﻿328 Disruptive Voltage of Thin Liquid Films. 



u knee/' one set at this important point being only 50 per cent. 

 o£ the other set. Again, since Earhart's \ alues are used to 

 give the constants in the formula, there is naturally a certain 

 degree of coincidence. But this is not all : the most recent 

 results by Hobbs (loc. cU.) are opposed to SchwedofPs theory 

 in each of the following particulars : — 



(1) The critical potential gradients for different gases are 

 identical below the knee. 



(2) The critical potential gradients vary very greatly 

 according to the metal used in the electrodes. 



(3) Although Earhart did not take note of the fact, his 

 results indicate a short horizontal portion to the curve, the 

 potential gradient there being nil. Hobbs has thoroughly 

 established the horizontal portion for every gas and for every 

 kind of electrode. 



As Hobbs points out, his results in conjunction with the 

 previous work of Peace (Proc. Roy. Soc. p. 99, 1892), Strutt 

 (Phil. Trans, p. 377, 1900), Carr '(Proc. Roy. Soc. p. 374, 

 1903), which showed in every case minimum spark-potential 

 in a gas (unless the voltage is very small), seem to indicate 

 that at atmospheric pressure the carriers of the discharge are 

 the ions of the gas for distances greater than 6jj, ; while they 

 are the ions of the metals of the electrodes for distances 

 less than 2^/j, ; for distances between 2^/uu and 6//, the P.D. 

 for discharge remains constant, and discharge occurs not at 

 the extreme points of the opposing surfaces, but between 

 points at the distance corresponding to the minimum sparking- 

 potential, i. e. about 350 volts in air. 



J. J. Thomson had previously suggested (' Conduction of 

 Electricity through Gases,' p. 386, Camb. Univ. Press) that 

 the metal corpuscles might play a part in these effects. A 

 corpuscle about to leave the electrode feels an electrostatic 

 attraction towards the electrode; if the electric intensity 

 between the electrodes exceeds this, the corpuscle escapes. 

 This critical field works out, from the principle of electric 

 images, to be 8 X 10 3 , which is the order of field in Earhart's 

 experiments. Thus Thomson accounts for the fact that the 

 first part of the curve is straight. This theory, however, 

 may need some modification or amplification in view of the 

 fact, mentioned above, discovered by Hobbs that each metal 

 has its own gradient. Hobbs' curves indicate that great 

 gradient goes with great atomic weight, i. e. the heavy metals 

 lose their corpuscles with greatest reluctance. 



