Ignition of Gases by Hot Wires. 623- 



585° C. of carbon monoxide 651°, of methane about 700°, 

 and these were practically the same in air as in oxygen. 



Observing the wire through a microscope in a combustible 

 mixture the surface appears quite dark until a certain value 

 of the current is reached when it glows suddenly, in some- 

 cases to an apparent white heat. There is no corona effect 

 or glow in the gas itself. The whole action appears to be 

 inside the surface of the wire. When there is no com- 

 bustible gas present the wire when visibly hot merely glows 

 steadily at a temperature reached as slowly as the current is 

 raised. 



When there is surface combustion the outside layer is 

 undoubtedly radiating freely, yet the resistance indicates a 

 mean temperature much lower than the optical temperature. 

 The surface of the platinum wire is polished and a reflector. 

 It is conceivable that the heat generated by surface com- 

 bustion never penetrates the metal but is reflected from the 

 surface as soon as made. If all the energy of combustion 

 were set free as radiation, the gas, through which radiation 

 passes without effect, would never be ignited, and this would 

 account for the fact of wires glowing white hot bv surface 

 combustion not igniting explosive mixtures surrounding 

 them. The rate of combination is raised by increasing the 

 current, and as the action becomes more intense the heat set 

 free cannot all be got rid of as radiation, the true temperature 

 of the surface begins to rise rapidly and explosion follow*. 



The process of ignition by hot platinum wires would then 

 appear to be that at a temperature about 200° C. an active 

 discharge begins to take place from the surface which when 

 sufficiently intense starts gaseous combination spreading into 

 surface combustion over the exposed area. This combustion 

 depends for its maintenance upon a continued supply of heat 

 from within or of ions passing through the surface double 

 layer, for on stopping the current in most cases it ceases. 

 The rate of surface combustion depends to some extent upon 

 the true temperature of the surface metal. When this ex- 

 ceeds a value which possibly is that at which free gases 

 combine, as distinct from surface combustion, the mixture 

 explodes. Until this value is reached it is possible to have 

 surface temperatures, certainly in appearance as high as the 

 melting-point of iron, derived from the sustained combina- 

 tion of the gas in renewed contact with the wire, the energy of 

 which is dissipated as radiation and does not ignite the gas. 



Coating platinum wires with aluminium phosphate, which 

 gives a vigorous emission of positive ions at low tempera- 

 tures, suppresses surface combustion and retards ignition of 



