1887.] On the Discharge of Electricity through Gases. 371 



X. " Experiments on the Discharge of Electricity through 

 Gases. (Second Paper.)" By Arthur Schuster, F.R.S., 

 Professor of Applied Mathematics in Owens College. Re- 

 ceived May 18, 1887. 



Three years ago I gave a sketch of a theory of the passage of 

 electricity through gases, in which it was assumed that in the 

 gaseous discharge an atom carries a definite molecular charge just as 

 in electrolysis (see Bakerian Lecture, 'Roy. Soc. Proc.,' vol. 37, 

 p. 317). I showed how this molecular charge if constant might be 

 measured, and I have since that time worked continuously in arrang- 

 ing for the necessary experiments. 



I have, with the help of a grant from the Royal Society, mounted a 

 battery which gives me an electromotive force of 1800 volts, which I 

 hope will be sufficient for my purpose. The experiments which I have 

 in view, involve the accurate measurement of the electric potential at 

 different points of a vessel through which a discharge is passing ; but 

 before these could be undertaken a number of intermediate questions 

 had to be settled by experiment. As these have in this way estab- 

 lished some definite points which I believe to be of importance, I 

 venture to bring them before the Society. 



In thinking over the phenomena presented to us in vacuum tubes, 

 I always felt a difficulty owing to our ignorance of the conditions 

 which hold at the surface of bodies, either suspended in or near the 

 discharge, or even at the boundary of the vessel through which the 

 discharge is passing. It is evident enough, that if there is a flow of 

 electricity on the surface of a non-conduetor that flow must be 

 tangential, but it is not so clear whether we are justified in concluding 

 from this that there can be no normal forces at such surfaces, for it 

 is not necessary that the flow should always take place along the lines 

 of force. Imagine, for instance, a discharge to consist of particles 

 charging at one pole, then moving, under the action of electric forces, 

 to the other pole ; and let the vacuum be sufficiently good that few or 

 no encounters take place while it passes from one pole to another. 

 Then bring an electrified body near the discharge. That body may 

 deflect the particles conveying it, but unless its electrification is 

 sufficiently large, it will not draw up the discharge to its surface, so 

 that the normal forces on the electrified body which has been intro- 

 duced need not be neutralised by the discharge. 



The question is one altogether for experiment to decide. Supposing 

 we suspend two pieces of gold leaf, as in an electroscope, at any place 

 in a partially exhausted vessel, and render them divergent by electri- 

 fication, they should collapse as soon as the discharge begins to pass, 

 if tangential forces only can permanently exist at their surface. This 

 I have tested by experiment and found to be the case. 



