from Hot Platinum. 295 



These figures are at any rate interesting as indicating the 



order of agreement between the different observations. As a 



matter of fact the probable error of any observation is much 



greater than its deviation from the mean, owing to sources of error 



which affect all the observations, so that one has to be very careful 



in drawing conclusions from them. 



<£> . 



The signification of the constant b = - p which occurs in the 



exponential factor is equally important, since <E> is the work done 

 by an ion in overcoming the discontinuity in the potential at the 

 surface layer. We obtain b from the equation 



loga (J7-h l0g« 0. 

 6 = 1—1 ' 



where C, C and 6, 6' are corresponding currents and absolute 

 temperatures. Substituting the values of C and C for = 1571, 

 0' = 1378 respectively we get the average value of b from 1378 to 



1571 absolute as 4'93 x 10 4 . Since R is equal to (-n^i) x lO" 16 



2 

 we have <X> = 4 # 93 x x 10 -x2 = e8(f), where e is the charge on 



an ion and S<£ is the discontinuity in the potential at the surface 

 of the metal. From this we obtain 8$ = 1*365 x 10 -2 electro- 

 static units 



= 4*1 volts. 



This is therefore the discontinuity in the potential at a platinum- 

 vacuum surface and it is of the right order to give the contact 

 e.m.f. as the difference of its value for different metals. The 

 author intends to make further experiments on other conductors, 

 notably iron and carbon, with the hope of confirming this part of 

 the theory. 



The preceding theory only claims to represent the main 

 features of the radiation from a hot platinum wire. In the first 

 place it is evident that the number of ions emitted per unit area 

 cannot be regarded in the strict sense as an exact function of the 

 temperature. This number will evidently be altered by anything 

 which changes the state of the surface, so that we should expect 

 the current to depend on the previous treatment of the wire. 

 Variations under this head do certainly seem to occur, but the 

 amount of disturbance produced by them remains to be in- 

 vestigated. 



In conclusion I wish to thank Professor Thomson for numerous 

 suggestions during the course of this work. 



