﻿786 Dr. Norman Campbell on 



the spark-gap being interpreted by the tables given by 

 Landolt and Bernstein. 



The current was measured by the method, adopted in all 

 my recent work, which depends on the use of an electro- 

 meter and a high resistancs. It was always of the order of 

 10 -12 amp. : if it was steady, it could be easily measured to 

 1 part in 1000. 



4. Let I T be the positive current carried to C by the 

 primary positive particles, I 2 and I 3 the positive currents- 

 carried away from it by the reflected positive particles 

 and the liberated electrons respectively. Then B = I 2 /Ii 

 will be termed the reflexion coefficient of the rays, 

 P=— I3/I1 will be termed their ionizing power. If the 

 incident positive particles carry each a single electronic 

 charge, then P will be the average number of electrons 

 produced by each positive particle. 



If ?\ is the current received by B, C, and D connected 

 together, then ti=Ji, If i 2 is the current received by G 

 when B and D, connected together, are kept at a potential 

 slightly higher than that of C, so that all the electrons 

 liberated at C, as well as the reflected primary particles, are 

 absorbed in B and D, then ? 2 = I 1 — I 2 — 1 3 . If, on the other 

 hand, B and D are kept at a potential lower than C, the 

 difference of potential being greater than that of the fastest 

 electrons liberated at C, then none of the electrons can leave 

 C and 2*3 = 1! — 1 2 . These statements will be true only if the 

 difference of potential (v) between B and C is not great 

 enough to alter the path of the primary or reflected positive 

 particles. An examination of the relation between i. 2 or i s . 

 and v shows that, if V was not less than 400 volts, this 

 condition could be fulfilled to a high degree of accuracy. 

 In measuring i 2 and i z , vwas +4 and —40 volts respectively; 

 any uncertainty in P in the results owing to the condition 

 not being fulfilled exactly certainly does not amount to more 

 than 2 per cent. We may then take 



p=X^, 



R = 



h 

 When V is less than 400 another method of measuring P 

 was adopted. C was pushed forward till it occupied the 

 position C indicated by the dotted lines. D was maintained 

 at a potential 400 volts higher than that of A, B, and C, 

 which were all at the same potential. This difference of 

 potential was, of course, sufficient to ensure that no reflected 

 rays fell on 1) ; it appeared from preliminary experiments 



