A before, El, a cell of constant electromotive force, Es the standard cell, and G the galvanometer. Es is first connected up, as shown, and the slider B manipulated until no deflection results upon the galvanometer G. Note the resistance a, substitute E, the battery or E.M.F. under test, for Es, and repeat the Es al operation, obtaining a second value al; then E= a The Measurement of High Potential Differences.— There are several methods of ascertaining the E.M.F. or P.D. of a current when the latter is much above the normal, as in high tension work, for instance, of which the following is probably the most satisfactory. с C G2 C3 C4 CS C6 C7 C9 НННННННЕ FIG. 57. 8 Let it be required to know the E.M.F. between the points A and B, Fig. 57; a number of equal condensers, c, c1, c2, etc., are connected in series between the points A and B, and the electromotive force E between the terminals of one of them, c, is ascertained by any of the usual methods, and multiplied by the number of condensers in the series. Departing from the subject of electromotive force determination, we come to the next important item on our list of electrical measurements, viz.— (9) The Determination of Current Strength. One of the most direct methods for the determination of current strength in ampères consists in passing the current to be measured through a Siemen's electro-dynamometer, the construction and principle of which useful instrument we will now proceed to discuss. Referring to Fig. 58, if we have two rectangular circuits A B C D, a b c D, one lying in a plane at right angles to the other, and connected in series at D, as shown, and, moreover, if one of the rectangles a b c D be fixed, and the other suspended by a frictionless suspension as indicated in the figure, and a current be passed through the system by way of the terminals 1 and 2, it is obvious, from certain well-known laws governing the action of neighbouring currents upon one another, that the movable rectangle A B C D will be attracted on one side and repelled on the other, and will, in consequence, tend to set itself in the same plane with the fixed rectangle a b c D, as represented in the figure. From the same laws it follows that this force, tending to produce motion of the movable rectangle A B C D, varies as the square of the current, and it is this principle, practically applied, which constitutes the Siemen's electro-dynamometer. The instrument, as commercially constructed, is represented in the accompanying illustration, and consists of a fixed composite coil, composed of a few turns of thick wire, and a number of turns of thin wire. These two coils are connected together at one end, and to a common terminal which is the centre one of the three indi cated in the figure. The remaining ends of the thick and thin coils are brought respectively to the two outer terminals, the object of the two coils being to increase the effective range of the instrument according as to whether the current to be measured is large or small. The fixed coil, which corresponds with a b c D, Fig. 58, is supported on a suitable stand as shown, and is surrounded by a stout wire rectangle, lying in a plane at right angles to it, and suspended at its upper extremity from a thumb Siemen's Electro-Dynamometer for Current Measurement. screw by means of a thin fibre running through the centre of a delicate spiral spring, which is also rigidly attached both to the rectangle and to the aforementioned thumb-screw. Superimposed upon these intersecting coils is a horizontal graduated dial, the periphery of which is divided into degrees, and around the outer edge of which, between the limits of two stops placed a short distance apart, and embracing at their centre the zero, or 0° point of the scale, plies an index finger, also rigidly attached to the movable coil. A second radial index attached to the thumb-screw also indicates upon the circumference of the scale, whilst three levelling screws and a suitable level or plumb-line complete the apparatus. Electrical connection with the movable coil or rectangle is made by means of two mercury cups, into which its lower extremities dip. The mode of usage is as follows:-The apparatus having been set up and levelled until the movable coil is free to move in either direction, but remains stationary with both indices at zero, the current to be measured is passed through by way of one pair of terminals or the other, according to its probable value, and a deflection of the movable coil until checked by one of the stops is the result. The thumb-screw at the top is then turned in the opposite direction until the pointer or index attached to the movable rectangle is brought back to zero by the consequent torsion of the spiral spring. The angle through which the radial pointer has been turned to secure this result is then read off upon the horizontal dial, and the current passing is indicated upon a table of degrees and corresponding currents specially prepared for the instrument. This table is constructed by the manufacturers in the first instance by passing a current of known value through the instrument, and noting the number of degrees of torsion required to bring the movable index to zero. When this has been ascertained, the remainder of the table can be deduced by simple rule of three. As may readily be imagined, the electro-dynamometer is most accurate when used for large currents, which require a considerable degree of torsion to counteract their deflective effects upon the movable rectangle, as in such cases the percentage of error is very small compared with that attendant upon the measurement of correspondingly small values. The direct deflection method of current measurement is a comparatively simple one, and depends for its accuracy on the corresponding definition by the observer's eye, of the galvanometer readings. It involves the employment of a low resistance galvanometer, the resistance of which is known, a standard cell or accumulator from which a current equivalent to that to be measured can be taken without disturbing its constancy, and a variable resistance of sufficient dimensions to carry the current under test without appreciable heating. The resistance of the standard cell must either be known, or of so small a value as to be negligible in calculating subsequent results. The galvanometer is first joined up in the circuit through which is flowing the current which it is required. B to measure, and its deflection duly noted. It is then dis- Expressed as a formula, let E be the E.M.F. of the Let C be the current which it is required to measure. Rg Ꭱ Rc required). The difference of potential deflection method for the determination of current strength is indicated in Fig. 59, where a b is a low resistance introduced into the path of the current to be measured, which is flowing from A to -A |