Properties of Potassium-Sodium Alloy. 289 



it, but actually creates au electromotive force and current 

 in the galvanometer circuit, which is in such a direction as to 

 indicate that negative electricity is caused to move, by the 

 action of the light, through the vacuum from the surface 

 of the alloy to the platinum plate *. 



It has been pointed out by Professor Sir J. J. Thomson in 

 his book 'The Discharge of Electricity through Gases' (see 

 p. 73J that air traversed by ultra-violet light behaves like 

 an electrolyte ; and it is also stated that Stoletow has shown 

 by direct experiment that two different metals immersed in 

 such illuminated air are brought down to the same potential 

 just as when they are immersed in a liquid electrolyte. 

 Again, it is said that when plates of two different metals are 

 opposed to each other and ultra-violet light allowed to fall 

 on one of them, whilst the terminals of a battery of electro- 

 motive force E are connected to them so as to force a current 

 across the space between them, the total E.M.F. in the 

 circuit is not E but is E + M/M', where M/M' is the contact 

 difference of potential between the metals. There is, 

 however, here an unexplained discrepancy. If metallic 

 potassium or sodium were placed with a platinum plate in a 

 liquid aqueous electrolyte, and the two metals connected by a 

 wire outside so as to construct a voltaic cell, the current 

 through the cell, that is the movement of positive electricitv, 

 would be from the potassium or sodium to the platinum 

 through the electrolyte, and therefore the movements of 

 negative electricity would be in the opposite direction. In 

 the case of the photoelectric cell, the movement of negative 

 electricity in the cell is from the K-Na alloy to the platinum. 

 Hence the physical operation of the photoelectric cell is not 

 identical with that of an ordinary voltaic cell. Stoletow has 

 described (Physikalische Revue , i. p. 7(35, 1892) experiments in 

 which two plates of different metal-, one perforated with holes, 

 were placed parallel and at a little distance from each other, and 

 ultra-violet light allowed to fall through the openings of the 

 perforated plate on to the other plate. The plates, when con- 

 nected together, were fouud to produce a current of elec- 

 tricity in the connecting circuit. Prof. Sir J. J. Thomson, 

 however, remarks (loc. cil. p. 73) that for this to happen in 

 accordance with voltaic principles, the perforated plate must 

 be made of the more electropositive metal, for then only 



* In the case of one tube of alloy prepared as above, my assistant, 

 Mr. G. 13. Dyke, noticed that the galvanometer deflexion was largely 

 increased for a time by tilting' the tube so as to make momentary contact 

 between the alloy and the platinum, but the increased effect is not 

 permanent. 



