﻿790 Dr. Norman Campbell on 



the latter : the authors quoted do not state the lowest value 

 for the ionizing power which they were able to detect. It 

 was probably lower than that which could be detected in 

 these experiments, but it is impossible to say whether this 

 difference is sufficient to account for the difference in the 

 limits of appreciable ionization. 



9. The main interest of the experimental results lies in 

 fig. 2, which shows that the ionization increases to a 

 maximum at about 38,000 volts, and thereafter falls rapidly. 

 This conclusion is so surprising that something should be 

 said about the reliability of the measurements. There are 

 three possible sources of error. 



(1) The measurements made with ihe largest values of V 

 were generally somewhat irregular and some of them were 

 rejected altogether. The measuring apparatus was shielded 

 effectively from the effects of the induction-coil ; but as 

 soon as B or C w T as connected to it, the electrometer 

 fluctuated, even when S was not heated and there was no 

 appearance of a discharge from S to A. This effect may 

 have been due to electrostatic induction through the tube 

 opening into B, or, more probably, to charges creeping 

 along the inner surface of the glass vessel with which A, B, 

 and D were in contact only at a few points. The measure- 

 ments of / 1? when B aud D were connected to the measuring 

 apparatus, w T ere much more irregular than these when 

 only C was so connected ; indeed, measurements of i x were 

 practically impossible when Y was greater than 35,000 volts. 

 Though this difficulty was troublesome, there is no reason to 

 believe that it had any effect but to make the observations 

 vary somewhat widely about the true mean. All the 

 measurements which in other respects seemed at all reliable 

 agreed in showing a decrease in P for the highest values of 

 the V. 



(2) It is uncertain that the energy of the rays falling on C 

 is actually that corresponding to the potential indicated by 

 the spark-gap. There appears to be no method of removing 

 this source of uncertainty other than measurements in 

 a magnetic field which will permit the velocity of the 

 rays to be found. Such measurements are about to be 

 undertaken. But it is to be noted that the decrease in P 

 will not be wholly illusory unless the average energy of 

 the rays decreases notably as the equivalent spark-gap 

 increases. 



(3) It has been mentioned that at the highest values of V, 

 i x could not be measured. The values of P plotted are those 



