Page:Collected Physical Papers.djvu/26

6 side by side, the sensitive surface being 1 × 2 cm. The springs are prevented from falling by a glass slide in front. The spirals may be compressed by means of a brass piece which slides in and out by the action of a screw. The resistance offered by this portion of the circuit can, therefore, be gradually varied. An	electrical 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. The resistance of the receiving circuit is thus almost entirely concentrated at the sensitive contact-surface, there being little useless short-circuiting by the mass of the conducting layer. When electric radiation is absorbed by the sensitive contacts, there is a sudden diminution of resistance, and the galvanometer is violently deflected.

A pair of insulated wires from the ends of the receiver are led out to a galvanometer placed at a distance. The leading wires are shielded from radiation by enclosing them inside two coatings of tin-foil. As an additional precaution, the galvanometer and the voltaic cell are also enclosed in a metallic case with a slit in front of the galvanometer mirror. A spot of light reflected from the mirror is received on a scale. By adjusting the electromotive force of the circuit, the sensitiveness of the receiver may be increased to any extent desirable.

This is most simply effected by the following arrangement of a modified Daniell cell and a shunt. A small