76 
mr. j. c. Mclennan on electrical conductivity in gases 
An experiment somewhat analogous to this is described in Section IX. The 
apparatus used is shown in figs. 4 and 5. The diameters of the electrodes C and I) 
were each about 1 centim. and, as already stated, the distance between the window 
and the centre of each of the chambers was about 2 centims. 
By applying the equation (2) to this experiment, and taking r — 1'5 centims. and 
d — 1 centim., it follows that the saturation current would be a maximum when 
e* = 1*66 . . . 
or A. = - 5. 
From Lenard’s values, Table IX., it will be seen that this value corresponds 
approximately to a pressure of about 120 millims. of mercury. The observed results, 
however, Table II. and fig. 6, indicate a maximum of about 75 centims. Further, 
the calculated values of the current from equation (l) exhibit a more rapid rise than 
that actually observed. 
But the difference in the results is not surprising. The field within the receiver 
was far from uniform, being disturbed by the proximity of the walls of the chamber. 
The presence of the narrow tube through which the rays were conducted into the 
receiver also produced irregularities. On this account it was impossible to define, 
even approximately, the region from which the saturation current was drawn. 
Moreover, the actual paths of the rays, as Lenard has pointed out, are largely 
influenced by the pressure of the gas traversed. Even at best, then, the calculated 
results can scarcely be regarded as more than a rough approximation. 
12. Summary of Residts. 
1. The conductivity impressed upon a gas by cathode rays is similar to that 
produced by Rontgen and uranium rays, and can be fully explained on the hypothesis 
that positive and negative ions are produced by the radiation throughout the volume 
of the gas traversed. 
2. When cathode rays are allowed to fall upon insulated metallic conductors 
surrounded by air at atmospheric pressure, 
(a.) such conductors if initially uncharged gain a small limiting negative charge, 
( b .) positive charges are completely dissipated, 
(c.) negative charges drop to a small limiting value, 
( d .) the loss of charge is due to the action of the ionized air surrounding the 
conductor, and the value of the limiting negative charge is determined by 
the extent of the conduction in this air. 
3. The ionization produced in a gas by rays coming from the aluminium window 
in a Lenard discharge tube is due to cathode rays and not to Rontgen rays. 
