Ignition of Gases by Hot Wires. 619* 



later. A curve, fig. 3, through these cuts the position of 

 methane at 2 to 21 amperes. Platinum wires '01 cm. 

 diameter soften and break at about 2*0 amperes. It is 

 just possible, therefore, to ignite methane in a 10 per cent, 

 mixture by wires on the point of melting. 



This difference between hydrogen and the other members 

 of the paraffin series is peculiar to platinum-wire ignition. 

 With tungsten wires there is no difference between the 

 igniting currents of hydrogen, methane, carbon monoxide, 

 or coal gas. The ignition of hydrogen and the higher 

 paraffins begins at temperatures well below redness. Ethane 

 and methane require high temperatures. Tungsten is always 

 at red-heat before ignition begins. 



5. Influence of Heat of Combustion of the Gas. 



It is evident from the figures of Tables II. and III., 

 that the calorific value of the combustible gas does not per- 

 ceptibly modify the conditions of ignition, nor is it a question 

 of adjustment of percentage so that the calorific value of unit 

 volume remains constant, for ethyl alcohol, ether, and pentane 

 were used in the same strength of mixture, and have the same 

 slope of line though their calorific values are widely different. 

 The curve of fig. 3 would at first sight suggest that igniting 

 current is inversely proportional to calorific value. It 

 follows closely the relation (I — ■69)n =5'25, where n is 

 the number of oxygen atoms required for complete burning 

 of one molecule of the combustible gas. Calorific value is 

 proportional to n , so that the curve might equally well be 

 drawn on a heat base. The absence of any such relation 

 when tungsten wires are used would, however, seem to rule 

 the latter out, so that the more difficult ignition of the lighter 

 paraffins by platinum wires may arise rather from difficulty 

 of their " sorption," and the emission by platinum at low 

 temperatures of ions derived from the paraffin molecule. 

 The absorption and adsorption of the paraffins by platinum 

 should be further investigated from the point of view of 

 surface combustion. 



6. Influence of Heat generated by the Current. 



Ignition is a, matter of intensity of action rather than of 

 dissipation of a certain quantity of heat. The heat generated 

 per second in unit length of a wire of diameter J and con- 

 ductivity a is H ^-iriTTiPo-. For the same gas mixture 



a constant for any particular metal provided that ignition 



