as unity.
[table]

Μ. Marianini has obtained a great number of interesting results respecting the conducting power of water holding in solution different acids, alkalis, or salts, compared with that of distilled water, at the temperature of 3° of Reaumur. The following is a selection from his results.
[table]

The conducting power of solutions increases as the quantity of salts dissolved, but more slowly as the solution approaches to saturation. The preceding table shows that the acid solutions have the greatest, and the alkaline and neutral solutions, the least conducting power.

The relation between the contractibility of non-metallic bodies in the solid state, and that of the same bodies in the liquid state, has been investigated by Dr Faraday, with his usual ability and success. Having found that a thin plate of ice stopped the electric current, while the same current passed when the ice was converted into water, Dr Faraday examined a great number of non-metallic solids, and found that they assumed the conducting property during liquefaction, and lost it during congelation; but what was remarkable, all those bodies underwent decomposition when [7:21:676]fluid, with the single exception of the periodide of mercury, which, though it insulated when solid, and conducted when fluid, was not decomposed in the latter state. Dr Faraday found also a great variety of bodies, which acquired no conducting power in the fluid state, such as sulphur, phosphorus, &c ., and they were not decomposed in this last state.

The relation between conduction and decomposition is a very important one; but no less so is the relation of the conducting power for electricity to that for heat. “As the solid becomes a fluid,” says Dr Faraday, “it loses almost entirely the power of conduction for heat, but gains in a high degree that for electricity; but as it reverts back to the solid state. It gains the power of conducting heat, and loses that of conducting electricity.”

Dr Faraday has given the following summary of the conditions of electric conduction in bodies:—

“1. All bodies conduct electricity in the same manner from metals to lac and gases, but in very different degrees.

“2. Conducting power is in some bodies powerfully increased by heat (such as in sulphuret of silver, fluoride of lead, periodide of mercury, and corrosive sublimate), and in others diminished, yet without our perceiving any accompanying essential electrical difference, either in the bodies, or in the changes occasioned by the electricity conducted.

“3. A numerous class of bodies, insulating electricity of low intensity, when solid, conduct it very freely when fluid, and are then decomposed by it.

“4. But there are many fluid bodies which do not sensibly conduct electricity of this low intensity; there are some which conduct it, and are not decomposed, nor is fluidity essential to decomposition.

“5. There is but one body yet discovered (periodide of mercury, to which Dr Faraday subsequently added corrosive sublimate), which, insulating a voltaic current when solid, and conducting it when fluid, is not decomposed in the latter case.

“6. There is no strict electrical distinction of conduction which can, as yet, be drawn between bodies supposed to be elementary, and those known to be compounds.”^[28. Researches, § 444-449. ]

Sir Humphry Davy^[29. Phil. Trans. 1821, p. 431. ] has shewn, that, as a class, metals have their conducting power diminished by heat; and Mr Snow Harris has proved, that heat does affect gaseous bodies, or at least air.^[30. Id. 1834, p. 230. ] Sect. II.— On the Intensity and Direction of Voltaic Currents.

The two electricities of the pile, when disengaged by the chemical action of its elements, tend continually to reunite and form a neutral fluid, by entering the conducting bodies in their vicinity. The quantity of electricity which remains free, constitutes the tension of the pile, or the intensity of the current, as we have already explained.

The tension of the pile is affected by various causes. When its two extremities are united by a metallic arc, the tension at first diminishes rapidly, but the diminution becomes slower and slower, till it reaches its limit, beyond which the tension no longer decreases, however great is the length of time during which the circuit is closed.

The loss of tension in a given time increases with the number of voltaic couples, and the pile is longer in reaching the limit beyond which the tension does not decrease.

The loss of tension is more rapid when the liquid exercises a more powerful chemical action on the oxidable metal of the voltaic couple, and the longer it is in reaching the limit of decrease. In these experiments, which we owe to Marianini, the conducting power of the metallic arc has no influence.

The electric tension lost under the preceding circum stances, is again restored by opening the pile, that is, reversing the metallic arc which united its extremities; but it requires more time to recover its primitive tension, the longer the circuit has been closed.

In studying the change produced upon the tension when the circuit is not closed by a metallic arc, M. Marianini found, that a battery with new plates loses less tension in a given time, than one with oxidated plates, the new apparatus reaching its limit sooner than the old one. In piles consisting of gold and zinc couples, tension diminishes more rapidly than in the ordinary pile, and reaches its limit in a very short time; whi1st in piles with lead and zinc, the tension diminishes less rapidly than with copper and zinc.

The direction as well as the intensity of electric currents, depends on the degree of chemical action exerted by the liquid on one of the metals, particularly the most oxidable one. When the liquid and the metals are therefore known, the direction of the current can be predicted. The metal most acted upon takes away from the liquid negative electricity. Zinc, for example, is more attacked by brine than copper, and therefore takes from the brine its negative electricity; but if the liquid is a solution of sulphuret of potassium, which affects the copper more than the zinc, the copper will take from the solution its negative electricity, and the current will take an opposite direction. Hence, in a pile with brine, the zinc extremity will give positive electricity; whereas, in a pile with a solution of sulphuret of potassium, the zinc extremity will give negative electricity.

M. Delarive obtained the following interesting results, by immersing different voltaic couples in nitric acid of different strengths. Each metal in the two columns is positive in relation to the one which precedes it.

In Concentrated Nitric Acid.

Oxidated iron.

Silver.

Mercury.

Lead.

Copper.

Iron.

Zinc.

Tin.

In Dilute Nitric Acid. Silver. Copper. Oxidated iron. Iron. Lead. Mercury. Tin.

Zinc.

Μ. Becquerel mentions arsenic and iron as a remarkable example of a change of polarity produced by the chemical action of the fluid. With a voltaic couple of iron and arsenic, the iron is strongly positive compared with the arsenic, when they are immersed in diluted acid, which acts slightly upon the arsenic; but when they are immersed in potash kept in fusion, the arsenic, upon which the potash acts powerfully, becomes positive.

M. Delarive has illustrated this branch of science with a number of valuable experiments on the changes produced upon electric currents, while passing through liquid conductors, interrupted by metallic plates. The following are the results at which he arrived :—

1. One or more metallic laminae (platina), placed perpendicularly to the direction of the electric current, in a liquid conductor, diminish the intensity of the current which traverses them.

2. This diminution is almost nothing when the electric current is very energetic, and proceeds from a pile containing a great number of couples; but the intensity diminishes the more rapidly in passing through the same number of plates, as its original intensity is less energetic.

3. If one or two electrical currents of the same intensity, the one an original one, and the other one which has previously passed through several metallic plates, the first will have its intensity much more diminished by the interposition of a plate than the second. Hence, these currents have [7:21:677] the same property, (not by polarisation, as stated by BecquereD, as light and heat which have passed through absorbing media.^[31. Μ. Becquerel, to whose excellent work we are indebted for the latest and best information on this subject, asks, “if we will from analogy say, that the current, in traversing different plates, acquires a polarisation similar to that of light.” Certainly not. When while light passes through a certain thickness of any coloured medium, or any similar number of coloured plates. It loses a certain portion of its rays, say 9-10ths, and the transmitted light is red. Now, this red light may be transmitted through the same number of the same plates, and yet not lose 1-10th of its light; but there is no polarisation. The first plates absorbed all the rays but the red. and having the property of transmitting the red rays freely and more copiously than any other, they passed through in greater abundance. As these transmissions are all at a vertical incidence, polarisation can have nothing to do with the matter. ] We have no doubt, therefore, that currents which have passed through plates of metals, will have thus acquired, or have possessed previous to their separation, other properties than that of passing more freely through other metallic plates.

From the preceding results, M. Delarive has explained the difference between the effects of a pile with a small number of couples, and a pile with a larger number. The first produces more easily the effects which took place when the circuit is closed by a very good conductor, while the second may be best employed in producing phenomena which take place in the circuit of an imperfect conductor, such as a fluid, the transmitted currents in the latter case having acquired, in passing through a greater number of plates, a greater facility of traversing an imperfect conductor.

As the intensity of the electric current had been found to increase with the surface, acted upon by the fluid, M. Delarive endeavoured to determine the law of increase. He found that an increase of surface facilitated the transmission of the current; that the augmentation of intensity produced by a greater extent of surface, increased in a greater ratio than the surface when the current is feeble, and in a less ratio when the current is intense; so that we gain more by increasing the surface when the current is weak than when it is strong.

With regard to the influence of fluid conductors in diminishing the intensity of the electric current, Μ. Delarive found that nitric acid diminishes the intensity least, then muriatic acid, then sulphuric acid. Nitric acid, pure and greatly diluted, produces a less diminution than concentrated acid. The contrary takes place with sulphuric acid, which is a bad conductor. The silver solution came next, then potash and ammonia, which differ little from each other.

For further information on the subject of this section, see Marianini, Saggio di Experience Electromotriche, Venice, 1825; Ann. de chirn. er de Phys. tom. xxxvi. p. 33; xxxvii. p. 256; xxxviii. pp. 49, 337; xlv. p. 2; Delarive’s Esquisse Historique des Principales decouvertes faites dans l’Electricite, Geneva, 1823; or in the Bibl. Universelle for 1833. See especially Becquerel’s Traite de l’Elect, et Magnet, tom. iii.

Sect. III.— On the Production of Light, Heat, and cold by Voltaic Electricity—the Ignition of Wires.

The phenomena of light and heat, and the ignition of metals and wires by ordinary Electricity, having been fully described under that article, we shall here confine ourselves to the analogous phenomena produced by voltaic currents.

Soon after the discovery of the pile, Van Marum, Pfaff, and Tromsdorff discovered that thin leaves and fine wires of metal placed between the poles of a pile, became incandescent. and even burned while conducting the electrical current; and some time afterwards, M.M. Fourcroy, Vauquelin, and Thenard observed that piles with large plates deflagrated metals more powerfully than piles with a great number of plates of smaller surfaces.

It was in England, however, that the calorific and luminous effects of the pile were principally developed. In 1813, the immense battery of the Royal Institution, composed of 2000 couples, and exposing 28,000 square inches, enabled Sir H. Davy to produce light and heat of the highest intensity. When the ends of the wire from each pile terminated in two charcoal points, the most dazzling light passed from the one to the other, and continued for several hours. Platina, sapphire, quartz, and lime, &c., when exposed to this source of heat, were instantly melted, and the diamond and charcoal disappeared, as if they were completely volatilised; and these effects were produced in vacuo as well as in air.

By means of the splendid battery described in a preceding part of this article, Mr Children obtained many important and curious results. His experiments commenced in 1809, but it was in 1815 that he brought into play the powerful instrument which we have already mentioned. He found that metallic wires connecting the two piles of the battery became red-hot in the following order :—

Platinum. Copper. Zinc. Iron. Gold. Silver.

And hence Mr Children concluded, that the conducting power of these metals was in the inverse order, silver being the most conducting, and platinum the least.

With this battery, five feet six inches of platinum wire, 0∙11 inch in diameter, were brought by Mr Children to a red heat throughout, so as to be visible in day-light.

Eight feet six inches of platinum wire, 0∙44 inch in diameter, were heated red.

A bar of platinum 1 / 6 th of an inch square, and 2∙25 inches long, was heated red, and fused at the end.

The oxides of tungsten, uranium, cerium, titanium, and molybdenium, were fused. Having filled an opening in an iron wire with diamond powder, the diamond disappeared, and the iron was converted into steel.^[32. ‘ See Phil. Trans. 1815. ]

At the same time that Mr Children was constructing the greatest voltaic battery ever made, Dr Wollaston was occupied in constructing the smallest. He took a small thimble, and having removed the bottom, he flattened the remaining cylinder, till its sides were about 1 / 6 th of an inch distant. He then placed between these two surfaces a small plate of zinc, which did not touch either side of the thimble. With a platinum wire about 1 / 40 th of an inch long, and 1 / 3000 dth of an inch in diameter, he united externally the plate of zinc with this thimble; and when this little galvanic couple was immersed in acidulated water, the platinum wire became red-hot, and was melted! This important result led Dr Wollaston to the valuable conclusion, that in order to obtain powerful calorific effects, we must increase the surface of the copper in negative metal.

In repeating the experiments of Davy on the light developed by charcoal points, Μ. Brandes discovered that this light, like that of the sun, affected the combination of chlorine and hydrogen, and the decomposition of muriate of silver and other bodies.

By means of the powerful voltaic battery, which Dr Hare calls a deflagrator, and which we have already described, this able chemist obtained some splendid results. A brilliant light, equal to that of the sun, was produced between charcoal points; and plumbago and charcoal were fused by Professors Silliman and Griscom. By a series of 250, baryta was deflagrated; and a platina wire, 3 / 16 ths of an inch in thickness, “was made to flow like water.” In the experiments with charcoal, the charcoal on the copper side had no appearance of fusion, but a crater-shaped cavity[7:21:678] was formed within it, indicating that the charcoal was volatilised at this side, and transferred to the other, where it was condensed and fused, the piece of charcoal at this pile being elongated considerably. This fused charcoal was four times denser than before fusion.

Owing to its superior conducting power, a continued voltaic current will maintain, in a state of incandescence, a greater length of silver wire than of platinum or iron; but if we form a wire of short pieces of silver and platinum wire alternately, the platinum portion will become red-hot, while the silver ones remain cold. In this case, the current which passes readily along the silver wire, encounters the degree of obstruction in the platinum which produces the red heat. This fact is no doubt connected with the very remarkable one observed by M. Peltier, in the passage of weak currents through metallic circuits, where cold was produced at the points of junction of certain crystallizable metals.

Sect. IV.— On the chemical Effects of Voltaic Electricity.

In a preceding article we have given a full account of the general chemical effects of ordinary electricity. We shall therefore confine ourselves at present to the chemical effects of the voltaic pile. No sooner was this apparatus made known in England, than Messrs Nicholson and Carlisle applied it to chemical inquiries. Although Volta had inferred from the shock, that the action of the pile was electrical, yet it was to the above inquirers that we are indebted for determining by means of the revolving doubler, that the silver end of the battery was in a negative, and the zinc end in a positive state of electricity. In the course of their experiments, they observed a disengagement of gas, which smelt of hydrogen, from water which happened to be in the circuit; and on the 2d of May 1800, they discovered that water was decomposed into its elements, viz. oxygen and hydrogen, when the water formed part of the circuit between the positive and negative ends of the pile. Mr Cruickshanks of Woolwich confirmed this result, and found that hydrogen was always emitted from the silver or copper end of the pile, and oxygen from the other. He discovered also the important fact, that metals could be revived from their solutions, under the same circumstances, the reduced metal being deposited at the end of the wire; and he succeeded in decomposing some of the neutral salts. Dr Henry decomposed the nitric and sulphuric acids and ammonia; and he reduced the oxymuriatic to the state of muriatic acid.

The attention of our illustrious countryman, Sir H. Davy, was about this time attracted to the subject. So early as 1802, he had made experiments on the chemical agency of the pile; but in 1806, in his first Bakerian Lecture, he was led to the conclusion, that chemical attraction and repulsion were produced by the same cause, acting in the one case on particles, in the other on masses, and that the same property, under different circumstances, was the cause of all the phenomena exhibitedby differ entvoltaic combinations. In October 1807, he decomposed potash and soda, and proved that they were oxides of two new metals, potassium and sodium. With a voltaic battery of 2000 plates, he decomposed several of the earths, and discovered their metallic bases, barium, strontium, calcium, and magnesium. In attempting to decompose the proper earths, he was less successful. He succeeded in proving, however, that they consist of bases united to oxygen, but the completion of the inquiry was left to Wohler, Bussy, and Berzelius, who found that all the bases of these earths, except silica, were metallic, and capable of uniting with iron.^[33. See our articles Chemistry and Davy, for a full account of Sir Humphry Davy’s electro-chemical researches. ]

Our narrow limits will not permit us to give an account of the successive labours of different philosophers, in effecting decompositions by the voltaic battery. We shall content ourselves with giving a brief account of the researches of our distinguished countryman, Dr Faraday, to whom this branch of science owes its greatest acquisitions.

The phenomena of electro-chemical decomposition have been generally ascribed to two opposite powers residing in the two poles of the voltaic battery. Grotthus^[34. Ann. de Chim. 1806, tom. lviii. p. 61, ] regards the pile as an electric magnet with attracting and repelling poles, the one attracting hydrogen and repelling oxygen, and the other attracting oxygen and repelling hydrogen. The force exerted upon each molecule of the body is supposed to be inversely as its distance from the poles, and a succession of decompositions and recompositions is supposed to exist among the intervening molecules. Sir H. Davy adopts the idea of attractions at the poles, diminishing to the middle or neutral points, and he thinks a succession of decompositions and recompositions probable. Messrs Riffault and Champre regard the negative current as collecting and carrying the acids on to they positive pole, and the positive current as doing the same, with the bases toward* the negative pole. Μ. Biot attributes the effects to the opposite electrical states of the decomposing substances in the vicinity of the two poles. M. de la Rive considers the portions decomposed to be those contiguous to both poles, the current from the positive pole combining with the hydrogen or the bases which are there present, and leaving the oxygen or acids at liberty, but carrying the substances in union with it across to the negative pole, where it is separated from them, entering the conducting metal, and leaving on its surface the hydrogen, or its bases. Dr Faraday regards the poles as exercising no specific action, but merely as surfaces or doors by which the electricity enters into or passes out of the substance undergoing decomposition. He supposes that “the effects are due to a modification of the electric current, and the chemical affinity of the particles through or by which that current is passing, giving them the power of acting more forcibly in one direction than in another, and consequently making them travel by a series of successive decompositions and recompositions in opposite directions, and finally causing their expulsion or exclusion at the boundaries of the body under decomposition in the direction of the current, and that in larger or smaller quantities, according as the current is more or less powerful.

In resolving a compound body into its elements, liquidity is an essential condition of the body. A plate of iron, the 16th of an inch thick, placed between the two sides of the pile, will stop completely the most powerful electrical current.

When the elements of a body are separated by electric action, the current communicates to each a definite direction, the oxygen travelling towards the zinc, and the hydrogen towards the platina pole.

By an irresistible body of evidence, Dr Faraday has established the important proposition, “that the chemical power of a current of electricity is in direct proportion to the absolute quantity which passes and this is true of all bodies capable of electro-chemical decomposition.

The same eminent philosopher has also deduced, from a variety of facts, the following conclusion, “that the quantity of electricity, which, being naturally associated with the particles of matter, gives them their combining power, is able, when thrown into a current, to separate these particles from their state of combination; or, in other words, that the electricity which decomposes, and that which is evolved by the decomposition of a certain quantity of matter, are alike.” According to this theory, “the equivalent weights of bodies are simply those quantities of them which contain equal quantities of electricity, or have naturally equal electric[7:21:679] powers. It being the electricity which determines the equivalent number, because it determines the combining force; or if we adopt the atomic theory or phraseology, then the atoms of bodies which are equivalents to each other in their ordinary chemical action, have equal quantities of electricity naturally associated with them.”

When exposed to the voltaic current, bodies are decomposed with different degrees of facility. Dr Faraday found by experiment, that the following bodies were decomposed with currents of different intensities, those at the top of the list being decomposed by currents of lowest intensity.
[table]

All compound bodies are not decomposable by electric currents. The following bodies are not decomposed under ordinary circumstances
[table]

The following bodies are not decomposed :—
[table]

All solid non-conductors which become conductors when liquified by heat, with the exception of periodide of mercury, are decomposed.

Mr Faraday is of opinion that all binary compounds, one of whose elements goes to the negative, and the other to the positive pole, are decomposable, but not ternary compounds.

The following bodies, being non-conductors of electricity, are not decomposed by it.
[table]

The perchlorides of mercury and of antimony, being conductors when liquid, are decomposable, but periodide of mercury’, though a conductor when liquid, is not decomposable.

The following bodies are not decomposable by voltaic electricity:—
[table]

The following table contains a list of the elementary constituents of decomposable bodies, with their electrochemical equivalents^[35. Faraday’s Experimental Researches, p. 247, § 846. ]:—

Elements which go to the Positive Pole.
[table]

Elements which go to the Negative Pole.
[table]
[table]

In the course of his electrical researches, Dr Faraday^[36. Id. Id. p. 195, § 564, &c. ] discovered the very remarkable fact, that metals and other bodies had the power of promoting the combination of gaseous bodies. When a plate of clean platinum was put into a mixture of oxygen and hydrogen gas, the two gases gradually disappeared, in consequence of having united and formed water. When the platinum plate was made very clean, by placing it in sulphuric acid. It acted with such energy on the gases, that the tube became warm, the platina became red-hot, and the residue of the gases was inflamed. A solution of tartaric or acetic acids gave the platinum plate the power of producing explosion in the mixed gases, but strong sulphuric acid was most certain and powerful. Gold and palladium, when acted on by hot oil of vitriol, possess also the power of combining oxygen and hydrogen.

Dobereiner had previously discovered the remarkable property possessed by platinum, which, in the state of a fine black powder, or spongy, became hot, and ignited a jet of hydrogen. This is the well known instantaneous light apparatus. The following is the theory of these remarkable facts. The particles of hydrogen repel each other, so do those of oxygen; but the strong adhesion of the gaseous particles to the platina suspends, as it were, upon its surface, the above repulsive forces, and brings the particles of oxygen and hydrogen within the influence of their mutual affinity.

Spongy platinum also decomposes ammonia, and its salts when mixed with atmospheric air, and passed over the metal at 572° of Fahr. Non-metallic bodies, such as pounded glass and charcoal, have, at 600° Fahr., the same property as platinum.

Sect. V.— On Electro-chemical Decompositions by weak Electric currents.

The precipitation of metals from their solutions, by the presence of other metals, has been long known. A plate of copper, for example, will throw down metallic silver from a solution of the nitrate. Ritter, Sylvester, and Bucholz found that these precipitations were owing to electric currents, and obtained some interesting results. It is to Μ. Becquerel, however, a distinguished member of the lnsti-tute of France, that we owe the successful investigation of this curious subject. In 1826, he found that, by very feeble electric forces, metals easily reducible were precipitated from their solutions, by plates of the same metal as that held ln solution. In 1627, he formed chlorides and iodides in the same way, and also double chlorides and double iodides. In 1829, he succeeded in forming sulphureis, iodides, and bromides by similar methods; and when the electric energy of the apparatus was exceedingly feeble, and the decomposition slow, the sulphurets had time to assume a regular crystalline form, and he thus obtained crystals of almost all the metals. In a similar manner, he obtained distinct crystals of the metallic iodides.

In 1830, M. Becquerel found that new compounds were formed during these processes, by the reaction of the oxide of the metal at the positive pole of a solution. By the reaction of two solutions, one of which was the sulpho-carbonate of potash, and the other the sulphate of copper, and by employing an arc of copper and lead, the copper plunging into the sulphate, and the lead into the sulpho-carbonate, he succeeded in depositing on the lead small octahedral crystals of sulphur, with rhomboidal faces, exactly si.
[7:21:680]

milar to the natural crystals of this substance. In this manner, by a skilful combination of solutions and metallic arcs, and a profound knowledge of the reactions which arise from the contact of these different substances, he succeeded in obtaining, in a crystalline state, compounds which had never been procured under that form.

Μ. Becquerel has been equally successful in reducing the bases of certain oxides, and in obtaining immediately from their solutions, in their metallic, and even crystallised state, iron, zirconium, glucium, and magnesium. In 1832, he also obtained crystals of metallic oxides, such as those of the anhydrous black oxide of copper, of the red oxide, of the protoxide and peroxide of lead, and the oxide of cobalt.

The same distinguished philosopher has employed the effects of weak electric forces to explain the process of cementation, by which iron is converted into steel, by the combination of carbon with all the interior molecules of the iron; and that remarkable mineralogical process by which the elements of many rocks are transferred from within to without, and replaced by others without any disintegration. The process of cementation is the consequence of the opposite electric state of the carbon and the iron, which, with the aid of high temperature, produces electric currents that convey the atoms of carbon from molecule to molecule to the very interior of the iron. In like manner, pieces of iron, which have been buried in the metallic state, are almost wholly oxidated; and ancient copper medals have been found changed entirely into the protoxide of copper.

By studying this class of phenomena, Μ. Becquerel has been led to an explanation of the phenomena of phosphorescence, which he ascribes to the recomposition of the natural electricities of each molecule, which have been separated by heat or some other cause. The restoration of phosphorescence to bodies that have lost this property, or that never possessed it, by repeated electrical discharges, as ascertained by Mr Pearsal, confirms the ingenious explanation of M. Becquerel.^[37. See Ann. de Chimie et de Phys. tom. xxxiv. p. 152; xxxv. 126; xlii. 225; xliii. 131; xlvii. 5, 13; xlviii. 337; lii. 181; liii. 105, 243. See also Becquerel’s Traité de l’Electricité, &c. tom. iv. v. ] '

Our countryman, Mr Andrew Crosse of Bromfield, by processes differing from those of Becquerel, arrived at similar results, not long after the publication of the discoveries of the French philosopher. There can be no doubt, however, that Mr Crosse was unacquainted with these discoveries; and that his results were entirely independent of them. By means of voltaic electricity, he obtained the following substances, namely, calcareous spar, arragonite, quartz, red oxide of copper, arseniate of copper, blue carbonate of copper, phosphate of copper, sulphuret of copper, carbonate of lead, sulphuret of silver, carbonate of zinc, chalcedony, oxide of tin, yellow oxide of lead, the sulphurets of antimony and zinc and iron, protoxide of iron, and crystals of sulphur.

Sect. VI On the coloured Bings formed on Polished Metallic Plates by Voltaic currents.

This new class of phenomena are remarkable from their beauty and singularity, as discovered by Μ. Nobili. They may be produced by a small battery like that of Wollaston, with twelve elements of an inch square, in the manner shown in fig. 46. A small apparatus, not shown in the figure, is constructed so as to move up and down the pincers R S, which hold two pieces of large platinum wire P N, pointed at their extremity, the one communicating with the positive, and the other with the negative pole of the battery. A polished metallic plate A B, intended to receive the coloured rings, is placed horizontally in a saucer or plate which contains the fluid to be used, suppose a solution of sulphate of copper.

When the solution of copper is poured into the saucer above the silver plate AB, and the point of one of the wires N brought as near as possible to n, while the other point p is plunged in the solution, there will be formed round the point n, four or five concentric circles, alternately bright and dark. When the point ρ is used, there will be formed round it three small circles of copper. The two extreme circles are of a deep red colour, and the middle one of a higher colour; sometimes four or five are formed, which alternate like the preceding.

When a solution of acetate of lead is used, and the plate of gold or silver is positive, while the point is negative, the concentric rings are as brilliant as the coloured rings of Newton. When we increase the number of negative points, there are formed as many systems as there are points of concentric wires or rings which never cross each other, but which, when they meet, extend outwards so as to form only one exterior ring.

The effects are the reverse with a solution of acetate of copper, that is, the concentric rings are formed only when the plates are negative, while positive plates exhibit nothing remarkable.

When the fluid is urine and the plate silver and positive, several orders of very brilliant-coloured rings are formed round a dark centre. Μ. Nobili has obtained very remarkable effects of colour by using different animal substances, such as milk, white and yoke of an egg, saliva, &c. and vegetable substances, such as the juice of carrots, onions, parsley, grapes, apples, &c. Animal and vegetable substances yield colours more beautiful and brilliant than ordinary chemical solutions. The colours produced by the leaves of plants are more marked than those arising from the decomposition of the roots. In some chemical solutions the phenomena are equally beautiful on the positive as on the negative plate; but when the circles meet, the two figures experience, as it were, a sort of compression, and when the coloured rings are impressed only on one plate, they may be made to disappear, if not wholly, at least in part, by inverting the direction of the current.

Μ. Nobili has drawn the following conclusions from a great number of experiments:—1. Certain electro-negative substances possess the property, in some circumstances, of attaching themselves to the surface of some of the less oxidable metals, in layers so thin and regular as to exhibit under an infinity of varied forms, the beautiful phenomena of coloured rings. 2. That when electro-negative substances do not deposit themselves in thin layers on polished metals, they attack their surface, not uniformly, but at regular intervals.

The most varied and remarkable appearances were obtained by Μ. Nobili when the polished metal was positive; but he succeeded in obtaining equally remarkable phenomena when the metal was negative, either by augmenting the force of the current, or by using compound solutions.

He took, for example, a mixture of acetate of copper and nitrate of potash, and upon a negative plate of silver he formed a series of concentric rings, the centre of which retained the metallic brilliancy. The two next circles were green, then came rings of white, red, and green, then a zone of copper of a fine red colour. This zone was surrounded with a blue circle, marked with radiating lines, like a graduated circumference. Its rays extending even to the circle of copper: Then came a second copper zone wider than the first, but equally brilliant, surrounded with a circle of beautiful green, which terminated the figure. The same appearances were obtained on gold and platina.^[38. See Ann. de Chim. &c. tom. xciv. p. 210, and Becquerel s Traité, &c. tom. iii. pp. 274—287. ]
[7:21:681]

Sect. VII— On the Physiological Effects of Voltaic Electricity.

The effect of voltaic electricity on animal bodies is analogous to that of ordinary electricity, a subject which we have already treated at considerable length.^[39. Se Electricity, vol. viii. p. 609, &c. and p. 638, &c. ]

The galvanic shock is not conveyed through the skin of the human hand in the same manner as the electric shock. This arises from its low intensity, in consequence of which it can be transmitted only through good conductors. The best way is to grasp with both hands wet, two silver spoons, or two metallic belts, and by means of them form a connection between the poles of the battery.

A luminous spark is produced by voltaic electricity, when the eye forms part of the circuit. This may be done by placing a piece of silver between the gums and the upper lip, and inserting a silver probe into the nostrils. If a piece of zinc is then laid upon the tongue, and the two metals brought into contact, the flash will be seen. This, and other affections of the eye, were observed by Ritter, who declares that when the positive pole was inserted, in his eye he saw objects darker and less, and when the negative pole acted upon the eye, he saw the same objects brighter and larger. But according to Purkinje, the only difference is, that the positive pole excites less light than the negative pole. Purkinje also observed, that by the application of the positive pole to the eye, a yellow light was excited, while the negative pole excited a violet light, brighter and more abundant than the yellow. He also observed that the luminosity was excited principally at the base of the optic nerve, and at the foramen centrale of the retina. With the positive pole the base of the nerve exhibited a bright violet light, and the dark foramen was surrounded with a double rhomboidal limb of yellow light; but with the negative pole the base of the nerve was black, and the foramen was violet, and surrounded with a violet rhomboidal limb, at a little distance from the foramen. He noticed also, that when the voltaic circuit was broken, the preceding colours passed into their opposite or complementary ones.

If a living leech, or an earthworm, is placed upon a crown piece, laid upon a piece of zinc of a larger size. It experiences no uneasiness while it touches the silver only; but when it stretches itself, and touches the zinc. It instantly draws itself back, as if it had received a shock.

The influence of voltaic electricity upon the muscles of animals after death, is very remarkable. This subject has been recently investigated by Marianini, Nobili, Peltier, and Becquerel; but our limits will not permit us to give even the shortest account of their labours.^[40. See Becquerel’s Traité, &c, tom. iv. p. 211—255, for full and interesting details on this branch of the subject. ] We must content ourselves with mentioning a few interesting facts. If the negative and positive wires are inserted in the ears of an ox or sheep taken from the body of the animal recently killed, strong convulsive motions will be excited in the muscles of the face, whenever the circuit is completed, provided the battery have an hundred pair of plates. Lite seems to be restored, and the animal to be under great suffering. The eyes open and shut, and roll in their sockets; the pupils dilate; the nostrils expand and vibrate; and the jaws move as in mastication. If a horse is subjected to powerful galvanic action, when recently killed, the struggles of its limbs can scarcely be restrained by several persons.

Similar experiments were made in Glasgow in 1811, by Dr Ure, on the body of a criminal after execution. He used a battery of 270 pair of four inch plates. When the spinal marrow and sciatic nerve were made the points of communication with the positive and negative poles, the whole body shudderea as with cold. The left side was most powerfully convulsed; and upon moving one of the rods from the hip to the heel, the knee being previously bent, the leg was thrown out with such violence as nearly to knock over one of the assistants. By acting upon the nerves connected with the respiratory system, a laborious breathing instantly commenced, and the chest heaved and sunk. When a communication was made between the super-orbital nerve and the heels, “most extraordinary grimaces,” says Dr Ure, “were exhibited, by running the wire in my hand over the edges of the plates in the last trough, from the 220th to the 278th pair. Thus, fifty shocks, each greater than the preceding ones, were given in two seconds. Every muscle of his countenance was simultaneously thrown into fearful action. Rage, horror, despair, and anguish, and ghastly smiles, united their hideous expression in the murderer’s face, surpassing far the wildest representations of a Fuseli or a Kean. At this period, several of the spectators were obliged to leave the room, from terror or sickness, and one gentleman fainted.” The last experiment made by Dr Ure consisted in transmitting the voltaic current from the spinal marrow to the ulnar nerve. “The fingers now moved nimbly, like those of a violin performer. An assistant who tried to close the fist, found the hand to open forcibly in spite of his efforts. When one rod was applied to a slight incision on the top of the fore finger, the fist being previously clenched, the fingers extended instantly; and from the convulsive agitation of the arm, he seemed to point to the different spectators, some of whom thought he had come to life." In these experiments the positive wire communicated with the nerve, and the negative with the muscles.

M. Becquerel mentions he effect produced by a pile of 100 plates, upon the head of a person who had been guillotined. The two poles of the pile communicated with the two ears, wetted with salt water. The muscles of the face experienced the strongest contractions, and the action of the eye-lids was extremely distinct. Aldini obtained analogous, though feebler effects, in experiments on a body after a natural death. Experiments of a similar kind have been made upon insects and fishes. Μ. Zanotti of Bologna, having killed a cigala, (grasshopper,) he placed it in contact with the two extremities of the pile, when it immediately moved, and emitted the sounds which are peculiar to it. M. Becquerel mentions also, that a fish whose head had been cut off half an hour before, struck the table with its tail, when excited by the voltaic current, and its whole body leaped about the table.

Sect. VIII.— On the Secondary Agency of Electric currents.

Dr Faraday has shown that many cases of voltaic decomposition of substances held in solution by water, such as nitric acid, ammonia, &c., are due not to the direct action of the current, but to the secondary agency of the elements of decomposed water. He, however, conceived that the hydracids in solution are directly decomposed by the current. Mr Connell has endeavoured to show that the decomposition of these substances also is secondary. This he did by connecting them by asbestus with distilled water, and making the acid negative and the water positive, when it was found that no chlorine or iodine was carried to the positive pole until after some time, when the acid itself had been carried over; whereas, when the battery was reversed so as to cause evolution of oxygen in the acid solution, chlorine or iodine immediately appeared by the combination of the nascent oxygen with hydrogen of the hydracid. From an extensive series of experiments on solutions in water, alcohol, and other solvents, Mr Connell has been [7:21:682]led to the general conclusion, that “when solutions of primary combinations of elementary bodies in water, and in those liquids, such as alcohol and pyroxylic spirit, which contain water as such as an essential constituent, are submitted to voltaic agency, the dissolved substance is not directly decomposed by the current, but only the water of the solvent.” The rule of course does not include solutions of combinations of the second order, such as ordinary salts, consisting of an acid and an alkali, but those of primary compounds of elementary bodies, such as acids, alkalies, &C.^[41. Transactions of the Royal Society of Edin., vol. xiii., xiv , and xv. ]

When absolute alcohol, holding a minute quantity of pure caustic potash in solution, such as 1 / 500 , is subjected to moderate voltaic agency, hydrogen gas is evolved from the negative pole, and no elastic fluid from the positive; and if a powerful battery, such as 200 pairs of four-inch plates, is employed, the same effect is obtained with absolute alcohol holding nothing in solution. A long investigation showed that this result is due to the direct voltaic decomposition of the water entering into the constitution of absolute alcohol; its hydrogen being evolved at the negative pole, and its oxygen being engaged in producing various secondary changes on the hydro-carbon of the alcohol. The effect of the potash is merely to give conducting power to the alcohol, and to favour the secondary action by its affinity for some of the secondary products. The addition of a minute quantity of potassium answers better than that of the hydrate of potass, because we thus, by the oxidation of the potassium, in effect add anhydrous potass, and avoid any addition of water, although in reality the water of the hydrate has no influence whatever on the result. Even 1 / 10000 of potass has a very decided effect in promoting the voltaic agency, and various other saline bodies when dissolved promote the action by increasing the conducting power. These experiments. It is conceived, prove directly what had previously been very generally inferred, that water as such enters into the constitution of absolute alcohol. A similar conclusion was drawn from similar experiments on the analogous substance, pyroxylic spirit.

It was found that pure rectified ether showed no symptom whatever of decomposition, when acted on by 200 pairs of four-inch plates; nor did the previous solution in it of as much caustic potass as it could take up, lead to any action of the voltaic current. It was therefore concluded that ether is not a hydrate.^[42. Transactione of the Royal Society of Edin, vol. xiii, and xiv., and Lond. and Ed. Jour. Dec. 1841. ] Sect. IX.— On the Application of Voltaic Electricity to the Arts.

There is perhaps no science, not even excepting chemistry, which has made such donations to the fine and useful arts as voltaic electricity. Those which depend on galvanism are the art of protecting the copper sheathing of ships; the galvano-plastic art, or that of multiplying works of art in metals, electro-metallurgy, and the reduction of the metals, the electrotype, or the art of copying and multiplying engravings, and galvanic etchings.

1. Protection of copper Sheathing. This art, invented by Sir H. Davy, has been already sufficiently described in our article, Davy, vol. 7, p. 662.

2. The Art of Multiplying Works of Art in Metal. This beautiful art seems to have been invented about the same time by Mr Jacobi of St Petersburg, and Mr Spencer of Liverpool. Mr Jacobi, who announced his discovery in October 1831, called it the galvano-plastic process, and Mr Spencer had, previous to the knowledge of Mr Jacobis' labours, executed medals in copper, which were called electrotypes or voltatypes. Both Mr Jacobi and Mr Spencer had confined their invention to the deposition of copper upon metallic bodies; but Mr Murray announced in January 1840, that non-conducting substances, such as plaster of Paris, wax, wood, &c ., might have metallic copper thrown down upon them by previously metallising their surface with black lead.

The single cell apparatus for taking casts of coin and medals is shewn in fig. 47, which consists of an outer vessel A, containing a saturated solution of sulphate of copper, and some undissolved crystals of the sulphate suspended near the upper surface of the fluid. The medal m, to be copied, is suspended in the copper solution. The inner vessel B, made of porous earthenware, contains the usual dilute acid, which acts upon a rod or plate of zinc Z, the upper end of which is connected with the bent wire w which suspends the medal m . When the medal has remained two or three hours in the copper solution, the copper will be found to have deposited itself on every part of its surface, so as to afford a perfect intaglio or hollow impression of the medal. If the surface of the medal or any part of it is greased, no copper will be deposited on the greased part. The obverse and reverse of the coin or medal being thus copied, and the two retained at their proper distance, the next step is to place them at m in the copper solution, and we obtain in the same manner a raised impression from the intaglio one, accurately resembling the original.

Instead of obtaining the intaglio cast directly in copper it is thought best to take it either in fusible or type metal, or in some non-conducting substance, such as sealing wax, bees’ wax, rosin, plaster of Paris, stearine, &c. When nonconducting substances are employed, those which are absorbent, such as plaster of Paris, must be prevented from absorbing the fluid, by rubbing the surface of the intaglio mould with tallow or spermaceti; they are then to be metallised by covering their surface with black lead.

When we wish to form gold and silver medals, a gold or silver surface is necessary, as non-conducting bodies are not well fitted for this branch of the art. For a gold medal, a strong nitro-muriatic solution of gold should be used, and the medal to be copied should, according to Mr Smee, be connected with the zinc end of a series of from four to twelve batteries. A very fine platinum wire, according to the same author, immersed in the solution^[43. In Mr Smee’s ingenious battery, the negative plate which be makes of platinum or silver is roughened with solid paper; or in the case of silver, with nitric acid, and this roughened surface is covered by galvanic agency, with the finely divided black powder of platinum for the purpose of increasing the solution of hydrogen For these purpose· platinized silver is now manufactured for sale. See Smee’s Elements of Electro metallurgy. Lond. 1841, p. 1718. ] to a trifling depth, must be united to the platinised silver of the battery, and the deposit of gold will then take place. For silver medals a solution of the nitrate, sulphate, or acetate, may be used. The solution should be weak at first, and then gradually increased. If we do not wish to have the whole medal of solid gold and silver, a thin layer may be deposited, and the rest completed by copper. On the same principles medals of platinum or palladium may be formed from their solution. Great care must be taken to prevent bubbles of air from forming on the mould.

The difficulty of coining large medals gives great value to this art. Mr Smee mentions a very fine medal of Boulton, about four inches in diameter, which required no less than 300 blows to insure a perfect impression. Seals [7:21:683] plaster casts, and various other works of art, may be copied in copper and other metals by the above process.

3. On the Multiplication of Engraved Copper Plates. The difficulty of procuring good and pure copper plate for engraving, has been entirely removed by this new art. A prepared copper plate with a good surface may have copper deposited on its surface, so that the deposited plate has the same perfect surface, with the additional advantage of consisting of pure copper. It is advisable, however, to hammer and prepare with charcoal the deposited plate to give it elasticity, &c., and such plates have been found superior to all others for the purposes of engraving.

The method of copying engraved copper plates, of the most delicate execution, is shewn in fig. 48, where D is a vessel, a gallipot for example, about eight inches high and six inches internal diameter. The dotted line, E E, is a copper cylinder six and a half inches high and five in diameter, and Ο Ο O is a porous cylinder, which may be made of brown paper when a quick action is wanted, but, in general, a thin unglazed gallipot is preferable. A cylinder of zinc, Z Z, as large as possible, is then placed within the porous cylinder without touching it, nearly at the distance of one-8th of an inch. A perforated cover, S S of earthenware, is made to rest either on the copper cylinder EE, or upon a rim in the gallipot about an inch from its top. The object of it is to hold crystals of sulphate of copper for keeping the solution in a state of saturation. Wires X, Y, are soldered to the cylinders of copper and zinc, and these are connected with the wires in the other vessel by a binding screw s , fig. 49, used by Mr Spencer, uniting the two wires M, N. The square cell A B, contains the engraved plate b, to be copied, and connected by the wire, b x, with the zinc cylinder of the battery; and c b is the plate to be oxydised and to be attached to the zinc cylinder of the battery. The secondary cell, or precipitating trough, which may be made of earthenware, wood, or glass, is filled with a saturated solution of sulphate of copper. In this way a reverse copy of the plate itself is obtained in relief, and from this copy, or relief, another copy in intaglio is to be taken by the same method. It is considered preferable, however, to take a perfect mould from the engraved plate in white wax or plaster of Paris. When this mould is rubbed with black lead, an electrotype plate is then deposited upon it. In like manner steel plates may be copied by first taking moulds from them in lead, wax, or plaster. It is a curious fact, that the deposited plate is always superior to the engraved plate. Mr Palmer has succeeded in thus copying the works of our finest engravers. For farther information on this subject, see Spencer’s Instructions for the Multiplication of Works of Art in Metals, &c., in Griffin’s Miscellany, Glasgow, 1840; Jacobi’s Die Galvanoplastik, Petersburgh, 1840; Annales de chimie et de Physique, September 1840, tom. lxxv. p. 24; and Smee’s Elements of Electro-Metallurgy, Lond. 1841.

3. Voltaic Etching. In this new art, which is fully described by Mr Smee, the copper plate having the design drawn upon the etching ground, as in ordinary etching, and having its back and sides coated with wax, is connected, by means of a wire, with the silver plate of one of Mr Smee’s batteries. “A piece of copper,” says Mr Smee. “of the same size as the plate, should then be connected to the zinc, when both the copper plate and the piece of zinc are to be placed in a solution of sulphate of copper. Immediately copper will be reduced from the solution on the negative plate, and copper from the etching plate will be dissolved to keep up the strength of the solution. Whatever is favourable to the increase of electricity causes the copper to be more quickly acted upon, and whatever diminishes the galvanic current retards the solution of the metal; so that the nearer the etching plate, forming the positive pole, and the piece of copper, forming the negative, are approximated, the more rapid will be the action. In the same way the intensity of the battery also affects the rate at which the plate is bitten in. The negative plate of copper, however, should not exceed in size the copper plate on which the etching is executed, or else there is a risk of some of the lines being more deeply bitten in; and, in like manner, if any considerable part of the plate has a great deficiency of lines, compared with other parts, that part must be stopped out rather before the other, to ensure a uniformity of depth, or else the negative copper opposite the part must be so bent, that it is at a greater distance. The advantages of galvanism for etching are, the absence of poisonous nitrous fumes, which are evolved in the ordinary process; the greater uniformity of action which takes place than when acids are used, and that the rapidity of biting may be regulated to the greatest nicety. The lines may be made of any depth, and are sharper and clearer than when acid is used; and lastly, no bubbles are evolved, which the engraver well knows are apt to tear up the ground, or to cause unequal action.”^[44. Smee’s Electro-Metallurgy, p. 138, 139. ]

4. Voltaic Gilding arid Plating. We owe the art of gilding upon silver and brass, by electricity, to M. de Larive, who was led to it by witnessing the dreadful effects which are produced at Geneva by the use of mercury in gilding. Gold, platinum, palladium, silver, copper, and carbon, when their surfaces are smooth and chemically clean, and freed from adhering air, may be gilt by means of a feeble voltaic current which deposits the gold from a weak nitro-muriate solution of that metal, and in this way a coating of any thickness can be obtained. By a similar process, metals may be platinated or palladinated by using the nitro-muriate solution of those metals. Metals may, in like manner, be covered with nickel by means of its nitrate.

By similar means fruit, vegetables, leaves, seeds may be coated with copper; and crystallised copper may be deposited on wicker work, baskets, &c., after they are blackleaded, or upon articles of earthenware. Mr Smee has succeeded in coating copper with almost every other metal; but for an account of De Larive’s and Mr Smee’s processes we must refer to the Bibliotheque Universelle, April 1840, the Comptes Rendus, &c., 1840, No. 14, p. 578, and to the work of Mr Smee, already quoted, book iii.

PART II—ON ELECTRO-MAGNETISM.

Various insulated facts and experiments, observed and made by Franklin, Van Marum, Cavallo, Ritter, Mojon. and Maschmann, led to the belief that electricity produced magnetical effects; and this opinion was strengthened by the magnetical changes which had been repeatedly observed in compass needles struck by lightning. It was not however, till 1820, that electro-magnetism was discovered by Professor H. C. Oersted of Copenhagen. In the month of July of that year, after obtaining several feeble magnetical effects from wires conducting the galvanic current, he at last succeeded, by using larger wires, in establishing the [7:21:684]fundamental law, that the magnetical effect of the electrical current has a circular motion round the current.

Soon after this important discovery was made, M. Ampere established a second fundamental law of electro-magnetism, that the two conducting wires from the poles of the battery, when conveniently suspended, attract each other when they transmit electrical currents moving in the same direction, and repel each other when the currents which they transmit have opposite directions.

On the 25th September 1820, M. Arago communicated to the French Institute the important discovery, that the electrical current possesses, in a very high degree, the power of developing magnetism in iron and steel. Sir H. Davy communicated a similar fact to Dr Wollaston on the 12th November 1820, and Dr Seebeck laid before the Royal Academy of Berlin a series of experiments on the same subject. Μ. Savary of Paris has more recently found that steel needles, placed at different but small distances from a wire conveying an electrical discharge, are not all magnetised in the same direction.

The most important addition to voltaic electricity, since the discovery of Oersted, is that of Dr Seebeck, who found that electro-magnetic currents can be produced by heat alone, a subject which will be treated in a separate chapter on Thermo-electricity.

When we join the two poles of a galvanic battery by a metallic wire, this wire is called the conductor, or the uniting wire, and the galvanic circle is said to be closed when this wire is single and unbroken, or when it consists of two wires in contact. When these two wires are not in contact, the circuit is said to be open, in which case the wires have no action upon magnetic needles.

Let A B, fig. 50, be the conducting wire of a closed galvanic circuit, along which electricity is carried from A to B, A being the positive end, and B the negative end: then, if a delicate magnetic needle is suspended near A B. its direction is changed in the following manner:

1. When the needle is above the wire. Its north pole will go from the observer as at d, in the upper part of the ellipse, c d e f, fig. 50.

2. When the needle is below the wire. Its north pole will approach the observer, as at f in the lower part of the ellipse.

3. When the needle is in the same horizontal plane as the wire, and stands between the observer and the wire. Its north pole is elevated, as at c.

4. When the needle is in the same plane, but on the other side of the conductor. Its north end is depressed, as at e.

Hence. It appears, that the direction of the magnetic current is c d e f, when the electrical current is in the direction A B.

If the uniting wire is bent into parallel directions, as in fig. 51. the two exterior surfaces of the branches AC, BD, will exercise similar actions on a needle NS, and so will the two interior surfaces, the actions at e and f being similar, and also those at g and h.

From these experiments, Professor Oersted concluded, that the magnetical action of the electric current describes circles round the conductor, and hence he gave the name of r evolving magnetism to this magnetical action.

This action of revolving magnetism was at first opposed by Professor Schweigger, on the ground that if it were true, a magnet might be made to revolve round the uniting wire. Dr Wollaston drew the same conclusion, but for the purpose of producing such a revolution. Before he had effected his purpose, however, Dr Faraday went a step farther, and found experimentally not only that a magnet could be made to revolve round the uniting wire, but that a moveable uniting wire might be made to revolve round a magnet. An apparatus for exhibiting these remarkable properties is shewn in fig. 52. A wire a, from the voltaic battery, passes into the glass vessel M, through a hole in its bottom, so as to communicate with mercury contained in the vessel. The lower end of a small magnet b, of the form of a cylinder, is fixed by a thread to the bottom of the vessel, so that it floats almost vertically in the mercury. A wire Ced, communicating with the other end of the battery, by means of the brass pillar C, dips with its lower end d into the mercury in M; and as soon as the voltaic current is established in the direction of the arrows, or a d e C , the pole b of the magnet will revolve round the fixed conductor deC.

The revolution of the conductor round a magnet is exhibited in the same figure, where N is a glass vessel containing mercury, and having a small cylindrical magnet F fixed to its bottom, and projecting a little above the surface of the mercury. The wire d, being attacl>ed by a hook to the horizontal arm C, will commence its revolutions round, as soon as the voltaic current passes in the direction of the arrows, or x F, d C. If we make the current pass in the direction a d e C F x , from the zinc to the platinum end of the battery, both the above revolutions will go on simultaneously. When the current was made to pass in the opposite direction, the direction of the rotation was likewise changed.

The rotation of a magnet round its own axis was first effected by M. Ampere. The magnet was made to float vertically in mercury, by a platina weight at its lower end. When the electrical current descended through the upper half which stood above the mercury. It was carried off by the mercury without entering the other halt of the magnet. Had a positive current entered the other half, after passing through the first half. It would have tended to make the upper pole revolve from left to right, and the under pole fiom right to left, and these contrary forces would have balanced each other; but when it is prevented from entering the lower half, the positive current produces a rotation in the magnet from left to right. Mr Watkins^[45. Popular Sketch of Electro-Magnetism. ] has constructed the apparatus in fig. 53, for shewing this experiment in a' better way. A flat bar magnet M is supported vertically by the bent wire WSW, fixed to the stand AB. The lower end of the magnet, which is pointed, rests in an atrate cup C, while its upper end is a pivot, turning in a hole in the screw S. At the centre and lower end of the magnet are circular grooves containing mercury, [7:21:685] into which dip small bent and pointed wires, fixed to the magnet, as seen in the figure. When the voltaic circuit is completed in the usual manner, the current passes only through the lower half of the magnet, and being a moveable part of the circuit. It turns round on its two pivots with a velocity depending on the strength of the magnet, the power of the battery employed, and the freedom from friction at the pivots. A current from another battery might be passed from the top of the magnet to its centre, which, by producing a rotation in the same direction, would increase the velocity of revolution.

Upon the same principles, a conductor may be made to revolve round its axis. An instrument for shewing this was invented by Professor Barlow, and which has been improved by Mr Watkins, by applying it to the horse-shoe magnet.^[46. Popular Sketch of Electro-Magnetism. ]

The rotation of liquid conductors may likewise, as Sir H. Davy has shewn, be produced by the pole of a magnet. If mercury is placed in a shallow dish, between the two poles of a battery, a magnet placed either above or below the mercury, will cause the mercury to revolve round the points from which the currents issue. The rotation of the flame produced by the passage of a powerful voltaic charge between two charcoal points, arises from the same cause. Professor Daniell gives the following pleasing method of shewing this effect. He makes a powerful horse-shoe magnet part of the conducting wire of a constant battery of a moderate number of cells; the flame which may then be drawn from one of its poles will revolve in one direction, while that from the other will revolve in the opposite direction.^[47. Introduction to Chemical Philosophy, § 815. ]

Soon after the discovery of electro-magnetism, Μ. Ampere made the important discovery, that the conductors attract each other when they are transmitting electrical currents having the same direction, but repel each other when the currents have opposite directions. This may be proved experimentally by the apparatus in fig. 54, invented by Μ. Ampere. It consists of a bent wire, ABCDEFGH, the parts of which at B and G, are kept insulated by a non-conducting substance m, to which they are tied. The extremities A, H, with steel points dip into iron cups of mercury, K, M, at the ends of the brass wires, JK, LM, fixed into a piece of wood, NO. When the electric current enters at J, passes along the conductor ABCDE, &c., and issues at L, the conductor is put in motion by means of a magnet. When the south pole of a magnet is directed against the side BCD. It will repel the conductor, but will attract it when directed against the opposite side.

The conductor in the above apparatus may also be moved by the earth’s magnetism. For this purpose the plane, CDEF, must stand perpendicularly to the magnetic meridian. When the current enters at A, the vertical part FE will be placed towards the west, but if the current enters at H, the part FE will be placed towards the east.

Ampere’s electro-dynamic cylinder is shewn in fig. 55, where M is the extremity of a wire, with a steel point resting in a cup of mercury. The wire, after descending to A, passes horizontally through a glass tube AB, and is then wrapped round it to form a helix or spiral, returning to A. It then passes to C, where it is wrapped in a similar manner round the glass tube CD; and when it reaches the end D. It returns through the tube to C, when it descends vertically with its steel point into another mercury cup N. This instrument is a complete imitation of a magnetic cylinder, and, while an electric current is passing through it. It possesses all the properties of a magnet, and may in every case be substituted for one.

Another electro-dynamic cylinder, invented by Ampere, and improved by Mr Marsh, is shewn in fig. 56. It consists of a coil or helix of wire AB, the ends of which return along its axis to its middle point C, and are there fixed to the wires, n, p, of a small voltaic battery MN, which consists of a single plate of zinc z, surrounded by a plate of copper cc, and floating in a basin of diluted acid, in which it can freely move. This spiral ACB, will place itself in the magnetic meridian, when acted upon by the magnetism of the earth, and will likewise yield to the action of another magnet placed near either of its poles.

Various forms have been given to these electro-dynamic cylinders. In some the coils all lie in one plane, as in Fig. 57, where one face of the coil has north, and the other south polarity, the magnetic poles being as it were situated in the centre of each disc.

When the helix is constructed, as in fig. 58. Its power is so great, that a small steel bar SN, placed within it, and supported perpendicularly, will, as soon as the connection is made with the voltaic battery, by means of the mercury cups P, p, start up, and place itself in the air, where, like Mahomet’s coffin, it will remain suspended without any visible cause, and in opposition to the power of gravitation.

We owe also to M. Ampere the very interesting apparatus of a small voltaic battery made to revolve round a magnet. This is shewn in fig. 59, where ABCD, abed exhibits a section of two cy-lindersof coppers oldered to a copper bottom, so as to hold a fluid. This double cylindrical vessel is suspended by a bent wire a F b (having a cavity at F,) upon the north pole N of a vertical magnet NS. A light cylinder of zinc zz is also suspended by a bent wire zEr, and a steel pivot at E upon the same pole N of the magnet. The cylinder zz can therefore revolve upon this pivot. When the cylinder ABD dbac CA, is filled with dilute acid, so as to constitute a small battery, the cylinder zz will revolve[7:21:686] from left to right when N is the north end or south pole, and from right to left when N is the south end or north pole. Owing to the attraction of the fluid, the cylinder of zinc is often drawn to one side, and prevented from moving; but this may be avoided by making the space Ac sufficiently wide. Mr Watkins has ingeniously applied this contrivance to the poles of a horse-shoe magnet, as in fig. 60. It consists of a horse-shoe magnet AB, fixed to a stand SS. Above each pole is suspended a double cylindrical copper vessel, with a bent metallic wire fixed to the top of the inner cylinder, and a vertical wire pointed at each extremity, fixed in the middle of the bent wire. The lower ends of the vertical wires of each cylinder rest in the holes at each pole of the magnet. Within the above double copper vessels are placed two hollow cylinders of zinc, having similar bent wires with holes in the lower side of each, in which holes the upper ends of the vertical wires are inserted. When the copper cylinder is filled with dilute acid, the voltaic action begins, all the four cylinders revolving round their respective axes. The copper cylinders turn slowly and heavily, from their weight, in opposite directions to one another, and the zinc cylinders, with great velocity, in opposite directions to the copper ones. Very delicate suspensions are necessary to ensure the rotation of the copper cylinders.

A very simple apparatus for showing the magnetic state of a single coil, is shown in fig. 61, where Z and C represent the elements of a small galvanic battery of one zinc and one copper plate attached to a cork which floats on dilute acid. Each plate is half an inch wide, and two inches long. A piece of copper wire W, with silk thread wrapped round it, is bent into a ring, one end of which is soldered to the zinc, and the other to the copper plate. An electric current now passes in the direction of the arrow, and the ring W becomes a flat magnet, having its poles in the centre of its two surfaces, the one being north and the other south. This floating magnet will, when acted upon by a real magnet, exhibit the usual magnetic attractions and repulsions. Mr Marsh has improved this apparatus by doubling the copper plate, as in fig. 62, and converting it into a vessel for holding the dilute acid. The plates are then placed in a glass cylinder which may float in water.

A very beautiful apparatus for exhibiting helical rotations has been constructed by Mr Watkins, and is shown in fig. 63. A horse-shoe magnet, with its poles uppermost, is fixed upon a wooden box S. Two helices of copper, having slender bars across their summit, with needle or steel points in their centre, move in conical holes drilled in the poles of the magnet, with small platina cups to hold a small portion of mercury. The lower extremities of each helix terminate in steel points, which dip into the mercury in wooden cups below, screwed to the legs of the magnet. A wire likewise goes from the lower end of each cistern, and being bent upwards, terminates in a small cup with mercury. The brass rod R, fixed to the stand, carries a forked piece MN, the ends of which are two points which dip in the mercury in the platina cups. On the top of the coil, another mercury cup is placed on the fork MN; and when the voltaic current is made to pass through the apparatus, the helical coils will revolve rapidly in opposite directions, the directions changing with the disposition of the wires which connect them with the voltaic battery.

We have already mentioned the fine discovery of Μ. Arago, of the power of electrical currents to develope magnetism in iron and steel. Μ. Arago found that the uniting wire of a powerful voltaic battery attracts iron filings often with such force as to form a coating round the wire ten or twelve times thicker than itself. This attraction, as he found, did not originate in any magnetism previously possessed by the iron filings, which he ascertained would not adhere to iron; and that it was not a case of common electrical attraction, was evident from the fact that copper and brass filings were not attracted by the uniting wire. Μ. Arago likewise found, that the iron filings began to rise before they came in contact with the uniting wire, and hence he drew the conclusion, that the electric current converted each small piece of iron into a temporary magnet. In following out this view, the French philosopher converted large pieces of iron into temporary magnets, and also small steel needles into permanent ones. Sir H. Davy and Dr Seebeck obtained analogous results without knowing what had been previously done in France. Μ. Savary of Paris obtained also some very important results relative to the magnetic action of the uniting wires at different distances, but we have already given a brief account of them, as well as of the experiments of Professor Erman, in our article, Electricity, vol. viii. p. 574.

The next step in the progress of discovery, was that of making magnets of extraordinary power by means of a voltaic battery. This seems to have been first accomplished by Professor Moll of Utrecht and Professor Henry of Princetown College, who was able to lift thousands of pounds’ weight by his apparatus, but as we have already given a full account of the construction of such magnets, and of the experiments of Μ. Quetelet of Brussels, and Mr Watkins of London, in our article Magnetism, in Sect, xiii. p. 761, vol. viii., we must refer our readers to that part of the work.

Since these important discoveries, however, were made, an electro-magnet of extraordinary power has been constructed by the Rev. N. J. Callan,^[48. See Sturgeon’s Annals of Chemistry, &c., July 1837. ] Professor of Natural Philosophy at Maynooth. It has the form of a horse-shoe, and is thirteen feet long, two and a-half inches in diameter, and weighs fifteen stone. The distance between its poles is seven inches, and a copper wire one-sixth of an inch in diameter, is wrapped round the bar from one pole to the other. The total length of this thick wire is 490 feet, but it is divided into seven parts, each 70 feet in length. A copper [7:21:687]wire, about the fortieth of an inch in diameter, is soldered to one of the thick wires, about a foot from one of its extremities, and is wrapped round the horse-shoe bar in the same direction as the thick wire, and in one continuous helix. When the opposite ends of the seven thick wires are connected with the opposite poles of a voltaic battery, the horse-shoe bar is converted into a magnet of extraordinary power, and when the battery communication is broken, an electric current of singular intensity is established in the long coil of small wire. The armature or keeper of Mr Callan’s magnet was a horse-shoe bar of iron 20 inches long, two and a-half in diameter, and weighing 28 lbs. Its poles were seven inches apart, and the apex of the arch seven inches high. Such was the power of the magnet that it was found impossible to separate the keeper from it by any force acting in a direction perpendicular to the touching surfaces. The calorimotor, consisting of a single pair of plates, with 18 square feet of copper, and 16 of zinc, was found by Mr Noad more effective in exciting the magnetism than a Wollaston battery of 100 double pairs highly excited. When the connexion was broken between the battery and the charcoal points fixed to the thick wires, the succession of sparks formed a continued blaze of brilliant light, and when a succession of sparks was sent rapidly through a large fowl, they produced instant death.

The idea of applying the powerful agency thus developed in a bar of iron, as a mechanical power, naturally suggested itself; but there is reason to believe that Mr Thomas Davenport of Brandon, in the county of Rutland, and state of Vermont, was the first person who thought of applying it in producing rotatory motion. This uneducated individual, by trade a blacksmith, having, in 1833, accidentally seen one of Professor Henry’s electro-magnets, purchased it with the idea of employing it as a mechanical power. In July 1834, he is said to have constructed a voltaic engine; and on the 16th March 1837, he took to Newhaven two machines; one a rotatory machine, composed of revolving electro-magnets, with fixed, permanent magnets; and the other, a rotatory machine, composed entirely of electro-magnets in its fixed and revolving members, which being wholly made of soft iron, may be magnetised in an instant by a very small battery.^[49. Silliman’s American Journal of Science, April 1837, vol. xxxii∙ No. 65, p. 217. ] Professor Henry,^[50. Ibid July 1831, vol. xx. p. 340. ] ’ however, had previously, and so early as 1831, produced a reciprocating motion by magnetic attraction and repulsion, aided by electromagnetic action; and as the contrivance seems to have been overlooked in this country, we shall lay before our readers his own drawing and description of it.

In Fig. 64, AB is an electro-magnet of soft iron, about

'seven inches long, and moveable on an axis at the centre

S. Its two extremities, when placed in a horizontal line, are about one inch from the north poles of the upright magnets C and D. G and F are two large tumblers containing dilute acid, in each of which is immersed a plate of zinc, surrounded with copper. l, m, s, t, are four brass thimbles soldered to the zinc and copper of the batteries, and filled with mercury.

“The electro-magnet AB is wound with three strands of copper bell wire, each about twenty-five feet long. The similar ends of these are twisted together, so as to form two stiff wires, which project beyond the extremity B, and dip into the thimbles s, t.

"To the wires q, r, two other wires are soldered, so as to project in an opposite direction, and dip into the thimbles l, m. The wires of the electro-magnet have thus, as it were, four projecting ends; and by inspecting the figure it will be seen that the extremity m, which dips into the cup attached to the copper of the battery in G, corresponds to the extremity r connecting with the zinc F.

“When the batteries are in action, if the end B is depressed until q r dips into the cups s , t, AB instantly becomes a powerful magnet, having its north pole at B. This of course is repelled by the north pole D, while at the same time it is attracted by C. The position is consequently changed, and o p comes in contact with the mercury in l, m. As soon as the communication is formed, the poles are reversed, and the [>osition again changed. If the tumblers be filled with strong dilute acid, the motion is at first very rapid and powerful, but it soon almost entirely ceases. By partially filling the tumblers with weak acid, and occasionally adding a small quantity of fresh acid, a uniform motion, at the rate of seventy-five vibrations in a minute, has been kept up for more than an hour. With a large battery, and very weak acid, the motion might be continued for an indefinite length of time.

“The motion here described is entirely distinct from that produced by the electro-magnetic combination of wires and magnets. It results directly from the mechanical action of ordinary magnetism—galvanism being only introduced for the purpose of changing the poles.”

Professor Green, to whom Professor Henry first exhibited this machine in motion, recommended the substitution of electro-magnets for the two vertical ones C, D. Though Professor Henry described this apparatus as a toy, yet he distinctly states, that in the progress of discovery, the same principle, or some modification of it on a more extended scale, might hereafter be applied to some useful purpose.

These contrivances have been followed by several others of great ingenuity. The Rev. J. W. Macgauley exhibited a working model of an electro-magnetic machine, to the British Association at Dublin in 1835; and in the sixth report of the Association he mentions his having “in his possession a machine of not inconsiderable power.” Mr Sturgeon of Woolwich mentions that he had a galvanic machine in use on his premises, for pumping water, and for other mechanical purposes.^[51. Report on, vol. v. p. 24. ] Mr Jacobi has some time ago employed electro-magnetic machinery for impelling a boat on the Neva at St Petersburg; and Mr Davidson of Aberdeen has made a similar application to a turning lathe.

A series of beautiful instruments, of great practical value, have been invented for increasing minute voltaic effects, by electro-magnetic action. The first of these was constructed by Professor Schweigger of Halle, immediately alter the discovery of electro-magnetism. It is exhibited in fig. 65 where a magnetic needle SN, is placed or suspended within several bendings of the uniting wires ABCDE. Now, as each of the branches of this wire acts upon one of the poles of the needle in the same direction, the effect will be quadrupled; and hence the direction of the needle becomes a means of measuring any minute voltaic effects produced in the uniting [7:21:688]wires. The power of multiplication does not, as Dr Seebeck proved in 1820, increase with the number of windings in the uniting wire, as the resistance to transmission increases with the length of the wire, thus diminishing the conducting power of the wire. Professor Orested improved the multiplier^[52. For a perspective drawing on a large scale, of this instrument, see Edinburgh Encyclopaedia, art. Thermo-Electricity, plate 522, fig. 11. ] by adding a bent magnet, as shewn at JKL, fig. 66, which can be placed so as to repel the nearest end of the needle, or index, or to attract it. In the position in the figure the first of these effects is produced; but by turning the angle of the bent magnet towards the needle, the second effect is produced. By causing the pillar which carries this magnet to approach to, or recede from the needle, the directive power of the needle may be made scarcely sensible. In this state the instrument will shew the difference in the voltaic effects produced by two pieces of metal, which differ only by 1 / 100 th of alloy when a powerful liquid is used. Professor Cumming, we believe, first suggested the idea of neutralizing the directive force of the needle, arising from the earth’s magnetism, which he did by placing a magnetized needle immediately beneath the moveable or index needle.^[53. Trans. Cambridge Phil. Soc. vol. i. p 279 ]

M. Nobili has improved this instrument by using two needles, as in fig. 66; but he fixed the neutralizing needle S'N', to the moveable one NS, placing the one above the other, with their poles reversed. The two needles are fixed in a piece of straw’ GH, and suspended by a silk fibre at G. The needles were twenty-two lines long, three wide, and one-fourth thick, and GH was five lines. The wire was a copper one, one-fifth of a line in diameter, and thirty feet long, and covered with silk. It made seventy-two evolutions round the frame.

M. Lebaillif has extended this principle by using four needles in place of two, each pair being exactly the same as in fig. 66, the one being brought near the upper surface of the coil, and the other near the under surface. In this instrument the increased weight of the needles may compensate any additional sensibility possessed by the approximation of the needles to the sides of the coil. The instrument is shewn in fig. 67, where ab, ab are the four needles, mnop the square bobbin, around which the wires are coiled, one or two feet of their length at each end being left free, as at gi, gf, that the electric current may enter at the one, and issue from the other. Instead of a single wire 300 feet long, M. Lebaillif employs five parallel ones, each end of which is stripped of the silk, and united by strong pressure into a bundle. The electric current thus divided into five parts, flows in five channels, which, according to M. Pouillet, transmit a proportionally larger quantity of electricity, while the diminished intensity produced by a great length of wire, is avoided.^[54. Pouillet’s Elemens de Phys.. Exp liv. v. chap. i. sect. 412. and Library of Useful Knowledge, art. Electro-Magnetism, p. 44. ]

A torsion galvanometer invented by Dr Ritchie,^[55. Phil. Trans. 1830. p. 218. ] is shewn in fig. 68. Having covered a fine copper wire with a thin coat of sealing wax, he rolls it about a heated cylinder, an inch or two in diameter, ten, twenty, or any number of times. The opposite sides of the circular coil are then pressed together, till they become parallel, and about one or one and a half inches long. The coil W is then fixed on a proper sole, and the ends of the wires connected with two metallic mercury cups C, C. A graduated disc of paper is then placed horizontally on the upper half of the coil, having a black line drawn through its centre, parallel with the middle line of the coil. A small magnet SN, made of a common sewing needle, is then fixed to the lower end of a fine glass thread, while the upper end is securely fixed with sealing wax, in the centre of a moveable index I, as in the torsion balance. This is inclosed in a glass tube T, fitted into a disc of thick plate glass, which forms the upper surface of the box. When a voltaic current passes through the coil W, the needle SN is deflected. The glass thread is then twisted by turning the index I, till the needle is brought back to its former position, and the number of degrees of torsion will be an accurate measure of the quantity of electricity, whose deflective power over the needle is exactly balanced by the torsion of the glass fibre.

A very ingenious galvanoscope, for ascertaining merely the existence and direction of an electric current, is described in the Library of Useful Knowledge.^[56. . Art. Electro-magnetism, part ii. chap, viii. p. 44, fig. 82. ] It is shewn in fig. 69, where M is the needle, T the suspending fibre, placed between four vertical spiral coils, the centres of which are brought very near the poles of the needle. The same voltaic current is made to circulate through all the four spirals, which have their turns such as to produce repulsion of the contiguous pole on the one side, and attraction of the same pole on the other side. The wire of the four spiral discs proceeds from the mercury cup P, and terminates in another cup N. In this admirable instrument, the current is brought as near as possible to the needle, so that its action is very powerful. The whole force of the four discs is quadruple that of a single one, as they all concur in giving the needle a deviation in the same direction.

Gold-leaf has been employed in the formation of the gold-leaf galvanoscope, which is similar in construction to Bennet’s gold-leaf electrometer. The strength of the current is indicated by the curvature of the strip of gold-leaf fh, fig. 70, which is held loosely by forceps at f and h, each forceps terminating in a mercury cup, the one, P, being above, and the other, N, below. The gold-leaf is enclosed in a glass case, the middle of which is placed equidistant between the poles M, m, of a horse-shoe magnet. When the electric current passes through the gold-leaf, [7:21:689] the leaf is bent, or attracted and repelled laterally by the poles of the magnet, according as the current ascends or descends, the broad surface of the leaf becoming convex towards the magnet in the one case, and concave in the other. As the degree of curvature is easily ascertained, Dr Roget considers this instrument as affording “the most delicate test possible of the existence and direction of a weak voltaic current.”^[57. Cumming’s Manual of Electro-dynamics, p. 178. ]

We shall now conclude this part of the article with an account of a very ingenious contrivance of Ampere’s for quickly altering the direction of the electric current in voltaic batteries. Two grooves R, R, fig. 71, are made in the table TT, some lines in depth, and also four similar cavities νv', tth, communicating diagonally by means of the plates of copper ll, mm, which are kept separate at their crossing by a non-conducting substance. Mercury is then placed in the grooves and cavities, after they have been varnished with mastic. If the positive wire of a battery is immersed in the groove R, and the negative one in the groove R', the current will not flow until a metallic communication is made between each of the grooves, and one of the cavities. To do this, b, bl are two plates, fig. 72, for transmitting the current; the plate b may become positive or negative, according as the cavity R communicates with t, and R' with tl, or when R communicates with v, and R' with v'.

In the first case, the current follows the direction R t , bb', t 'R', in the second it goes from R to v, then traverses the plate ll', and afterwards goes from b' into bt, and from v' into R. Now, these communications may be easily made or interrupted, by means of a wooden rod BB', which turns round its axis in the holes o, o'. Four metallic arcs bbl, dd', are fitted to this rod, so that by merely raising or depressing it, the communications are changed. When b and bl are depressed, R and v communicate through rbc, and R and v ' by r'b'c', and when d and dl are depressed, R and t, and R' and t' communicate by means of the corresponding arcs.^[58. Becquerel, Traité D'Electricité, &c. tom. iii. pp. 9, 10. ]

Mr Edward Clarke has improved this instrument, and, we believe, given it the name of electripeter. It is shown in fig. 73, where a, a, a', a' are four mercury cups, communicating with wires beneath the stand SS'. Large mercury cups A, A', B, B', are similarly con structed for conveying the current from the battery to a machine to be set in motion. The wires CC' are moveable about a horizontal axis. Suppose them to be in the position in the figure, and that the current is passing from A to B, and back again, from any apparatus from B' to A, then, by merely pressing the other ends of the wires CC' into their respective cups a,a, the direction of the current will be immediately changed, and it will pass from A beneath the stand, to B', and back from B to A. By retaining the wires horizontally which keep their ends out of the cups, the passage of the current will be stopped.^[59. Noad’s Lectures, p. 319. ]

Among the applications of Electro-magnetism, two of the most interesting are the Electro-magnetic Telegraph, and the Electro-magnetic Clock, invented by Professor Wheatstone, of King’s College, London.

Wheatstone’s Electro-magnetic Telegraph.

Although the idea of conveying signals along wires by means of electric currents, must have occurred to many persons, yet the value and success of the invention must depend on the principles and methods by which that idea is carried into effect. Professor Wheatstone and Mr Cook have taken out a patent for this invention, and, in March 1840. It was in practical operation on the Great Western railway, throughout a distance of 14 miles from Paddington to West Drayton. It is also in operation on Blackwall railway. Professor Wheatstone has recently made an entirely new arrangement for his telegraph, which possesses great advantages over the old one: It is extremely portable; and any child can both read and send the messages with scarcely a minute’s instruction. It requires only a single pair of wires, and 30 or 40 letters can be successively sent by it in a minute. The telegraph with its accompanying alarm, is included in a case not larger than that of the smallest table clock. From the utility, simplicity, and cheapness of this new contrivance, we are convinced that its application will not be confined to long telegraphic lines, but will also be extensively employed in public and private establishments.

Wheatstone’s Electro-magnetic Clock.

To the same ingenious author we owe one of the most beautiful inventions of modern times, the electro-magnetic clock, the detailed construction of which has not yet been published. The following is a brief abstract of the account of it which was read at the Royal Society on the 26th November 1840.

The object which Professor Wheatstone had in view was to enable a single clock to indicate exactly the same time in as many different places distant from each other as may be desired. In an observatory, for example, every apartment may be furnished with a cheap and simple clock, scarcely liable to derangement, and giving the time so accurately, that it will beat dead seconds audibly with as great precision as the standard astronomical timepiece with which it is connected. Hence the necessity is avoided, in such scientific establishments, of having several clocks, and of being at the trouble of winding them up and regulating them individually. “In like manner, in public offices and large establishments, one good clock will serve the purpose of indicating the precise time in every part of the building where it may be required, and an accuracy ensured which it would be difficult to obtain by independent clocks, even putting the difference of cost out of consideration. In the electro-magnetic clock, which was exhibited in action in the apartments of the Society, all the parts employed in a clock for maintaining or regulating the power are entirely dispensed with. It consists simply of a [7:21:690] face with its second, minute, and hour hands, and of a train of wheels which communicate motion from the action of the second hand to that of the hour hand, in the same manner as in an ordinary clock train; a small electro-magnet is caused to act upon a peculiarly-constructed wheel (scarcely capable of being described without a figure) placed on the second’s arbor, in such a manner that whenever the temporary magnetism is either produced or destroyed, the wheel, and consequently the second’s hand, advances a sixtieth part of its revolution. It is obvious, then, that if an electric current can be alternately established and arrested, each resumption and cessation lasting for a second, the instrument now described, although unprovided with any internal maintaining or regulating power, would perform all the usual functions of a perfect clock. The manner in which this apparatus is applied to the clocks, so that the movements of the hands of both may be perfectly simultaneous, is the following :—On the axis which carries the scape-wheel of the primary clock a small disc of brass is fixed, which is first divided on its circumference into sixty equal parts; each alternate division is then cut out and filled with a piece of wood, so that the circumference consists of thirty regular alternations of wood and metal. An extremely light brass spring, which is screwed to a block of ivory or hard wood, and which has no connection with the metallic parts of the clock, rests by its free end on the circumference of the disc. A copper wire is fastened to the fixed end of the spring, and proceeds to one end of the wire of the electro-magnet; while another wire attached to the clock-frame, is continued until it joins the other end of that of the same electro-magnet: A constant voltaic battery, consisting of a few elements of very small dimensions, is interposed in any part of the circuit. By this arrangement the circuit is periodically made and broken, in consequence of the spring resting for one second on a metal division, and the next second on a wooden division. The circuit may be extended to any length; and any number of electromagnetic instruments may be thus brought into sympathetic action with the standard clock. It is only necessary to observe, that the force of the battery and the proportion between the resistances of the electro-magnetic coils, and those of the other parts of the circuit, must, in order to produce the maximum effect with the least expenditure of power, be varied to suit each particular case.”

In Professor Wheatstone’s paper, of which the above is an abstract, he has pointed out several methods of effecting the same purpose. In one of them he substitutes Dr Faraday’s magneto-electric currents, in place of the voltaic battery, and he likewise describes a modification of his clock which will enable it to exercise its controlling power with a weaker electric current than when constructed on the plan above described. Professor Wheatstone has likewise pointed out other most important purposes to which his invention is applicable.

PART III—MAGNETO-ELECTRICITY.

In the preceding chapter, we have detailed the leading phenomena produced by electric currents, or electricity in motion, for no magnetic effects are produced by accumulated electricity. We come now to give an account of the new science of magneto-electricity, which we owe to Dr Faraday. Although certain effects of the induction ^[60. By induction Dr Faraday intends to express the power of electrical currents “to induce any particular state upon matter in their immediate neighbourhood otherwise indifferent." Experimental Researches, p. 1. ] of electrical currents had been discovered. It had always appeared to Dr Faraday unlikely that these could be the only effects which induction by currents could produce; and whatever theory of the phenomena might be adopted. It still seemed to him “very extraordinary, that as every electric current was accompanied by a corresponding intensity of magnetic action, at right angles to the current, good conductors of electricity, when placed within the sphere of this action, should not have any current induced through them, or some sensible effect produced equivalent in force to such a current. With these views, and under the expectation of obtaining electricity from ordinary magnetism, he investigated experimentally the inductive effect of electrical currents.”

If the uniting wire of a voltaic battery is placed parallel to the wire connecting the two ends of a delicate galvanometer, the most powerful current along the uniting wire will produce no deviation in the needle. But if the current along the uniting wire is stopped, by breaking the circuit, a momentary deviation of the needle takes place, as if a wire passed in the same direction as that of the voltaic current. When the needle has become stationary, a similar impulse is given to it in the opposite direction, by restoring the circuit. Dr Faraday found that similar effects took place, when the current along the uniting wire being uninterrupted, the uniting wire was made to approach or to recede suddenly from the wire of the galvanometer, the approximation inducing a current in the direction contrary to the inducing current in the uniting wire, and the division inducing a current in the same direction as the inducing current.^[61. Experimental Researches, p. 5. ] To this inductive action of the voltaic current Dr Faraday has given the name of volta-electric induction.

As the preceding effects were clearly produced by a transverse action of the current, in the first case at the instant where the current was annihilated or generated, and in the second by the mechanical motion of the uniting wire, Dr Faraday expected to obtain similar results, by the sudden induction and cessation of the same magnetic force, either by means of a voltaic current, or by that of a common magnet. By various experimental arrangements, Dr Faraday verified these anticipations; but in order to connect his experiments on volta-electric induction with the present ones, he constructed a combination of helices upon a hollow cylinder of pasteboard. The wire was 1-20th of an inch in diameter, and the different spires were prevented from bending by a thin interposed twine. Each helix was covered with calico. Eight lengths of copper wire were used, or nearly 220 feet of wire. “Four of these helices were connected end to end, and then with the galvanometer; the other intervening four were also connected end to end, and the battery of 100 pairs discharged through them. In this form, the effect on the galvanometer was hardly sensible, though magnets could be made by the induced current. But when a soft iron cylinder, 7 / 8 ths of an inch thick, and 12 inches long, was introduced into the pasteboard tube, surrounded by the helices, then the induced current affected the galvanometer powerfully. It possessed also the power of making magnets with more energy apparently than when an iron cylinder was used. When the iron cylinder was replaced by an equal cylinder of copper, no effect beyond that of the helices alone was produced.” “Similar effects,” continues Dr Faraday, “were produced with ordinary magnets. Thus, the hollow helix just described had all its elementary helices connected with the galvanometer, by two copper wires each five feet long; the soft iron cylinder was introduced into its axis; a couple of bar magnets, each 24 inches long, were arranged, with their opposite poles at one end in contact, so as to resemble a horse-shoe magnet, and then contact made between the other poles and the ends of the iron cylinder, so as to convert it for the time into a magnet; by breaking the magnetic contacts, or reversing them, the magnetism of [7:21:691] the iron cylinder could be destroyed or reversed at pleasure. Upon making magnetic contact, the needle was deflected; continuing the contact, the needle became indifferent, and resumed its first position: on breaking the contact. It was again deflected, but in the opposite direction to the first effect, and then it again became indifferent. When the magnetic contacts were reversed, the deflections were reversed. When the magnetic contact was made, the deflection was such as to indicate an induced current of electricity in the opposite direction to that fitted to form a magnet, having the same polarity as that really produced by contact with the bar magnet.”

But in order to show that it was by the approximation of the magnets that the momentary induced current was excited, Dr Faraday substituted for the soft iron cylinder a cylindrical magnet 8½ inches long, and ¾ths of an inch in diameter. He introduced one end of this magnet into the axis of the helix, and then, the galvanometer needle being stationary, the magnet was suddenly thrust in, the needle was then instantly deflected in the same direction as if the magnet had been formed by any of the two preceding processes. “Being left in, the needle resumed its first position, and then the magnet being withdrawn, the needle was deflected in the opposite direction. These effects were not great, but by introducing and withdrawing the magnet, so that the impulse each time should be added to those previously communicated to the needle, the latter could be made to vibrate through an arc of 180° or more.^[62. Experimental Researches, p. 11, or Phil. Trans. 1832, p. ]”

Although the law which governs the evolution of electricity by magneto-electric induction, is very simple, yet Dr Faraday has found it rather difficult to express it, except in reference to diagrams. We shall therefore give it in his own words. “If in fig. 74, PN represent a horizontal wire passing by a marked magnetic pole, so that the direction of its motions shall coincide with the curved line proceeding from below upwards; or if its motion parallel to itself be in a line tangential to the curved line, but in the general direction of the arrows; or if it pass the pole in other directions, but so as to cut the magnetic curves^[63. By magnetic curves, I mean the lines of magnetic forces, however modified by the juxtaposition of poles, which would be depicted by iron filings, or those to which a very small magnetic needle would form a tangent. ] in the same general direction, or on the same side as they would be cut by the wire if moving along the dotted curved line; then the current of electricity in the wire is from P to M. If it be carried in the reverse direction, the electric current will be from N to P. Or if the wire be in the vertical position, as at P'N', and it be carried in similar directions, coinciding with the dotted horizontal curve, so far as to cut the magnetic curves on the same side with it, the current will be from P' to N ’. If the wire be considered a tangent to the curved surface of the cylindrical magnet, and it be carried round that surface into any other position, or if the magnet itself be revolved on its axis, so as to bring any part opposite to the tangential wire; still, if afterwards the wire be moved in the directions indicated, the current of electricity will be from P to N; or if it be moved in the opposite direction, from N to P; so that as regards the motions of the wire past the pole, they may be reduced to two, directly opposite to each other, one of which produces a current from P to N, and the other from N to P.

“The same holds true of the unmarked pole of the magnet, except that if it be substituted for the one in the figure, then, as the wires are moved in the direction of the arrows, the current of electricity would be from N to

P, and when they move in the reverse direction, from P to N.

“Hence the current of electricity which is excited in metal when moving in the neighbourhood of a magnet, depends for its direction altogether upon the relation of the metal to the resultant of magnetic action, or to the magnetic curves, and may be expressed in a popular way, thus: Let AB (fig. 75) represent a cylinder magnet, A being the marked pole, and B the unmarked pole; let PN be a silver knife-blade resting across the magnet, with its edge upward, and with its marked or notched side towards the pole A; then in whatever direction or position this knife be moved edge foremost, either about the marked or the unmarked pole, the current of electricity produced will be from P to N, provided the intersected curves proceeding from A abut upon the notched surface of the knife, and those from B upon the unnotched side. Or, if the knife be moved with its back foremost, the current will be from N to P in every possible position and direction, provided the intersected curves abut on the same surfaces as before. A little model is easily constructed, by using a cylinder of wood for a magnet, a flat piece for the blade, and a piece of thread connecting one end of the cylinder with the other, and passing through a hole in the blade, for the magnetic curves; this readily gives the result of any possible direction.

“When the wire under induction is passing by an electro-magnetic pole, as for instance, one end of a copper helix traversed by the electric current the direction of the current in the approaching wire is the same with that of the current in the parts or sides of the spirals nearest to it, and in the receding wire the reverse of that in the parts nearest to it.

“All these results show that the power of inducing electric currents is circumferentially exerted by a magnetic resultant, or axis of power, just as circumferential magnetism is dependent on, and is exhibited by an electric current.”^[64. Experimental Researches pp. 32, 34. ]

Dr Faraday has made several experiments with the large compound magnet of Dr G. Knight, belonging to the Royal Society, and consisting of 450 bar magnets, each 15 inches long. The electrical effects which it exhibited were very striking. When a soft iron cylinder, 13 inches long, was put through the compound hollow helix, with its ends arranged as two general terminations, and these connected with the galvanometer; then, when the iron cylinder was brought in contact with the two poles of the magnet, so powerful a rush of electricity took place, that the needle whirled round many times in succession. When Dr Faraday brought the helix alone near to or between the poles, the needle was thrown 80°, 90°, or more, from its natural position.

Dr Faraday failed in obtaining evidence of chemical decomposition by the magnet, or any sensation on the tongue, or any effect on a frog, but he afterwards, by an armed loadstone of Professor Daniell’s, lifting 30 pounds, not only thought that he perceived a sensation on the tongue, and a flash before the eyes, but was able to produce a very powerful convulsion in the limbs of a frog, every [7:21:692] time that magnetic contact was made, the convulsive effect increasing with the suddenness with which the contact was broken and restored.

Dr Faraday, as we have now seen, was the first person who obtained, in November 1831, the electric spark from a magnet. The spark which he obtained was got from a soft iron magnet, made by the influence of a voltaic current. Professor Nobili^[65. Ann. de Chim. December 1831, and Antologia, November 1831. ] and Antinori afterwards obtained the electric spark from a soft iron magnet, made by the influence of a common artificial magnet; and Professor Forbes (March 1832) obtained the electric spark from a soft iron magnet, made by the influence of a natural loadstone.^[66. See Phil. Mug. June 1832, p. 401, and Lond. and Ed. Phil. Mag. November 1834. ] The method adopted by Professor Forbes is shewn in fig. 76, where A is a suspended natural magnet. A cylindrical keeper or armature, ab, has a helix, c, coiled round it, about 7½ inches long, and consisting of about 150 feet of copper wire, about 1-20th of an inch in diameter; the helix consisted of four layers in thickness, separated by insulating partitions of cloth and sealing wax. The branch bde of the wire terminates in the bottom of the glass tube h, containing mercury, with a pure surface. The other branch f of the helix communicates by means of the mercury cup i , with the iron wire g, the fine point of which is brought by the hand into contact with the surface of the mercury in h. and is separated from it the instant the keeper ab is brought into contact with the poles of the magnet; the spark is then produced in the tube h .^[67. Edin. Trans, vol. xii. ]

That the action of magneto-electricity is the converse of that of electro-magnetism, is well shown in the rotatory apparatus in fig. 77. It consists of a copper disc C, revolving round a horizontal axis by means of the handle H. A powerful horse-shoe magnet, AB, is so placed that the edge of the disc C, can revolve between its poles n, s. Two conducting wires w, w ’ , are so placed, that two of their extremities terminate in the mercury cups of a galvanometer g, while the other end of the first is kept in perfect metallic contact with the axis, and the other end of the second is in contact with the circumference of the disc at the point between the poles n and s of the magnet. When this disc revolves from right to left, an electric current proceeding from the centre to the circumference of the disc, is generated in the direction of the curves, and the needle of the galvanometer is deflected. If the disc revolve from left to right, the electric current moves in the opposite direction.

For further information on the subject of magneto-electric induction, see Mr Faraday’s recent papers in the Annales de Chimie et de Physique, tom. li. p. 404, &c., Lond. and Ed. Phil. Mag. October and November 1840, vol. xvii. p. 281 and 356, and Dr Golding Bird’s Elements of Natural Philosophy, Lond. 1839, p. 243.

Description of Magneto-Electric Apparatus.

After Dr Faraday’s great discovery of magneto-electric and volta-electric induction, various machines were constructed for experimental investigation and exhibition. M. Hippolyte Pixii of Paris exhibited to the Academy of Sciences in 1832, his magneto-electric machine. A powerful magnet was made to revolve with great rapidity before its keeper or armature, which had round it a coil of copper wire about three thousand feet long. By this means sparks and severe shocks were obtained, a feeble charge was accumulated in a Leyden phial, the gold leaves of an electrometer were made to diverge, and water was decomposed.

A very ingenious and complete machine was exhibited by Mr Saxton, at the meeting of the British Association in June 1833, as shewn in fig. 78. The magnet A is a horse-shoe one of great power, composed of many steel plates, closely applied to each other, or it may be a soft iron electro magnet of the same shape. A keeper, CD, of the purest soft iron, has each of its ends bent into a right angle, and is so mounted that the surfaces of their ends are exactly opposite and close to the poles of the magnet. In this position the keeper CD may be made to revolve round the horizontal axis EF, by means of the wheels C and E, and band GE fixed to the upright pillar B. Round each end, C, D, of the keeper, are coiled two series of copper wires, covered with sdk, so as to form compound helices. The ends of these wires, which have the same direction, are joined together, and are likewise connected with a circular disc, revolving with the keeper in a cup of mercury, with which in every position of the disc it is in metallic communication. The other ends of the wires are joined, and passing together without metallic contact through the spindle EF, terminate in a slip of copper, with two opposite points, as at i , at right angles to the axis. These points alternately dip into, and rise above the mercury, in another cup, K, which may be connected with the first by means of a copper wire. Now, whenever the ends of the keeper are opposite the poles of the magnet, the keeper becomes a temporary magnet, and it ceases to be so when the line joining them is at right angles to the line joining the two poles. The instantaneous generation and extinction of the magnetic force, which takes an opposite direction in the keeper according as its opposite ends are close to the same poles, and induce corresponding opposite electric currents in the copper wire, provided the circuit is complete by the immersion of the points at i. The arrangement of the points at i is such, that they just rise from the mercury as the ends of the keeper come opposite to the poles of the magnet; and hence the sudden breach of the circuit makes the current pass in the form of a brilliant spark. If a fine platinum wire, instead of the dipping points, forms the communication between the revolving disc and spindle. It may be kept at a red heat. Its light slightly intermitting from the alternation of the currents. If a communication is formed between[7:21:693] the two cups of mercury by two copper cylinders grasped in the hands, a strange sort of shock will be experienced, which is sometimes almost intolerable. Chemical decompositions are also readily effected, and the amount will be proportioned to the quantity of electricity in circulation.

The magneto-electric machine has been greatly improved by Mr E. M. Clarke, magnetical instrument maker, London. It is represented in fig. 79, where A is the battery of bent bar magnets resting against the vertical board B, and by means of a bar of brass C, with a bolt and screw-wheel, the magnets can be drawn firmly to the board B, or taken from it. One of the keepers or armatures D is screwed into a brass mandrill between the poles of the magnets, and it is made to revolve by the multiplying wheel E. This armature has two coils of fine copper wire 1500 yards long wrapped round its cylinders, the beginning of each coil being soldered to the armature D, from which also projects a brass stem carrying the break piece H, which can be fastened in any required position by a binding screw. A hollow brass cylinder K, to which the ends of the coils are soldered, being insulated by means of a piece of hard wood attached to the brass stem. An iron wire spring O presses at one end against the cylinder K, and is kept in contact with it by a screw in a brass strap M in the wooden block L. A square brass pillar P fits also a square opening in the other brass strap N on the other side of the block L. A metallic spring Q rubs gently upon the break piece H, and is retained in perfect metallic contact with it by a screw in the pillar P, the two straps of brass M, N, are connected by a piece of copper wire T, and in this state the parts D, H, Q, P, N are in connexion with the commencement of each coil, and the parts K, Ο, M with the termination of each coil. The perfect metallic contact thus obtained by the spring and break, enables Mr Clarke to dispense entirely with the use of mercury, which is at all times a troublesome accompaniment of machinery.

But the great superiority of Mr Clarke’s machine arises from his employing two different armatures, and thus being enabled to produce the separate effects of quantity and intensity to the full extent of the power of his battery. Having, in November 1834, tried the effects of coils of wire of different thicknesses, he found that the thick copper bell wire gave brilliant sparks, but no perceptible shock, while very fine wire gave powerful shocks, but very feeble sparks. By means of the intensity armature, which is that shewn in fig. 79, the various experiments made with a number of separate galvanic plates may be performed, while the intense agony produced by its shocks is intolerable: It can, at the same time, electrify the most nervous person without occasioning the least uneasiness. It decomposes water and the neutral salts. It deflects the gold leaves of the electroscope, charges the Leyden jar; and by an arrangement of wires from the mercury box to the battery, the electricity is made visible, passing from the magnetic battery to the armature, and sparks and brilliant scintillations of steel can be obtained.

The quantity armature differs greatly from the intensity one, as is shewn in fig. 80, which exhibits the method of producing the spark. The weight of the iron in the cylinders is much greater than in the intensity one, the copper wire is much thicker, and its length is only forty yards. By this armature all the experiments can be made which are usually performed by a single pair of voltaic plates of large surfaces, or by a calorimotor; but it will not do for any of the intensity experiments. It produces such large and brilliant sparks, that a person can read small print from the light it produces. Il ignites gunpowder and platina wire, without enclosing the wire in a hermetically sealed glass case. It deflagrates gold and silver leaf, and produces brilliant scintillations from a small steel file. It produces also rotatory motions in delicately suspended wire frames round the poles of a vertical horse-shoe magnet,^[68. See Loud, and Edin. Phil. Mag., Oct. 1836, No. 54, vol. ix. p. 262, and Noad’s Lectures, p. 344. ] and all the other effects of voltaic electricity.

Several very curious and unexpected results were obtained on a magneto-electric machine of very large dimensions, which Mr Clarke exhibited at the meeting of the Electrical Society. The battery was separated into two parts connected by the armatures, the quantity armature being at one side, and the intensity one at the other. The quantity armature had a short coil of thick insulated copper wire, and the intensity one had 15,375 yards of fine copper wire. The intensity arrangement, to the surprise of every body, gave no decomposition, but gave an excruciating, and even dangerous shock, while the quantity arrangement gave one cubic inch of the mixed gases in four minutes. Considering these unexpected results as owing to a compound action produced by the rotation of the two armatures, he arranged the magnets as in his first machines, the only difference being in the size of the new machine, and in the armatures being moved by a crank and treadles. The battery was composed of ten cut and polished steel bars, each four feet long, the whole weighing 156 lbs. According to Mr Noad,^[69. Lectures on Electricity, p. 352. ] the novel results of the experiments were the great amount of gas given by the quantity armature, viz. one cubic inch in one and a half minutes, and the trifling decomposing effect of the intensity armature. The intensity spark was long, straggling, and noiseless, like a spark at the striking distance from the prime conductor of an electrical machine, while the quantity spark had the usual stellar form, but was attended with a loud snapping noise, as in the discharge of a Leyden jar. Both the sparks, however, were equally luminous. By employing a secondary coil, Mr Noad has given shocks with the quantity armature, almost as powerful as those obtained from the intensity one, by using the form of coil first proposed by Professor Callan of Maynooth, and the contact-breaker of Dr Golding Bird.

The following arrangement (fig. 81 ) for producing powerful shocks, and strong chemical action by secondary currents, was first given by Dr Golding Bird. Upon a reel, with a hollow axis three inches long, wound about 60 feet of copper[7:21:694] wire, 1 / 12 th inch in diameter, covered with cotton thread. The two ends of the wire are connected with p,p', by means of binding screws. Over this primary coil is wound a second insulated copper wire, 1 / 80 th inch in diameter, and about 1500 feet long, and the two ends of this wire are connected with s , s', by means of binding screws. From the law of electro-dynamic induction. It is evident that, if the ends p,p, of the thick coil are connected with a single pair of voltaic elements, as at a , a current of electricity is set in motion in the thin coil, and, in breaking contact, a second current in another direction traverses the same coil, sufficiently intense to give a powerful shock, by grasping the handles d, d, communicating with the extremities s, s', of the thin coil. The intensity of the secondary or reduced current is greatly increased, by inserting a bar i , of soft wire, or what is better, a bundle of soft iron wires in the hollow axis of the steel, which becomes magnetic.

The ingenious method of breaking contact in this arrangement, which we owe to Dr Golding Bird, though shewn in fig. 81, is more distinctly represented in fig. 82. It consists of a base of wood, eight inches long and three broad, having at both ends a piece of hard wood, A, B, each piece having two holes excavated in it to hold mercury. The holes in A communicate with those in B, by thick copper wires D, D. A piece of soft iron wire EF, five inches long, and one-eighth inch diameter, supported with screws with milled heads, moves in a vertical plane upon the upright stem C. Round the wire EF are wound two helices of thin insulated copper wire in the same direction from right to left, so that the two ends of one helix may terminate in the copper points G, H, and those of the other in the points KNL. The small horse-shoe permanent magnets, shewn in fig. 82, are fixed on proper supports, near the ends of the bar EF, so that in depressing the end F of the bar. It may be opposite one, suppose the south pole of one magnet, and consequently the end E will be opposite the other, the north pole of the second magnet. On raising the end F, the contrary will take place, and to effect this the similar poles of the magnet should be in the same direction. Upon connecting the mercury cups in A or B, as shewn in fig. 81, with the small voltaic battery at a, the wire EF will become a temporary magnet, if the ends of either helix are allowed to dip in the mercury; and if connection with the battery is properly made, the ends or poles of the temporary magnet will be repelled by the poles of the permanent magnet to which they are opposed; the bar EF will therefore move, and make the ends of the second helix dip in the other cups of mercury,—repulsion will again take place, and so on ;—in this way, about 300 oscillatione of EF can be obtained in a minute. Upon connecting the ends p,p, of the thick helix with a single voltaic pair, by means of this contact-breaker, a series of powerful induced currents will be obtained from the extremities s, s’, of the larger helix. This connection is best made, as in fig. 82, where R is a section of the reel, S one end of the short helix, connected with a cup of mercury in the piece B, Z the other end of the short helix, connected with one plate of the battery, while the wire C connects the other cup of mercury in B with the other plate of the voltaic pair. When the points G, H, K, L, leave the mercury, very brilliant sparks are produced. A loud snapping noise accompanies them, and a vivid combustion of the mercury, clouds of the oxide of mercury being largely evolved. If the ends P, R, fig. 82, or s , s' , fig.81, have platinum points, and are immersed in water, acidulated with sulphuric acid, torrents of minute bubbles of oxygen and hydrogen are evolved; and if, instead of water, the points are pressed upon paper, moistened with iodide of potassium, iodine and oxide of potassium are separated. Solutions of sulphate of potash and soap, chloride of potassium, sodium, antimony, and copper, are also rapidly decomposed. Dr Page was the first person who suggested the application of permanent magnets for the purpose of breaking contact, though Dr Bird afterwards, and without knowing this, made the same application.^[70. See Lond. and Edin. Pbil. Mag., Jan. 1838, vol. xii. p. 18; Noad’s Lectures, p 364; and Dr Golding Bird’s Elements of Nat. Phil. chap. xvii. ]

We shall now conclude this chapter with a brief notice of some very recent investigations of Dr Draper of New York, on the electromotive power of heat. The apparatus which he employed is shewn in fig. 83, where AA is a glass vessel about three inches in diameter, with a neck wide enough to receive a mercurial thermometer b, and the extremity of a pair of thermo-electric wires about a foot long, and the 1 / 16 th of an inch in diameter, soldered at s with hard solder. The free extremities of these wires dip into the glass cups d, d, filled with mercury, and immersed in a trough e, containing water and pounded ice. By means of the copper wires f, f, 1 / 6 th of an inch in diameter, the apparatus is connected with the mercury cups of the galvanometer, the coil of which is of copper wire, 1 / 8 th of an inch thick, and making only twelve turns round the ar-tatic needles, whose deviations are determined by the torsion of a glass thread, as invented by Dr Ritchie. When a copper and iron wire are used, they indicate temperatures with a promptitude and accuracy quite surprising. In [7:21:695]using this apparatus, the vessel AA is filled two-thirds with water, the bulb of the thermometer being in the middle of the vessel, and the soldered extremity s of the wires in contact with it. The wires f, f are then placed in the cups, and the trough e filled with water and pounded ice, and carefully surrounded with flannel. The water in AA is then brought to the boiling point with a spirit-lamp, and kept at that temperature till the astatic needles and thermometer are steady. For temperatures above 212°, Dr Draper substitutes for the glass vessel a tubulated retort containing quicksilver. Dr Draper’s experiments were all made with the metals in the form of wires, and he obtained the following general results:—

1. Equal increments of heat do not set in motion equal quantities of electricity.

2. The tension undergoes a slight increase with increase of temperature.

3. The quantity of electricity evolved at any given temperature is independent of the amount of heated surface, a point being as efficacious as an indefinitely extended surface.

4. The quantities of electricity evolved in a pile of pairs is directly proportional to the number of the elements.

Dr Draper has been led to the following forms of construction, which give peculiar advantages to thermo-electric combinations.

In fig. 84, A, let a be a bar of antimony, and b one of bismuth, soldered at c d, and let the temperature be raised at d, a current is excited, which does not pass round the bars a ft, but in a shorter and readier path, through the metals between c d, circulating as shewn by the arrows. Nor will the whole current pass round the bars till the temperature of the soldered surface has become uniform. The combination A will therefore be improved, by giving it the form in fig. B, a part being cut out at the dotted lines. In this form the whole current will be immediately forced to pass along the bars, and in such a pair the temperature will change very quickly. Dr Draper considers the form in fig. C, as the best for a thermo-electric couple. In this form a is a semi-cylindrical bar of antimony, ft one of bismuth, united together by the opposite corners of a lozenge-shaped piece of copper c. From its extent of surface, the copper becomes readily hot and cold, and may be made very thin. With a pair of bars, three-fourths of an inch thick, and a circular copper plate c , with both surfaces blackened, Dr Draper repeated the greater part of those experiments, which M. Melloni made with his multiplier. Dr Draper found that thermo-electric currents, evolved by pairs of different metals, do not differ specifically, like the rays of light and heat.^[71. See Lend, and Edin. Phil. Mag., June 1840, vol. xvi. p. 431. ]

We regret that our limits do not permit us to give an account of some interesting experiments of Mr Noad, on the effects of strong and weak electrical currents,^[72. Lectures, p. 356. ] on long flat coils of considerable breadth of surface, and of various inventions made in the United States by Dr Henry, Dr Page, and other eminent philosophers, an account of which will be found in the recent numbers of Professor Silliman’s American Journal of Science.

CHAP. IV ON THERMO-ELECTRICITY.

While investigating the influence of heat in voltaic combinations, Dr Seebeck of Berlin was led to the important discovery that magnetism was developed in two metals forming a circuit, when the equilibrium of temperature in that circuit was disturbed.

If A B C D, for example, fig. 85, be a metallic circuit, consisting of an arch of bismuth, A B C, and an arch of copper, A D C, then if one of the junctions, A, is heated, an electrical current is established, passing into the heated junction from the bismuth to the copper. From many experiments, Dr Seebeck found that, in various circuits formed with bismuth and other metals, the current always passes from the bismuth to the other metals, the bismuth losing positive electricity, or becoming negative with all the other metals.

The order of the metals, beginning with galaena, in which they are negative in reference to those which precede them, is given in the following table, which, excepting some additions and alterations, was drawn up by Professor Oersted.
[table]

Although Dr Seebeck found that most of the metals which stand near each other in the above series produce feeble thermo-electricity, and those more distant a more powerful effect, yet this law did not always hold. Tellurium, for example, is less thermo-electric with bismuth, and most of the other metals, than antimony is; and with silver it is more effective than most of the metals above it. Antimony, too, is more effective with cadmium than with mercury; while iron is very feebly thermo-electric with most of the other metals, especially nickel and cobalt.

The effects of the sulphurets Dr Seebeck found to be remarkable. Sulphuret of lead becomes negative even in contact with the bismuth. The sulphurets of iron, arsenic, cobalt and arsenic, and copper, all of which have a maximum of sulphur, stand near to bismuth; while all the sulphurets, with a minimum of sulphur, have nearly the same power as antimony. The sulphuret of copper, with a minimum of sulphur, occupies a place even below antimony. Concentrated nitric and sulphuric acids stand above bismuth; while concentrated solutions of potass and of soda are below antimony and tellurium.

It has been considered probable that the specific heat [7:21:696]and the conducting power of the metals perform a part in the thermo-electric phenomena; but this is not established by observations yet made.

The following table, by Professor Cumming, shews the relations of the thermo-electric and voltaic series, and of the series of conductors of heat and electricity. To these we have added two columns on the optical properties of the metals.

series of conductors.
[table]

The structure or the crystalline arrangement of the particles of bodies seems to exercise some influence over their thermo-electric powers. In a thermo-electric combination of zinc and silver, for example, the electricity increases with the temperature up to about 250° of Fahrenheit, when it ceases altogether, and by increase of temperature the electric current is re-established in an opposite direction.

In order to measure the thermo-electric power of different binary combinations of metals, from the same differences of temperature, a compound circuit must be formed of all those which we desire to compare. The junctions of the metals must be kept at the temperature of melting ice, excepting the junction which is to be made active, and which is to be plunged into hot oil. In this way the mere conducting power of the circuit is the same in every experiment, and the results obtained become strictly comparable.

The following table, given by Becquerel,^[76. Traité Exp. de L'Electricite, tom. ii. p. 53. ] exhibits the quantities of the currents for a difference of temperature of 36°, of pairs of eight metals differently arranged. The lengths of the metals were 7·88 inches, and their diameter about the 200th of an inch. The sign + indicates the metal from which the electric current proceeds.
[table]

If we compare the numbers in the last column, we shall find, as Μ. Becquerel states, that, for a temperature of 36°, each metal acquires such a degree of thermo-electric power that the intensity of the current, produced by the contact of the two metals, is equal to the difference of the quantities which represent each of these actions in each metal. Thus, if we call the power of each metal p, we shall have, in the case of the iron and copper junction, p. iron — p. platina = 36∙07∙ Substracting the first from the second, we have p. copper— p. platina = 8∙11, instead of 8∙55, given by experiment. The iron and tin junction gives 31∙24, and that of copper and tin 3∙50. The difference in that of iron and copper is thus 27·74, in place of 27·96 by experiment. The intensity of the thermo-electric current being, therefore, equal to the difference of the thermo-electric action produced in each metal by the same temperature, we shall obtain the powers of each of these metals as follows: Calling the power, or thermo-electric action of iron at 36° Fahrenheit, x, we shall have,
[table]

Hence, if x were known, we should obtain p upon the supposition that the thermo-electric powers are proportional to the radiating powers of the metals. M. Becquerel has obtained the following numbers :—
[table]

These values will suit any thermo-electric circuit, and all cases where the thermo-electric power increases with the temperature, that is for all temperatures below 122° Fahrenheit.

Μ. Nobili^[77. ‘ Biblioth. Univers, tom. xxxvii. p. 54. ] formed similar circuits, with substances whose conducting power was inferior to that of the metals. Having made cylinders of porcelain clay, about two and a half inches long, and three and a half lines in diameter, he coiled round the ends of each of them cotton steeped in a conducting liquid, by which they were made to communicate directly with the galvanometer. One end of the cylinder [7:21:697]was brought to a point, and after it was made red hot by a spirit lamp, he pressed it against the cold extremity of the other cylinder, when he found that a current was established from the hot extremity to the cold one. This effect, as M. Becquerel states, arises from the mutual reaction of the two portions of water of different temperatures.

It did not escape the sagacity of Dr Seebeck, that the thermo-electric current might be increased, by forming a compound thermo-electric current, and arranging the metallic couples in a series analogous to those in the voltaic circuit. Having met, however, with some obstacles in this part of his inquiry, he discontinued the investigation, which was taken up without their knowing that he had entered upon it, by Baron Fourier, and Professor Oersted. They first employed a hexagonal combination of three pieces of bismuth, and three of antimony, soldered together. One side of the hexagon was placed in the magnetic direction, and a compass put below it. One of the junctions was then heated, then two, not adjacent. then three, always leaving one junction not heated between the two heated ones. By heating one junction the needle deviated some degrees, still more by heating two, and still more by heating three junctions. When three junctions were cooled with ice, the other three having the ordinary temperature of the atmosphere, effects still more distinct were produced. When three alternate junctions were heated, and the other three cooled with ice, the needle deviated sixty degrees.

In a rectangular circuit, with twenty-two bars of antimony, and twenty-two of bismuth soldered together, the same effects were obtained. After dissolving one of the junctions, a little mercury cup was soldered to each of the disjoined bars, so that the circuit could be re-established by different means. A copper wire, four inches long, and l-25th of an inch in diameter, nearly re-established the current; and it was completely re-established by two parallel pieces of the same ware. A wire of the same diameter, but three feet long, was found a tolerably good conductor; but a platina wire, about sixteen inches long, and 1-50th of an inch in diameter, scarcely transmitted a fortieth part of the effect. Acids, and solutions of alkalies, and other metallic oxides, though good conductors in the voltaic or hydro-electric circuit, insulated entirely the thermoelectric current. The same effect was produced by two discs of silver, separated by the thinnest blotting paper, moistened with sulphate of copper. In these experiments, the intensest current produced no chemical effects, no ignition of the wires, and no electric condensation; but a prepared frog was made to palpitate.

In thermo-electric currents, which differ only in lengths, the shortest is the most powerful, a circuit of double length having little more than half the effect. In order to find the law of increased effect, as depending on the number of junctions, Professor Oersted composed circuits of equal length with different numbers of junctions.

In fig. 86, is shewn a simple circuit consisting of one bar, a a, of antimony, and one, b b, of bismuth, and in fig. 87, a complex circuit of the same length and materials. When one of the junctions in fig. 86 was heated or cooled, and two of the junctions at the extremities of the diagonals in fig. 87. heated or cooled to the same degree, the deviation of the needle was 22° in the first case and 30° in the second.

In like manner, open circuits, as in figs. 88, 89, having each the same length, but double that of the preceding two, had, the one one junction, and the other three junctions, heat-or cooled equally, the first gave a deviation of about 14°, and the other nearly 32°.

“In several complex circuits,” says Professor Oersted,^[78. Edinburgh Encyclopaedia, Art. Thermo-Electricity , vol. xviii. p. 585. ] “it is found that the heating or cooling of one junction only produces twice the angular deviations of that added by the addition of one active junction more. The effect of one active junction, when the others are at rest, is, by experiment, found to be twice the effect of all the arrangements divided by the sum of the elements + one. The effect of each addition of a new active junction is only half this quantity, and seems even to be in a decreasing ratio when the number of junctions is great.”

From these and other observations. It appears that the thermo-electric current produces a prodigious quantity of electricity, but in a state of very feeble intensity, while the voltaic current has a very great intensity. The former is impaired by the resistance opposed to it by a long multiplying wire, while the latter is increased in surmounting this resistance. Μ. Pouillet has endeavoured to compare the intensity of these two currents, by passing the hydro-electric current through a platinum wire long enough to reduce it to an intensity which will just balance the thermo-electric current. In one case he found that 590 feet of platinum wire, ∙006 of an inch in diameter, including the resistance of the battery, reduced a hydro-electric current, produced by twelve pair of plates with double coppers, to an equilibrium with that of one thermo-electric pair of bismuth and copper, in a circuit of 65∙6 feet of copper wire, ·039 of an inch in diameter, with a difference of temperature of 76° Fahrenheit. By computing the rotation between the electro-motive forces and the resistance in these two cases, he found that the hydro-electric current had an intensity 114,000 times greater than that of a single pair of bismuth and copper, produced by a difference of temperature, between the two junctions, of 108’ of Fahrenheit.^[79. Becquerel’s Traité, tom. v. p. 27. ]

In order to compare the conductibility of metals for thermo-electro currents, M. Pouillet^[80. Elem. Phys. Exp. liv. v. chap. v. § 426. ] employed two equal thermo-electric currents. The first was weakened by making it traverse the metallic wire submitted to experiment, and the second was weakened precisely the same quantity, by traversing lengths, more or less great, of another wire, which served as the term of comparison. The following table shews the results of this comparison, the conductibility of pure mercury being reckoned 100.
[7:21:698]
[table]

Hence it appears that palladium is the best conductor of thermo-electricity, and mercury the worst, having sixty times less conductibility than palladium. That which has a slight effect on the conductibility of mercury produces a prodigious variation in that of iron or steel. Even the heat of the hand produces very sensible effects, and what M. Pouillet justly thinks still more wonderful, the heating to redness of some millimetres of the length of a wire of iron or steel, is sufficient to make its conductibility three or four times less.

When thermo-electric currents are produced by a single element, and the thermo-electric power remains the same, M. Pouillet found that the intensity of the current which it produces, is inversely as the length of the circuit, and directly as the conductibility of the wire or rod which forms the current. He found, also, that in a thermo-electric circuit, composed of wires of different sections, the elementary force of the current is the same in all powers, if we take equal intervals on these different wires the direct currents will be found to have different intensities, which are nearly in the inverse ratio of the sections of the wires in the intervals of deviation. M. Pouillet succeeded, also, in establishing the curious fact, which had been recognised by M. Marianini,^[81. Ann. de Chim., &c. tom. xlii. p. 131. ] that several electric currents propagate themselves in the interior of bodies, as if they were alone, like light and heat.

The first persons who succeeded in constructing thermoelectric piles, were MM. Nobili and Melloni, who employed them successfully in their experiments on radiant heat. This instrument, however, has since been improved by Melloni, and we shall, therefore, describe it in preference after Μ. Becquerel. M. Melloni constructed his thermo-electric pile of fifty small bars of bismuth and antimony placed in a bundle, as shewn in fig. 90, the length of the bundle being 30 millimetres, and its section 96 centimetres square. The two terminal faces are blackened. The bars of bismuth, which alternate with those of antimony, are soldered at their extremities, and separated throughout their whole lengths by an insulating substance. The first and the last bar have each attached to them a copper wire, abutting against one of the pins c , c' , of the same metal, passing through a piece of ivory fixed in the ring AA'. The interval between the interior surface of this ring and the elements of the pile is filled with insulating matter. The free extremities of the two wires communicate with the ends of the wire of a multiplier, the needle of which indicates when the temperature of the anterior face of the pile rises or falls above that of the posterior face. Two metallic tubes B, B, polished without, and blackened within, are fitted to the two ends of the pile, to protect them from lateral radiations. The multiplier is shewn in fig. 91, where AB c is the frame enveloped by the copper wire, whose extremities abut against the metallic tubes FF', fig. 92. This frame is fixed on a horizontal stage DE, which can turn in its own plane, and round its centre, by means of a toothed wheel and pinion placed below, and moved by the milled head G.

MN is the support of the astatic system of the two magnetic needles, suspended by the silk fibre VL, and the cylinder of glass RS covers the apparatus, and rests on the base KL Fig. 92 is a section of the apparatus, by a plane passing through the support, and one of the tubes of communication. The needles are 53 millimetres long, the diameter of the copper wire is 0.76 mm., and it is doubly covered with silk, and makes 150 circumvolutions round the frame, which is 6 millimetres high, having its length a little greater than that of the needles.^[82. Becquerel’s Traité, &c. torn. iii. p. 425. ]

We have already seen that thermo-electricity possesses the same general characters as common and voltaic electricity. Although Oersted failed in obtaining chemical action from his thermo-electric combinations, yet Professor Botto^[83. Bibliothèque Universelle, Sept. 1832. ] of Turin subsequently decomposed acidulated water. His apparatus consisted of a metallic wire or chain, composed of 120 pieces of platinum wire, each an inch long, and 1 / 100 th of an inch in diameter, alternately with the same number of pieces of soft wire of thesamedimensions. This chain was wrapped spirally round a wooden rule 18 inches long, so that the joints were placed alternately at each side of the ruler, receding from the wood at one side to the distance of four lines. By using a spirit-lamp the same length as the helix, and a Nobili’s galvanometer, a very energetic current was shewn to [7:21:699]exist, and the decomposition of acidulated water was increased, by substituting copper in place of platinum poles, in which case hydrogen only was set free. An increased temperature augmented the current and the decomposition. Μ. Botto obtained still more powerful effects by a pile of bismuth and antimony, consisting of 140 elements forming a parallelopiped, with a base of 2¼ inches, and a height of 1 inch.

A distinct electric spark has also been obtained from the thermo-electric pile, by the Chevalier Antinori of Florence. Professor Linari^[84. L’Indicatore Sanese, No. 50, Dec. 1836. ] l of Sienna verified this result with a Nobili’s pile of 25 elements and temporary magnet, with an electro-dynamic spiral 805 feet long. With this apparatus he obtained a brilliant spark visible in open day, whenever the contact was broken. With this pile and temperatures from freezing to boiling water, he readily decomposed water, and also nitrate of silver. The same thermo-electric current magnetised an unmagnetic needle, and produced the phenomenon of the palpitation of mercury. Professor Wheatstone verified these experiments in 1837, by a thermo-electric pile of 33 elements of bismuth and antimony, forming a bundle three-fourths of an inch in diameter, and 1 1 / 5 th long. The poles were connected by two thick wires, with a spiral of copper ribbon 50 feet long and 14 inch broad, the coils being insulated by brown paper and silk. One face of the pile was heated by red-hot iron brought near it, and the other cooled by ice. Two strong wires connected the poles of the pile and the spiral, and the contact was broken, when necessary, in a mercury cup, between one extremity of the spiral and one of these wires. A distinct spark was seen in open day whenever the contact was broken.^[85. Lond. and Edin. Phil. Mag. vol. x. p. 415, May 1836. ]

The thermo-electric pile has been greatly improved by Mr Watkins, who employs a flat copper ribbon coil. In piles varying from 15 to 30 pairs of elements, he obtains brilliant sparks, by merely pouring hot water on one end, while the other has the temperature of the air.

Professor Andrews of Belfast has recently succeeded in developing thermo-electric currents, by simply bringing two metallic wires at different temperatures into contact with a fused salt, between which and the wires no chemical action takes place. This result he first obtained by means of fused borax. He took two similar wires of platina, and connected them with the extremities of the copper wire of one of Gourjon’s galvanometers, and fused a small globule of borax in the flame of a spirit-lamp on the free extremity of one of the platina wires, and having introduced the free extremity of the other into the flame, he brought the latter, raised to a higher temperature than the former, into contact with the fused globule. When this was done, the needle of the galvanometer was instantly driven with great violence to the limit of the scale. The direction of the current was always from the hotter platina wire, through the salt, to the colder wire. Professor Andrews obtained a permanent electric current in the same direction, by simply fusing the globule between the two wires, and applying the flame of the lamp in such a manner, that the wires, at their points of contact with the fused salt, had different temperatures. Fused carbonate of soda gave similar, but more powerful currents than borax. Carbonate of potash, chloride and iodide of potassium, sulphate of soda, chloride of strontium, heated glass, &c. produced similar currents; and even boracic acid, though such an imperfect conductor, deflected the needle 40°.

The currents thus produced have an intensity inferior to that of the hydro-electric currents, and they are capable of decomposing with great facility water and other electrolytes.

Before the salts were actually fused, Professor Andrews found that electrical currents were generated whose directions no longer followed the simple law, but varied in the most singular and perplexing manner, passing first from the hot to the cold wire, then by more heat from the cold to the hot, and by more heat still from the hot to the cold wire.

Professor Andrews obtained similar currents, by interposing certain minerals between unequally-heated platina wires. Mica, heated very strongly, caused a deflection in the needle of 7°, and Stilbite a deflection of 25°, the current being in both cases from the hot to the cold wire^[86. Lond. and Edin. Phil. Mag. vol. x. p. 433, June 1837. ].

Thermo-electric rotations were, we believe, first produced by Professor Cumming, by means of a very simple apparatus. He formed a rectangle of silver and platina, as shewn in fig. 93. The three upper sides are formed of silver, and the lower of platina. When suspended, as shewn in the figure, and when one of the junctions was heated. It revolved from left to right, when the pole of a magnet was presented to another junction. When the rectangle was suspended upon the loadstone itself, and heat applied to one of the junctions, the rectangle soon began to turn.

When a chain or wire, consisting of alternate links, or pieces of platinum and silver, is made part of the voltaic circuit, the links or portions of platinum wire will become red-hot, while those of silver remain dark, and comparatively cold.

In studying the effects of thermo-electric currents, Μ. Peltier made the interesting discovery, that cold, instead of heat, is produced at the points of junction of certain crystallisable metals. The instrument by which he obtained these interesting results is shewn in fig. 94, where A B are two thermo-electric couples in bismuth and antimony, C a copper wire which unites the antimony a' of the upper couple to the bismuth b of the lower couple. D, E, copper wires communicating with the galvanometer G of 84 coils, and completing the circuit between the upper bismuth B'', and the lower antimony a'. F, Hare the free extremities of a", b", which form a pair of pincers, which press against each other by a spring. The bar JK is formed by a bar of antimony α"', and of bismuth b"', which ought to traverse the electric current. L, M, M ’ are conductors of the pile P, N a plate of copper, with a graduated circle and magnetic needle O, for measuring the quantity of electricity which passes through the entire circuit M M ’, N, K, b'", a"', L, P. The galvanometer G indicates the electricity produced by the variations of temperature of the ends F, H, resulting from those of the bars J, K, the closed circuit of this electricity being a' , C, a''' , E', c , D, b'' , a' . The ball A of an air thermometer (with its capillary tube E plunged in a vessel d of coloured alcohoD is crossed by a compound [7:21:700]bar of α iv , b iv , of bismuth and antimony united at S'. A graduated scale is placed behind the thermometer tube E'. The current of the pile P is received by copper conductors F, G.

If we now place wires of different metals between the thermo-electric pincers F, H, and vary the intensity of the current, we shall observe a rise of temperature when the conductors are homogeneous, the heat being the same throughout the whole length of the wires, with the exception of their extremities, where it increases or diminishes according as the pincers which retain the wire are worse or better conductors than itself. When copper pincers are used, a depression of temperature is felt at the distance of two or three centimetres, according to the intensity of the current. M. Peltier has found that when the current has double the intensity, or when the section of the conductor is one-half, the temperature is tripled. The results obtained by Μ. Peltier tend to prove that the two electricities produce heat by their union, however feeble be their intensities.^[87. Μ. Becquerel’s Traité, &c., tom. iii., p. 165, and v., p. 286. ]

We shall now conclude this article with a description of two thermo-electric instruments, viz. Μ. Peltier’s thermo-electric hygrometer, for determining by the change of temperature whether a solution or a chemical combination has taken place when two bodies are brought into contact, and Mr Locke’s thermoscopic galvanometer. M. Becquerel had long ago shewn that when a simple solution takes place, no electrical effect is produced, but that when two substances chemically combine, positive electricity passes from an alkali to an acid, and negative electricity from an acid to an alkali. The nature of the electric wires therefore determine by means of a multiplier, whether combination or simple solution has taken place. But as a change of temperature also takes place, M. Peltier has employed this as the means of deciding whether solution or combination has taken place, cold being produced in the one case, and heat in the other. This hygrometer is shewn without the galvanometer in fig. 95, where A is a wooden disc for supporting the thermo-electric couples, being itself supported by a rod and a bar; B, B, B, three couples of bismuth or antimony forming the thermoscopic support; W, W, the wires leading to the multiplier; D, a platina capsule filled with distilled water, which is to be placed on the couples; E E, a cylinder of card; and F F, a glass receiver open at top, but surrounded with paper to prevent radiation. When distilled water is placed in the capsule. Its spontaneous evaporation produces a depression of temperature which varies ordinarily from 40° to 60°^[88. The degrees of the galvanometer we presume. Μ. Becquerel’s Traité, &c., tom. v., p. 243. ]. As this apparatus is very sensible, the needle of the multiplier arrives rapidly at 90°; but this inconvenience is removed by placing in the circuit supplementary conductors, which diminish the intensity of the current, and bring back the needle to the first 20°. Tables are then formed which give the ratio between the deviations of the needles, the intensities of the current, and the differences; a very extended scale is thus formed which may begin at 150° above zero, and descend indefinitely. To compare this instrument, we have only to determine the extreme dryness produced with muriate of lime placed in a close vessel. When the saturation of the air produces no evaporation, the capsule remains at the surrounding temperature, and the needle at zero. When we have determined the force corresponding to the number of degrees given by extreme dryness, we divided this force into 100 parts, corresponding to 100 degrees of ordinary hygrometers.

Dr Locke, professor of chemistry in the medical college of Ohio, has lately constructed a new thermoscopic galvanometer, the peculiarity of which is the massiveness of the coil, which affords a free passage to currents of the most feeble intensity, and enables them to deflect a very heavy needle. The coil is made of a copper fillet about 50 feet long, one-fourth of an inch wide, 9 1 / 3 thick, and weighing between 4 and 5 pounds. This coil is not made in a pile at the diameter of the circle in which the needle is to revolve, but is opened out, the several turns lying side by side, and covering almost the whole of that circle above and below. It is wound closely and in parallel turns on a circular piece of board 11½ inches in diameter, and half an inch thick, covering the whole of it except two small opposite segments of about 90° each. The board being extracted, leaves a cavity of its own shape, to be occupied by the needle. The copper fillet is not covered with silk or any other coating, but the turns of it are separated at their ends by veneers of wood just so far as to prevent contact throughout. The coil is supported on a wooden ring with brass feet and levelling screws, and surrounded by a brass hoop with a flat glass cover, in the centre of which is inserted a brass tube for suspending by a silk fibre, one of Nobili’s double astatic needles, each part being about 11 inches long, one-fourth of an inch wide, and 1 / 60 th thick. The lower part plays within the coil, and the upper part above it, and the thin white deal placed upon it. This instrument is peculiarly fitted for experiments in a class. It is very sensible to a single pair of thermo-electric metals, to the action of which it seems peculiarly adapted. With a battery of five couples of bismuth and antimony, the radiation of a person 12 feet distant, without any reflector, and when the temperature of the air was 72°, moved the needle sensibly. If a thermo-electric pile, massive in proportion to the coil, is used, this thermoscope would exhibit the experiments of Melloni satisfactorily to a large class. A more detailed account of this instrument will be found in the London and Edinburgh Philosophical Magazine, October 1837, vol. xi., p. 378.

For further information on thermo-electricity, and the other subjects of which this article treats, the reader is referred to Μ. Becquerel’s admirable work, entitled Traite Experimentale de L'Electricité et du Magnetisme, et de leurs rapports avec les phenomenes naturels. Five volumes; i. and ii., Paris 1834; iii., 1835; iv., 1836; and v, 1837.