of the cathode in a discharge tube. 187 



The direct results of these experiments are : 



(i) That an electrode contains gas which cannot be driven 

 off by an ordinary heating but which is easily evolved when the 

 electrode is run as a cathode. 



(ii) That an electrode after running as cathode for some 

 time emits more slowly moving and more homogeneous cathode 

 rays than it does when fresh. 



(iii) That it is not possible to restore a "fatigued" cathode to 

 its original state by causing it to absorb hydrogen, oxygen, nitrogen, 

 carbon dioxide, carbon monoxide or helium. 



It was thought that measurements of the cathode fall of 

 potential might throw some light on (ii). 



The cathode fall was measured only in the case of aluminium 

 cathodes in air. 



A platinum exploring point was used in conjunction with an 

 Ayrtou- Mather electrostatic voltmeter. 



The instrument always took from half to three-quarters of a 

 minute to reach its final reading owing both to the smallness of 

 the collecting point and the large capacity of the electrometer 

 quadrants. The arrangement could therefore not be used to 

 measure rapid variations in the cathode fall. 



In most cases the cathode fall started from a low value, rising 

 rapidly in about three minutes to a maximum and then gradually 

 falling to a final steady state. No value can be attached to the 

 first rapid rise to a maximum as very often the time taken was 

 comparable with the time of lag of the instrument. The value of 

 the maximum recorded by the electrometer was very variable with 

 different cathodes, even when they were cut out of the same sheet 

 of metal. The limits of variation were between 280 and 400 

 volts. In only one case was the upper limit reached, usually 

 300 volts was about the value. The final value to which all 

 cathodes converged was 265 volts and the time taken was of the 

 magnitude of 10 minutes. 



Thus the cathode fall for a new aluminium cathode is rather 

 greater than for one which has been run some time, the difference 

 usually being about 20 °j^ but sometimes being as much as 60 " j^. 



This is what the qualitative experiments might have led 

 one to expect ; for a new cathode has a wider dark space than 

 a run one. 



The effect of absorbed gases was tried but with negative 

 results. 



Running the cathode as anode also produced no change that 

 could be detected in the cathode fall. This again is what the 

 previously described experiment might be taken as predicting ; 

 for the effect could only be a momentary one and could not be 



