632 On the Ignition of Gases by Hot Wires. 



as nickel, or alloys haying a melting-point of the same order,, 

 to make electric lamps for use in coal mines which when 

 broken ' in an inflammable atmosphere of methane and air 

 will not ignite the gas. The resistance of such lamps would 

 be less than carbon or tungsten lamps and this would neces- 

 sitate some change in the lighting voltage, but combined 

 with an alternating current supply, which makes ignition 

 still more difficult in case of a filament breaking, safety can 

 be assured. Xor would a partial break, of the tip of the 

 lamp for example, which admitted a slow stream of gas be 

 any more dangerous, for it lias been shown above that hot- 

 wire ignition is independent of gas pressure. 



To apply this to places, on board ship for example, where 

 gases such as hydrogen or petrol vapour have to be guarded 

 against, would not be so easy, for in these cases surface com- 

 bustion begins at temperatures below incandescence, and it is 

 probable that sound mechanical protection will be preferred 

 both in mines and battery rooms to dependence upon the use 

 of such safety-lamps alone. 



14. Su 



minariu 



Ignition by hot wires occurs at a definite rate of gener- 

 ation of heat in unit length which in a steady state at 

 low temperatures is removed by convection and is nearly 

 independent of diameter. Ignition is independent of gas 

 pressure, of the heat of combustion of the mixture, of the 

 proportion of gas present, and for any given metal inde- 

 pendent of the nature of the combustible molecule so far as 

 it is affected by change of diameter : hydrogen and platinum 

 are exceptions. That is, since previous heating of the gas 

 does not affect ignition temperatures, it is on the whole 

 independent of every external physical or chemical variable. 



The source of ignition must therefore be sought in some 

 phenomenon depending strictly upon the rate of generation 

 of heat by the passage of the current, but occurring if not 

 within the surface layer of the metal itself so near to it that 

 the ordinary gas laws do not come into action. Oxygen is 

 the only active common component of all the mixtures, and 

 bv a process of elimination it can be inferred that the 

 mechanism of hot-wire ignition is an attack upon oxygen 

 either within the wire or by positive ions of combustible gas 

 just ejected from it, for the temperatures at which surface 

 action begins are lower than those which start combination 

 of gases separately heated to a point at which they explode 



