Theory of IJiermoelectric and Thermionic Effects. 743 

 quantity at a point outside the platinum, we see that 



V , ne 



*~ nB, ° f9V\ 



v P = v N e , \A6) 



where V is the contact potential difference between the two 



metals, and n is the number of molecules in 1 c.c. of a perfect 



gas at 0° C. and 760 mm. If we neglect differences in the 



phenomenon of electron reflexion in the case of the two 



metals, the saturation currents will be proportional to the 



corresponding values of v, so that 



V .fie 6-55 xlO 4 V .ne 



+ —-UT, , ;. — 4- 



., N = ., P<; K - Kd =6-9xlO'x0*X* 6 xe * B - e .(24) 



Consider the case of sodium and potassium at 350° 0. 

 In the case of sodium the contact potential difference with 

 platinum is said to be =2*4 volts. Thus V = 2*4 x 10 8 e.m. 

 units. We also have (9 = 623, ne==-'^21 e.m.u., and 

 ™R = 3'72xl0 3 erg/°C. 



Thus for sodium at 350° 0. the expected currents are 

 comparable with 



ax — 1*5 X 10 " 17 amp./cm. 2 



For potassium at the same temperature taking V = 2*7 x 10 8 

 e.m.u. the expected currents work out at 4'3 X 10 ~ 15 amp./cm. 2 

 At this temperature the currents from the alkali metals are 

 well below the limits of what can be measured on a galva- 

 nometer. If we assume that carbon is 3 volt electro- 

 negative to platinum, we have for the number of amperes 

 per sq. cm. from this substance at 2000° C, by a similar 

 calculation, the value vo c =2'1 x 10" 4 . 



It appears, therefore, that the experiments of Pring and 

 Parker, and of Fredenhagen, have removed two outstanding- 

 objections to the view that the emission of electrons from 

 pure platinum in a vacuum or in oxygen or nitrogen at a 

 low pressure is independent of chemical action in the ordinary 

 sense. 



There are, in my opinion, many other grave objections to 

 such a view. These have been stated elsewhere *, and it 

 would take too much space to recapitulate them here. 



I should like to state that I have never been averse to the 

 view that electronic emission may be conditioned by chemical 



* Phil. Trans. A. vol. cci. p. 545 (1903) ; ibid. vol. ccvii. pp. 24, 27, 

 53, 60 (1906). Cf. also H, A. Wilson, " Electrical Properties of Flames, 

 &c," p. 5 and p. 15 ; University of London Press, 1912. 



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