Page:Collected Physical Papers.djvu/360

340 ohms in the dark. An approximately equal resistance is placed in the second arm of the bridge. A rheostat having a large number of turns of fine wire with a sliding contact is used for the two variable arms of the bridge, diagrammatically represented by a straight line. Under exact balance the galvanometer deflection is reduced to zero. The balance is upset when the selenium cell is exposed to light and the resulting deflection gives a measure of the intensity of light.

After previous adjustment of the balance in the dark the electric circuit is completed by the closure of key K₁;