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XXXIX. The Ignition of Gases at Reduced Pressures by 

 Impulsive Electric Sparks. By W. M. Thornton, Pro- 

 fessor of Electrical Engineering in Armstrong College, 

 J¥ewcastle-on-Tyne*. 



1. Introduction. 



IGNITION by discbarge across a fixed spark-gap is a 

 complex function oE the nature of the gas, the mode of 

 production of the spark, and the gas pressure. The discharge 

 is intermittent f and in part oscillatory, the first spark of the 

 train being a bright active capacity discharge, the remainder 

 tailing off in brightness and igniting power. Faterson and 

 Campbell's researches deal fully with most of the variables. 

 Their result that the first spark of the train alone can deter- 

 mine ignition is of the first importance. The object of the 

 present work was to examine whether the singular variations 

 of inflammability previously observed in coal gas { were 

 regular phenomena of ignition of gases at reduced pressure, 

 and if so to determine their cause. 



A large induction-coil was used, with the primary con- 

 denser short-circuited, and the current broken by a switch 

 of very rapid action. Since the speed and nature of the 

 primary break is copied in the manner of rise of the 

 secondary voltage, a clean rapid separation by a large quick- 

 break switch gives a sharper and higher voltage than a break 

 made by a slow scraping contact, and ignition phenomena are 

 known to depend upon the rapidity of break. Platinum 

 poles 2 millimetres diameter with rounded ends were used 

 throughout and kept bright. 



The gases examined were confined to hydrogen, methane, 

 ethane and propane, carbon monoxide, and coal gas as a 

 mixture of some of these. 



2. Nature of Disruptive Sparks. 



Ionization in gases has three well-marked stages which 

 occur in succession as the electric field is increased ; an 

 ;ipproach to saturation, the electronic state, and ionization 

 by collision. The last begins with values of X/jo, volts per 

 centimetre per millimetre of mercury pressure, greater than 

 50. Higher values than this were used here, at least twice 

 as great. Before the electronic state is reached the current 



* Communicated by the Author. 



t C. C. Paterson and N. Campbell, Proc. Phys. Soc. vol. xxxi. Part IV. 

 June 15, 1919, p. 177. 



See also J. I). Morgan, 'Electric Spark Ignition,' Chap. II. 



X "The Reaction between Gas and Pole in the Electrical Ignition of 

 Gaseous Mixture/' Proc. Roy. Soc. A, vol. xcii. p. 10 (1915). 



Phil. Mag. S. 6. Vol. 40. No. 237. Sept. 1920. 2 A - 



