C3 



Ionization of Platinum by Cathode Rays. 297 



13. Before discussing the significance o£ these observations, 

 some other experiments may be noted briefly. 



A great deal of time was devoted to an attempt to de- 

 termine whether the ionization of the platinum varied with 

 its temperature. No such variation is to be expected, for 

 everything tends to show that it is not the free electrons of 

 the metal which are ejected in ionization, while the work 

 of Richardson proves that the work which electrons have 

 to do in emerging from the surface does not vary with the 

 temperature. But as the initial experiments showed an 

 apparent variation, the matter had to be investigated 

 thoroughly. 



P was disconnected from the electrometer and heated by 

 the transformer, while A was connected to the electrometer, 

 the differences of potential between the various conductors 

 being unchanged. With this arrangement the (negative) 

 current received by the electrometer increases with an 

 increase of the ionization of P, whereas in the other arrange- 

 ment it decreases. If Iw', I20' are novv the currents flowin 

 when the speed of the rays is ~\V and 20 volts respectively 



•and if Z' = - 20 T , Z' will measure the ionization, as Z did 



iw 

 previously. If P is at a temperature of more than 1000°, 

 there will be a thermionic current, for which a correction can 

 easily be made, since it is independent of the action of the 

 rays *. All the experiments were made after state B had 

 been reached ; in this condition the thermionic current at 

 1000° (approximately) had been reduced to less than 10~ 6 

 of its value in state A; the total current from the 2 cm. 2 of the 

 platinum at that temperature was less than 10" 10 amp., and 

 was small compared with the current due to the rays. 



It was found that heating P invariably increased I w ' 

 and I 20 ', and almost always increased Z', indicating an 

 apparent increase in ionization. The effect cannot be attri- 

 buted to the evolution of gas from P because (1) the presence 

 of gas, while increasing I', would decrease 7/, (2)eveui if the 

 pump was not going the values of I' and Z' were unaltered 

 when P was cooled again. However, it was found that the 

 increase in I' was only temporary; after half an hour (during 

 which the pump had to be kept, going, since P always evolved 



* Of course the object of the experiment is to lind whether the 

 the thermionic current is independent of the rays. The statement in 

 the text is better expressed thus : — If it be assumed that the thermionic 

 current is independent of the rays and the measurements, interpreted on 

 that assumption, show that the ionization is independent of the tempe- 

 rature, then it is really so independent. 



