On the Pressure on the Poles of an Electric Arc. 511 



l£ e be the coefficient of restitution, and /=2(l+ e )> f lB 

 unity when the collisions are perfectly elastic, but a very 

 small reduction in/ below this value has the effect of making 

 a large reduction in the velocity of agitation. The following 

 are the values of/ found by Pidduck corresponding to the 

 observed value of k: — 



Z/p -2 2 'JO 150 



k-l observed... 1-8 20 55 210 



./' -9993 -9988 -99 -06 



Thus as the velocity of the electrons increases, the elasticity 

 tends to diminish, but with the smaller velocities the mole- 

 cules may be considered to be perfectly elastic. 



LX. On the Pressure on the Poles of an Electric Arc. 



To the Editors of the Philosophical Magazine. 



Gentlemen, — 



IN his latest publication under the above title Prof. Duffield 

 (Froc. Roy. Soc. June 1920), discussing his results, 

 tries to show that the motion of ions within an electric arc 

 and the electric wind could not be expected to produce a 

 pressure upon the poles of the arc. Having myself contri- 

 buted to your Magazine a few papers on the electric wind, 

 will you once more give me your hospitality in order to 

 clear up some points in connexion with this question ? 



I am very much indebted to Sir J. J. Thomson for having 

 shown to me some time ago (in a way similar to that used by 

 Prof. Duffield) that ions moving under an electric force 

 towards an electrode (evidently of opposite sign) cannot 

 produce a pressure upon it, the pull on this electrode during 

 the motion of the ions being balanced by the impact of the 

 ions on the electrode. Prof. Duffield, however, has entirely 

 overlooked the fact that ions moving/row an electrode (of the 

 same sign) produce a reaction upon this electrode which is 

 not compensated by any other force. Consequently, a strong 

 pressure upon the poles of an electric arc is to be expected 

 as a result of the motion of the ions within the arc. Moreover, 

 when the experiments are carried out not in a vacuum (as in 

 the case of Prof. Duffield's experiments), the energy of the 

 ions is partly transferred by collision to the neutral gaseous 

 molecules, which results in the formation of a stream of the 

 surrounding atmosphere in the direction of the electric field 

 — a phenomenon known as the electric wind. Experiments 

 show that only a small part of the momentum of this stream 



