awav 



630 Profs. Cooke and Richardson on Absorption of 



experiments k was 1*29 X 10" 8 , i.e. 1:29 x 10" 8 amp. gave 

 1 scale-division. Since "di\/ii was at most about 0*0005, it 

 may be neglected in this equation. Thus 



^R = l-29xlO- 8 -^/G + a[l + 7r+^]^D E . (4) 



+ l-29xlO- 8 ii^/G + ari + 7r+^])D E i. (5) 



We shall now consider the change in the rate of supply 

 oE energy which occurs when the thermionic current is 

 turned on and off. Were it not for disturbing effects this 

 would be a simple matter. From equation (1) we see that 

 when a thermionic current of strength T flows from the 

 filament, the sum of the work done and the energy carried 



(£ + 2-0 I. The whole of this energy 



cannot be regarded as lost by the filament, since the electrons 

 do not flow into it at the absolute zero of temperature but 

 at the temperature O of the rest of the electrical system. 

 We shall therefore write the increment ^E' in the energy 

 supply due to the thermionic current as 



■ d W = -T^ + 2^[0-0 Q ]\. ... (6) 



The terms involving and O are not exact. They are cal- 

 culated on the supposition that Maxwell's law of distribution 

 of velocities can be applied to the electrons. This is only 

 approximately true when a current is flowing, the deviation 

 depending on the nature of the collisions which the electrons 



undergo. However, 2 — only amounts to about 5 per cent. 



of cj) at 2000° K, so that the error involved here cannot be 

 very great. It is of the same order as the work done by 

 the thermionic current against the Thomson effect in the 

 filament and the Peltier effect at the junctions. This we are 

 also disregarding, as its magnitude is unknown although it 

 must be much smaller than <£. 



The important disturbing effects which have to be con- 

 sidered are the Joule heating effect due to the flow of the 

 thermionic current inside the filament before it escapes, and 

 the direct effect it produces on the galvanometer Gr due to 

 incomplete compensation. Consider the former first. The 



