188 Mr Whiddington, Some fatigue effects of the cathode, etc. 



detected owing to the great time lag of the instrument. No 

 collecting point device could be expected to indicate changes in 

 potential so rapid as this. 



Some quite thin aluminium plate '01 cm. thick was tried in 

 the cathode fall tube. It was found that the cathode fall rose 

 almost immediately to the final value 265 volts instead of starting 

 from a high value and then slowly diminishing with time. 



This observation suggested that the time taken for a cathode 

 to give its final value for the cathode fall might depend on the 

 volume of the cathode, the time being greater, the greater the 

 mass of metal involved. 



Such time effects were looked for but could not be quantita- 

 tively worked out owing to two main causes, firstly to the fact 

 that different cathodes did not start from the same high value of 

 the cathode fall although they all reached the same final value, 

 and secondly to the fact that even two electiodes of the same 

 thickness and size cut out of the same sheet of metal and starting 

 from almost the same high value took very different times to reach 

 the same final limit. 



In general however it seemed certain that for heavy cathodes 

 a considerable time was required to reach a steady state, while 

 light cathodes reached it immediately, without any gradual fall. 



This result led at once to the trial of very thin cathodes in 

 the ordinary discharge tube. They did not show any fatigue 

 effect, the dark space being sharply defined from the commence- 

 ment. 



It is possible that some gas is present in the new metal which 

 is expelled by running as a cathode which is responsible for these 

 fatigue effects. If the gas is one of those mentioned above it 

 certainly cannot be made to enter the material of the cathode in 

 the ordinary way. On the other hand it may be one of the rarer 

 gases or even an unknown one. 



It is possible that photographic methods using large masses 

 of metal for the cathode may reveal this gas. But in these 

 experiments the only gases evolved appeared to be oxides of 

 carbon and hydrogen. 



I have pleasure in thanking Prof. Sir J. J. Thomson for 

 suggesting the work and for his kind interest while it was being 

 carried out. 



