DISCHARGE OF ELECTRICITY FROM HOT PLATINUM. 
257 
air at low pressures, it can discharge positive electricity to an extent appreciable on 
a galvanometer, but the current rapidly falls off and in a few seconds becomes 
inappreciable. A wire which has been cleaned by boiling HNO 3 usually only shows 
this effect to a very slight extent or not at all. 
In air at high pressures the permanent positive leak is, of course, appreciable on a 
galvanometer. It increases with the pressure at constant temperature. In pure air 
at atmospheric pressure, using a wire cleaned with HNOg in the way described, the 
positive leak is about double the negative leak when the temperature is about 1550° Ct 
The two leaks are nearly equal at a pressure of about 200 millims. at the same 
temperature. 
(3.) The Variation of the Negative Leah from Hot Platinum with the Temperature. 
The results in the preceding section show that the number of negative ions or 
corpuscles which start from the platinum is independent of the air pressure, provided 
that this is not more than a few millims., or that if the pressure is large the P.D. used 
is sufficient to drag away all the corpuscles which get out. In the present section the 
variation of the number of these corpuscles which escape from the platinum with the 
temperature will he considered. 
The wire used was 13'0 centims. long and OH millim. in diameter, and it was well 
cleaned with HNOg after it had been mounted as described above. The apparatus 
was filled with pure air and pumped down to a low pressure several times with the 
wire red hot. When it was thought that all gases evolved by the wire were got rid 
of, the apparatus was pumped down and the measurements made. If these precautions 
were not taken, the leaks were always much larger and very irregular. 
The following table contains the results obtained with a particular wire treated in 
this way. Balancing resistances in P.O. box at 18’5° C. before experiments 1000 and 
1669 ohms, and after the experiments 1000 and 1669 ohms.* 
Balancing resistances 
200 ohms and 
Negative leak in galvanometer scale divisions. V 
= 40 volts. 
= 0 ■ 1 millim. 
= 0 • 2 millim. 
Mean. 
1600 
(1) 5-0 
(8) 5-0 
5 
1625 
(2) 11-0 
(9) 11-0 
11 
1650 
(3) 25 
(10) 23 
24 
1675 
(4) 50 
(11) 48 
49 
1700 
(5) 108 
(12) 100 
104 
1725 
(6) 207 
(13) 202 
205 
1750 
(7) 412 
(14) 410 
411 
* In making a series of measurements, it is necessary to measure the resistance of the wire when cold 
from time to time, because at high temperatures the wire disintegrates and its resistance consequently 
rises with long continued heating. 
VOL. CCIT. — A. 2 L 
