Gases at Reduced Pressures by Transient Arcs. 457 



X 



— and of the time of duration of the arc. The velocity which 

 p 



•an ion or a number of ions can acquire while the arc lasts 

 depends on its duration, so that T enters directly and the 

 term aT t /> 2 expresses the fact that when the break is extremely 

 rapid ignition becomes more an energy effect, dependent upon 

 i alone, the ionization then being negligibly small. It is 

 again shown by the relation between ajp and X//j for air* 

 that ionization approaches an upper limit as X/p is raised, 

 so that when /.> is very small the term does not become great. 

 There is then a constant to be added to p, so that the final 



term is aTj(p +po) 2 and j i + j—r — v 2 [ (p — b) = C/T expresses 



all the essential facts of break spark ignition. It may be 

 remarked that this closely resembles Clausius' equation for 

 the curve of change of state, though the present work is 

 clearly in too early a stage for any equation to have more 

 than general interest as collecting the facts for a typical 



case. 



6. Two phases of Ignition. 



Such an expression as that given above is only valid when 

 the accelerating forces, molecular attraction in the change 

 of state, activation in the case of ignition, are developed 

 gradually. The action of spark ignition is too local for this 

 to be generally applicable except in the case of a gas with 

 high velocity of translation such as hydrogen. If in any 

 other case the critical stage is reached suddenly, as might 

 be expected, the term added to the current would come as 

 suddenly into effect. The result would be that for a small 

 change of pressure the igniting current should fall sharply, 

 with almost discontinuity, as it does in methane, ethane, and 

 propane. At pressures below this ignition proceeds with the 

 ionization term fully active. There are as it were two distinct 

 phases of ignition, and this is specially marked in ethane and 

 propane. It is not in the facts of ionization alone or of the 

 thermal changes in combustion that a sudden change in the 

 conditions of ignition arises, but in their combination, having 

 regard to the influence of the mean free time in allowing an 

 ion to acquire sufficient velocity for collision to become 

 •decisive. 



* Loc. cit. fig. 50. 

 Phil Mag. S. 6. Vol. 40. No. 238. Oct. 1920. 2 H 



