Light on the Polarised Electrode. 42^ 



plate being connected with one extremity of a galvanometer, and 

 a gridiron of silver-wire placed in front of the plate with the 

 other. This experiment I showed at a lecture at the London 

 Institution in 1843 ; and it was subsequently used as an illustra- 

 tion of the convertibility of force, in my essay on the ' Correlation 

 of Physical Forces.' 



I tried some further experiments at the time without obtaining 

 results of any importance, but as galvanometers at that period 

 had not reached the degree of delicacy they have since attained 

 in the hands of ]\I. Ruhmkorff, I determined this summer to 

 resume the inquiry; and the results which I have obtained I 

 now proceed to describe. The galvanometer used in the following 

 experiments is by Ruhmkorff, formed of 5-14 feet of fine copper 

 wire, and though not as delicate as the very long-wired instru- 

 ments used by M. DuBois Reymond and others, it has proved 

 sufficiently delicate for most of the effects I aimed at. 



The idea with which I started, was to arrange two plates of 

 platinum in an electrolyte in such manner that a bright beam 

 of light should impinge on one while the other was in darkness, 

 and yet to allow free eletrolytic communication. After making 

 a somewhat complex apparatus which did not answer the purpose, 

 the following simple means of effecting my object was adopted. 

 In a cell similar to those used for the nitric-acid battery, the 

 outer cell being of thin glass and the inner one of porous ware, 

 two platinum plates were placed, each of 6 inches by 2, — 4 inches 

 by 2, or the immersed portions of the plates, being platinized or 

 coated with a deposit of black platinum. Both the outer and 

 inner cell were filled with distilled water slightly acidulated with 

 sulphuric acid ; and some tow steeped in the same solution was 

 stuffed into the upper part of the porous cell around the pla- 

 tinum, so that this latter plate was perfectly excluded from light. 

 The extremities of the two plates were metallically connected. 

 A brass cylinder, covered at the top, was placed over the whole, 

 its lower circumference resting on a circular pad of paper so as 

 to exclude light. 



The apparatus, thus disposed, was set aside for ten days so as 

 to allow the local currents to subside. At the expiration of this 

 period the apparatus was taken into bright sunlight, the position 

 of the plates so arranged that the one in the outer or glass cell 

 should be opposite the sun, the terminals connected with the gal- 

 vanometer, and the temporary deflection occasioned by polariza- 

 tion allowed to sTibsidc, or rather to reach a fixed point, for there 

 was always a slight deflection. 



The brass cylinder which excluded light from the apparatus 

 wan now removed ; and the galvanometer needle instantly deviated 

 to 10', the platinum exposed to sunlight being positive to that in 



