﻿378 Dr. Norman Campbell on the 



Gehrts. These other experiments indicated a maximum of 

 reflexion at about 5 volts, which was especially marked in 

 Gehrts' work. I am unable to explain this discrepancy. 

 Gehrts' method of measuring i 2 was more satisfactory than 

 my own, but in order to produce a maximum in my curves 

 it would have to be supposed that i Y had a maximum between 

 1 and 2 volts. On the other hand, the rays which I used in 

 these experiments were probably more homogeneous than 

 those employed by Gehrts or those employed in my earlier 

 work. 



Accordingly it does not appear to be at all certain that 

 there is any change of the ionization potential with the 

 treatment of the metal. ]f there is a change it consists of 

 a reduction of the ionization potential as the ionization is 

 decreased. And this result might seem in accordance with 

 the idea that in state B the surface of the metal is exposed, 

 whereas in state A it is covered with hydrogen. For it is to 

 be expected that the ionization potential of metals should be 

 less than that of hydrogen ; the ionization potential of 

 gaseous mercury is below 5 volts. On the other hand, if 

 this explanation is adopted, it is not easy to see why the 

 ionization potential should be reduced still further in passing 

 from B to B'. 



Careful comparative experiments have been made along 

 the whole curve up to 400 volts : there is no indication 

 whatever of any other kink in the curve suggesting the 

 presence of an ionization potential greater than 12 volts. 



6. Now let us turn to the part of the curve corresponding 

 to greater values of "V'. It is clear at once from the com- 

 parison of the curves B and B' that the expectation that, in 

 the change from one of these states to the other, the 

 ionization should be changed at all values of V in the same 

 proportion is not fulfilled. For whereas the maximum 

 value of R occurs in B at about the same position as in A, 

 in B' it occurs (if at all) at some much higher value. (The 

 steady potential available was not great enough to determine 

 the position of the maximum for B', but it appeared to occur 

 between 400 and 9000 volts.) The following table gives 

 the ratios of R (measuring the ionization) for different 

 values of V in the three states. For each value of V the 

 value of R in state B is put equal to 1 ; the figures corre- 

 sponding to states A and B' are the ratios of the ionizations 

 in these states to that in state B. 



V 40 80 200 300 400 9000 26000 



R (state A) 116 135 14.3 1-52 162 161 166 



R( „ B) 1-00 100 1-00 1-00 1-00 1-00 l'OO 



B( „ B'; 0-30 0:38 049 056 0-67 079 0-87 



