Page:Collected Physical Papers.djvu/359

Rh When an electric current is maintained in the circuit, exposure of the selenium cell to light causes an increase of the galvanometer deflection. Several difficulties are, however, encountered in employing it for a continuous record of fluctuation of light for the whole day. The resistance of selenium undergoes a change under the polarising action of an electric current, the variation increasing with the strength and the duration of the current. The effect of polarisation is however negligible, if the current be feeble and of short duration. There is a possibility of another difficulty arising from the effect of daily variation of temperature on the normal resistance of the selenium cell. Allowance for this could be made by taking a continuous record of the effect of hourly variation of temperature on the resistance of the cell kept in darkness. Finally means have to be devised for automatic record of galvanometer deflections under changing intensities of light.

The difficulties encountered in obtaining automatic record have been removed by the following devices:—

(a) The Wheatstone Bridge for balancing electric resistance of the selenium cell in dark and its upset on exposure to light.

(b) The arrangement of three electric keys which are automatically put on and off in regular sequence and at pre-determined intervals.

(c) The Self-recording Galvanograph.

This is diagrammatically represented in B (fig. 106). The resistance of the particular selenium cell S is 76,000