58 
mr. j. c. McLennan on electrical conductivity in gases 
necessary either to have a constant source of rays, or else to he able to ascertain the 
relative intensities of the rays used with the different gases. 
One method which suggested itself was the use in series of two chambers, such as 
that shown in fig. 1. By inserting a thin aluminium membrane between them a 
different gas could be put in each chamber, and a single pencil of rays could be used 
to produce the ionization in both chambers. With this arrangement it was thought 
that the ionization obtained in the first chamber might perhaps bear a constant 
ratio to that produced in the second. But this relation was not found to hold, and 
further, as the cathode rays are rapidly absorbed, the amount of ionization obtained 
in the first chamber was so very much greater than that in the second, that even 
if the ratio had been fairly constant the method would not have been at all 
satisfactory. 
This led to a trial of two receivers in parallel. Although the cathode rays on 
issuing from the window diverge very greatly, mechanical difficulties made it im¬ 
practicable to receive part of the issuing rays in each chamber, and so recourse was 
had to the use of two windows. With a single large disc as cathode, a stream of 
rays was received in each of the chambers. The ratio of their intensities, however, 
as measured by the ionizations they produced, did not remain constant but varied 
quite irregularly. The explanation of this is probably found in a paper by A. A. C. 
Swinton,* where he points out that the carriers are shot off in a hollow cone from 
the cathode, and that the dimensions of such a cone of rays vary with the degree of 
exhaustion in the tube. Besides, the aluminium windows were opposite to eccentric 
points on the cathode, and the ratio of the intensities of the two pencils was in this 
way greatly influenced by slight variations in the directions of the rays within the 
tube. A cathode formed of two small discs was then tried, and the results obtained 
were very satisfactory. The ratio of the discharges from the windows was in this 
case quite constant, and it was therefore possible to make measurements with con¬ 
fidence. The main difficulty of the investigation was in this way overcome, and the 
method was applied to obtain among other things a knowledge of—1, the absorption 
of the rays ; 2, the ionizations produced by them in air at different pressures ; and 
3, the relative ionizations in different gases. 
6. Description of Apparatus used. 
A diagram of the apparatus is shown in fig. 4, and the way in which the connec¬ 
tions were made is exhibited in fig. 5. The exciting tube was slightly over 
3 centims. in diameter. The two discs of the cathode were each about a centimetre 
in diameter, and they were placed with their centres directly in front of the 
aluminium windows. That portion of the apparatus in which the ionizations were 
* ‘Proc. Roy. Soc.,’ vol. 61, p. 79 (1897). 
