Incandescent Metals, etc. 297 



carriers. When the temperature is sufficiently high we get 

 carriers of both signs, the amount of negative being at first much 

 less than that of positive. 



The amount of both positive and negative carriers increases 

 rapidly with the temperature of the wire, and at very high 

 temperatures we get in the gas approximately the same amount 

 of each. 



The preponderance of the positive at low temperatures may be 

 due to the ionisation being produced by the hot wire only in a 

 very thin layer of gas close to itself, and the negative ions owing 

 to their very small initial mass are discharged to the wire, the 

 positive coming off to some extent in the gas. When the layer in 

 which ionisation takes place gets thicker as the temperature of the 

 wire is increased, the negative carriers are no longer all discharged 

 to the wire, and when the temperature is sufficiently high the 

 amount of negative discharged in this way is small compared with 

 the total ionisation. 



A similar excess of positive at low temperatures is observed 

 with other incandescent metals, as with platinum ; iron, German 

 silver, and brass wires gave the same result. Also C0 2 gave a 

 result similar to air. 



The velocity under electric force of the negative and positive 

 carriers was determined as described in the paper referred to 

 above. The velocity was found to vary greatly with the tempera- 

 ture of the wire, the velocity diminishing as the temperature of 

 the wire increased. The greater disintegration of the wire at the 

 higher temperatures rendered the conditions more favourable for 

 uncharged masses collecting round the carrier. It was also found 

 that the negative carrier had a greater velocity under a given 

 electric force than the positive, the excess in air being about 20°/o- 



Ionisation produced by an Incandescent Wire in Air at 

 Reduced Pressure. 



3. All the results mentioned above refer to the case where 

 the gas surrounding the wire is at atmospheric pressure. 



To experiment at reduced pressures the following apparatus 

 was used : 



AA is a glass tube in connection with a mercury air-pump so 

 that the pressure in the tube can be reduced as desired. Inside 

 the tube an aluminium cylinder G, 3'8 cms. long and 2 cms. 

 diameter, is placed and supported by a thick wire passing through 

 an ebonite plug at D so that the cylinder C can be insulated when 

 desired. A fine platinum wire W is stretched along the axis of 



VOL. XI. PT. iv. 22 



