Galvanometers Next: Worked Examples Up: Magnetism Previous: Gauss' Law for Magnetic Galvanometers We have talked a lot about potential differences, currents, and resistances, but we have not talked much about how these quantities can be measured. Let us now investigate this topic. Broadly speaking, only electric currents can be measured directly. Potential differences and resistances are usually inferred from measurements of electric currents. The most accurate method of measuring an electric current is by using a device called a galvanometer. A galvanometer consists of a rectangular conducting coil which is free to pivot vertically in an approximately uniform horizontal magnetic field --see Fig. 31 . The magnetic field is usually generated by a permanent magnet. Suppose that a current runs through the coil. What are the forces exerted on the coil by the magnetic field? According to Eq. ( 152 ), the forces exerted on those sections of the coil in which the current runs in the horizontal plane are directed vertically up or down. These forces are irrelevant, since they are absorbed by the support structure of the coil, which does not allow the coil to move vertically.

Equation ( 152 ) also implies that the force exerted on the section of the coil in which the current flows downward is of magnitude , where the length of this section, and is directed out of the page (in the figure). Likewise, the force exerted on the section of the coil in which the current flows upward is also of magnitude , and is directed into the page. These two forces exert a torque on the coil which tries to twist it about its vertical axis in an anti-clockwise direction (looking from above). Using the usual definition of torque (i.e., torque is the product of the force and the perpendicular distance from the line of action of the force to the axis of rotation), the net torque acting on the coil is (183) where is the horizontal width of the coil, and is its area. Note that the two vertical sections of the coil give rise to equal contributions to the torque. Strictly speaking, the above expression is only valid when the coil lies in the plane of the magnetic field. However, galvanometers are usually constructed with curved magnetic pole pieces in order to ensure that, as the coil turns, it always remains in the plane of the magnetic field.

It follows that, for fixed magnetic field-strength, and fixed coil area, the torque exerted on the coil is directly proportional to the current . Figure 31: A galvanometer. The coil in a galvanometer is usually suspended from a torsion wire. The wire exerts a restoring torque on the coil which tries to twist it back to its original position. The strength of this restoring torque is directly proportional to the angle of twist . It follows that, in equilibrium, where the magnetic torque balances the restoring torque, the angle of twist is directly proportional to the current flowing around the coil. The angle of twist can be measured by attaching a pointer to the coil, or, even better, by mounting a mirror on the coil, and reflecting a light beam off the mirror. Since , the device can easily be calibrated by running a known current through it. There is, of course, a practical limit to how large the angle of twist can become in a galvanometer. If the torsion wire is twisted through too great an angle then it will deform permanently, and will eventually snap. In order to prevent this from happening, most galvanometers are equipped with a ``stop'' which physically prevents the coil from twisting through more than (say) .

Thus, there is a maximum current which a galvanometer can measure. This is usually referred to as the full-scale-deflection current. The full-scale-deflection current in conventional galvanometers is usually pretty small: e.g., . So, what do we do if we want to measure a large current? What we do is to connect a shunt resistor in parallel with the galvanometer, so that most of the current flows through the resistor, and only a small fraction of the current flows through the galvanometer itself. This is illustrated in Fig. 32 . Let the resistance of the galvanometer be , and the resistance of the shunt resistor be . Suppose that we want to be able to measure the total current flowing through the galvanometer and the shunt resistor up to a maximum value of . We can achieve this if the current flowing through the galvanometer equals the full-scale-deflection current when which reduces to (185) Using this formula, we can always choose an appropriate shunt resistor to allow a galvanometer to measure any current, no matter how large.

For instance, if the full-scale-deflection current is , the maximum current we wish to measure is , and the resistance of the galvanometer is , then the appropriate shunt resistance is (186) Most galvanometers are equipped with a dial which allows us to choose between various alternative ranges of currents which the device can measure: e.g., 0- mA, 0-1A, or 0-10A. All the dial does is to switch between different shunt resistors connected in parallel with the galvanometer itself. Note, finally, that the equivalent resistance of the galvanometer and its shunt resistor is (187) Clearly, if the full-scale-deflection current is much less than the maximum current which we wish to measure then the equivalent resistance is very small indeed. Thus, there is an advantage to making the full-scale-deflection current of a galvanometer small. A small full-scale-deflection current implies a small equivalent resistance of the galvanometer, which means that the galvanometer can be connected into a circuit without seriously disturbing the currents flowing around that circuit. Figure 32: Circuit diagram for a galvanometer measuring current.

A galvanometer can be used to measure potential difference as well as current (although, in the former case, it is really measuring current). In order to measure the potential difference between two points and in some circuit, we connect a galvanometer, in series with a shunt resistor, across these two points--see Fig. 33 . The galvanometer draws a current from the circuit. This current is, of course, proportional to the potential difference between and , which enables us to relate the reading on the galvanometer to the voltage we are trying to measure. Suppose that we wish to measure voltages in the range 0 to . What is an appropriate choice of the shunt resistance ? Well, the equivalent resistance of the shunt resistor and the galvanometer is , where is the resistance of the galvanometer. Thus, the current flowing through the galvanometer is . We want this current to equal the full-scale-deflection current of the galvanometer when the potential difference between points and attains its maximum allowed value . It follows that which yields (189) Using this formula, we can always choose an appropriate shunt resistor to allow a galvanometer to measure any voltage, no matter how large.

For instance, if the full-scale-deflection current is , the maximum voltage we wish to measure is V, and the resistance of the galvanometer is , then the appropriate shunt resistance is (190) Again, there is an advantage in making the full-scale-deflection current of a galvanometer used as a voltmeter small, because, when it is properly set up, the galvanometer never draws more current from the circuit than its full-scale-deflection current. If this current is small then the galvanometer can measure potential differences in a circuit without significantly perturbing the currents flowing around that circuit. Figure 33: Circuit diagram for a galvanometer measuring potential difference. Galvanometer | Article about galvanometer by The Free Dictionary Galvanometer | Article about galvanometer by The Free Dictionary Related to galvanometer: ballistic galvanometer , Tangent galvanometer galvanometer (găl'vənŏm`ətər), instrument used to determine the presence, direction, and strength of an electric current in a conductor. All galvanometers are based upon the discovery by Hans C.

Oersted Oersted or Ørsted, Hans Christian , 1777–1851, Danish physicist and chemist. He was professor at Copenhagen from 1806. ..... Click the link for more information. that a magnetic needle is deflected by the presence of an electric current in a nearby conductor. When an electric current is passing through the conductor, the magnetic needle tends to turn at right angles to the conductor so that its direction is parallel to the lines of induction around the conductor and its north pole points in the direction in which these lines of induction flow. In general, the extent to which the needle turns is dependent upon the strength of the current. In the first galvanometers, a freely turning magnetic needle was hung in a coil of wire; in later versions the magnet was fixed and the coil made movable. Modern galvanometers are of this movable-coil type and are called d'Arsonval galvanometers (after Arsène d'Arsonval, a French physicist). If a pointer is attached to the moving coil so that it passes over a suitably calibrated scale, the galvanometer can be used to measure quantitatively the current passing through it.

Such calibrated galvanometers are used in many electrical measuring devices. The DC ammeter ammeter , instrument used to measure the magnitude of an electric current of several amperes or more. An ammeter is usually combined with a voltmeter and an ohmmeter in a multipurpose instrument. ..... Click the link for more information. , an instrument for measuring direct current, often consists of a calibrated galvanometer through which the current to be measured is made to pass. Since heavy currents would damage the galvanometer, a bypass, or shunt, is provided so that only a certain known percentage of the current passes through the galvanometer. By measuring the known percentage of the current, one arrives at the total current. The DC voltmeter voltmeter, instrument used to measure differences of electric potential, commonly called voltage, in volts or units that are multiples or fractions of volts. A voltmeter is usually combined with an ammeter and an ohmmeter in a multipurpose instrument. ..... Click the link for more information. , which can measure direct voltage, consists of a calibrated galvanometer connected in series (see electric circuit electric circuit, unbroken path along which an electric current exists or is intended or able to flow.

A simple circuit might consist of an electric cell (the power source), two conducting wires (one end of each being attached to each terminal of the cell), and a small lamp (the ..... Click the link for more information. ) with a high resistance. To measure the voltage between two points, one connects the voltmeter between them. The current through the galvanometer (and hence the pointer reading) is then proportional to the voltage (see Ohm's law Ohm's law [for G. S. Ohm], law stating that the electric current i flowing through a given resistance r is equal to the applied voltage v divided by the resistance, or i=v/r. ..... Click the link for more information. ). Galvanometer A device for indicating very small electric currents. Although the deflection of a galvanometer results from current in the moving coil, the voltage in a closed circuit producing this current is frequently the quantity of interest to the user. In this mode, galvanometers are used to detect a null or an unbalanced condition in a bridge or potentiometer circuit. Electronic instruments that employ amplifying circuits to achieve sensitivities approaching the nanovolt level are also used as balance detectors in bridge circuits.

However, in applications where extreme sensitivity and high rejection of ac signals are required, a galvanometer in combination with a photocell amplifier is to be preferred. Galvanometers may also be used ballistically to integrate a transient current, as from the discharge of a capacitor, or a transient voltage, as produced when a coil moves relative to a magnetic field. See Current measurement , Voltage measurement The d'Arsonval galvanometer is the most common type and is widely used. Its indicating system consists of a light coil of wire suspended from a copper or gold ribbon a few thousandths of an inch wide and less than 0.001 in. (0.025 mm) thick. This coil, free to rotate in the radial field between the shaped pole pieces of a permanent magnet, carries a small mirror which serves as an optical pointer and indicates the coil position by reflecting a light beam onto a fixed scale. Current is conducted to and from the coil by the suspension ribbons. The torque which deflects the indicating element is produced by the reaction of the coil current with the magnetic field in which it is suspended.

Voltage sensitivity is of importance in applications where the galvanometer serves as the detector of unbalance in a bridge or potentiometer network. The energy of motion in the indicating system must be dissipated and the system brought to rest in order for the deflection to be evaluated. This process, common to all indicating systems in which equilibrium must be achieved between a driving torque and a restoring torque, is called damping. See Damping The sensitivity of modern galvanometers ranges up to 0.04 in. (1 mm) of deflection, on a scale 40 in. (1 m) distant from the mirror, for a current of 0.00001 microampere. Such a galvanometer may have a coil resistance of 800 ohms and a critical damping resistance of 100,000 &OHgr;. The voltage response of this instrument amounts to 0.04 in./microvolt (1 mm/μV) at critical damping. A galvanometer designed for voltage sensitivity, has a coil resistance of 20 &OHgr;, a critical damping resistance of 30 &OHgr;, and a response of 0.04 in.

(1 mm) for 0.05 μV in the critically damped circuit. It will be seen from these examples that a large response to current is associated with large coil resistance and high critical damping resistance, whereas voltage response is associated with low coil resistance and low critical damping resistance. The limitations reached in improving a galvanometer's resolution by increasing its scale length or using multiple reflections can be overcome by operating the galvanometer in its equilibrium position by using a negative-feedback network. In this system the beam of light reflected from the galvanometer mirror is directed onto a pair of photocells connected in series opposition (see illustration). As the galvanometer coil rotates, the light received on one photocell increases while the other decreases, and an amplified output current is produced whose sign and magnitude depend on the direction and amount of the coil deflection, respectively. The photocell output current is monitored by an external indicator and is approximately 1800 times the galvanometer input current. This photocell output also provides a negative-feedback current to servo the galvanometer at its equilibrium position, so that the linearity and input resistance of the system are increased.

In such an arrangement it is frequently possible to approach rather closely the theoretical limit of resolution of angular motion imposed by the brownian motion of the coil, or by the Johnson noise of the circuit connected to the galvanometer. In a low-resistance galvanometer this theoretical resolution may be about 0.001 μV. See Brownian movement , Kinetic theory of matter Simplified schematic of photocell galvanometer amplifier A galvanometer is an extremely efficient low-pass filter, and it is still the most sensitive low-level detector at input impedances of 1 k&OHgr; or less. At higher input impedances, chopper-stabilized amplifiers, called nanovoltmeters, exhibit better resolution, noise, and drift characteristics than galvanometers. These instruments modulate the input dc signal by using a chopper (mechanical contact, transistor, or photocell switching), then amplify the signal by traditional ac techniques to achieve sensitivities of 10 nV or less. State-of-the-art digital voltmeters are now available with nanovolt resolution and are useful as low-level detectors.

See Ammeter , Electrical measurements , Electrometer , Voltmeter Galvanometer a highly sensitive electrical measuring instrument that responds to very low current or voltage values. Galvanometers are most commonly used as zero current indicators—that is, devices to indicate the absence of current or voltage in an electrical circuit. After preliminary determination of the constant for an instrument (the value of a scale division), they are also used to measure small currents and voltages. Both DC and AC galvanometers exist. The first DC galvanometers were made in the 1820’s and operated on the principle of the permanent-magnet system of instruments. They consist of a magnetic needle suspended on a fine thread within a coil of wire. When no current flows in the coil, the needle orients itself along the local magnetic meridian. The appearance of a current causes the needle to deviate from its initial position. In the 19th century a great variety of galvanometer designs with a movable magnetic needle were developed and were extensively utilized for scientific studies of electromagnetic phenomena. For example, in 1886, F. Kohlrausch used such an instrument to determine with great accuracy the electrochemical equivalent of silver. In 1881 the French scientist J.

A. d’Arsonval created a suspended-coil galvanometer, in which the moving part is a current-carrying coil located in the field of a permanent magnet. Depending on the design of the moving part of such galvanometers, they are classified as frame types, in which the moving part is a frame with several turns of wire; loop types, in which the moving part is a single-turn wire loop; and string types, in which the moving part is a wire stretched like a string. An example of a frame galvanometer is shown in Figure 1. A frame positioned in the field of a permanent magnet has an indicating pointer attached to its axis. Current flowing through the turns of the frame interacts with the permanent magnetic field and produces a torque that causes the movable part to turn and move the pointer correspondingly along the scale. To increase sensitivity, the pointer of such a galvanometer is replaced by the miniature mirror of an optical indicating device. (This is shown in Figure 2.) A beam of light from the illuminator falls on a mirror and, upon being reflected from it, arrives at the scale.

The scale is placed at a distance of 1.5 to 2 m from the galvanometer, so that even very small angular displacements of the mirror produce noticeable deflections of the luminous spot on the scale from its zero position. Galvanometers with an illuminated scale, in which the illuminator and the scale are located in the same housing with the galvanometer mechanism, are a version of this type of galvanometer. To obtain sufficient length of the light beam, it undergoes multiple reflection from several fixed mirrors. Figure 1. Frame galvanometer When a current pulse of short duration passes through a galvanometer’s winding, the moving part is ballistically deflected from the zero position and subsequently returns to this position after several oscillations. If the pulse duration is substantially shorter than the period of the moving part’s natural oscillations, the first and greatest deflection of the indicator is proportional to the amount of electricity carried by the pulse. To measure the amount of electricity for relatively long pulses, ballistic galvanometers are made, in which the moment of inertia of the moving part is much greater than in the ordinary types. With ballistic galvanometers it is possible to measure the amount of electricity in pulses lasting up to 2 sec.

For the detection of small alternating currents and voltages, AC vibration galvanometers and galvanometers with AC-to-DC transformers are used. The operating principle of vibration galvanometers is the same as for DC galvanometers; the two types differ only in that the moment of inertia of the moving part in vibration galvanometers is very small. The mechanism of a vibration galvanometer with a moving magnet is shown in Figure 3. A movable vane made of magnetically soft steel is located between the poles of a permanent magnet and in the field of an electromagnet (between poles n and m). The vane, together with a small mirror, is attached to a bronze band. The alternating current being measured Figure 2. Mirror galvanometer passes through a winding on the electromagnet and creates an alternating magnetic field, which is superimposed on the constant magnetic field from the permanent magnet. The resultant field changes its direction with the frequency of the alternating current and causes the vane to vibrate; the sharp image of the illuminated slit on the scale thereby becomes blurred into a luminous band. The width of the band is proportional to the strength of the alternating current in the winding of the electromagnet.

The maximum sensitivity of a vibration galvanometer is produced when the natural vibration frequency of the movable parts equals the frequency of the alternating current, and for this reason all such galvanometers have devices for changing the natural vibration frequency so that the moving part can be tuned to resonate with the test current. These types of galvanometers are manufactured for operation up to frequencies no higher than 5 kilohertz (kHz). Figure 3. Vibration galvanometer A thermogalvanometer is an AC galvanometer with a thermoconverter. The galvanometer has the mechanism of a permanent-magnet galvanometer, with a single-turn moving frame. The halves of this turn are made of different metals, forming a thermocouple. A heater located close to one of the junctions carries the alternating current to be measured. The thermocurrent generated in the frame deflects it from the zero position. This galvanometer can be operated at frequencies above 5 kHz. The basic characteristic of a galvanometer is its sensitivity or its inverse—the galvanometer constant.

Modern mass-produced DC galvanometers can detect currents of about 5 × 10-11 amperes and voltages on the order of 5 × 10-8 volts. The constants of AC vibration galvanometers are on the order of 1 × 10-7 amperes per division. REFERENCES Galvanometers Galvanometers Figure 1: General scheme of a galvanometer. Galvanometer is the historical name given to a moving coil electric current detector. When a current is passed through a coil in a magnetic field, the coil experiences a torque proportional to the current. If the coil's movement is opposed by a coil spring, then the amount of deflection of a needle attached to the coil may be proportional to the current passing through the coil. Such "meter movements" were at the heart of the moving coil meters such as voltmeters and ammeters until they were largely replaced with solid state meters. Figure 2: Moving coil of the galvanometer. The accuracy of moving coil meters is dependent upon having a uniform and constant magnetic field. The illustration shows one configuration of permanent magnet which was widely used in such meters. Figure 3: Operation of a galvanometer.

The galvanometer is used to measure very low currents, such as those in bridge circuits. In modified form, the galvanometer has the highest sensitivity of any of the various types of meters in use today. A simplified diagram of a galvanometer is shown in Figure 3. It is different from other instruments used for the same purpose because its movable coil is suspended by means of metal ribbons instead of a shaft and jewel-bearing arrangement often used in other instruments. The movable coil is wrapped around the aluminum frame of the galvanometer. The coil is suspended between the poles of the magnet by means of thin, flat ribbons of phosphor bronze. These ribbons provide a conduction path for the current between the circuit being tested and the movable coil. The ribbons allow the coil to twist in response to the interaction of the applied current through the coil and the magnetic field of the permanent magnet. They also provide the restoring force for the coil. Basically, the restoring force is that force necessary to return the movable frame to its resting position after a reading. The ribbons restrain or provide a counterforce to the magnetic force acting on the coil.

When the driving force of the coil current is removed, the restoring force provided by the ribbons returns the coil to its zero position. Figure 4: D'Arsonval type galvanometer Moving-coil Galvanometer. In this type of instrument, known also as the D'Arsonval form of galvanometer, the suspended system is a coil of fine wire which hangs in a strong magnetic field due to a permanent steel horseshoe magnet. In Figure 4 is shown a vertically placed horseshoe magnet, between the poles of which is hung a light rectangular coil of many turns of fine wire, the plane of the coil being parallel to the direction of the lines of force. The coil is suspended by a fine ribbon of phosphor-bronze which also serves to connect one end of the suspended coil to the outer circuit while the other connection is made through a spiral wound strip of the phosphor-bronze ribbon attached to the lower end of the coil. A cylindrical mass of soft iron is fixed midway between the poles of the magnet so that as the suspended coil turns its vertical branches move in the gaps between the core and pole pieces.

This arrangement secures a strong uniform field, across which the wires of the coil pass, and when a current is sent through it, it is deflected. A small mirror mounted just above the coil and moving with it, enables the deflection to be determined by the telescope and scale, or reflected spot of light method. The moving-coil galvanometer has the advantage that it is not affected by changes in the earth's magnetic field, and can be used near dynamo machines and where there is considerable magnetic disturbance. Also the coil damps strongly or comes almost immediately to rest when the wires leading to it are touched together, forming a short circuit, as it is called. This damping is due to electromagnetic induction. Forces in a tangent galvanometer Tangent Galvanometer. In the tangent galvanometer there is a circular coil having one or more turns of wire, at the center of which a magnetic needle is either balanced on a point or suspended by a fine fiber of silk or quartz. The instrument is placed so that the plane of the coil is vertical and in the magnetic north and south plane (Figure 5(A)).

When a current is sent through the coil the needle turns to one side or the other, and the strength of the current is proportional to the tangent of the angle of deflection. The force due to the current in the coil is at right angles to the plane of the coil at its center and the strength of the field at that point in a given coil is proportional to the strength of the current (Figure 5(B)). Let G represent the strength of field at the center due to the coil when unit current is flowing, then IG will be the strength of field when the current strength is I. Let OA in Figure 5(B) represent the plane of the coil and O the point where the needle is placed, then when no current is flowing the needle points in the direction OA, being acted on only by the horizontal component H of the earth's magnetic force. The magnetic force F due to the current in the coil is IG and at right angles to H, therefore, the resultant force R is the diagonal of the rectangle whose sides are IG and H, and where x is the angle which the resultant force makes with H. But the needle must point in the direction of the resultant force, and so x is the angle through which the needle turns.

Therefore and if H and G are known the current may be determined by measuring the angle x. In case of a tangent galvanometer the magnetic force F due to the coil is expressed by IG. But if the current is measured in electromagnetic units, And since the length of n turns of wire of radius r is, The galvanometer coil constant G can be calculated from this formula when the coil of the galvanometer has so large a radius compared with the length of the needle that the poles of the needle may be regarded as at the center, and when the cross section of the coil is so small that all the turns bear ncarly the same relation to the needle. If G is determined in this way, r being measured in centimeters, and if H is found in C.G.S. units system, the current will be also found in C.G.S. electromagnetic units by the use of the formula: To obtain the current strength in amperes, we must take as the value of the coil constant: By this method the strength of a current is determined in amperes directly from the fundamental units of length, mass, and time, for we have already seen how the measurement of H is based on these units.

A tangent galvanometer in which the constant is determined in this way directly from measurements of the coil is known as a standard galvanometer. Ballistic galvanometer is designed to deflect its indicating needle (or mirror) in a way that is proportional to the total charge passing through its moving coil or to a voltage pulse of short duration. Any conventional galvanometer may also be employed as a ballistic type, but the latter has smaller torque and higher inertia in the coil. Pictures below show some examples of galvanometers used in the 19th - beginning of 20th centuries: Figure 6: Pye Coulomb Balance This unusual device balances out the attraction between the two balls and a magnetic field created by the horizontally mounted coil of wire. It is capable of measuring very small amounts of current. Figure 7: Deprez D Arsonal galvanometer "Item of the collection of Fons Vanden Berghen, Halle, Belgium". Figure 8: Early Thompson Mirror Galvanometer Manufactured by Elliott Bros., London. This tripod-based all brass Thompson mirror galvanometer number 502 has overhead compensating magnets extending up from the top.

This unit is the same kind as the one on Thomas Edison's desk while he was experimentg with early telegraph systems. Thomas Edison using a Thompson galvanometer Figure 9: Mirror Galvanometer Manufactured by Knott Apparatus Company, U.S.A. The Ivory label reads: Designed for International Correspondence Schools, Scranton, PA. Manufactured by L. E. Knott Apparatus Company, Scientific Supplies, Boston, MASS. Figure 10: Elliot mirror-galvanometer used as a sensitive receiever on undersea cables for telegraph connections. "Item of the collection of Fons Vanden Berghen, Halle, Belgium". Figure 11: Abraham Rheogram Prism Synchroscope This extremely rare prism synchroscope has an early synchronous-motor driven horizontally rotating triangular prism and mirror system that provides the time-base for the light beams from the two low-mass mirror galvanometers. This provides a dual-trace plus time-base display. Original finished wooden case. Figure 12: Galvanometer Manufactured by Cambridge Instrument Co. Ltd, England.

A large beautiful complex brass galvanometer with hair thin meter needle, compensating magnet, and mechanism sticking up out of center of a large round brass base with leveling screws. Figure 13: Paschen Type Galvanometer Made by the Cambridge and Paul Instrument Company. This company (originally the Cambridge Scientific Instrument Company) was founded in 1881 by Horace Darwin, Charles Darwin's youngest son, to supply the laboratories of Cambridge University. The Darwin family association with the company continued until the 1970s. The galvanometer was purchased for Caltech's freshman physics lab in the fall of 1921. It is a Paschen type--designed by the German spectroscopist Friedrich Paschen, based on the Thomson astatic galvanometer. Figure 14: Early Gramme Ring Horizontal Coil Galvanometer Very early and primitive horizontally mounted multi-tapped coil surrounding a horizontally oriented compass. Figure 15: Tangent Galvanometer Made by Philip Harris Ltd., Birmingham, England. A large 4" diameter horizontally oriented compass indicator is located in the center of a 7" vertically oriented coil of wire all mounted on a wooden stand with leveling screws.

The coil taps are marked 2,50,& 500. Figure 16: Tangent Galvanometer, Helmholtz type Made by Philip Harris Ltd., Birmingham, England, ca. 1913. The galvanometer is graduated in degrees of the horizontal rotation. "Item of the collection of Fons Vanden Berghen, Halle, Belgium". Figure 21: Tangent Galvanometer Made by Stoelting Company. Very simple and primitive tangent galvanometer consisting of a compass mounted in a wooden block with a coil of wire wound around the block in a vertical plane. Figure 22: Galvanometer for use in laboratory and for field testing used for early telegraph applications. "Item of the collection of Fons Vanden Berghen, Halle, Belgium". Figure 23: Galvanometer Early General Radio Model 129. The galvanometer uses two internal horseshoe magnets and shows signs of having been partly manufactured by hand. A view of the early General Radio Galvanometer with cover removed. ( see acknowledgment ) The reflecting galvanometer is a sensitive instrument for measuring current.

It consists of a magnet coil with an attached mirror suspended between the poles of a magnet by a thin gold ribbon above and a coiled gold wire below. When an electric current is introduced into the magnet coil it induces a magnetic field, causing the coil to rotate in proportion to the current. The mirror allows the user to see very small deflections, and thus to measure very small currents, by observing a distant reflected scale (one meter or more away) through a telescope. The ballistic galvanometer is specialized in having a wide coil characterized by a long period. It is designed to measure the quantity of electricity (charge) discharged through the instrument over a short period of time. Applications include the measurement of capacitance, inductance by comparison with a standard, and magnetic field intensities using a search coil. galvanometer | measurement instrument | Britannica.com measurement instrument bathythermograph Galvanometer, instrument for measuring a small electrical current or a function of the current by deflection of a moving coil . The deflection is a mechanical rotation derived from forces resulting from the current.

The most common type is the D’Arsonval galvanometer , in which the indicating system consists of a light coil of wire suspended from a metallic ribbon between the poles of a permanent magnet. The magnetic field produced by a current passing through the coil reacts with the magnetic field of the permanent magnet, producing a torque, or twisting force. The coil, to which an indicating needle or mirror is attached, rotates under the action of the torque; the angle through which it rotates to balance the torsion of the suspension provides a measure of the current flowing in the coil. The angle is measured by the movement of the needle or by the deflection of a beam of light reflected from the mirror. The ballistic galvanometer is designed to deflect its indicating needle (or mirror) in a way that is proportional to the total charge passing through its moving coil or to a voltage pulse of short duration. Any conventional galvanometer may also be employed as a ballistic type, but the latter has smaller torque and higher inertia in the coil. Learn More in these related articles: Astatic Galvanometer Galileo telescope and IMSS Florance Italy Thomas Edison's first light bulb used to demonstrate his invention at Menlo Park.

This photo shows you what one of Galileo's telescopes looked like when it was new. Click on photo of our replica of Galileo's telescope to make it larger, click on that photo to make it even larger . Click here to visit our page on making this very fine and very precise telescope for Griffith Observatory. IMSS in Florence Italy, A museum that is a must visit for everyone. No Words are Powerful Enough To Express The Importance Of Basic Scientific Research To Each Of Us And Our Future. We must fight for its rights! Perhaps we should recognized, sponsor and establish a yearly world celebration for the 12 most significant scientific instruments. one a month. It would be educational. We suggest the first two candidates should be: 1, Galileo's Telescopes I nternationally famous, broadened our perspective by introducing us to the Universe. 2, Nobili's Astatic Galvanometers ; gave us our first meaningful connection to the micro universe of the electron.

completely revolutionizing our finger tip control of great power: tapping the energy of the atom, communicate around the world and to the planets, looking into our genes putting our telescopes on other planets etc.etc.etc.etc.etc.etc.etc.etc.etc.etc.etc.etc.etc.etc.etc.etc.. Both instruments deal with the flow of fundamental particles; ( photons with telescope) (electrons with galvanometer) from a historical point of view they both are key examples of how basic scientific research really works not our popular view, but the dull tedious work funding, marketing, and laboratory that goes into every discovery all of this is hidden from us because of the lack of suitable words ( Remember No Words are Powerful Enough) Galileo's telescopes, and early examples of astatic galvanometers are on display at the IMSS in Florence Italy The telescope is out in front one of the first things one see's. The little astatic galvanometers that showed us how to have finger tip control of enormous power is almost hidden in a darkish room off to one side. Hmmm? Both these instruments were made by Natural Philosophers who were interested in searching out Natures secrets.

The technologist developing these instruments started out with simple off the shelve materials wood, glass, and wire to make their instruments. They used these primitive tools and their current knowledge to design and build them. They used them to do basic research. and made significant fundamental discoveries, discoveries that made enormous differences in our everyday world, discoveries that helped in improving the instruments (tools) which in turn made more discoveries keeping the information loop growing to even more discoveries. How can we possibly ignore such an instrument or instruments and the scientist and technologist using them? What little we know about them has and can at times been swamped by authors attention to the personal habits and eccentric behavior. The instruments and discoveries total ignored or poorly described. Galileo's telescope which show us views of the magnificent universe we live in gave us the first hint of the number of stars out there that we never could have seen with the unaided eye. They gave us a poetic understanding of who we really were and are. click here to visit our page on making this fine and very precise instrument for Adler Planetarium.

Nobili 's astatic galvanometers like the telescope, which magnifies the effect of photons on our eyes, magnifies effects of electrons flowing by the action of a compass needle in a circuit. The astatic galvanometer is responsible for switching on the technological world we live in. Like the telescope the astatic galvanometer gave us the view of the universe ,but in this case the micro universe, of the electrons so that we could benefit from this knowledge to harness the energy of electron to our everyday needs. The astatic galvanometers were the first instruments, after the discovery of the initial connection of electricity and magnetism, to give us quantitative and sensitive measurements to guide us through the micro universe of the electrons how to use them in new inventions and instruments this in spite of the fact that we can't see them as we do with photons. Electrons are the messengers connecting our ears, eyes, nose, and sense of touch to our brains. They also do the shuffling of the activities of the brain. They do the chemistry of the universe. There are more electrons in our bodies than atoms and molecules, and there are many more electrons in the universe then stars.

With this endorsement why has there been such little recognition of the instrument and the technologist that opened the door to the universe of the electron with the astatic galvanometer and as Galileo opened the door of the astronomical universe with his photon telescope? Why has this very important instrument and its inventors been woefully neglected? Is it because there is apparently little to no emotional people drama in the personal lives of the scientist who worked on and used them?. Are there no stories strong enough to write a book that would sell? Is it because electrons acts in a very anti social manner, always trying to get away from each other. Silly questions perhaps; however, wouldn't it be helpful to us if we had a better grasp of story of the electron and its varied and busy life? Don't they ever get tired? Isn't it sad that electrons are not treated like the super stars that they are? What would happen if they went out on strike? 1, The rest of this this web site introduces us to Nobili astatic galvanometer adj. 1. Unsteady; unstable. 2. Physics. Having no particular directional characteristics. --astatically adv.astaticism n. galvanometer n.

An instrument used to detect, measure, and determine the direction of small electric currents by means of mechanical effects produced by a coil in a magnetic field. --galvanometric or galvanometrical ad --galvanometry n. A short technical description It is a very sensitivity electric meter which was developed c 1825 out of work in basic research on electromagnetism where researchers were discovering that an electric current flowing in a wire generated a magnetic field that would make a compass needle move. This type of meter is a laboratory instrument that can measure very tiny electron current or voltage with great accuracy. Currents can be measured in the millionths of an ampere range. This simple instrument has been coupled to a range of sensors that have given us most of the major discoveries in the physical sciences. They even ultimately helped to gather data in non electrical field of research and to make improvements in other current measuring instruments that helped in replaced astatic galvanometer with even better current measuring instruments. A few examples of the galvanometer and astatic galvanometer, concepts and uses. A Demonstration Galvanometer (Tangent) Above a very simple classroom Oersted demonstration galvanometer. showing the presence of a magnetic field generated by an electric current.

A current source to be measured can be connected in three configurations, current can go through 1st through ,bottom 2nd, top and 3rd, around the coil. This is not a astatic galvanometer because it only has one needle. The unit is initially is set up so the needle and coil point in a north south direction. The needle interacts with the magnetic field of the earth and that generated perpendicularly by the current flowing through the coil. The needle deflect as the tangent of the of the angle of the needle to the north south line. It is thus often called a tangent galvanometer. It also gives the direction of current flow by the direction the needle moves, toward the east or west direction. This galvanometer has a range in amperes. The more turns the coil has the stronger the magnetic field becomes for the same current thus increasing its current sensitivity of deflection of the needle. Some meters have coils with thousand of turns in the coil in an attempt to measure small currents. Unfortunately the electrical resistance of the coil goes up and the current for a given voltage goes down. Also the needle always has the earth magnetic field to content with so there is a limit to the sensitivity it can achieve in this configuration.

There are a number of ways of reducing these affect. The best solution was mounting another compass needle above or below the first but reversing it .i.e. north over south and south over north poles. Its called the astatic galvanometer. The needle is usually suspended with a light thread of silk which gives the needle set a slight torsion to over come. More advanced meters had a mirror attached to the needle. A collimated source of light is brought onto the mirror and as it rotated with the current, the movement of the spot of light moves. The amount it moves depends on the distant the receiving scale is from the mirror. The further the distance of the scale from the mirror the more sensitivity the instrument is. It was not unusual for one of the authors to use spot distance upward 30 feet. One disadvantage was the magnets had considerable weight and therefore inertia giving it the habit of oscillating back and forth like a pendulum. Some meters used a copper coil form others a copper sheet very close to the needles as a Faraday break to dampen the swinging. The movement of the needle and its magnetic field generated a current in the copper sheet. This current generated it own magnetic field that worked against the needle magnetic field reducing swing of the movement.

The movement was generally placed in a glass covered chamber to protect it from drafts. These instrument had high sensitivity and one their early jobs was in the studies of electrophysiology by their inventor. Thousands and thousands electro mechanical gadgets bosomed forth from the astatic galvanometer It was at the root of most of our early discoveries and inventions that we now take for granted. Below are just a few. When two wire of different metals are joined together at one end and the junction heated it generates a voltage that can be measured across the open end. It acts as a thermal battery. In the reverse sense if one route current thru this junction in the correct direction it will cool the junction. A demonstration apparatus that combines a square box like coil with different metals and a pair of magnetized needles forms an apparatus which is an astatic galvanometer for showing the thermal electric Seebeck phenomena. The top of the box coil is copper, the bottom bismuth. At their junctions they form thermocouples. Heating one will generates a voltage difference between the dissimilar metals causing a current to flow around the loop.

This current generates a magnetic field that acts on the astatic apposing compass needles causing them to rotate, thus indicating the heat generated a current flow in the box coil. the picture above shows one from these instruments which is in our collection. Below note the astatic galvanometer is center stage of a typical laboratory set up to measurement resistance of a special coil such as Dr. A. Bell's Telephone , Dr. A. Bell's Metal Detector and T Edison work in electrical instruments including the Edison effect leading ultimately to the vacuum tube the late 1800's. Note above; Physicist explored the fundamental laws of electromagnetic radiation spectrum using astatic galvanometers wired up to the output current from a thermal pile, a radiation detector. This system shown on the left was used to measure the radiation from emitting bodies like the light from a star, the radiation from a radio transmitter, a blackbody, a furnace, or even a Easter Lilly. Another version of the operation of astatic galvanometer for the readers consideration and a bit of the development history by Instruments of Natural Philosophy "T.B. Greenslade Jr Before the invention of the astatic galvanometer the existing galvanometers had a serious defects measureing small currents.

Instrument such as the tangent galvanometer the galvanoscope and other simple galvanometers the needle responded to the vector sum of the horizontal component of the magnetic field of the earth and the magnetic field produced by the current being measured. As long as the galvanometer is being used for qualitative measurements, it is important to turn the apparatus so that the compass needle points toward magnetic north when no current is applied to the coil. Because the earths magnetic field is comparably strong only sizeable electric currents could be measured. The development of the astatic magnetic needle by Leopoldo Nobili (1784-1835) in 1825 took the opposite path by eliminating the effect of the earth's magnetic field on the needle. A pair of needles is mounted parallel to each other, but with the poles reversed. This combination has a net magnetic dipole moment of zero and thus has no preferred direction in the earth's magnetic field. The lower of the magnetic needles is inside the coil which carries the current under test, and alone experiences a torque due to the resulting magnetic field. .

Visit Link 1 and Link 2 At the IMSS in Florance Italy to see some of the earliest example of these instruments and a brief history of their development. These instruments can be beautiful and playful. Below one of our astatic galvanometer we recently made as functional display piece. It measures current in the 1/1000 ampere range. Please Click Here For More Detail copyright 12/30/2006Jim & Rhoda Morris All photos and written material are by Jim & Rhoda Morris unless noted otherwise. Free personal and educational use and reproduction is encouraged; all commercial rights are reserved Random notes to be expanded on 1-15-07- connections----------Photons and electrons can both treated as particles . They both flow. The movement of electrons on the surface of a star generates photons. some of which are collected by the objective lens of Galileo's telescope the photons are processed by the lenses and are directed into the observer's eye where they are converted back to electrons movement. This is further processed electrically and sent to the brain where more electrons are processed into an image which is further process with more electrons into an interpretation of what we see.

This information is further process by even more electrons such as into the movement of our fingers jotting down notes in a notebook. Electrons electrons, they are everywhere doing everything but they're not given credit for anything. 1-17-07 Neither Galileo nor Nobili invented their instruments, Both instruments were amplifiers of a phenomena discovered by earlier work. Their instrument were used to make quantitative measurements of phenomena that could not be made with unaided humans senses. both made break through improvements in their instruments which let us see and measure things that or forever changed our world! 1-18-07 junk stuff there are about 6 * 10^-9 grams of electrons per second in 1 ampere??? or 6*10^-3 grams per second per micro-amps or 6 milligrams of electrons???? Hydrogen is 1 gram per mole.

there are 6.02 * 10^ 23 atoms per mole one electron is 1/1825 of the weight of a hydrogen atom therefore 1 mole of electrons = 1/1825 of a gram =0.000547 grams or 0.547 milligrams so 5.47 * 10^-4 grams/ 6.2 * 10^23 electrons per mole = 0.88 * 10 ^-19 grams per electron Voltmeters and Ammeters Voltmeters and Ammeters About Watch and Favorite Watch Watching this resources will notify you when proposed changes or new versions are created so you can keep track of improvements that have been made. Favorite Favoriting this resource allows you to save it in the “My Resources” tab of your account. There, you can easily access this resource later when you’re ready to customize it or assign it to your students. Voltmeters and Ammeters Voltmeters and ammeters are used to measure voltage and current, respectively.

Learning Objective Compare circuit connection of an ammeter and a voltmeter Key Points A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit . An ammeter is a measuring device used to measure the electric current in a circuit. A voltmeter is connected in parallel with a device to measure its voltage , while an ammeter is connected in series with a device to measure its current. At the heart of most analog meters is a galvanometer , an instrument that measures current flow using the movement, or deflection, of a needle. The needle deflection is produced by a magnetic force acting on a current-carrying wire. Terms shunt resistance a small resistance R placed in parallel with a galvanometer G to produce an ammeter; the larger the current to be measured, the smaller R must be; most of the current flowing through the meter is shunted through R to protect the galvanometer Register for FREE to remove ads and unlock more features! Learn more Full Text Voltmeters and ammeters measure the voltage and current, respectively, of a circuit. Some meters in automobile dashboards, digital cameras, cell phones, and tuner-amplifiers are voltmeters or ammeters.

A brief introduction to voltmeters and ammeters for introductory physics students. Voltmeters A voltmeter is an instrument that measures the difference in electrical potential between two points in an electric circuit. An analog voltmeter moves a pointer across a scale in proportion to the circuit's voltage; a digital voltmeter provides a numerical display. Any measurement that can be converted to voltage can be displayed on a meter that is properly calibrated; such measurements include pressure , temperature, and flow. (a) To measure the potential difference in this series circuit, the voltmeter (V) is placed in parallel with the voltage source or either of the resistors. Note that terminal voltage is measured between points a and b. It is not possible to connect the voltmeter directly across the EMF without including its internal resistance, r. (b) A digital voltmeter in use Ammeters An ammeter measures the electric current in a circuit. The name is derived from the name for the SI unit for electric current, amperes (A). In order for an ammeter to measure a device's current, it must be connected in series to that device . This is necessary because objects in series experience the same current.

They must not be connected to a voltage source -- ammeters are designed to work under a minimal burden, (which refers to the voltage drop across the ammeter, typically a small fraction of a volt). An ammeter (A) is placed in series to measure current. All of the current in this circuit flows through the meter. The ammeter would have the same reading if located between points d and e or between points f and a, as it does in the position shown. (Note that the script capital E stands for EMF, and r stands for the internal resistance of the source of potential difference. ) Galvanometers (Analog Meters) Analog meters have needles that swivel to point at numbers on a scale, as opposed to digital meters, which have numerical readouts.The heart of most analog meters is a device called a galvanometer, denoted by G. Current flow through a galvanometer, IG, produces a proportional movement, or deflection, of the needle. The two crucial characteristics of any galvanometer are its resistance and its current sensitivity. Current sensitivity is the current that gives a full-scale deflection of the galvanometer's needle -- in other words, the maximum current that the instrument can measure.

For example, a galvanometer with a current sensitivity of 50 μA has a maximum deflection of its needle when 50 μA flows through it, is at the scale's halfway point when 25 μA flows through it, and so on. If such a galvanometer has a 25-Ω resistance, then a voltage of only V = IR = (50 μA)(25 Ω) = 1.25 mV produces a full-scale reading. By connecting resistors to this galvanometer in different ways, you can use it as either a voltmeter or ammeter to measure a broad range of voltages or currents. Galvanometers as Voltmeters A galvanometer can function as a voltmeter when it is connected in series with a large resistance R. The value of R is determined by the maximum voltage that will be measured. Suppose you want 10 V to produce a full-scale deflection of a voltmeter containing a 25-Ω galvanometer with a 50-μA sensitivity. Then 10 V applied to the meter must produce a current of 50 μA.

The total resistance must be: $R_{tot} = R + r = \frac{V}{I} = \frac{10V}{50\mu A} = 200 k\Omega,$ or: $R = R_{tot} - r = 200 k\Omega - 25 \Omega \approx 200 k \Omega.$ (R is so large that the galvanometer resistance, r, is nearly negligible. ) Note that 5 V applied to this voltmeter produces a half-scale deflection by sending a 25-μA current through the meter, and so the voltmeter's reading is proportional to voltage, as desired. This voltmeter would not be useful for voltages less than about half a volt, because the meter deflection would be too small to read accurately. For other voltage ranges, other resistances are placed in series with the galvanometer. Many meters allow a choice of scales, which involves switching an appropriate resistance into series with the galvanometer. Galvanometers as Ammeters The same galvanometer can also function as an ammeter when it is placed in parallel with a small resistance R, often called the shunt resistance .

Since the shunt resistance is small, most of the current passes through it, allowing an ammeter to measure currents much greater than those that would produce a full-scale deflection of the galvanometer. Suppose, for example, we need an ammeter that gives a full-scale deflection for 1.0 A and that contains the same 25-Ω galvanometer with 50-μA sensitivity. Since R and r are in parallel, the voltage across them is the same. These IR drops are: IR = IGr so that: $IR = \frac{I_G}{I} = \frac{R}{r}.$ Solving for R, and noting that IG is 50 μA and I is 0.999950 A, we have: $R = r \frac{I_G}{I} = (25 \Omega) \frac{50 \mu A}{0.999950 A} = 1.25 \times 10^{-3} \Omega.$ Galvanometers (E23) Galvanometers The Tangent Galvanometer The simplest and earliest method of measuring electric current uses the tangent galvanometer.

A flat coil of insulated wire is wound on a rigid frame, held by a stand in a vertical position. In the empty space in the middle of the coil a magnetic compass is mounted horizontally, and initially the coil is lined up so that is plane is vertical and in the north-south magnetic direction, the same as the magnetic needle. Now insert the coil in the circuit being measured. The magnetic field of the current now tries to swing the needle 90 degrees, to point perpendicular to the coil. The position at which it settles will represent a balance between the north-south field of the Earth and the east-west force of the coil. That angle better be close to 45 degrees: If the effect of the coil overwhelms that of the Earth, accuracy is poor, though it can be improved by adding a known by-pass resistor so that (knowing also the resistance of the coil) only a known fraction of the current passes the coil. If the force of the coil is barely enough to move the needle off the north-south direction, the instrument is not sensitive enough. This early instrument was not too practical, though students in labs may practice with it.

It depends on the Earth's magnetism (whose accurate measurement was a separate task--first achieved by Gauss ), which is variable and usually off-horizontal, its sensitivity depends on the coil and the current being measured still requires a calculation. Still, it provided an early application of Oersted's discovery. Measuring the Magnetic Force The force between two very short current segments dL1 and dL2 separated by a distance r decreases 1/r2 (the letter "d" in "dL" is calculus notation for "a very small part of L", and the directions of the vectors also count). However, the force between fairly long parallel currents L1 and L2 (attraction when parallel, repulsion if opposed) only decreases like 1/r--one half at twice the distance, not one quarter-- the result of adding up an enormous number of small forces from each part of L1 acting on each of an enormous number of parts of L2. Ampére actually measured the force between parallel currents, by comparing it against known weights in an ingenious "Ampére Balance". An instrument based on this force is the moving-coil galvanometer or D'Arsonval galvanometer, used to measure currents and voltages.

Its main components (be sure to identify the main ones before continuing!) are a strong permanent U-shaped magnet with concave pole pieced and a small rectangular pivoted coil ABCD, around which fine insulated wires are wound. The coil itself is shaped like a small flat box without top or bottom, and the pivot EE' crosses its middle, allowing it to rotate (to some extent) in the space between the pole pieces. Other parts are insulated wires connected loosely to the coil (near the pivot, so as not to interfere with rotation, a pointer attached to the axle, with its tip in front of a dial indicating the current strength, a thin spiral spring defining the rest position of the pointer, and a small peg at the edge of the dial, stopping the pointer from moving too far--within (typically) 40-80 degrees of its rest position. The pole pieces end just short of the coil, and they channel the magnetic field lines to be perpendicular to the coil's positions. A cylinder of soft iron fills most of the space inside the coil and helps even out the magnetic field (and give more inertia to the coil).

The operation is best understood if the rotating coil is viewed from above; let the current flow around the frame, in the direction (say) from A to B. With the magnet's polarity as drawn, by Faraday's rule of the right hand the magnetic force on the current will lift side AD and push down side BC. Since these are on opposite sides of the axis, the net effect would be to rotate the frame and its attached pointer, until the resistance of the spring stops it, at a position depending on the strength of the current. The forces on sides DC and AB do not contribute any rotation, they are parallel to the axle (plus, the currents are opposite so the forces cancel). A sensitive galvanometer is useful for measuring either currents ("ammeter") or voltages ("voltmeter")--and the higher its sensitivity, the less it interferes with the circuit where measurement is made. Suppose the full deflection corresponds to 10 microamperes (μA). Working as an ammeter, the galvanometer uses a choice of bypass resistors ("shunts") allows bigger currents to be measured.

For instance, one having only a millionth of the resistance of the coil (or rather 1/999,999 of it--though lab instruments are not accurate enough for the difference to matter) multiplies the readings of the dial a million times--a deflection of a full dial now indicates 10 amperes. The instrument (including shunt) is inserted in series with the current being measured, and for very small currents, with no shunt, the resistance of the galvanometer must be taken into account. On the other hand, suppose voltage between two points is measured. The instrument is connected "in parallel" between those points (DRAWING) and includes a large resistor. The resistor greatly limits the current drawn from the main circuit, and thus minimizes the interference generated by the measurement. For instance, suppose our galvanometer with full deflection at 10 μA) has an added resistor of 10,000,000 Ω ahead of it. Now a voltage of 100 Volt will create a full-scale deflection, 20 volt a deflection of 1/5 of the dial, and so forth.

The above are the most traditional uses, and a laboratory "multimeter" will have a master switch to choose resistors in series or parallel, multiplying the dial's range by various powers of 10. It will also have a rectifier and filter for measuring AC, also internal batteries for measuring resistance, with appropriate settings of the master switch, and insulated cables for connecting it. This design, from the 19th century, is still widely used, although electronic multimeters are also being sold, based on solid-state devices and probably more rugged. Afterthought: Electric Motors Suppose we modify the instrument: omit the needle, the dial and its limiting peg, and attach the coil firmly to the axle. Would this create an electric motor? No, not yet... but we are getting close! The wires leading to the coil should not get twisted, so they can't lead simply to the outside. Instead, cover a section of the axle with an insulating cylinder, and on it two metal rings, each divided in half by a small gap. The halves on one ring are connected to two ends of the coil in one order, on the other ring, in opposite order, and small leaf springs connect them to a battery.

Suppose the coil is connected so that the force rotates the axle clockwise. When the wires are pulled towards the poles of the magnet almost as far as they can go, the leaf springs encounter the gap and reverse the connection. Now the sections AD and BC are repelled by the pole closest to them, and attracted to the opposite one, and if a small flywheel keeps them spinning past the gap, the rotation will continue. Some DC motors operate like this, but with greater efficiency. Why one square coil and not two perpendicular to each other for stronger force (the split ring will need more gaps), or even more? Why not fill the space inside the coils with iron, which concentrates the magnetic force? Why not replace the permanent magnet with an electromagnet, which can provide 4 regions of concentrated magnetic field lines, or more--to pull-repel the coil several times in the cycle? Why leaf springs and not carbon rods ("brushes") pushed by springs, for better and broader contacts? Do all these and you will be well on the way to designing electric machinery. This section is not meant as an introduction to electric technology, so we stop here; but electric machinery has become a huge field, which keeps expanding.

Electricity nowadays can drive cars, railroad engines, tanks and many other useful mechanisms, of all sizes.

Question: What would be measured on a Galvanometer?
Answer: Electric Current