Page:Collected Physical Papers.djvu/251

Rh moves through its own length past a fixed index, the sliding contact moves through the whole length of the potentiometer. If the applied E. M. F. at the terminals A and B be, for example, 1 volt, each inch of paper along the platform would then represent $1⁄20$th or 0·05 volt, the smaller sub-division representing 0·005 volt. This or any other suitable terminal value of E. M. F. can be obtained with the help of the storage cell S, a rheostat r and an auxiliary voltmeter V, applied to the terminals A and B.

The galvanometer, as was said before, is also adjusted by means of a shunt, so that a deflection of one division is a convenient small fraction of an ampere. The absolute values of the E. M. F. and current being known, the resistance of the sensitive substance at any point of the curve can easily be determined.

In the completed apparatus the platform is mounted on small wheels and moves on rails with perfect smoothness. A string connected with one end of the platform is carried round a large winding wheel and a second string connected with the other end of the platform carries a counterpoise. When the wheel is turned at a uniform rate in one direction, the platform carrying the paper also moves at a uniform rate, at the same time producing a continuous increase of E. M. F. acting on the circuit. When the wheel is turned in the opposite direction, the counterpoise reverses the motion, and the impressed E. M. F. undergoes a continuous diminution. In this way it is possible to produce either an increase, or a decrease, or a rapid cyclic variation of E. M. F. I shall first describe in detail the rising part of the curve thus obtained, and then deal with peculiarities of complete curve when carried through a cycle.