Page:Collected Physical Papers.djvu/344

324 against the surface of the leaf by means of a glass rod; or the surface is struck with a glass hammer. In both these cases, A end of the leaf becomes galvanometrically negative, the direction of the current of response being the same as when A is stimulated by light.

Having given a simple demonstration of the fundamental reaction, I describe the photo-electric cell made of two pieces of leaf. In the experiment just described, the resistance of the circuit is very great, on account of the high resistance of the two non-polarisable electrodes, and the resistance offered by the leaf. The non-polarisable electrode, moreover, is a source of trouble; an attempt was therefore made to discard it, and employ other means for diminishing the resistance of the circuit. For the following experiments we employ the leaf of Musa sapientum which is divided into two longitudinal halves by a slit along the thick midrib. Two pieces of leaves are thus obtained about 10 × 10 cm. which are hung parallel and separated from each other in a rectangular glass vessel filled with normal saline; the distance between the two leaves is 3 cm. Two gold wires are thrust through the length of the two divided midribs; they serve as external electrodes of the photo-voltaic cell, which lead to the galvanometer G. The glass trough is placed inside a rectangular wooden chamber with two hinged doors on opposite sides, by which the leaf A or B could be alternately exposed to light (fig. 96). When the doors are closed, A and B are in darkness; they are practically iso-electric, there being no current in the galvanometer. But exposure of A to light gives rise to a, difference of potential between A and B, A becoming galvanometrically negative, the resulting deflection being in one