60 LEAKAGE IN ULTRA-VIOLET LIGHT [CH. vi. 



J. J. Thomson devised a most ingenious method 

 of carrying out this experiment that of discharge 

 by means of ultra-violet light in a metrical 

 manner, and of deducing from it the electrochemical 

 equivalent of the charged particles, that is to say 

 the amount of matter which each contains com- 

 pared with the electric charge which each carries. 



FIG. 8. J. J. Thomson's apparatus for measuring the magnetic 

 deflexion of the electric charge thrown off clean negative metal by 

 ultra-violet light in a vacuum. The negative electrode is a clean zinc 

 plate AB which can be raised or lowered, the other is wire gauze CD 

 connected to an electroscope. Ultra-violet light enters through the 

 quartz plate EF, and then a magnetic field is applied. 



To this end he employed the usual arrangement 

 of a small negatively charged zinc plate on which 

 ultra-violet light from a distant arc-lamp could 

 shine ; the light passing through a plate of quartz, 

 and also through a parallel piece of wire gauze 

 connected with an electrometer. The distance be- 

 tween the zinc plate and the metallic gauze was 

 variable, and the experiment consisted in observing 

 how much electricity reached the gauze from the 



