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Rh uniformly pressed against the potentiometer wire. Care was taken to maintain a uniformly good electrical contact. Any sudden introduction of the E. M. F. was avoided with special care, as, owing to the self-induction of the circuit, an unknown induced E. M. F. might act on the sensitive substance. The slide-contact was therefore never broken; starting from zero of the scale (E. M. F.=0) the slider was moved at a uniform rate to the right, thus continuously increasing the E. M. F. without any sudden variation. Movement of the slider to the left produced a continuous diminution of E. M. F. The voltaic cell was never cut off from action till the slider returned to zero.

Method of Observation

One observer moved the slider at a uniform rate and called out the successive potentiometer readings; the corresponding deflections of the galvanometer were read off by the second observer, and a third took down in parallel columns the applied E. M. F. and the corresponding galvanometer readings. From these data the characteristic curves could be subsequently obtained. This method of work was, however, very tedious. It took a long time and the co-operation of three persons to complete one experiment, and the curves could only be obtained from a series of observations which were not absolutely continuous. I had, therefore, to devise a recording apparatus by which a continuous curve could be obtained in a simple and direct manner.

Recorder of Conductivity Variation

If a platform be fixed to the slider of the potentiometer, then the movement of the platform in one