TRAVERSED BY CATHODE RAYS. 
65 
reason their effective intensity was very largely determined by the pressure of the 
gas traversed. 
While a diminution in the pressure would not affect the original intensity of a 
pencil of rays issuing from the window, it would, owing to a decrease in the absorp¬ 
tion, increase the ionizing power of this pencil at the centre of the receiver. In this 
way, although the available amount of matter to be ionized was lessened by lowering 
the pressure, it could happen that the resultant ionization, as measured by the satu¬ 
ration current, would at first exhibit increasing values. This in all probability 
accounts for the numbers obtained in Table II. 
Now from this point of view such a condition would only hold down to a stage 
when the two influences produced equal effects. The ionization would then be a 
maximum, and would afterwards fall off with diminishing pressures. Although the 
numbers obtained for the saturation current do not show definitely that a maximum 
value was obtained for the ionization, still there are indications from them, as the 
curve shown on fig. 6 illustrates, that the maximum value was reached at a pressure 
of about 75 millims. of mercury. 
ionizations 
Arbitrary Scrle 
4 
3 
2 
/ 
600 700 600 500 400 300 ZOO 100 0 
Pressures in MMS. 
Hie/. VI. 
As indicated in Table II., the conditions of the experiment made it impossible 
to measure the ionization in air at pressures much below 50 millims. At about 
40 millims. pressure a sudden large increase was obtained in the value of the satura¬ 
tion currrent, whch was found to be due to the influence exerted by the applied field 
in breaking down the gas. At these low pressures the electric intensity, which was 
1000 volts a centimetre, was sufficient to dissociate the attenuated gas and to produce 
a discharge on its own account between the electrodes. This was shown by simply 
connecting the electrometer to one of the electrodes, C for example, and applying the 
potential difference without exciting the discharge tube. On then exhausting the 
VOL. CXCV.—A. 
K 
