254 Mr M c Clelland, On the Conductivity of Gases 



gas, and we have the positive carriers only coming away from the 

 wire at low temperatures, and at higher temperatures those of 

 both signs ; when the wire is at a bright red heat the amount of 

 negative is approximately the same as that of positive. 



(21) These results agree with former experiments on the 

 subject. Elster and Geitel found that when a platinum wire is 

 heated to luminosity in air, the air acquires a charge of positive 

 electricity. They found that the potential to which a body could 

 be raised when placed near the wire depended on the temperature 

 of the wire and reached a maximum when the wire was at a bright 

 yellow heat, and was very small when the wire was at a very high 

 temperature. This would follow because the amount of positive 

 and negative carriers in the gas is nearly the same when the wire 

 is very hot. Branly found that with the wire at a dull red heat 

 the gas from it will only discharge negative electricity, but at 

 higher temperatures both positive and negative. 



(22) When the temperature of the wire is comparatively low 

 the ionisation, no doubt, takes place in a very thin layer close to 

 the wire, and the ions being very close to the wire are attracted 

 towards it. Since the mass of the negative carrier is initially very 

 much smaller than that of the positive, the negative, when the 

 layer of ionisation is very thin, may be -entirely discharged to the 

 wire while the positive carriers come off to some extent with the 

 gas which is drawn away from the wire. As the temperature of 

 the wire is increased the layer of gas in which the ionisation takes 

 place will get thicker, and when the thickness is sufficiently great 

 all the negative ions will no longer be discharged to the wire, and 

 we will have both positive and negative in the gas; when the 

 temperature is very high the number of carriers discharged to 

 the wire from a very thin layer close to it will be small compared 

 with the total ionisation, so that the amounts of positive and 

 negative in the gas taken from near the wire will be nearly the 

 same. 



(23) The property possessed by an incandescent wire of being 

 able to discharge a negatively charged body near it before it can 

 discharge one positively charged, as its temperature is gradually 

 raised, does not depend on the sort of wire used. Iron, German- 

 silver, and brass wires were tried and gave the same result as the 

 platinum. When any of these wires is heated the gas drawn from 

 it discharges negative electricity so soon as the wire is luminous, 

 but a considerable further heating is required before positive 

 electricity can be discharged. The following numbers refer to 

 a German-silver wire, and the results with the others were 

 similar 



