1915] on Electrons and Heat 447 



This ix>sition is streugtlieaed when we examine the way in which 

 the electron emission depends on the temporature of the hot body. 

 This may readily be done by surrounding the hot wire with a cylin- 

 drical electrode to catch the electrons, which then flow through a 

 galvanometer whose deflection measures their number. The hot wire 

 is arranged to lie in one arm of a Wheatstone's bridge ; so that its 

 temperature may be deduced from its resistance. Innumerable 

 experiments with different substances have shown that this emission 

 increases with great rapidity as the temperature rises, ju^t as doe3 

 the corresponding phenomenon in the case of evaporation. The 

 correspondance is, in fact, exceedingly close. We may take the rate 

 of emission of molecules from the surface of an evaporating liquid 

 to be proportional to the vapour pressure. The proportionality is 

 not exact, but it is sufficiently so for our purpose. The crosses on 

 the next slide represent value 5 of th3 vapjur pressure of water, on 

 the vertical scale, plotted agiinst the corresponding temperatures 

 from 0° C. to 90° C, on the horizontil scale ; whilst the circles repre- 

 sent tne emission currents from platinum plotted similarly against 

 temperature over the range 1000^ C to 1250° C. All the points lie 

 on the same continuous curve within the limits of experimental 

 error. To bring about this coincidence, it is, of course, necessary to 

 plot the temperatures on quite different scales in the two case?, but 

 the agreement demonstrates in a simple way the similarity of the 

 laws which govern the temperature variation in both cases. 



Numerous cases of electron emission have now been examined, and 

 it has invariably been found, provided there is no reason to suspect 

 changes in the chemical nature of the emitting surface, that the 

 relation between the current i and the absolute temperature T is 

 expressed by a very simple equation. This is 



i = A Ts exp - m 



or 



logz-JlogT = logA-^, 



where A and h are constant quantities for any particular substance. 

 The theory underlying this equation shows that the quantity b is very 

 nearly equal to half the energy change, expressed in calories, when 

 one gram molecular weight of the electrons is emitted. Pursuing 

 th3 analogy with evapDration, this qumbity miy be called the mole- 

 cular latent hea'i of evaporation of the ele3bron5. It is not, however, 

 with the theory underlying this equation that I particularly wish to 

 CDiicern you now ; but I do wish to impress ths fact that this formula 

 is not an empirical affair covering a smdl ranga of temperature and 

 current. The most recent maasuremants,* mida with tungsten, have 



* c/. K. K. Smith, Phil. Mag. vol. 29, p. 102 (1915). 



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