Page:Collected Physical Papers.djvu/345

Rh direction. Exposure of B gives rise to a responsive deflection in the opposite direction. The two leaves serve as the two plates in a voltaic cell; but unlike ordinary voltaic cell with elements of different metals,

the two plates of the vegetable cell are made of two halves of the same leaf, the electromotive force being generated by the excitatory action of light on one of the two half leaves. The advantages of this method of obtaining electromotive response are: (1) that the troublesome employment of the non-polarisable electrodes with their high resistance is dispensed with; (2) that the area of the surface of the leaf exposed to light is considerably increased; (3) that the electric resistance of the circuit is greatly decreased, since the interposed resistance is that of normal saline about 3 cm. thick with a broad section of 100 square cm.; and (4) that alternate and opposite responses may be obtained by successive exposures of the two leaf-plates to the parallel beam of an arc lamp, this being easily secured