required to start a Gaseous Explosion* 735 



ignition of gases by break-sparks is a convenient means for 

 examining this, and the present note is a comparison of the 

 energies of the least igniting break-sparks of continuous and 

 alternating current with those of condenser discharge between 

 platinum poles, which may be taken, as the. standard. 



The igniting currents of general gases at different volt- 

 ages and in various strengths of mixture have been given in 

 a previous paper*. For each mixture there is a definite 

 least igniting spark, different for continuous and alternating 

 currents. The electrical energy may be less than the true 

 energy which starts the explosion, if it is increased by that 

 of combustion of the gas in immediate contact with the spark 

 once this is formed. But even with mixtures of carbon 

 monoxide in air there is a ring of carbon deposited around 

 the point of contact of the poles after the passage of sparks 

 which do not cause explosion. It is therefore possible that 

 there is no true combustion until the explosion occurs, and 

 that instead of the gas helping ignition it may at first retard 

 it by the energy absorbed in breaking up the combustible 

 molecules in contact with the spark. 



When the time-constant L/r of an electric circuit is always 

 the same, as it is when the same type of resistance is used 

 throughout, the energy -JLi 2 of the break-flash is propor- 

 tional to the power of the circuit, for since L = rc, and 



V 



r — - , ^Li 2 = IcYi, where V and i are the circuit voltage 



and current. In the present case the time-constant c is 

 0*0016 second. The igniting power of a break-flash measured 

 by the ratio of the current to the number of atoms of com- 

 bustible gas in unit volume has been found to vary in the 

 case of continuous currents as the inverse cube of the voltage f, 

 other factors being constant. When, however, these are 

 being varied there will be a component of the current which 

 will change as V" 3 , and of the corresponding power Yi as 

 Y~ 2 . Below a certain voltage it is exceedingly difficult to 

 ignite a gas by break-sparks except with currents of destruc- 

 tive magnitude. 



2. Continuous Current. — Curves of energy of ignition for 

 continuous current breaks in methane and coal-gas with 

 various metals for poles are given in figs. 1 & 2 (Pi. XIII.). 



* " The Electrical Ignition of Gaseous Mixtures/' W. M. Thornton, 

 Eoy. Soc. Proc. A. vol. xc. p. 272 (1914). 

 + Loc. cit. fig'. 8. 



