﻿to maintain a Current in a Gas at Low Pressures. 221 



the pressure of only 0*5 millimetre, the mean free path of the 

 positive ions on the kinetic theory, assuming that they are 

 atomic in size, is nearly "4 millimetre, and that of the negative 

 ion about 1*6 millimetre, which is not very small in com- 

 parison with the total distance between the plates. For 

 higher pressures when the value of /3 is very small, the deter- 

 mination of /3 in this way must necessarily be more accurate; 

 but in the range under investigation it seems very doubtful 

 whether absolute confidence can be placed in this equation as 

 expressing the true state of affairs, even though it appears to 

 fit in remarkably well with the facts. It may be that the 



assumed curve for - was too low, and consequently the results 

 p 



are more favourable than theoretical grounds would warrant. 



Leaving the sparking potential, and coming to the values 

 of the potential required to maintain a current, in the first 

 place, above the critical pressure, a complete discussion of 

 results similar to those obtained in the present work has been 

 given by Professor Townsend *. He clearly shows that the 

 difference in the velocities of the positive and the negative 

 ions, tending to cause a surplus positive charge in the gas 

 near the electrode, will fully account for the observed differ- 

 ence between the sparking potential (which is really the 

 potential required to maintain a minute current) and the cor- 

 responding potential to maintain larger currents. The latter 

 should be lower, at least for pressures that are above the 

 critical value. Thus the curves obtained (fig. 3) fit in exactly 

 with the theory, and lend further confirmation to the previous 

 work. 



Below the critical pressure, however, some other expla- 

 nation of the effect observed must be given. It seems pro- 

 bable that it is masked by another and more marked effect 

 which was not got rid of in the course of the experiments. 

 At high pressures the mass of the carriers is fairly large, the 

 ions being composed of groups of molecules travelling together 

 with a comparatively small velocity. At low pressures and 

 high forces, on the other hand, such as are obtained below the 

 critical pressure, the mass may be very much smaller, in all 

 probability of the order of the mass of a molecule. Hence 

 the velocity is probably very much greater than the velocity 

 at the higher pressures. The charge in the gas for a given 

 current would then be very much less than is obtained with 

 the same current above the critical pressure. We should 

 therefore expect that in this case the drop from the sparking 

 * J. S. Townsend, Phil. Mag. ix. p. 289 (March 1905). 



