456 Prof. W. M. Thornton on the Ignition of 



In an arc the gap is saturated with electrons from the 

 circuit, and these have high temperature velocity apart from 

 that given by the field. A lower voltage fall than 87 would 

 then be sufficient to ionize a molecule by collision with an 

 electron *, and in flame the velocity of combination is such 

 that ions are produced with no electric field. 



In the case of normal ionization by collision change of 

 gas pressure does not affect the total current passing between 

 plates. The velocity and rate of diffusion are both increased. 

 As shown by the influence of traces of moisture, ignition is 

 started by internal combination of a few molecules rather 

 than by general action. The variation of ignition, if in- 

 fluenced in any case by ionization, is not a function of the 

 ionization current taken as a whole ; but it is known that 

 when the velocity of an electron reaches a certain value 

 ionization by collision begins suddenly. The velocity 



X D 



U= — . e— , where m is the mass of the ion carrying the 



m p J ° 



current, and D a coefficient expressing the dimensions of 



X 



this mass which varies with the field and pressure f. As - 



D P 



increases m diminishes and — rises in value. For small 



m 



changes of pressure D/m may be taken as a first approxima- 



"S 7 " ~V A 7 " 2 



tion proportional to — , and writing it =k— , U = ke( — ) , 



P 6 P \PJ 



and here k, e and X are constant. Thus as p falls U increases 

 until, as a critical value is approached, ionization begins. 



When ignition is made easier by such a strong ionization 

 by an electric field in addition to that caused by the high 

 temperature of the arc, a term of the form a/p" 2 added to the 

 circuit current expresses the influence of ionization by 

 collision on the igniting current. This applies to ions moving 

 in gases at relatively high pressures, such as half an atmo- 

 sphere. When, however, X has values of the order of 1000 



volts per cm. and p = l mm., U varies as a/— J, and "the 



whole character of the motion then changes." 



Over the range of pressure within which ignition is 

 possible D/m is in all probability a more complex function of 



* E. Rutherford and R. K. McClung, Phil. Trans. A, 196. p. 25 (1901). 

 See Townsend, ' Electricity in Gases/ p. 263. 



t Townsend, loc. cit. p. 290. £ Townsend, p. 312. 



