4 [heavy of Thermoelectric and Thermionic Effects. 741 



Subject to these conditions a calculation similar to that 

 ^iven in my paper on the " Theory of: Contact Electromotive 

 Force, &c." (Phil. Mag. vol. xxiii. p. 275), but extended so 

 as to take account of a possible difference in the rate o£ 

 energy transference by the current, shows that 



From this (18) follows by comparison with (4) above. 



It appears, therefore, that (16) and (17) are consistent 

 with one another. If, for example, ?0=z0 o + a#, it is still 

 true that 



^ = ne-4^feU-^ IR \ . '. . . (20) 

 1 a_ 



= A0 2 * =-*am. , , , , ■ , „ (21) 



Thiis |f w (=L<b above) has a temperature coefficient, the 

 determination of n from the temperature variation of the 

 electronic emission would lead to a yalue which is false by 

 the unknown factor e'^ ; as I have previously maintained *'. 

 On the other hand, equation (21), as Professor Wilson 

 pointed out f , puts a limit on the value of a. In fact, since 

 N ck 0i e~ b/d very nearly, it can be shown that the order of 

 magnitude of a cannot be greater than unity. 



The same conclusion is reached by considering equation 

 (15). The various terins on the right when divided by II 



are of the order unity or less, The term 0^1— ~n — °) « 



zero if the force during collisions varies as any power of the 

 mutual distance, and in general there is no reason to expect 

 it to be larger than the other terms. Thus we have every 



reason for believing that y. —— is comparable with unity. 



This has an important bearing on the results of some of 

 the experiments which have been made on the emission of 

 electrons by different hot bodies. In my old experiments J 

 with carbon and sodium the currents obtained were much 

 larger than those calculated from the emission from platinum 



* Phil. Trans. A. vol. cci. p. 543 (1903). 

 t Phil. Trans. A. yol. pcii. p. 2,60 (1903). 

 + Phil, Trans. A. vol. cci. p. 497 (1903). 



Phil. Mag. S. 6. Vol. 24. No. 143. Kov. 1912, 3 C 



is 



