Incandescent Metals y 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°/ 0 - 
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 : 
A A 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 C, 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 G can be insulated when 
desired. A fine platinum wire W is stretched along the axis of 
22 
VOL. XI. PT. IV. 
