Ignition of Gases by Hot Wires. 615 



ear rent is then made and raised quickly, a voltmeter con- 

 nected across the wire being constantly observed, so that 

 change of resistance can be measured and the temperature 

 -estimated. It is found that there is for every diameter and 



BAROMETER 



GAS MIXTURE 



9 VACUUMPUMP 



metal a particular current which when made suddenly, not 

 approached slowly, just causes ignition. This current is very 

 sharply defined and is not quite the same for old and new 

 wires : the former ignite gas rather more easily than fresh 

 wires, though the variation is at most a few per cent. It 

 does not seem to depend upon the mechanical state of the 

 wire so long as its diameter does not change with use, as in 

 the case of tungsten which undergoes a permanent extension 

 of as much as 20 per cent, after heating, contracts in diameter, 

 and becomes brittle. 



3. Influence of Metal and Diameter of Wire. 



Wires of pure platinum, nickel, iron, tungsten, molybde- 

 num, gold, and silver were obtained in three sizes, about 

 O'l, 0*2, and 0*3 millimetre diameter, and formed into 

 lengths or loops 3 centimetres long. In these sizes copper 

 melts too readily to be used. The exact length of wire did 

 not matter so long as 1 to 1J centimetres of it glowed 

 brightly. The same results were obtained with straight as 

 with curved wires. The gases were hydrogen, methane, 

 •ethane, pcntane, methyl and ethyl alcohol, ethylene, ethyl 



2 U 2' 



