Reduced Pressures by Impulsive Electric Sparks. 349 



partly of the lower thermal energy per unit volume of the 

 mixture. The curves were all obtained with the same break, 

 and the causes of any singularities arc to be looked for in the 

 gas and not in the spark alone. The second significant 





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feature is that all the curves have sudden changes or points 

 of inflexion at about 0'4 of an atmosphere. Now it has been 

 shown elsewhere that this pressure is a critical one in all ignition 

 by transient sources, as distinct from hot wires, and since there 

 is ionization in all these cases and no other electrical action 



