Fig. 522.--Queen tangent and sine galvanometer. This instrument properly adjusted can be used as a standard instrument for laboratory work. The brass ring is 12 inches in diameter, and the grooves in which the wire is wound are carefully turned so as to be of true rectangular cross section, thus allowing the constant of the instrument to be accurately calculated and compared with the constant as obtained by other methods. The compass box is 5 inches in diameter and is so held in position that it may be raised or lowered, rotated on its vertical axis, shifted out of the plane of the coil, etc., thus enabling the operator to acquire proficiency with the instrument and to meet all cases of derangement possible. The dial is graduated to single degrees, and the needle is suspended by a very light cocoon fibre. The whole instrument can be turned about its vertical axis, and a quadrant graduated in degrees upon the base allows the amount of rotation to be accurately measured, and the laws of the sine galvanometer investigated. The instrument is wound to measure .25 ampere to 8 amperes.
Ques. What is the objection to the scale with tangent values?
Ans. It is more difficult to divide an arc into tangent lines with accuracy than into equal degrees.
Ques. What disadvantage has the tangent galvanometer?
Ans. The coil being much larger than the needle, and hence far away from it, reduces the sensitiveness of the instrument.
The Sine Galvanometer.--This type of instrument has a vertical coil which may be rotated around a vertical axis, so that it can be made to follow the magnetic needle in its deflections.
In the sine galvanometer, the coil is moved so as to follow the needle until it is parallel with the coil. Under these circumstances, the strength of the deflecting current is proportional to sine of angle of deflection.
Fig. 523.--Central Scientific Co. universal tangent galvanometer. This instrument may be used as a tangent, Gaugain, Helmholtz-Gaugain, sine, cosine, Wiedemann or detector galvanometer. The coils, which slide on a beam parallel to the one carrying the needle box, are wound on brass rings 12 inches in diameter. On each ring are wound two coils of 48 turns each, connected to separate binding posts, and double wound so as to be of equal resistance. The coils and needle box are each provided with an indicator for reading their position on the scale. The needle box is swivelled and removable and one coil may be rotated about its vertical axis and its position read on a disc graduated in degrees. Currents may be measured ranging from .000002 ampere to 100 amperes.
Ques. Describe the construction of a sine galvanometer.
Ans. A form of sine galvanometer is shown in fig. 524. The vertical wire coil is seen at M. A needle of any length less than the diameter of the coil M, moves over the graduated circle N. The coil M, and graduated circle N may be rotated on a vertical axis, and the amount of angular movement necessary to bring the needle to zero, measured on the graduated circle H.
Ques. How is the current strength measured?
Ans. It is proportional to the sine of the angle measured on the horizontal circle H, through which it is necessary to turn the coil M, from the plane of the earth's magnetic meridian to the plane of the needle when it is not further deflected by the current.
Fig. 524.--Sine galvanometer. It differs from the tangent galvanometer in that the vertical coil and magnetic needle are mounted upon a standard free to revolve around a vertical axis, with provision for determining the angular position of the coil. The needle may be of any length shorter than the diameter of the coil. In the figure the parts are: M, coil; N, graduated dial of magnetic needle; H, graduated dial by which the amount of rotation necessary to bring the needle to zero is measured; E, terminals of the coil; O, upright standard carrying coil and graduated dial of magnetic needle; C, base with levelling screws.
Ques. How is the sine galvanometer operated?
Ans. In using the instrument, after the needle has been set to zero, the current is sent through the coil, producing a deflection of the needle. The coil is then rotated to follow the motion of the needle, the current being kept constant, the rotation being continued until the zero on the upper dial again registers with the needle. The current then is proportional to the sine of the angle through which the coil has been turned, as determined by the lower dial.
Ques. Has the sine galvanometer a large range?
Ans. For a given controlling field, it does not admit of a very large range of current measurement, since, for large deflection, on rotating the coil the position of instability is soon reached.
TABLE OF NATURAL SINES AND TANGENTS
| Angle | Sin. | Tan. |
| 0° | .0000 | .0000 |
| 1 | .0175 | .0175 |
| 2 | .0349 | .0349 |
| 3 | .0523 | .0524 |
| 4 | .0698 | .0699 |
| 5 | .0871 | .0875 |
| 6 | .1045 | .1051 |
| 7 | .1219 | .1228 |
| 8 | .1392 | .1405 |
| 9 | .1564 | .1564 |
| 10° | .1736 | .1763 |
| 11 | .1908 | .1944 |
| 12 | .2079 | .2126 |
| 13 | .2250 | .2309 |
| 14 | .2419 | .2493 |
| 15 | .2588 | .2679 |
| 16 | .2756 | .2867 |
| 17 | .2924 | .3057 |
| 18 | .3090 | .3249 |
| 19 | .3256 | .3443 |
| 20° | .3420 | .3640 |
| 21 | .3584 | .3839 |
| 22 | .3746 | .4040 |
| 23 | .3907 | .4245 |
| 24 | .4067 | .4452 |
| 25 | .4226 | .4663 |
| 26 | .4384 | .4877 |
| 27 | .4540 | .5095 |
| 28 | .4695 | .5317 |
| 29 | .4848 | .5543 |
| 30° | .5000 | .5774 |
| 31 | .5150 | .6009 |
| 32 | .5299 | .6249 |
| 33 | .5446 | .6494 |
| 34 | .5592 | .6745 |
| 35 | .5736 | .7002 |
| 36 | .5878 | .7265 |
| 37 | .6018 | .7536 |
| 38 | .6157 | .7813 |
| 39 | .6293 | .8098 |
| 40° | .6428 | .8391 |
| 41 | .6561 | .8693 |
| 42 | .6691 | .9004 |
| 43 | .6820 | .9325 |
| 44 | .6947 | .9657 |
| 45 | .7071 | 1.0000 |
| 46 | .7193 | 1.0355 |
| 47 | .7314 | 1.0724 |
| 48 | .7431 | 1.1106 |
| 49 | .7547 | 1.1504 |
| 50° | .7660 | 1.1918 |
| 51 | .7771 | 1.2349 |
| 52 | .7880 | 1.2799 |
| 53 | .7986 | 1.3270 |
| 54 | .8090 | 1.3764 |
| 55 | .8192 | 1.4281 |
| 56 | .8290 | 1.4826 |
| 57 | .8387 | 1.5399 |
| 58 | .8480 | 1.6003 |
| 59 | .8572 | 1.6643 |
| 60° | .8660 | 1.7321 |
| 61 | .8746 | 1.8040 |
| 62 | .8829 | 1.8807 |
| 63 | .8910 | 1.9626 |
| 64 | .8988 | 2.0503 |
| 65 | .9063 | 2.1445 |
| 66 | .9135 | 2.2460 |
| 67 | .9205 | 2.3559 |
| 68 | .9272 | 2.4751 |
| 69 | .9339 | 2.6051 |
| 70° | .9397 | 2.7475 |
| 71 | .9455 | 2.9042 |
| 72 | .9511 | 3.0772 |
| 73 | .9563 | 3.2709 |
| 74 | .9613 | 3.4874 |
| 75 | .9659 | 3.7321 |
| 76 | .9703 | 4.0108 |
| 77 | .9744 | 4.3315 |
| 78 | .9781 | 4.7046 |
| 79 | .9816 | 5.1446 |
| 80° | .9848 | 5.6713 |
| 81 | .9877 | 6.3138 |
| 82 | .9903 | 7.1154 |
| 83 | .9925 | 8.1443 |
| 84 | .9945 | 9.5144 |
| 85 | .9962 | 11.43 |
| 86 | .9976 | 14.30 |
| 87 | .9986 | 19.08 |
| 88 | .9994 | 28.64 |
| 89 | .9998 | 57.29 |
Ques. What is the position of instability?
Ans. The position of the needle beyond which the rotation of the coil will cause it to turn all the way round.
Ques. How may the range be increased?
Ans. By an adjustable controlling field or a shunt.
Ques. What advantage has the sine galvanometer over the tangent instrument?
Ans. Its advantage is in the case where the relative values of two or more currents are required to be measured, or where the constant of the instrument is obtained by comparison with a standard measuring instrument and not calculated from the dimensions of the coil, because all galvanometers thus used follow the sine law independently of the shape of the coil, while only circular coils will follow the sine law.
Fig. 525.--Differential galvanometer. It consists of two coils of wire, so wound as to have opposite magnetic effects on a magnetic needle suspended centrally between them. The needle of a differential galvanometer shows no deflection when two equal currents are sent through the coils in opposite directions, since, under these conditions, each coil neutralizes the effect of the other. Sometimes the current is so sent through the two coils, that each coil deflects the needle in the same direction. In this case the instrument is no longer differential in action. If, when this condition obtains, the magnetic needle be suspended at the exact center of the line which joins the centers of the coils, the advantage is gained by obtaining a field of more nearly uniform intensity around the needle. When the needle is suspended by a silk fibre, a final and most delicate adjustment can be obtained by raising or lowering one of the levelling screws slightly, so as to tilt the needle nearer to or farther from one of the coils.
The Differential Galvanometer.--This is a form of galvanometer in which a magnetic needle is suspended between two coils of equal resistance so wound as to tend to deflect the needle in opposite directions. The needle of a differential galvanometer shows no deflection when two equal currents are sent through the coils in opposite directions, since under these conditions, each coil neutralizes the other's effects. Such instruments may be used in comparing resistances, although the Wheatstone bridge, in most cases, affords a preferable method.
Ques. What is the special use of the differential galvanometer?
Ans. It is used for comparing two currents.
Ques. What is the method of comparing currents?
Ans. If two equal currents be sent in opposite directions through the coils of the galvanometer, the needle will not move; if the currents be unequal, the needle will be deflected by the stronger of them with an intensity corresponding to the difference of the strengths of the two currents.
Ques. How are the coils adjusted?
Ans. This is done by coupling them in series in such a way that they tend to turn the needle in opposite directions, and when a current is passing through them, they are moved nearer to the needle or farther from it until the needle stands at zero with any current.
If the coils be not movable, a turn or more can be unwound from the coil giving the greatest magnetic effect until a balance is obtained, the wire so unwound can then be coiled in the base of the instrument.
Ballistic Galvanometer.--This type of galvanometer is designed to measure the strength of momentary currents, such for instance, as the discharge of a condenser. In construction the magnetic system is given considerable weight, and arranged to give the least possible damping effect.
If a momentary current be passed through a ballistic galvanometer, the impulse given to the needle does not cause appreciable movement to the magnetic system until the current ceases, owing to the inertia of the heavy moving parts, the result being a slow swing of the needle.
Fig. 526.--Queen dead beat and ballistic reflecting galvanometer. As illustrated, the coils are easily removable and enclose a heavy block of copper fixed in a central fork. In a cylindrical hole bored in this block hangs the bell magnet which with its mirror is suspended by a long cocoon fibre, and the eddy currents induced in the copper bring the system quickly to rest after a deflection. By lifting the copper block out of the frame the instrument is made ballistic. The instrument is made with coils of any desired resistance up to 1,000 ohms.
Ques. What name is given to the swing of a ballistic galvanometer needle?
Ans. It is called the kick.
Ques. How is the current measured?
Ans. As the needle swings slowly around it adds up, as it were, the varying impulses received during the passage of the momentary current, and the quantity of electricity that has passed is proportional to the sine of half the angle of the first swing or kick.
If a reflecting method be used with a straight scale, the observed deflection depends upon the tangent of twice the angle of movement of the needle. For small deflections, however, the change of flux can be taken as directly proportional to the observed deflection.
Fig. 527.--Thompson galvanometer with mirror reflecting system for reading the deflections of a galvanometer needle by the movements of a spot of light reflected from a mirror attached to the needle or movable magnetic system.
Use of Mirrors in Galvanometers.--In order that small currents may be measured accurately, some means must be provided to easily read a small deflection of the needle. Accordingly, it is desirable that the pointer be very long so that a large number of scale divisions may correspond to small deflections. In construction, since sensitive galvanometers must be made with the moving parts of little weight, it would not do to use a long needle, hence a ray of light is used instead, which is reflected on a distant scale by a small mirror attached to the moving part.
In the Thompson mirror reflecting galvanometer, as shown in fig. 528, a small vertical slit is cut in the lamp screen below the scale, and the ray of light from the lamp, passing through the slit, strikes the mirror which is about three feet distant, and which reflects the beam back to the scale. It should be noted that the angle between the original ray of light and the reflected ray is twice the angle of the deflection of the mirror; the deflections of the ray of light on the scale, however, are practically proportional to the strength of currents through the instrument. The mirror arrangement as shown in fig. 528, requires a darkened room for its operation, but such is not necessary when a telescope is used as in fig. 529. Here the scale readings are reflected in the mirror and their value observed by the telescope without artificial light.
Fig. 528.--Telescope method of reading galvanometer deflections by reflection of scale reading in mirror. Here two mirrors are used, but in most cases the telescope is pointed directly toward the mirror on galvanometer shown in fig. 527, because the two mirror system, as illustrated in the figure, is used on portable galvanometers since it is the more compact.
Damping.--This relates to the checking or reduction of oscillations. Thus, a galvanometer is said to be damped when so constructed that any oscillations of the pointer which may be started, rapidly die away. Galvanometers are frequently provided with damping devices for the purpose of annulling these oscillations, thus causing the moving part to assume its final position as quickly as possible.
Sometimes the instrument is fitted with a damping coil, or closed coil so arranged with respect to the moving system that the oscillations of the latter give rise to electric currents in the closed coil, whereby energy is dissipated. Again, air vanes are employed, but anything in the nature of solid friction cannot be used.
Figs. 529 and 530.--Galvanometer lamp and scale for individual use. The scale is etched on a ground glass strip 6 centimeters wide by 60 centimeters long with long centimeter divisions and short millimeter divisions the entire length, reading both ways from zero in the center. It is mounted in an adjustable wooden frame. A straight filament lamp (110 volts) is enclosed in a metal hood japanned black to cut out all reflected light. This form of filament makes a single brilliant line on the scale, enabling closer readings than the "spot of light" arrangement. The lamp hood is adjustable to any desired height on the support rod.
D'Arsonval Galvanometer.--This instrument has a movable coil in place of a needle, and its operation depends upon the principle that if a flat coil of wire be suspended with its axis perpendicular to a strong magnetic field, it will be deflected whenever a current of electricity passes through it.
Ques. Describe the construction of a D'Arsonval galvanometer.
Ans. The essential features are shown in figs. 532 and 533. The coil, which is rectangular in section is wound upon a copper form, and suspended between a permanent magnet by fine wires to the points A and B. The magnet has its poles at N and S. It is a soft iron cylinder fixed between the poles in order to intensify the magnetic field across the air gaps in which the coil moves.
Fig. 531.--Queen reading telescope. This arrangement is utilized to measure the deflections of a galvanometer having suspended mirror moving system. It consists of a reading telescope mounted as illustrated with a millimeter scale, having a length of 50 centimeters. In use, the image of the scale is seen in the galvanometer mirror through the telescope. The eye piece of the telescope has a cross hair which acts as a reference line so that by noting the particular division on the scale when the galvanometer is at rest, the amount of deflection can be readily observed when the galvanometer is deflected. The instrument has all the necessary adjustments to set it up quickly and for bringing the cross hair and scale in focus. It is generally placed at a distance of one meter from the galvanometer mirror.
Ques. Explain its operation.
Ans. An enlarged horizontal cross section of the galvanometer on line XY is shown in fig. 533. The current is flowing in the coil as in fig. 532, up on the left side and down on the right. The position of the coil when no current is flowing is indicated by n' s'. By applying the law of mutual action between magnetic poles, it is seen that when the current is applied, the poles developed at n' s' will move into the position n'' s''. See fig. 119.
Ques. How is the coil affected by a change in the direction of the current?
Ans. The polarity of the coil is reversed and consequently the direction of the deflection.
Figs. 532 and 533.--Diagrams showing essential features of construction and principle of operation of D'Arsonval galvanometer.
Ques. Upon what does the sensitiveness of the instrument depend?
Ans. Upon the strength of the field of the permanent magnet, the number of turns in the suspended coil, and the torsion of the wires by which it is suspended.
Ques. When is this galvanometer called "dead beat"?
Ans. When the construction is such that the moving part comes quickly to rest without a series of diminishing vibrations.
Figs. 534 to 536.--Queen horizontal magnet D'Arsonval galvanometer with telescope and scale. It is very sensitive and is used in many electrical measurements, including commercial testing, such as measuring insulation of cables, fault location, etc. It is not affected by surrounding magnetic disturbances, and may, therefore, be used in proximity to dynamos and switchboards. The instrument has a pair of binding post terminals, one of which connects to a bottom spiral of the system and the other forms a junction with the top of the tube holding the system, forming a complete circuit through the coil. The tube containing the system may be readily removed from the magnet and another tube having a different system inserted as is required for various kinds of electrical measurement. The entire system with its suspension may be inspected by the removal of a thumb screw. To inspect interior of tube first be sure that the screw B is turned so that the coil is clamped. Entirely remove screw C, and, holding the outside tube near the window, press firmly with the finger on the extreme top of the suspension support. The inside rib, with complete suspension, will draw from the tube, and the working parts can be fully inspected. Carefully return same to its original position in tube, setting tight the screw C. The galvanometer is designed so that the coil is clamped in position when the galvanometer must be transported. The insulation of the galvanometer terminals and binding posts is such as to guard against any possible leakage. As a further protection, each levelling screw is provided with a hard rubber insulator. This feature is essential since, in making insulation measurements, the operator wishes to be assured that the deflection being obtained is the result of leakage upon the cable or wire being measured and not leakage between the galvanometer terminals. The galvanometer is provided with an attached telescope and scale for noting the deflections. The deflections produced by this galvanometer are proportional to the current. To facilitate quickly setting up the instrument, two way levels are provided.
Ans. The instrument is made dead beat by winding the coil on a copper or aluminum frame, so that when in operation, currents are induced in the frame by the motion of the coil in the magnetic field; these currents oppose the motion of the coil.
Ques. For what service is the D'Arsonval galvanometer adapted?
Ans. It is desirable for general use as it is not much affected by changes in the magnetic field. It may be made with high enough period and sensibility to be satisfactory as a ballistic instrument, but for extreme sensibility an instrument of the astatic type is more generally used.
Galvanometer "Constant" or "Figure of Merit."--In order that a galvanometer shall be of value as a measuring instrument, the relation between the current and the deflection produced by it must be known. This may be obtained experimentally by determining the value of the current required to produce one scale division. The galvanometer constant then may be defined as the resistance through which the galvanometer will give a deflection of one scale division when the current applied is at a pressure of one volt.
Accordingly, the deflection as indicated on the scale must be multiplied by its constant or figure of merit, in order to obtain the correct reading. If the scale readings be not directly proportional to the quantity to be measured, the law of the instrument must also be considered.
Thus in a tangent galvanometer as previously explained
where I = current, φ the deflection or scale reading, and K the galvanometer constant.
Fig. 537.--Diagram showing method of connecting galvanometer shunt. By the use of a shunt the range of measurement of a galvanometer can be greatly increased.
Fig. 538.--Diagram of a form of universal shunt box for use with galvanometers of widely different resistances. The galvanometer, as indicated at G, is connected across the ends of a series of resistances AB. The main wires are connected, one to end A of the series and the other to a travelling point whose position is varied by means of plugs or by a dial switch.
Galvanometer Shunts.--The sensitiveness of a galvanometer used for measuring current may be reduced to any desired extent by connecting a resistance of known value in parallel with it. Thus, if it be desired to measure a current greater than can be measured directly by the galvanometer, a part of the current can be sent through the resistance or shunt, and the total value of the current calculated.
A galvanometer shunt bears a definite ratio to the resistance of the galvanometer, being usually adjusted so that only .1, .01, or .001 part of the current passes through the galvanometer.
The degree in which a shunt increases the range of deflection of a galvanometer is called its "multiplying power."
Fig. 539.--Ayrton-Mather universal shunt. This shunt may be used with any galvanometer. The total resistance is 10,000 ohms, with shunt powers of 1, 5, 10, 50, 100, 500, and 1,000. It is also fitted with positions in which the galvanometer is shorted and off. The coils are of constantan wire.
If .1 of the current flowing, passed through the galvanometer and .9 through the shunt, then the current in the circuit would be ten times that through the galvanometer. Accordingly the current in the galvanometer must be multiplied by the multiplying power of the shunt to obtain the true value of the current in the circuit.
In order to determine the resistance necessary to be used with a certain galvanometer, the resistance of the latter is to be divided by the multiplying power desired less one.
EXAMPLE.--What must be the resistance of a shunt for a galvanometer of 2,000 ohms resistance where only one fifth of the current is to pass through the galvanometer?
The multiplying power less one is
and the required resistance is
When it is essential that the total resistance of the circuit should not be altered by an alternation of the galvanometer shunt, a compensating box should be used which automatically inserts a resistance for each shunt in series with the shunted galvanometer to bring the total resistance up equal to the unshunted value. Thus the current in the main circuit is not altered.
CHAPTER XXVII
TESTING AND TESTING APPARATUS
The practical electrician frequently has to make tests of various kinds which require the rapid and accurate measurement of voltage, current and resistance. It is therefore essential that he understand the methods employed in testing and the operation of the instruments used.
Most tests are made with a galvanometer, and the devices, such as resistances, switches, etc., which are used in connection with the galvanometer may be obtained put up in a neat and substantial box together with the galvanometer, the combination being called a "testing set." Numerous forms of testing set are illustrated in this chapter.
The construction, use, and operation of the various types of galvanometer have been explained in chapter twenty-six. Ammeters and voltmeters, which are simply special forms of galvanometer, and which are largely used are fully described in the preceding chapter.
Pressure Measurement.--An electromotive force has been defined as that which causes or tends to cause a current; it is analogous to water pressure; potential difference corresponds to difference of level. The total electromotive force of a circuit is independent of resistance or current, and cannot be limited to mean the fall of pressure between any two points, as for instance the terminals of a battery.
If the pressure of a battery be two volts when measured on open circuit by a static voltmeter, there will still be two volts on closed circuit, but there will now be a loss of pressure through the internal resistance of the battery and the voltage across the terminals will be less than the total voltage. The static voltmeter, never closing the circuit, actually measures the total voltage.
Fig. 540.--Clark cell (Kahle's modification of the Rayleigh H form), the standard for the International volt. The cell has for its positive electrode, mercury, and for its negative electrode, amalgamated zinc. The electrolyte consists of a saturated solution of zinc sulphate and mercurous sulphate. The pressure is 1.434 volts at 15°C., and between 10°C. and 25°C. the pressure decreases .00115 of a volt for each increase of 1°C. The containing glass vessel consists of two limbs, closed at bottom and joined above to a common neck fitted with a ground glass stopper. The diameter of the limbs should be at least 2 cms., and their length at least 3 cms. The neck should be not less than 1.5 cms. in diameter. At the bottom of each limb a platinum wire of about .4 mm. in diameter is sealed through the glass. To set up the cell, place mercury in one limb, and in the other hot liquid amalgam, containing 90 parts mercury and 10 parts zinc. The platinum wires at the bottom must be completely covered by the mercury and the amalgam, respectively. On the mercury, place a layer 1 cm. thick of the zinc and mercurous sulphate paste. Both this paste and the zinc amalgam must be covered with a layer of the neutral zinc sulphate crystals 1 cm. thick. The whole vessel must then be filled with the saturated zinc sulphate solution, and the stopper inserted so that it shall just touch it, leaving, however, a small bubble to guard against breakage when the temperature rises. Before finally inserting the glass stopper a strong alcoholic solution of shellac is applied to the upper edge, after which the stopper is pressed firmly in place.
Ques. What error is introduced in measuring the pressure of a battery with an ordinary voltmeter?
Ans. Since the measurement is made on closed circuit the reading does not give the total pressure of the battery.
The error is very slight because the resistance of the voltmeter is very high and the current so small that the loss of pressure in the battery can be neglected.
Fig. 541.--Weston Cadmium Cell. It is made in two forms; one known as the Weston normal cell, in which the solution of cadmium sulphate is saturated at all temperatures at which the cell may be used. The other, known as the Weston standard cell, in which the cadmium sulphate solution is unsaturated at all temperatures above 4°C. The Weston normal cell, or saturated form is slightly affected by changes in temperature, but, on account of the fact that it can be accurately reproduced, it was adopted by the London Conference in 1908, as a convenient voltage standard. The value of its voltage suggested by the committee of the London Conference on Electrical Units and Standards, and adopted by the Bureau of Standards at Washington, Jan. 1st, 1911, is 1.0183 International volts at 20°C. At any other temperature its voltage is:
Et = E20 - 0.0000406(t-20) - 0.00000095(t-20)2 + 0.00000001(t-20)3
The Weston standard cell, or unsaturated form is practically unaffected by changes in temperature and is the form most commonly used for laboratory work and general testing. The average pressure of this form is 1.0187 Int. volts.
Ques. Define the International volt.
Ans. It is the electromotive force that, steadily applied to a conductor whose resistance is one International ohm, will produce a current of one International ampere, and which is represented sufficiently well for practical use by 1,000/1,434 of the voltage between the poles of the Clark cell at a temperature of 15° C., when prepared as in fig. 540.