260 
IMl. HAROLD A. WILSON ON THE 
Temperature. 
° 0 . 
1.375 
1408-5 
1442 
1476 
1510-5 
1545 
1580 
Current per square centimetre 
Current calculated. 
found. 
1-57 X10-s 
1-49x10-8 
3-43 X10-s 
3-33x10-8 
7-46 X10-s 
7-18 X10-8 
15-2 xlO-8 
15-3 xlO-8 
32-3 xlO-s 
31-8 xlO-8 
63-8 xlO'S 
64-5 X10-8 
128 xlO-s 
128-5 xlO-8 
Tlie calculated and observed currents agree very well, so that we may conclude that 
a in the formula is really zero or nearly so. It is easy to show that 
the value of a cannot be greater than unity in order for the observed and calculated 
results to agree as well as they do. Consequently, since Qo = 131,100, Q does not 
vary as much as 1 part in 100,000 per degree centigrade. 
The value found for Q is slightly more than double that found previously ( Phil. 
Trans.,’ A, vol. 197, p. 430) for the work required to ionize a gramme molecular 
weight of air in contact with hot platinum. 
All that it is necessary to assume to deduce the equation used to calculate Q eithei 
for tlie positive leak in air at atmospheric pressure, or for the negative leak m a 
vacuum, is that ions are produced in some way at the surface ol the hot platinum. 
The results just described cannot, therefore, lie regarded as any additional evidence 
in favour of the view that the negative leak from hot platinum is due to negative 
corpuscles coming out of the metal; they are, however, consistent with this view. 
The positive leak in air at atmospheric pressure can be represented liy the lorniula 
X 
The value of A' may be obtained from the observation that the positive leak at 
1550° C. is double the negative leak. This gives A' = 0'47. The temperature at 
which the two leaks with the particular wire used were equal is, therefore, given by 
by the formula 
6-9 X = 0-476>’e- 
- 60,000 '29 
which gives = 1895 or 1622° C. Above 1622° C. the negative leak with this wire 
was greater than the positive leak, and below this temperature less. 
At low pressures the positive leak is smaller. If we assume that it varies as the 
pressure, then it will be given at any pressure by the formula 
X 
— 60,000/29 
“ 760 
At 1 niillim. pressure this gives a' = 6’2 X 10 and the tempeiatuie at 
which the positive and negative leaks are equal conies out 112/ C. These results 
