Page:Collected Physical Papers.djvu/248

228 micrometer screw, together with the tube of filings, were appropriately fixed on a heavy base. This rested in turn on a steady pedestal, with one or two sets of pneumatic tyres interposed. Experiments were carried out with the sensitive substance in air, or immersed in kerosine.

2. The Galvanometer was appropriately shunted so as to give a deflection of one division of scale for a definite small fraction of an ampere. In some experiments, for example, one division of the scale was equal to 10$-6$ ampere. The calculation of resistance of the sensitive substance is very much facilitated if the scale values of the galvanometer and the potentiometer were suitably adjusted beforehand. The resistance of the shunted galvanometer and the potentiometer was practically negligible in comparison with the resistance of the sensitive substance, which usually varied from about 1,000 to 50,000 ohms.

3. The Potentiometer consisted of a thin nickeline wire of uniform section, 20in. in length. Each inch of the scale was further sub-divided into tenths. A storage cell with an interposed resistance was applied to the terminals of the potentiometer, and the resistance adjusted till the terminal E. M. F. attained a suitable value. This was found by the help of an auxiliary voltmeter applied to the terminals of the potentiometer.

The E. M. F. acting on the main circuit was derived from the potentiometer; one end of the circuit was connected with one terminal of the potentiometer and the other connected with a slider, the movement of which to the right or to the left increased or diminished the impressed E. M. F. The sliding contact was made by means of a bent flat metallic spring which