270 
DR. HAROLD A. WILSON ON THE 
pressure at liigher pressures up to a few luiiliiuetres. Tlie leaks iu the two sets ot 
oljservatious given above at O'OOIS inilliin., and at 0'112 millim., are nearly in the 
ratio of the pressures. If a large P.D. is used ionization hy collisions comes in at 
higher pressures just as with air, and the current rises with the pressure, at pressures 
of a few millimetres, more quickly with large P.D.’s than with small. The following 
table shows the variation of the leak at a constant temperature of 1340° C. with the 
pressure up to 760 millims., using a P.D. of 40 volts, which is too small to produce 
ionization hy collisions :— 
Pressure. 
Current. 
millims. 
auippres. 
766 
.3000 X 10-s 
4.50 
1500 X 10-'5 
156 
222 X 10-« 
14 
GO X10-« 
At Iiigh pressures tlie leak is again nearly proportional to the pressure. 
Fig. 7 shows the variation of the negative leak in hydrogen with the P.D. at 
pressures of 133 and 766 millims. The current at 133 millims. is nearly saturated 
with about 200 volts, but then l)eglns to rise inpidly owing to ionization by collisions 
coming m. The upper part of the curve can be represented approximately by the 
formula obtained in the section on the leak in air. 
The iiositive leak, like the negative leak, is much increased by the presence of 
hydrogen. Witli clean wires it is, nevertheless, inappreciable on a galvanometer, 
