632 * Profs. Cooke and Richardson on Absorption of 

 When there is no thermionic current the Joule heating 

 effect is I poC 2 dx, so that the extra effect due to turning 

 on the thermionic current is 



f 



po{i +$*jdx) 2 dx 



jo 





p C 2 dx 



= RC S 



•BRi 2^Ri 



L R 



Ho J 



P<A>(i + 



2 L 



R 



R 



)[(>] 



<fo? 



BRA 



IW 



d.\ 



(?) 



The middle term in (7) reverses when the direction of the 

 heating current C is reversed. The other two terms do not. 

 We shall see in a moment that the effect of the second term 

 can readily be eliminated. The magnitude of the third term 

 was negligible in all our experiments, being equal to only 

 1/500 of the first term in the most unfavourable case. 



In the above expressions BRi is a negative quantity. If 

 we consider ^R x to be positive when it represents a diminu- 

 tion in R, the diminution in the Joule heating effect due to 

 turning: on the thermionic current is 



Po { 



Cn 1- 



3Ri 

 R 



1 + 



Rr 



)[.(>] 



if we disregard the third term. The total diminution in the 

 rate of supply of energy is made up of this together with 

 the loss due to the escape of the thermionic current. 

 To the required accuracy it is therefore : — 



3H=T(<H^V*o)) + *c4^-2^] 



tie. 



... (8) 



The first R is starred to indicate that it is the gas constant 

 and not a resistance. The first two terms of (8) have the 

 same direction independently of the direction of C . Since 

 the deflexions of the Wheatstone's bridge galvanometer, for 

 a given change in the resistances, reverse when G is re- 

 versed, it follows that the deflexions due to the first two 

 terms of (8) reverse with C . The third term gives rise to 



dx 



