622 Prof. W. M. Thornton on the 



much below that of the burning gas. For example, it has- 

 been shown that a platinum wire '01 cm. diameter, heated 

 by a current of 1*2 amperes in a 30 per cent, mixture of 

 hydrogen and air, ignited the gas. The same wire was just 

 visibly red at 1*44 amperes in air. At 1*2 amperes the voltage 

 drop across the filament was 0*64 volt, which rose quickly 

 to 'S2, at which point it paused for a moment and the gas 

 exploded. The resistance of the wire therefore increased 

 from 0'53 ohm to 68 ohm, while the current remained 

 constant on account of the relatively large external resist- 

 ance in the circuit. The temperature at 1 2 amperes was 

 found to be between 120° C. and 140° C, the wire was 

 therefore quite dark. The rise of temperature corresponding 

 to a 2S per cent, change of resistance is 73° C. ignition 

 therefore began when the temperature of the wire didl not 

 exceed 213° G. 



In carbon monoxide the resistance changed by 46 per cent, 

 with 1*12 amperes steady in the wire, showing u rise of 

 120° C. and a final temperature of about 240° (J. In coal 

 gas the change was 23 per cent., giving a final temperature 

 of ignition of 200° C. These temperatures are of the same 

 order as those given by Bone as the starting point- of 

 surface combustion. 



Taking; now these latter wires and heating them with air 

 alone in the explosion vessel, it required a change of 37 per 

 cent, in their resistance as measured to raise them to just 

 visible redness in the dark. Yet at the moment of explosion 

 at the end of the above recorded rise of voltage, the w*ire had 

 the appearance of glowing brightly except for a millimetre 

 or two at each end where the leading-in rods cooled it.. 

 The length heated to redness was never less than half the 

 length of the wire. Taking' the worst case of a wire in 

 which the curve of temperature is a parabola falling to zero 

 at each end, the mean temperature is § of the maximum, and 

 in any other case it is I (2 4- ?') of it, where r is the ratio of 

 the length of the middle uniform part to the whole. In thin 

 wires r is very nearly unity. The temperature can then be 

 determined from the resistance of the wire with fair accuracy 

 at moderate temperatures, and if necessary Callendar's cor- 

 rection can be applied. 



In no case does the resistance of the bright platinum 

 wire indicate temperatures approaching those of Dixon 

 and Cow^ard * for ignition of gaseous mixtures heated by 

 external action. Their mean value for hydrogen in air was 



* H. B. Dixon and H. F. Coward, Cbem. Soc. Jour. xcv. pp. 514-543- 

 (March 1909). 



