634 Profs. Cooke and Richardson on Absorption of 



lack of balance expressed in terms of the change in R which 

 would produce the same effect. It is clear from what has 

 been said, that the observed deflexion Dt of the galvanometer 

 G when the thermionic current is turned on, is not clue 

 entirely to the change "dH t of the resistance arising from 

 the change in the rate of supply of heat. Instead of being- 

 equal to p^R t it is equal to p (B Rt -f ^) , where p\jr is the 

 deflexion arising from the lack of compensation. 



Since the alteration in the rate of supply of energy to the 

 filament due to either turning on the thermionic current or 

 shunting the resistance X\ is small, it will be proportional to 

 the corresponding change in temperature or resistance of the 

 filament. If D E is the galvanometer deflexion and BR e the 

 corresponding change in the resistance of the filament caused 

 by shunting x, we have 



3E 



T(, +2 g (< -,, ))+ BC,-(fi- 2 f)- 2 |[>C,(l-^ + Jg.)[i:i^> 



_ BR^__ Dt — p/r 



= 3R ~~D7~ 



(9) 



If the corresponding values of T, BR*, and D T when C is 

 reversed are denoted by dashes, we have, adopting the con- 

 vention that the positive direction of the deflexions reverses 

 with C , 



(l-27r)^(W^/^--+BR, 



(10) 



BR*' D T ' +p\jr 



dlie ~ D^ 



By addition of (9) and (10), remembering that Y = ~R Q i 1 

 and C = (l — 7r)ii, where tt is a small fraction whose square 

 may be neglected, we have, after transposing, 



.... (ii) 



