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XXXVII. On the Form of the Temperature Wave spreading 

 by Conduction from Point and /Spherical Sources ; with a 

 suggested application to the Problem of Spark Ignition. 

 By E. Taylor Jones, D.Sc, Professor of Physics in the 

 University College of JSorth Wales, J. D. Morgan, B.Sc, 

 and R. V. Wheeler, D.Sc, Professor of Fuel Technology 

 in the University of Sheffield* . 



IT is natural to assume that the power of igniting inflam- 

 mable gaseous mixtures possessed by an electric spark 

 depends essentially upon its ability to impart heat to the 

 gaseous mixture, and that in this respect it behaves similarly 

 to any other source of heat. Most, if not all, investigators 

 of the phenomena of the ignition of gases by electric sparks 

 have entered upon their work with this assumption. An 

 obvious inference is that the heat just necessary to ignite a 

 given mixture under standard conditions should be a constant 

 quantity, and that the heat contents of all sparks that are 

 just capable of igniting such a mixture, no matter how those 

 sparks are produced, should be the same. 



It has, however, been definitely established by experiment 

 that the thermal energy of a spark that is just capable of 

 igniting a given mixture varies considerably with the 

 electrical conditions under which the spark is produced. 

 This has led to the view that electric-spark ignition is not 

 mainly, if at all, a thermal process ; that it is, in fact, mainly 

 due to ionization. 



Inasmuch as the rate of reaction between oxygen and 

 inflammable gas is increased if the mixture is ionized, and the 

 attainment of the ignition temperature of a gaseous mixture 

 is mainly dependent upon this rate of reaction, it can readily 

 be understood that means of ignition that perform the dual 

 function of ionizing a mixture and imparting heat to it 

 (either successively or concurrently) may be more effective 

 than those which only impart heat, if such there be. 



The common means of ignition of gaseous mixtures — 

 heated wires or surfaces, or jets of flame — are powerful 

 sources of ionization, though they are usually supposed to 

 cause ignition by virtue of the heat they supply. The 

 debateable question would therefore appear to be : Are 

 electric sparks so much more potent sources of ionization 

 than, say, heated surfaces as to warrant the process of ignition 

 of gaseous mixtures by the former being labelled "ionic" 

 in contradistinction to a " thermal " process of ignition by 

 the latter ? Or, assuming that there are marked differences 

 * Communicated by the Authors. 



