702 



Lord Blvthsu'ood and Mr. Allen on 



J. C;. Beatlie, and M. S. de Sinolan*. It has also b3en 

 investigated by E. Rutherford f, who found that when the 

 plates were charged from an external source '' in consequence 

 of this action, for small electromotive forces the rates of 

 leak are diflPerent for positive and negative." 



An exactly similar effect produced by the action of X-rays 

 has been studied by Erskine Murray % and Perrin §. 



The object of the experiments described in the present 

 paper was to measure this potential-difference for variou-^ 

 metallic couples, employing a radium salt as the source of 

 radiation. 



Preliminary Ea'periinents. 



Some preliminary experiments were made with the appa- 

 ratus illustrated in fi^. I. A is a horizontal brass disk 5 cms. 



Fio-. ] . 



in diameter connected to one pair of quadrants of a Kelvin 

 electrometer. B is a similar disk whose distance from A can 

 be adjusted by the screw C. It is supported by varnished 

 ^lass rods, and is in metallic connexion with the insulated 

 quadrants of the electrometer. This plate was coated with 

 a solution of radium salt of no great activity, and the solution 

 was allowed to evaporate. 



In making an experiment the metal plate to be tested 

 was carefully polished and laid upon the lower brass plate. 

 The apparatus was then inclosed in a case of lead. This 

 cover and all the quadrants were connected to '^ earth.'^ An 

 observation was made of the deflexion of the electrometer 

 obtained by disconnecting the upper plate from " earth." 

 The results for various metals are given in Table I. 



* Phil. Mag. vol. xlv. p. 277 (1898), vol. xlvi. p. 115 (1898). 

 t Phil. Mag. vol. xlvii. p. loo (1899). 

 X Proc. Roy. Soc. March 1896. 

 § a R. vol. cxxiii. p. 496. 



