The mode of procedure is as follows. Let us suppose in the first instance that we know the resistance of the battery E, then r is omitted, and K and K1 being closed, R is adjusted until a balance or zero reading is obtained on the galvanometer G. Then the required E.M.F. E=Es volts, where Re is the resistance of the R+ Re R source of E.M.F. (E.). If, on the other hand, Re be an unknown quantity, include the subsidiary resistance r in the circuit, and proceed as before to adjust R with K and K1 closed, until no deflection results upon the galvanometer. Next vary r to rl, and R to R1, until a balance is again obtained with K and K1 closed; then (r−rl)+(R−R1) the E.M.F. to be determined, E = Es volts. R R1 The variable resistances R and r, in this test are conveniently provided by the proportional and adjustable arms of a Wheatstone bridge, the connections in such case being as indicated by Fig. 51, R being unplugged in the adjustable arm, and r, if needed, in the proportional arms. As will readily be conceived from the attendant dia K R FIG. 51. grams of connections, this method consists in opposing one E.M.F. (under test) to the other (standard), and so adjusting the resistances in circuit that these electromotive forces exactly balance one another, and no current, in consequence, flows through the galvanometer. Kempe's method of determining electromotive forces is somewhat ingenious and simple in its application. Its two phases are represented by diagrams A and B, Fig. 52, respectively. In the first instance, the standard E.M.F., Es, and the E.M.F. under test, E, are opposed to one another, and connected with the terminals of a Thomson reflecting galvanometer; the resulting deflection d, due to the preponderance of one E.M.F. over the other, is duly noted. The electromotive forces are then, as indicated in diagram B, connected up in series to assist one another, and the resultant deflection being rather large, a shunt s is introduced across the terminals of the galvanometer G. The introduction of this shunt, according to the law of divided circuits, materially lowers the total resistance of this particular circuit which we are now C R Es A dealing with, and, to cope with this decrease, what is known as a compensating resistance," R, is introduced into the circuit, R being of such a value as to render the total resistance of the circuit the same as it was before the introduction of the shunt s. The second deflection dl is also duly noted, together with the value of the shunt, and the multiplying power of the latter, m, having been obtained from the afore Lumsden's or Lacoine's method, so-called from the fact that both gentlemen named devised this system independently of one another at or about the same time, is represented diagrammatically in Fig. 53. Es and E represent, as before, the standard and E.M.F. under test respectively, G the galvanometer, and R, R1 adjustable and fixed resistances respectively. The two electromotive forces are connected in series as shown, and R is adjusted until a balance is obtained on the galvanometer. Then the respective electromotive forces Es and E will bear the same proportion to one another as Es R1 do the resistances R and R1, or E = Ꭱ Before proceeding with a description of any further tests for the determination of electromotive force, I must digress for a moment to describe a modification of the sets of plug resistances as usually constructed in the Wheatstone bridge form of instrument. The modification alluded to consists of a species of extended slide wire; it is obviously impossible, with the ordinary stretched slide wire, to command a resistance of any magnitude between its extremities, without producing it to an enormous and therefore practically inconvenient length. The modification consists of a series of resistance coils, a, b, c, etc., Fig. 54, connected to metallic blocks as shown, B Н FIG. 54. but, instead of being manipulated, as in the case of the Wheatstone bridge, by plugs, they are cut in or out of circuit by a sliding contact B, working along a 'bus bar A, so that any required resistance, within the limits of the apparatus, may be inserted between terminals 1 and 2 by moving the slider B along the bar without, at the same time, disturbing the value of the total resistance between terminals 2 and 3, a condition sometimes required in testing. This apparatus may conveniently be arranged in circular form, with a radial contact slider. Having described this modification, which we shall require very shortly, we will now proceed to deal with Fahie's method of determining electromotive forces. This is essentially a combination method for measuring simultaneously the E.M.F. and internal resistance of a given battery, and is an adaptation of Poggendorff's and Mance's respective methods for these determinations, both of which have already been singly dealt with in the preceding paragraphs. Fig. 55 represents a diagram of the connections for the test, in which a and b are the resistances on either side of the slider B in the apparatus depicted in the foregoing figure; c is an adjustable resistance, and G the galvanometer. E and Es are the battery under test and the standard respectively, whilst K is an ordinary circuit key employed to connect the two junctions, as shown. The mode of procedure is as follows:-K being open, the resistance c is manipulated until a balance is obtained on the galvanometer G. When such a balance has been obtained, the key K is manipulated in conjunction with the slider B until the latter is brought to such a position on the slide resistance a b, that the manipulation of K has no effect upon the resultant galvanometer deflection, then the resistance R of the bat a c tery E = and the required electromotive force E= Es (c + b). b The What is known as the Potentiometer Direct Method of determining electromotive force also involves the use of a slide resistance such as that described above. connections for this test, which is an extremely simple one, are represented in Fig. 56, where a b represent the two resistances on either side of the slider B as |