176 Profs. Richardson and Cooke on the Heat developed 



to the positive end of the battery B l9 which supplied the 

 potential difference necessary to drive the electrons from the 



Fig. 1. 



c. 



1 > 



lilament F to the strip S. 13j was in series with the rheostat 

 H and the potential could be tapped off from various points 

 of this by means of a sliding contact. The switch A enabled 

 the sliding contact to be connected with the negative end of 

 the filament F and the potential thus applied between F and 

 S was measured by means of the voltmeter Vj. The rocking 

 switch A was introduced so that this potential difference 

 could be changed from zero to any desired value almost 

 instantaneously, and thus the flow of the electrons into the 

 grid could be started or stopped when desired. 



The mechanism of the compensating device needs a little 

 further consideration. Imagining for a moment the therm- 

 ionic current from F to S to be turned off, it will be seen 

 that the condition that the battery C l5 C 2 should produce no 

 deflexion in the galvanometer G is the usual relation 

 S: Ri = R 3 : R 2 - Now suppose the thermionic current to 

 be turned on. There will be a current flowing into various 

 parts of the resistance S. The result will be that in general 

 the two points K and L will no longer be at the same potential. 



