Disintegration in a Vacuum Tube, 421 



Influence of Cathode Fall of Potential. 



This was first studied in detail by Holborn and Austin, 

 who found that metals could be arranged in two groups for 

 which the disintegration " y " expressed in milligrams per 

 half-hour for a current of: 0*6 miliiampere is given by the 

 expression 



y =_(V_495)xlO- 2 , 



where A is the atomic weight, " n" the valency, and " V ' : 

 the cathode-fall in volts, k for air is 0*0016 for the first 

 group and 0*0018 for the second. Kohlschutter has con- 

 firmed and extended this work. 



Now this result implies that the disintegration is zero 

 when the cathode-fall is 495 volts, and it lias been stated 

 that this is borne out by experience *. 



The values at low cathode-falls obtained in previous expe- 

 riments are meagre in number, but so far as they go they 

 show that the linear relationship breaks down in this region. 

 In any case they have been neglected in enunciating the law. 

 Since, however, considerable importance has been attached to 

 this " limiting " value of cathode-fall, the authors felt that 

 more systematic determinations of disintegration at low 

 values were desirable. This was possible despite the small rate 

 of disintegration in such cases* since for constant fall and 

 current the rate of disintegration is constant, and the dis- 

 charge could therefore be carried on for long periods of time. 



The results for a number of different currents are shown 

 in Curves 2. It will be seen that the relationship between 

 rate of disintegration (ordinates) and cathode -fall (abscissa) 

 is not linear at low values of the latter : the curves, however, 

 do appear to approach asymptotes which may reasonably 

 -cut the horizontal axis at about 500 volts. Thus the rate of 

 disintegration, though small, is appreciable at this value. 



The determination of the true limiting value of " V " is a 

 matter of considerable interest. The lowest value of " V " 

 obtainable is of course the normal cathode-fall. The appa- 

 ratus was not suitable for determining the true normal fall 

 accurately, because, except with small currents, the exploring 

 ware had to be placed somewhat close to the cathode. But 

 the conditions were arranged so that the glow covered the 

 cathode but contracted suddenly when the current was 

 slightly reduced; the rate of disintegration was then measured. 

 The reading of the electrometer at the time was generally 



* J. J. Thomson, ' Rays of Positive Electricity/ 1914. 



I 



