450 Pref. W. M. Thornton on the Ignition of 



compared with that produced by the helium radiation at the 

 higher electron velocity, a result which is in agreement 

 with that found by means of the current-E.M.F. curves. 



The experiments described in this paper thus emphasize 

 the importance of maintaining helium perfectly pure when 

 attempting to investigate the ionization of the gas by electron 

 collisions. The only satisfactory method when a glass 

 apparatus is used is to have a slow circulation of freshly 

 purified helium through the ionization chamber during the 

 experiments. It is possible that if fused silica were sub- 

 stituted for glass, contamination of the gas would be less 

 likely to occur. 



The curves which have been given indicate that the 

 minimum ionization velocity for electrons in helium is 

 25*0 volts, a rather lower value than that obtained by 

 Horton and Davies. In their experiments the correction 

 applied to the accelerating potential difference to give the 

 critical velocity was that found from the swiftest electrons 

 present, and, although a sensitive electrometer was used to 

 detect ionization, it is possible that the swiftest electrons 

 were not sufficiently numerous to produce a measurable 

 ionization current. The results given in this paper also 

 indicate that the minimum radiation velocity for electrons 

 in helium is about 21 volts ; but this value is no doubt too 

 high, for the point at which ionization of the impurities by 

 the helium radiation was detected depended on the amount 

 of impurity present, and was probably always higher than 

 the point at which radiation was first produced from the 

 helium atoms. 



L. The Ignition of Gases at Reduced Pressures by Transient 

 Arcs. By W. M. Thornton, Professor of Electrical 

 Engineering in Armstrong College, Newcastle-on-Tyne* . 



1. Introduction. 



THE momentary arc formed at the point of break of a 

 current-carrying circuit is an active source of ignition. 

 It differs essentially in character from disruptive discharge, 

 and consists mostly of a stream of charged particles passing 

 from the negative to the positive pole. At the latter there 

 is always a bright spot formed corresponding to the crater of 

 * Communicated by the Author. 



