# Popular Science Monthly/Volume 56/December 1899/Electricity from Thales to Faraday

Layout 4
* ELECTRICITY FROM THALES TO FARADAY.
By ERNEST A. LESUEUR.
IT is so common a notion nowadays that electricity had its birth and rise in the nineteenth century that it gives one a strange mental sensation to contemplate the fact that all the myriads of commercial applications that have of late years been developed in this field might have been made by the Chinese or the ancient Egyptians, so far as the potentiality of Nature for developing electrical phenomena is concerned. The writer used to know a delightful old gentleman in Vermont who once referred, as to a well-known fact, to Edison's having invented electricity. It is astonishing how closely his state of mind typifies that of a great many people.
In the form of the lightning, the aurora, and the shock of the electric eel or torpedo, electrical manifestations have been known ever since man commenced to observe those phenomena, but the fossil resin amber was the substance which eventually gave its name to the now tremendous agency. This material was observed, many centuries before our era, to possess the property of attracting light bodies to itself when rubbed with wool, and, being called ᾒλεκτρον (electron) by the Greeks, transmitted its name to the property or force which it thus brought into evidence. The fact is mentioned as ​early as 600 b. c, by Thales of Miletus, although he does not transmit to us the name of the original observer of the phenomenon. Homely as was the experiment, it marked a beginning in electrical research.
Not that scientific investigations in that or any line were pushed very assiduously in those days, for there is a great gap between the discovery of the property above alluded to and the acquisition of any more solid knowledge pertaining to electricity. The phenomenon was at that time set down in the list of natural facts, and no attempt appears to have been made to connect it with others. The inquiring spirit of the present age can hardly be brought into more striking relief than by a comparison of the, at present, almost daily advances in scientific knowledge with the fact that twenty-two hundred years elapsed between the discovery of the above-mentioned power of amber by the ancients and the later one that a very large number of other substances, such as diamonds, vitrefactions of ail kinds, sulphur, common resin, etc., possess the same property. A few other scattered facts were, however, also noted by the ancients: fire is said to have streamed from the head of Servius Tullius at the age of seven, and Virgil asserts that flame was emitted by the hair of Aseanius.
In examining, now, the history of the rise of electrical science we find, as just mentioned, the vast gap of over two millenniums between the discovery of the attracting power of rubbed amber and the mere extension of man's knowledge so as to include other substances. The philosophers Boyle and Otto von Guericke, who were active during the latter half of the seventeenth century, added a mass of new data in this line. Boyle, moreover, discovered the equivalence of action and reaction between the attracting and the attracted body, and that the rubbed amber or other "electric" retained its attractive powers for a certain period after excitation had ceased.
Otto von Guericke made a vast step forward by constructing the first electrical machine, in a crude form, truly, but which proved of the utmost service in adding to our knowledge of the properties of electricity. His machine was constructed very simply of a globe of sulphur mounted on a spindle, which could be rotated by means of a crank; the operator applied friction with the hand, his body receiving a positive charge, while the surface of the sulphur acquired a negative. The fact of the two electrifications being separated at the surface of the sulphur was not, however, known at the time; the only charge that Guericke observed being that appearing on the sulphur. The reason for this was that the latter, being a nonconductor, any electricity generated upon it was compelled to stay there, ​for a certain time at least, and consequently accumulatcd so as to be observable; whereas the opposite electrification flowing into the operator's hand continuously escaped to earth without giving any sign of its presence. Had the operator stood upon an insulating support, the electrification would have accumulated on his body as well as upon the sulphur. Guericke made the discovery that a light body, having been once attracted to an electrified surface, was almost immediately repelled from it, and could not be again attracted without having its imparted electrification removed by contact with an uncharged surface.
Sir Isaac Newton, about 1675, made an interesting application of a principle allied to this. He used a hollow, drum-shaped contrivance with glass ends and a very short axis, into which he put a number of fragments of paper. On briskly rubbing the outside of the glass with a piece of silk the paper was caused to "leap from one part of the glass to another and twirl about in the air." This was repeated in 1676 before the Royal Society, to the great edification of that learned body.
Newton made a considerable improvement in the electrical machine of Guericke by the substitution of a hollow globe of glass for Guericke's sulphur one. What is chiefly interesting about the improvement is the fact that Guericke's sulphur globe, of comparative weight and cumbrousness, was made by casting melted sulphur into a glass globe and then breaking off the glass. Guericke observed in the dark a peculiar luminosity of conducting surfaces when well charged by means of his machine; he compared it to the phosphorescent light observed when lump sugar is broken in the dark. It was what is now known as the brush-discharge effect.
In 1705 Francis Hawksbee discovered the peculiar phenomenon which he termed the mercurial phosphorus. It was produced by causing a stream of well-dried mercury to fall through an exhausted glass receiver. The friction of the particles of mercury against the jet piece and the glass caused an electrification which evinced itself in a phosphorescent glow. The receiver, indeed, had not to be by any means thoroughly exhausted, the phenomenon occurring at an air pressure up to about fourteen inches of the barometer.
The crackling noise and the spark accompanying electrical discharge suggested about this time the analogy of those miniature disturbances to thunder and lightning, but the identity of the two was not fully established until later.
Up to this time the fact that certain substances were capable of conducting electricity was not known, but in 1729 Stephen Gray, F. R. S., an enthusiastic investigator, made the discovery, and at the same time the cognate one that a large class of materials are ​nonconductors. The only source of electricity which was at the disposal of experimenters up to this time was the electrical machine, improved, as described, by Newton, which furnished intermittent currents (discharges) of infinitesimal quantity, as we should say now, but of extremely high pressure. This fact of the enormous pressure resulted in the electricity's forcing its way through very imperfect conductors, so as to cause our investigators to rank many of these latter with the metals. Thus Gray concluded that pack thread was a good conductor because it did not oppose sufficient resistance to prevent the flow of his high pressure (or, as we should now say, high voltage or tension) electricity. He tried wire as well, but did not realize it was a better conductor than the thread, although its conductivity was actually in the millions of times as great. In collaboration with his friend Wheeler he conveyed electrical discharges a distance of eight hundred and eighty-six feet, through presumably air-dry pack thread—an achievement which would almost be notable at the present time. He insulated the line by hanging it from loops of silk thread.
Gray hoped "that there may be found out a way to collect a greater quantity of electric fire, and consequently to increase the force of that power, which, si licet magnis componere parva, seems to be of the same nature with thunder and lightning."
About this time Desaguliers discovered that those materials which, upon being rubbed, develop electrical charges, are all nonconductors, and that, conversely, nonelectrics are conductors. The terms electrics and nonelectrics were applied to bodies respectively capable and incapable of excitation; the words idioelectrics and anelectrics were also used in respectively equivalent senses.
In France, Dufay discovered that the conductivity of pack thread was greatly improved by the presence of moisture, and he succeeded in conveying a discharge a distance of almost thirteen hundred feet. He suspended himself by silken cords and had himself electrified, and then observed that he could give a shock accompanied by a spark to any person standing on the ground.
He also established the fact of the two opposite kinds of electrification, and gave them the names of vitreous and resinous, from the fact that the former was developed by the excitation of glass and vitreous substances generally, and the latter from that of amber and resins. He observed that the distinguishing characteristic of the two was the fact that opposite charges attracted each other, while similar ones exerted mutual repulsion. Dufay and Gray died within three years of each other, both at the age of forty. Gray having added to the results already mentioned the discovery of the conducting powers of certain liquids and of the human body.
​Experimental research now began to spread into Germany and the Netherlands. The electrical machine was greatly improved by Professor Boze, of Wittenberg, and Professor Winkler, of Leipsic, who respectively added the prime conductor and the silk rubber to that important piece of apparatus. A Scotch Benedictine monk of Erfurt—Professor Gordon—substituted a glass cylinder for the sphere, and thereby brought the instrument in its essentials practically to the form in which it exists to-day. The improvement enabled the production of very large sparks, which were caused to produce the inflammation of various combustibles. Gordon went so far as to ignite alcohol by means of a jet of electrified water.
We now come to an epoch-making discovery—that of the condenser, or, in its conventional laboratory form, the Leyden jar. Professor Muschenbroeck, of the University of Leyden, was struck with the idea that it would be a good plan to try to prevent the dissipation of the electric charge by inclosing the conductor containing it in an insulating envelope. He therefore took a glass jar, partly filled it with water, and electrified the latter. His assistant, who was holding the bottle, accidentally touched the wire which made connection with the water, and received on the instant a shock much more violent than any that the electrical machine was capable of giving. This led to the discovery that as the charge of vitreous electricity had accumulated in the water, a corresponding charge of the opposite kind had gathered upon the outside of the glass and been "bound" there, as it is called, by the attraction exercised upon it by the charge on the inside. It had been enabled to get upon the glass by the fact of the assistant's hand having covered part of the surface of the latter, and, since he stood upon the ground, the electricity had quietly flowed from the latter up through his body to the outside surface of the glass.
The apparatus was quickly perfected by coating both the inside and outside of a jar with tin foil, applying the charge by means of a wire or chain to the inside coating and allowing the outer one to stand upon the earth or upon a conducting substance in electrical contact with the latter. The exaltation of spirit with which the discovery was hailed by the savants appears to have been extraordinary—one student who took a discharge through his body being reported to state that he would not have missed the experience for a fabulous consideration, and that he would not repeat it if it were to save his life. In reality the advance was enormous; it gave a means for literally bottling up electricity in quantities previously unthought of. The prime conductor of an electrical machine could not retain any considerable quantity of electricity for the reason that, a certain small intensity of electrification having been reached, ​the addition operated to upset the balance, so to speak, and the electricity escaped by a sudden (disruptive) discharge, or spark, or by the brush discharge already alluded to. With the Leyden jar, however, as fast as electricity was supplied to the inside coating it became "bound" there by the charge of opposite sign accumulating on the outside, and the limit of capacity of the jar was simply one of strength of the glass: if too much electricity was supplied, the stress of mutual attraction between the two charges relieved itself by destroying the jar.
Although Professor Muschenbroeck discovered the principle in the manner above referred to, it appears extremely probable that two other investigators, working independently, also did the same. One Cuneus and a monk named Kleist each claimed the honor of original invention of the condenser.
About 1747 the first gun was fired by electricity; this was accomplished by Sir William Watson, who also succeeded in kindling alcohol and gas by means of a drop of cold water and even with ice. The same experimenter reversed the ordinary procedure of causing the electric influence to pass from an electrified body to the one to be experimented upon, the latter being unelectrified, by electrifying the latter, and then producing the desired effect by approaching it to an unelectrified one.
A party of the Royal Society with Watson as chief operator instituted a series of researches on a grand scale to determine, if possible, the velocity of the electric discharge, and arrived at a number of conclusions which, however, were of a decidedly negative nature. The most important of these were as follows: That they could not observe any interval between the instant of applying the discharge to one end of the line and its reception at the other; that the destructive effects of discharge are greater through bad conductors than through good ones; that conduction is equally powerful whether occurring through earth or water.
Just previous to this there had been some brilliant experiments carried on in France, and the discharge had been conveyed through twelve thousand feet of circuit, including the acre basin of the Tuileries, but they had not been performed as systematically, or with the definite objects in view, as had the English experiments.
The following year the Royal Society continued its researches on a larger scale than previously, using 12,276 feet of wire, and found that even through that length the velocity was practically instantaneous.
Watson urged as a theory that electrical disturbances were caused by influx or efflux of a single electric fluid from the state of normal electrification, thus differing from Dufay in his opinion as to the ​existence of two fluids. He was led to this belief by observing that he obtained a larger spark between two oppositely electrified bodies than from either to the earth.
From this time on there appears upon the scene a host of workers in this field, one of the most prominent being the distinguished American, Benjamin Franklin. Somewhat previous to his remarkable work, or about 1750, Boze made certain discoveries in the matter of the surface tension of conducting liquids being diminished by electrification, and Mowbray and Nollet ascertained that the vegetation of flowers and of vegetating seeds was hastened by electrifying them.
Franklin (born 1706, died 1790) made the important discovery of the active discharge of electricity from an electrified body by points as well as the converse of it—i. e., that electricity was rapidly abstracted from a charged atmosphere by points. This enabled him to increase the efficiency of the electrical machine by adding a combshaped series of points to the collector of the prime conductor.
Up to this time, although the identity of lightning with electricity had long been suspected, it had not been at all established, and to Franklin may be said to belong the honor of doing so, although in this, as in the case of the invention of the Leyden jar, there appears to have been successful contemporaneous research elsewhere. Before performing his great experiment Franklin published a book strongly supporting the belief in the identity of the two. Once having conceived the idea of drawing electricity from the upper atmosphere, he unfortunately lost some time through waiting for the completion of the spire of a certain church in Philadelphia, from the top of which he hoped to be able to collect electricity by means of a wire, but finally hit upon the device which now fills much the same place in connection with his memory that the classical cherry tree does with Washington's—the lightning-collecting kite. This apparatus was very simply constructed, and had a pointed wire projecting a short distance above the framework. It was controlled, and electrical connection made, by an ordinary string which terminated in a short length of silk ribbon to protect the person from possible injury, and to give electricity a chance to accumulate in the system, by insulating the "line." At the end of the string proper Franklin fastened a metallic key. In company with his son he flew the kite during a thunderstorm which occurred in June, 1752; for some time no electric disturbance approached the neighborhood, and he was on the point of abandoning the experiment when he observed what he had been waiting for—the outer fibers of the string standing out from the latter by repulsive force—and, applying his knuckle to the key, he drew a spark. Subsequently, when the rain soaked the string and caused it to conduct much better, there was a fine supply ​of electricity, and Franklin charged a Leyden jar from the key, thus achieving the actual storage of "lightning."
He continued his investigations in atmospheric electricity, and discovered that the electrification of the clouds (or of the upper atmosphere) was sometimes positive and sometimes negative. The invention of the lightning rod is due to him.
Franklin sided with Watson in his belief in the single nature of the electric fluid.
As intimated above, atmospheric electricity appears to have been collected independently about the same time in Europe, and certain very daring and dangerous experiments were performed there. One sad occurrence, as a result, was the death of Professor Richman, in St. Petersburg, in 1753. Richman, in company with a friend, Sokolow, was taking observations on an electroscope connected with an iron rod which terminated in the apartment and extended in the other direction above the roof of the building. During the progress of their experiments a violent peal of thunder was heard in the neighborhood, and Richman bent to examine the instrument. In doing so he approached his head to within a foot of the end of the rod, and Sokolow saw a ball of fire "about the size of a man's fist" shoot from it to Richman's head with a terrific report. The stroke was, of course, immediately fatal, and what we now know as the return shock stupefied and benumbed Sokolow. The unfortunate event served as a warning to other daring experimenters.
Canton, another prominent worker in this field, discovered that the so-called vitreous electricity was not necessarily always developed by the friction of glass, as had hitherto been believed to be invariably the case. By applying different rubbers to glass he obtained either positive or negative at pleasure. This at once disposed of the idea that one kind of electricity resided in certain bodies and its opposite in others. Canton also made the interesting discovery that glass, amber, rock crystal, etc., when taken out of mercury, were all electrified positively. He was thus enabled to make the improvement in the electrical machine of coating its rubber with an amalgam rich in mercury, which greatly enhanced. its powers.
Among the numerous names now coming into prominence must be mentioned those of Beccaria, Symmer, Delaval, "Wilson, Kinnersley, Wilcke, and Priestley.
The first named, Father Beccaria, was a celebrated Italian physicist who did most valuable work in connection with atmospheric electricity, and who published several classical works on that and allied subjects. Among these may be mentioned his Lettre del Elettricitá, 1758, and Experimenta, 1772. He ascertained that water is not by any means a good conductor, as it had previously been ​supposed to be, and, by using pure water, lie caused the electric spark to become visible in it, a phenomenon capable of occurring only through media almost nonconducting. In these experiments he used thick glass tubes with wires led through the opposite ends, the latter being sealed, and the tubes filled with water. These were invariably shattered by the passage of the spark on account of the accompanying elevation of temperature, which caused expansion. He also established the facts that the atmosphere adjacent to an electrified body acquires electrification of the same sign by abstracting electricity from the body, and that the air then parts with its electricity very slowly. He advanced the theory that there is a mutual repulsion between the particles of the electric fluid and those of air, and that a temporary vacuum is formed at the moment of the passage of a disruptive discharge or spark.
Robert Symmer, in 1759, described some most entertaining experiments, making use of the opposite electrifications of superposed stockings of different materials or merely of different colors (the dye matters in the latter case causing differentiation). If, in a dry atmosphere, a silk stocking be drawn over the leg and a woolen one pulled over it, the two will be found, upon being removed, to be very powerfully electrified in opposite senses. If the four stockings of two such pairs be used and then suspended together, they will indulge in remarkable antics due to each of the silk stockings trying to attract both of the woolen ones, and vice versa, and, on the other hand, each of each kind repelling the other. The amount of electrical attraction and repulsion produced in this simple way in a dry atmosphere is remarkable. The experiment may also be performed with all silk stockings, one pair white and the other black.
Symmer advanced the theory of two fluids coexisting in all matter (not independently of each other, as had been previously supposed), which by mutual counteractions produced all electrical phenomena. His conception was that a body, positively electrified, did not exist in that condition because of the possession of a charge of a positive (as distinct from a negative) electric fluid which it had not held before, and did not hold in a normal state; nor that it possessed a greater share of a single electric fluid than it did in an unelectrified condition, as had been believed by Franklin and Watson, and by Dufay respectively; but that such a body contained both positive and negative electricities which, when the body behaved as "unelectrified," entirely counteracted each other, but which, on the other hand, caused a positive or negative charge to be evinced should either positive or negative electricity respectively preponderate.
Æpinus was the author of another notable theory, of which we must omit further mention for want of space.
​Disjointed observations connected with animal electricity had been accumulating for many centuries. The first chronicled note that refers to the subject dates back to 676 a. d. Whether or not entirely by chance, the Arabians named the electric eel, or torpedo, in a way that impresses us now as singularly felicitous, raad (the lightning). Toward the end of the last century Redi discovered that the shock was sometimes conveyed through the line and rod to the fisherman, and Kampfer compared the effects to those of electrical discharges. It does not appear, however, that the resemblance was actually believed to be more than accidental until Bancroft urged, in the last ten years of the eighteenth century, the view which was shortly proved. Investigation since has shown that several other aquatic animals possess this astonishing manifestation of vitality, notably the Gymnotus electricus (Surinam eel), the Trichiurus electricus, and the Tetraodon electricus. Humboldt gives an account of wonderful battles in South America between gymnoti and wild horses. In fact, the most expeditious method, if not the most humane one, of capturing these alarming creatures appears to be to drive horses into the pond inhabited by them, and to allow the eels to exhaust their strength by repeated electric discharges before endeavoring to bring them to land by other means.
Cavendish was one of the most noted experimental investigators in the electrical field during the latter third of the eighteenth century. His work was remarkably accurate, considering the lack of a proper equipment for taking observations incident to operations in those days. He computed the relative conductivities of iron and water as four hundred million to unity, and found that the addition of but one part of common salt to one hundred of water increased the conductivity of the latter a hundredfold. A twenty-six-per-cent solution of salt he found to possess only seven and one quarter times the conductivity of the extremely weak one mentioned. He also established the law that the capacity of condensers (of which the previously mentioned Leyden jar is an example) varies directly as the active area, and inversely as the distance separating the conducting surfaces. It was reserved for later investigators to make the grand discoveries which relate to electrochemical dissociation, but Cavendish succeeded in accurately determining the ratio of combination of the elements of water in a method which superficially suggests the inverse of electrolytic decomposition—i. e., by inducing the combination of hydrogen and oxygen by the electric spark in the instrument known as the eudiometer.
Hard on the heels of this work came news of Galvani's remarkable discovery (1790) of the fact that freshly amputated frogs' legs, on being touched along the lines of the muscles by dissimilar metals, ​were powerfully agitated. We can only speak of this discovery as the stumbling on to an isolated fact, for it was reserved for Volta to establish the generalization that a current is produced in the conductor joining dissimilar metals when the latter are both in contact with a suitable electrolyte (or liquid capable both of conducting electricity and of acting on one, and incidentally also sometimes both, of the metals). Meantime (Du Bois-Reymond observes), "wherever frogs were to be found, and where two different kinds of metal could be procured, everybody was anxious to see the mangled limbs of frogs brought to life in this wonderful way. Physiologists believed that at last they should realize their visions of a vital power, and physicians that no cure was impossible."
Volta first discovered merely the fact of electrification by contact. He wrote to Galvani: "I don't need your frog. Give me two metals and a moist rag, and I will produce your animal electricity. Your frog is nothing but a moist conductor, and in this respect it is inferior to my wet rag!" Nobili, nevertheless, in 1825 proved the existence of galvanic currents in muscles.
Later on Volta invented the "couronne des tasses" (crown of cups), thus at the same time adopting the general form of cell used, with modifications, to-day, and producing the higher electromotive force, or electrical pressure, consequent on the multiplication of the cells in a series battery.
Just before Volta's celebrated communication to the Royal Society, in 1800, Fabroni, of Florence, in discussing Galvani's phenomenon, went to the root of the matter by suggesting that the energy of chemical action was at the bottom of galvanic manifestations, and he was warmly upheld in this contention by Sir Humphry Davy, who, upon the publication of Volta's discoveries, constructed a most elaborate battery with which (apparently about 1806) he produced the arc light between carbon pencils.
In the year referred to, Davy published the results of a series of experiments of enormous significance, among other things of the isolation of the alkali metals, sodium and potassium, whose existence had hitherto not been dreamed of. The simple electrolytic decomposition of water had been accomplished by Nicolson and Carlisle in the last year of the eighteenth century. Sir "W. S. Harris says: "A series of new substances was speedily discovered, the existence of which had never before been imagined. Oxygen, chlorine, and acids were all dragged, as it were, to the positive pole, while metals, inflammable bodies, alkalies, and earths became determined to the negative pole of the battery. When wires connected with each extremity of the new battery were tipped with prepared and well-pointed charcoal, and the points brought near each other, ​then a most intense and pure evolution of light followed, which on separating the points extended to a gorgeous arc." It was at first supposed that the galvanic or voltaic electricity was distinct from the so-called "frictional" or "ordinary" electricity.
A distinguished contemporary of Cavendish was Coulomb, the value of whose work in developing certain exceedingly important mathematical laws with regard to action at a distance, surface densities, and rates of charge dissipation can hardly be overestimated. His name was given to the torsion balance which, since his day, has been the standard instrument for measuring electric and magnetic attractions and repulsions. The importance of his work has since been recognized by the perpetuation of his name in connection with the unit of quantity of electricity, as that of Volta has been honored by its use, abbreviated (volt), to designate the unit of electrical tension or pressure.
Certain highly instructive and interesting data were accumulated about this time by Volta, Laplace, Saussure, and the renowned chemist Lavoisier, in connection with the subject of electrification produced when evaporation, and the liberation of gases and vapors in general from any cause, occurs. The liquid, solid, or mixture liberating the gas was contained in a metallic dish and the resultant electrification of the latter examined qualitatively. Volta's observations led him to conclude that the electrification was always negative, but Saussure demonstrated finally that its sign was dependent on the material of the dish. These experimenters covered, between them all, a somewhat extensive field, examining, among other things, the electrification resulting from the ebullition of various liquids, from the ordinary combustion of fuel, and from the decomposition of acids by metals to liberate hydrogen.
About the end of the first decade of the century Poisson attacked the phenomena of electricity analytically, and succeeded in demonstrating the right of electrical investigation to rank among the exact sciences. Of his most important mathematical propositions is one in which, assuming as a working hypothesis the existence of two mutually attracting fluids, he deduced formulæ covering the distribution of these fluids on the surfaces of two conducting spheres, in or out of contact.
A great deal of work was done during the end of the last century and the beginning of the present one on what is now known as pyro-electrification. The Abbé Haüy discovered that fragments of tourmaline crystal exhibited opposite electrifications on opposite extremities of their lines of cleavage. It is this crystal also which has unusually remarkable powers of polarizing light, and which, under electro-magnetic stress, suffers modifications of the latter property. ​Haüy investigated the field with much diligence, and succeeded in cataloguing a large number of natural crystals by the side of tourmaline. The subject was amplified later by Sir David Brewster, who added a series of artificial crystalline salts to the list of pyro-electrical materials, among them, notably, hydro-potassic (and sodic) tartrate. The property was found not always to reside on these substances, but to be developed by heating them. Brewster found that even powdered tourmaline exhibited opposite electrifications on the opposite extremities of each tiny particle, causing the latter to act, so far as attractions and repulsions went, as infinitesimal magnets.
Our rapid and imperfect survey has now brought us to the threshold of the great activity in electrical work elicited by the tremendous discovery, made by Professor Oersted, of Copenhagen, of the existence of the electro-magnetic field. It happens that two of the most amiable and estimable individuals that have ever devoted their lives to scientific research stand out in this connection head and shoulders above all other investigators—Ampère and Faraday, the latter sixteen years younger than the former and destined to long survive him.