DISCHARGE OF ELECTRICITY FROM HOT PLATINUM. 
261 
afford an explanation of Elster and Geitel’s observation that an insulated conductor 
near a hot wire becomes positively charged in air at atmospheric pressure, but 
negatively charged at low pressures. 
The results described above show that the negative leak from a platinum wire is 
verv little affected by the presence of air, nitrogen, or water vapour. It was found 
that putting some mercury into the tube containing the hot wire, so that it was tilled 
with mercury vapour, increased the leak considerably at very high temperatures. 
The whole tube, of course, gets hot, so that tlie pressure of the mercury vapour must 
have been very considerable in this experiment. The following table gives the results 
obtained :— 
Temperature. 
Negati\'e leak before 
introducing mercury. 
Negative leak after 
introducing mercury. 
'’’C. 
divisions. 
divisions. 
1728 
120 
400 
1645 
;17 
78 
1566 
12 
13 
Thus above 1600° C. the presence of mercury in large quantities appears to increase 
the negative leak, but below this temperature it is without effect. The very small 
amount of mercurv vapour ordinarily present can, therefore, safely be assumed to 
have no effect below 1600° G 
With different pieces of pure platinum Avire cut from the same sample and treated 
in tlie same way Avith nitric acid fairly concordant residts Avere ol)tained. Nevertlie- 
less, long continued heating of tlie AAure diminishes the negative leak, provided the air 
is frequently changed. If the AAure is sinqjly left standing in air at a Ioav pressure for 
a long time the leak is often greater than before on again heating the Avire. The 
process of cleaning the Avire Avith nitric acid and frecjiiently changing the air in the 
apparatus diminishes the negative leak something like a thousand times usually. 
Thus the leaks given above are about one thousand times smaller than those given 
l)y Richardson at the same temperatures. 
These facts naturally suggest the question AATiether more thorough cleaning ol the 
Avire and puritication of the air Avould not still further diminisb the negative leak. 
To test this a Avire Avas mounted as before and boiled in HNGy for about an hour. It 
Avas then left in the HNO^ for 24 hours and a current passed from the Avire to the 
cylinder through the HNO 3 , so that the NOg ions Avere set free on the Avire. The 
tube was then washed out and the leak measured in pure air at a low pressure, as 
before. The following table gives the results obtained :— 
