368 Form of Temperature Wave spreading by Conduction. 



the above calculation that there is no advantage to be gained, 

 from the point of view of the distribution of heat by con- 

 duction, in enlarging the source of heat from a point to a 

 uniformly supplied spherical volume 1 mm. in diameter ; 

 and that both are inferior to a spherical surface source of 

 2 mm. diameter. It is clear, however, that much more 

 effective distributions of temperature can be obtained by 

 further increasing the size of the source of heat. The best 

 possible source of" ignition is obviously such a volume of the 

 inflammable mixture as can be raised instantaneously by the 

 given quantity of heat Q to its ignition temperature ; any 

 further spreading out of the source, either in space or in 

 time, can only result in a diminution of the volume that 

 is simultaneously raised to the ignition temperature, and no 

 improvement can be attained by altering the shape of the 

 volume initially heated. With Q — 0*001 calorie, and an 

 assumed ignition temperature of 700°, the volume of the 

 most effective source for ignition is 10*2 mm. 3 (that is to 

 say, the volume of a sphere of radius 0*1345 cm.), a figure 

 which represents the maximum incendivity according to the 

 scale of measurement defined. 



The results of the above, calculation show that, if a source 

 of ignition be regarded solely as a source of heat, the effec- 

 tiveness of a given quantity of heat in raising a sufficient 

 volume of an inflammable mixture to a given temperature 

 (by conduction alone) depends essentially upon the manner 

 in which that heat is communicated to the mixture. 



Differences in the heat contents of the least sparks of 

 different types capable of igniting a given gas mixture are 

 therefore to be anticipated. To quote a single example, it 

 has been found that the energy necessary to ignite a certain 

 coal-gas mixture by a capacity spark was '00025 joule, and 

 by an inductance spark '0006 joule. The ratio of these 

 figures is of the order of magnitude which would be antici- 

 pated from a purely thermal theory of ignition (cf. discussion 

 of Tables I. and II.). It will be apparent from the foregoing 

 that the observed differences in spark energy required for 

 ignition of a given gas are not in themselves sufficient to 

 warrant the assumption that ignition is due to ionization, 

 as such differences are consistent with a purely thermal 

 theory. 



