during the Absorption of Electrons by Platinum, 1*H) 



(2) The Direct Heating Effect of the Thermionic Current. 



In addition to the effects which have been mentioned, the 

 grid will be continuously heated by the current in the 

 Wheatstone's bridge circuit. When the thermionic current 

 is turned on, the conduction current flowing along the grid 

 at any point will be altered, so that there will be a change in 

 the heating effect due to the conduction current. We shall 

 now proceed to show how the rate of! heat production due to 

 this cause may be calculated. Suppose AB (fig. 6) repre- 

 sents a length of heated metal emitting electrons, and the 



Fig-. 6. 

 A Q 



B 



material of the grid is represented diagram matically by 

 the line OQ. Then at any instant a uniform current i 

 arising from the Wheatstone's bridge circuit will be flowing 

 along OQ. In addition to this there will be a thermionic 

 current flowing from AB into various points of OQ. Let 

 the thermionic current into OQ at any point be j per unit 

 length. Then the thermionic current into a length dx of OQ 

 at the point x is equal to jd.v. The total thermionic current 



into the grid will thus be J = \ Q j dx. In the arrangement 



that we have used part of the thermionic current flows out 

 of one end of the grid and part out of the other (see fig. 1) . 

 There will, therefore, be a point M in the grid where the 

 thermionic current contributes nothing to the value of the 

 current along it, so that the current at this point has the same 

 value i , whether the thermionic current is on or off. Let us 

 xake this point as origin and let x denote distance along the 

 strip, x being positive upwards. Let the co-ordinates of Q 

 and be x l and — x Q respectively. Then the current along the 



grid at any point c y = / H-j jdx. Let R be the resistance per 



unit length of the grid. Then the rate of heat production in 

 the length dx 



= Kd.vL+ \ "jdx) = R{v + 2i i >.*•+( Cj<l<)'~}d,: 

 ^ %, o ^ Jo \"o 



