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Dr. J. Robinson on 



over some weeks, during which time the vacuum was con- 



stantly maintained. 



The general arrangement of the apparatus was as follows: 

 The electrode A was insulated by an amber rod B, which 



was fastened by sealing-wax into the glass vessel. C is an 



Fig. 1. 



D 



earthed metal tube with a hole in it to let the light through. 

 The vessel was closed by a quartz plate D. Three different 

 sources of ultra-violet light were used on different occasions, 

 two quartz-mercury lamps, and an aluminium spark *. 



Experiments were made to investigate whether there is a 

 fatigue due to the action of ultra-violet light, and whether 

 such fatigue depends on the electric field. 



Experiments with a Zinc Electrode. 



The electrode was connected to one pair of quadrants of a 

 Dolezalek electrometer. When ultra-violet light falls on 

 the zinc, the latter charges positively. The electrode was 

 insulated and allowed to charge itself up positively under the 

 action of light, and its potential at definite intervals noted. 

 Firstly, this rate was measured when the light was allowed to 

 fall on the electrode which had been in the dark for a long 

 time, and was in a condition which will be described as fresh. 

 Secondly, light was allowed to fall on the electrode for a 

 certain length of time, say 5 minutes, after which time the 

 rate of charging was measured as before. Thirdly, light was 

 allowed to fall on the electrode for 5 minutes, but electrons 

 were prevented from leaving the electrodes during this time 

 by keeping it charged to a positive potential. Then it was 

 insulated, and the rate of charging measured as before. 



* The experiments were made partly at the Armstrong College. 

 Newastle-on-Tyne, and partly at the University of Sheffield. 



