﻿Ionization of Metals, hy Cathode-Rays. 3^7 



just zero; it appeared that the true speed of the rajs should 

 be V + v. But I am by no means sure that the introduction 

 of v really does give the true speed of the rays ; v was very 

 nearly constant (0*4 to 0*7 volt) throughout all the experi- 

 ments ; it varied very much less than would be expected 

 from the known volta differences of potential between the 

 metals employed. Moreover, the variation of v is certainly 

 very much less than would be expected from the large values 

 found by Seeliger for the potential differences in the 

 a double-layers" which?are produced at the surface of metals 

 when cathode rays fall on them, and are removed by making 

 the metal the cathode of a discharge. For nickel, Seeliger 

 found potential differences as large as 20 volts. No change 

 in v of this order of magnitude, and indeed no consistent 

 variation of v with the state of the metal has been detected. 

 Throughout the experiments no indication of the presence of 

 such double-layers has been found ; perhaps all the 

 ionization investigated takes place at the outer surface of 

 the layer ; but even in that case a variation in v ought to be 

 produced by its presence. I cannot suggest any reason why 

 V'-f-v should not give accurately the speed of the rays 

 maximum falling on P. 



(3) There is uncertainty arising from the difficulty of 

 measuring i l7 which has been noted, but this source of error 

 does not seem serious. An examination of the figures shows 

 that unless i x actually increases as V 2 increases and V 

 decreases (such a change is almost inconceivable), an error 

 in measuring i l could not account for the differences in the 

 positions of the minimum. If i Y falls more rapidly with 

 V than has been supposed, the difference in the position of 

 the minimum would be increased and not decreased. Such 

 an error may have been caused by the assumption, implied 

 in the method of measuring i± adopted, that all the rays 

 entering B fall on P, and that there is no appreciable scatter- 

 ing even at the lowest speeds. (It must be remembered 

 that owing to the method of varying V adopted, these low 

 speeds would only be found close to P.) 



A lack of homogeneity in the rays could hardly account for 

 the variation of the minimum, for it is likely that such hetero- 

 geneity as existed would be the same in all experiments. 



(4) The position of the minimum is determined, not only 

 by the variation of the ionization with V but also by the 

 variation of the reflecting power. It is to be observed that 

 the variation of the reflecting power at values of V less 

 than that required to produce ionization, does not agree with 

 that found in the earlier experiments or with that found by 



