CHAPTER VI. 



DETERMINATION OF ELECTROCHEMICAL EQUIVA- 

 LENT IN THE CASE OF ELECTRIC LEAKAGE 

 IN ULTRA-VIOLET LIGHT. 



THE same ratio of m to e, or a ratio of quite com- 

 parable magnitude, is obtained from phenomena 

 which at first sight appear to be distinct. 



One of these phenomena is the effect of ultra- 

 violet light in discharging negative electricity from 

 a clean metal or other surface ; a phenomenon dis- 

 covered by Hertz, and the investigation of which 

 was continued especially by Righi and by Elster and 

 Geitel. (See one of the appendices to my " Signalling 

 without Wires," published by the Electrician Co.) 

 If ultra-violet light, whether from a spark or from 

 a flame, fall upon a negatively electrified surface, 

 then in general there will be a leak of electricity 

 from that surface : which electricity can be received 

 by any body placed opposite the illuminated one, 

 and can be used to charge an electrometer of 

 known capacity, and so be measured. The writer, 

 assisted by Mr. Benjamin Davies, has made very 

 many experiments in this subject, which, how- 

 ever, have not yet been published. Now Elster 

 and Geitel made the notable discovery that the 

 application of a magnet affected the rate of leak, 



