346 Prof. W. M. Thornton on Ignition of Gases at 



is known to be carried by ions, with a group of molecules 

 surrounding each, which groups become smaller as the field 

 and velocity of ions increase until there is a state of free 

 electrons in equilibrium with inert molecules. Above this 

 ionization is caused by collision of electrons, moving with 

 .very high velocity in the eleclric field, with molecules. 

 This then is the state in which, on the eventual passage of a 

 spark, ignition occurs. The molecules of combustible gas 

 and air in and near an electric arc are both, according to the 

 work of Lowry* and Haber and Koenigt, ionized by contact 

 with it. They then form groups in which electric recom- 

 bination occurs. When in the present case of disruptive 

 discharge there is a complete bridge of ions between the 

 poles, a spark passes and ignition may or may not follow. 

 The dividing line is extremely sharp. A change of one per 

 cent, of the primary current frequently converts a spark, 

 apparently brilliant but still inert, into one that gives certain 

 ignition. Something more than ionization is needed. There 

 is a critical intensity of spark, that is a certain number of 

 ions produced per second to ensure ignition. Time enters as 

 a principal factor as it has been shown to do with the slower 

 transient arcs which form circuit break sparks. 



The spectrum of disruptive sparks contains lines both of 

 the gas and material of the poles, but the latter lines are 

 secondary and arise from finely divided metal carried over by 

 the first current rush from the poles. There is, however, 

 as seen by the luminosity, intense energy of vibration as w T ell 

 as translalional energy of electrons in the spark, and to start 

 an explosion of gas certainly requires a finite liberation of 

 energy at the source, first in the spark and then by com- 

 bustion of the gas, sufficient to continue the latter by self- 

 ignition after the spark ceases. 



3. Ignition hy Disruptive Sparks. 



There is then in ignition by jump sparks the following 

 sequence of events. First, ionization leading up to ionization 

 by collision probably of all gases present, increasing until a 

 spark passes which releases the electrostatic energy at the 

 terminals, and by introducing a conducting path across the 

 gap allows a current to pass equal to the momentary 

 resultant voltage divided by the resistance of the high- 

 tension windings of the coil. Around this current there is a 

 magnetic field, which as the induced voltage impulse dies down 

 decays with oscillations and prolongs the duration of the spark. 



* T. M. Lowry, J. C. S. 101. p. 1152 (1912) ; Faraday Soc. ix. p. 189 

 (1913). 



t Haber and Koeni^ - , Zeitsch. Electrochem. xiii. p. 725(1907'); xiv. 

 p. 689 (1908). 



