186 Prof's. Richardson and Cooke on the Heat developed 



stage again ends rather abruptly, and after that the current 

 increases very slowly with the applied voltage, of which, 

 however, it is again an approximately linear function. The 

 character of the curves can be best realized by reference to 

 figure 3, which shows the values of the currents, obtained in 



Fiff. 3. 









1 

 1 

















^-r— 



— Q ~~7' 



^— 



-^ 

















I 

 j 



1 



s> 



/ 







i 







<U 







* 











V 



/ 



















A 



















- - - -^ 



j 

















— ' 



-•"> 5 10 15. 20 25 30 35 40 15 50 



Scale of voltage. 



another experiment plotted against the corresponding electro- 

 motive forces. The values are shown thus O. It will be 

 seen that in this case approximate saturation is attained in 

 the neighbourhood of 14 volts. After that the increase in 

 the current is relatively small. In the experiment to which 

 the numbers in the preceding table refer, the current was 

 approximately saturated at 8 volts. 



The heating effect was next determined for a series of 

 different voltages, and as a rule these were all chosen so as 

 to exceed the voltage where approximate saturation occurred. 

 In this way it was possible to avoid the difficulty which 

 arises from the fact that when the current is varying rapidly 

 with the voltage the mean potential difference through which 

 the electrons fall does not correspond to that at the middle 

 of the filament. 



In order to explain the method of taking the observations, 

 a typical series for one particular voltage will now be given. 

 In this experiment the range of voltage was from to 8. 

 That is to say, when the thermionic current was " on " the 

 potential at the negative end of the filament as measured by 

 the voltmeter, V, was 8 volts. When it was " off " the 

 potential of this end of the filament as measured by the volt- 

 meter was volt. The sliding contact at H was, therefore, 



