Page:Collected Physical Papers.djvu/104

84 was made uniform as each spring transmitted the pressure to the next. When the contact surfaces had too long been acted on, fresh surfaces could easily be brought into operation by the simultaneous rolling of all the spirals.

The sensibility of the receiver to the radiation, I found, depends (1) on the pressure to which the spirals are subjected, and (2) on the E. M. F. acting on the circuit. The pressure on the spirals may be adjusted, as will be described later on, by means of a fine screw. The E. M. F. is varied by a potentiometer-slide arrangement. This is a matter of great importance, as I often found a receiver, otherwise in good condition, failing to respond when the E. M. F. varied slightly from the proper value. The receiver, when subjected to long-continued radiation, undergoes exhaustion. The sensibility can, however, be maintained fairly uniform by slightly varying the E. M. F. to keep pace with the fatigue produced.

The receiving circuit thus consists of a spiral-spring coherer, in series with a voltaic cell and a dead-beat galvanometer. The spirals are placed between two pieces of brass, of which the upper one is sliding and the lower one fixed. These two pieces are in connection with two projecting metallic rods, which serve as electrodes. An electric current enters along the breadth of the top spiral and leaves by the lowest spiral, having to traverse the intermediate spirals along the numerous points of contact. When electric radiation is absorbed by the sensitive surface, there is a sudden diminution of the resistance, and the galvanometer spot is violently deflected.