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



8. Ethane and Propane, and the cause of their oscillation. 



When there are no singularities such as those first dis-* 

 cussed there is the oscillation of inflammability found in 

 the neighbourhood of 0*4 atmosphere pressure. Such a 

 variation has been observed in the case of ignition by 

 transient ares and it is there much more pronounced than in 

 figs. 3 and 4, for ethane and propane. As the pressure 

 is lowered the arc goes through a remarkable change of 

 condition and is for a time unstable. Its length and dura- 

 tion suddenly diminish almost with discontinuity. As tlie 

 pressure is still further reduced the arc again lengthens and 

 changes type from a sharply defined discharge to a glow. 

 This eventually extends so that when the mechanical break 

 is about one and a half centimetres the arc persists as a 

 brightly luminous though cool discharge, showing striations 

 or pulsations of brightness as it is drawn out. 



From examination of some eight hundred photographs of 

 transient arcs at low pressure it became evident that as the 

 pressure is reduced the temperature of the point of the cathode 

 from which the arc starts is lowered until a point is reached 

 where it is insufficient to start a discharge of metallic vapour 

 or of large ions through which the arc can be maintained. 

 The extreme suddenness with which this state is reached 

 resembles that observed by Sir J. J. Thomson at lower 

 pressures*, where a small change of cathode temperature 

 suffices to lower the current to a hundredth of its previous 

 v;ilue. This point corresponds to an increase of difficulty of 

 ignition because of the smaller size and less duration of 

 the arc. 



Following this period of inaction there appears with almost 

 equal suddenness a discharge similar to an arc but without 

 the distinctive features of one started by a hot cathode. 

 It is rather the commencement of: the positive glow and is a 

 relatively low-temperature discharge in which ionization is 

 active. Disruptive spark discharge at low pressure does not 

 show any such changes of length, the product of spark length 

 and pressure is constant. When, however, the spark-gap is 

 fixed and small the magnitude of the primary current is a 

 measure of the current in the spark, and the temperature of 

 the points as affected by the magnitude of the current changes 

 with it. So far as this is the case the action is not unlike 

 that of a transient arc. 



The ordinates of the curves of ignition of ethane and 

 propane show sudden fluctuations at nearly the same point. 



* ' Conduction of Electricity through Gases/ p. 480. 



