Mr Campbell, The study of discontinuous phenomena. 185 



balanced currents very nearly equal. The effect of a lack of 

 perfect saturation is to diminish the current due to either source 

 when the potential of the electrode is of the sign corresponding 

 to a preponderance of the current due to that source : it sets a 

 finite limit to the potential which the electrode can attain, 

 diminishes the magnitude of the fluctuations and, in every manner, 

 acts like an insulation leak. 



Errors from this source, even if their possibility had occurred 

 to me, would have been rejected as unimportant, if I had not had 

 actual experience of them. I have found the false insulation leak 

 a very serious trouble in dealing with currents for which saturation 

 is likely to be far easier of attainment than in the case of a ray 

 currents ; these currents are those due to the photoelectric effect 

 in a very high vacuum. In dealing with these currents, the 

 saturation, tested by the ordinary method, is perfect at 50 volts : 

 tested by balancing two currents and then watching the decay 

 of a potential difference applied to the common electrode, saturation 

 is not perfect even with a p.d. of 500 volts, though it is much 

 more nearly perfect than at 50 volts. 



The existence of this false insulation current leak reduces 

 Geiger's method to that of Meyer and Regener. p has a finite 

 value which can be found by watching the decay of a p.d. given 

 to the electrode when the two currents are balanced. It should 

 be pointed out that this apparent insulation leak affects Meyer 

 and Regener's experiments also : in that case too the value of p 

 must be found when the current to be measured is flowing through 

 the Bronson resistance and the compensating P.D. applied. 



§ 17. A few remarks on Geiger's work are all that remains. 



(1) In the first place we may note that by taking some 

 100 observations only of the fluctuations Geiger, like Meyer and 

 Regener, introduced a probable error of some 10 7o- Since the 

 agreement which he found between theory and experiment was 

 of this order of accuracy, it might seem that in other respects 

 his work was satisfactory. But 



(2) It seems to me that Geiger employed a totally inadequate 

 and inaccurate theory in the interpretation of his observations. 

 He assumes, like Meyer and Regener, that the value of his 

 fluctuations depends only on the constants of his instruments, 

 but equation (33) shows that it is also a function of the time 

 of observation. Only the most general consideration is necessary 

 to show that in Geiger's method (assuming the absence of an 

 important insulation leak) the effects of the particles which came 

 off at a distant time are as important as the effects of those which 

 came off only just before the moment of observation: the mean 

 value of the fluctuations over any short period must depend upon 

 the distance of this period from the time when the electrode was 



VOL. XV. PT. II. 10 



