Discharge of Electricity through rarefied Gases. 785 



In conclusion, it is of interest to note the bearing of 

 this investigation upon the pressure observed upon the 

 poles of an alternating -current arc, which has been shown 

 elsewhere to agree well with that for the D.C. arc. We 

 now find the rates of carbon loss to be also approximately 

 equal ; hence the argument used in the case of the D.C. 

 arc, that the evaporation of carbon is of the wrong order of 

 magnitude to account for the effect, holds good in the alter- 

 nating-current arc also. The reaction in the latter arc, as 

 in the former, is believed to be occasioned by the cathodic 

 expulsion of electrons whose momentum is propagated across 

 the arc through the vapour to the anode, where it produces 

 an equal pressure. In the alternating-current arc the pressure 

 is therefore made up of a reaction consequent upon electron 

 emission during the half -period when it is the cathode, and 

 of the pressure consequent upon the reception of the 

 momentum from the other pole during the second half-period 

 when it is acting as the anode. 



Physics Laboratory, 

 University College, 

 Heading. 



XCI. Experiments on the Nature of Discharge of Electricity 

 through rarefied Gases. By S. Ratner, Research Student, 

 University of Manchester *. 



1. TN spite of the large amount of work contributed to the 

 A study of the nature of the discharge of electricity 

 through vacuum-tubes, the present theories fail to give a 

 clear and adequate representation of the phenomenon. The 

 most obscure point is the process by which, under a certain 

 potential difference, the current begins to flow through the 

 gas. It is usually supposed that the stray positive ions 

 which may be present in the tube, strike against the cathode 

 wdth sufficient energy to cause an electronic emission from 

 the surface-layer of atoms of the cathode, and that these 

 electrons, by collision with the gaseous molecules, produce 

 sufficient ions to carry the whole current. Certain experi- 

 mental facts, however, disagree with this theory. Thus, 

 when the vacuum is sufficiently high, the potential necessary 

 to start the discharge increases with further exhaustion of 

 the tube, although the energy imparted to the cathode by the 

 positive ions depends only on the potential through which 



* Communicated by Prof. W. L. Bragg. 



