Electrical measurementsA laboratory manual . s to be preferred. But for values oiX less than </, the present method gives greater sensi-bility. If, for instance, the battery have a resistance of10 ohms, each coil of the galvanometer 500 ohms, and xis 10 ohms, then the Heaviside method is seven times assensitive as the first method. 33. Wheatstones Bridge. — Wheatstones Bridgeis a combination of resistances most commonly employedto measure all except a very high resistance or a verylow one. It consists of six conductors connecting fourpoints, in one of which is a source of electromotiveforce
Electrical measurementsA laboratory manual . s to be preferred. But for values oiX less than </, the present method gives greater sensi-bility. If, for instance, the battery have a resistance of10 ohms, each coil of the galvanometer 500 ohms, and xis 10 ohms, then the Heaviside method is seven times assensitive as the first method. 33. Wheatstones Bridge. — Wheatstones Bridgeis a combination of resistances most commonly employedto measure all except a very high resistance or a verylow one. It consists of six conductors connecting fourpoints, in one of which is a source of electromotiveforce, which need not be constant; and another branchcontains a galvanometer. Let AJB CD (Fig. 18) bethe four points connected bysix conductors. Then sincethe fall of potential by thetwo paths between A and Dis the same, there must bea point B on the path ABDwhich has the same poten-tial as another point on thepath A CD. If these pointsare joined by a conductor, including a galvanometer, no current will flow throughit, and we have the relation. 46 ELECTRICAL MEASUBEMENTS. For let Ji be the current through Ri. It will alsobe the current through ^3, since none flows acrossthrough the galvanometer. Also let I2 be the currentthrough the other branch A CD. Then since the poten-tial difference between A and B is the same as betweenA and C, B.,I, = RJ, (1) Similarly, BJ, = RJ, (2) Dividing (1) by (2), f =f. This may also be written, R\ R^ ErR, or Ri: Ro:: R^: R^. The last equation might have been obtained by bal-ancing with the galvanometer connecting AD and the battery applied to the pointsBC. The conditions for abalance are, therefore, thesame after the galvanom-eter and battery have ex-changed places as before,and depend only upon theproportionality of the fourresistances. If the six conductors arearranged as shown in , and if
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