﻿376 Dr. Norman Campbell on the 



explicitly. The continuous curves give the relation between 

 R and V, the apparent speed of the rays, from to 400 

 volts (upper row of figures on the abscissa), the dotted curves 

 show the part corresponding to the smaller values of V' on a 

 larger scale (the lower row of values on the abscissa). 

 Curves A,B,B' refer respectively to the copper in state A, 

 in state B, and in the state which gave the smallest 

 ionization. 400 volts was the greatest speed obtainable with 

 the source of steady potential obtainable ; some other 

 observations were made with speeds up to 30,000 volts, the 

 potential \ 1 being obtained by an induction-coil, and V 2 

 being always 40 volts. These observations at higher speed 

 will be noticed later. 



An examination of the dotted curves would seem to 

 show that the result announced in the first paper, that the 

 ionization potential remained unchanged by the treatment to 

 which the metal is subjected, is incorrect. In curve A, the 

 minimum of R occurs at 12 volts, in B at 9 volts, in B' at 

 6 volts. And certainly the changes of this nature observed 

 with copper are greater than those observed in the earlier 

 or later experiments with platinum or in those with nickel. 

 But since copper shows all the changes which are being 

 investigated more markedly than the other metals, certain 

 considerations have to be taken into account before con- 

 cluding that this change in the position of the minimum 

 really indicates a change in the ionization potential. 



(1) The exact position of the minimum varies somewhat 

 in different series. For state A values between 10 and 

 12 volts have been observed ; in state B values between 

 8 and 10 volts; in state B f values between 5 and 9 volts. 

 It is clear, then, that the position of the minimum is not an 

 exact indication of the ionization potential, and consequently 

 that the progressive change of position of the minimum as. 

 the ionization is decreased may be capable of some other 

 interpretation. 



(2) The most probable of such interpretations would be 

 based on a difference between V and the true speed of the 

 rays. V is the difference of potential between W and P 

 imposed by the battery; if there were. at the surfaces of the 

 metal other differences of potential, V would not be the 

 true speed of the rays striking P ; and, if these differences 

 of potential varied, the same value of V might correspond 

 to different values of the speed. As was explained in the 

 first paper, an attempt to allow for such differences of: 

 potential was made by observing the value of v, tho; 

 potential between W and P when the current from P w r as; 



