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mr. j. c. McLennan on electrical conductivity in gases 
densers G 'and H. The discharge having been stopped, the potentials of the two 
condensers were then successively determined. 
As the effective capacity of the electrometer was the same fraction of that of each 
of the two equal condensers, the deflection readings were direct measures of the 
charges obtained. The charging of both condensers proceeded for the same time, 
and consequently the electrometer deflections were also direct measures of the 
saturation currents, and therefore of the ionizations in the two chambers. 
The method possessed the advantage of being independent of the time of charging 
and of the strengths of the rays coming from the two windows, provided only that 
the ratio of their intensities remained constant. In using the electrometer the 
needle was kept at a high potential, and one pair of quadrants always connected to 
earth. Though with this arrangement slow losses from the needle occurred, yet the 
short interval required for the two readings made the gradual change in the effective 
capacity of the electrometer inappreciable. 
In practice, the electrometer was initially connected to one of the condensers, and 
the tube allowed to run until a suitable deflection was obtained. After noting this 
reading, the electrometer, having been put to earth, was then connected to the other 
condenser and the second reading taken. In this way the ratio of the ionizations in 
the two chambers was obtained. 
From the experiments described in Section 2, it is clear that the signs of the 
charges obtained in the condensers depended on the signs of the charges given to 
a and b by the battery. In case these plates were positively charged, the 
charges collected were positive, and were due entirely to ionization. With a nega¬ 
tive field, however, the negative charges obtained included not only negative ions 
produced by the rays, but also the negative carriers, constituting the rays, that were 
stopped in their motion by the gas. For this reason the positive field was always 
used, and consequently the charges obtained gave a measure of the number of ions 
produced in the gas by the passage of the rays. 
8. Ionization in different Gases at the Same Pressure. 
To compare the ionization in a selected gas with that in air at the same pressure, 
the saturating electromotive force was applied to the plates a and b, fig. 5. The 
two chambers A and B were first filled with air at atmospheric pressure, and a series 
of readings taken, the mean of which gave the ratio of the saturation currents in 
the two chambers. The air was then removed from A, and the gas to be tested 
introduced. A set of readings similarly taken gave a ratio for the saturation 
current obtained with the given gas in A, compared with that obtained with air in 
B. The combination of these results gave the ratio of the saturation current in A, 
when filled with the given gas, to that in the same chamber when filled with air. 
This ratio was, consequently, the ratio of the ionization produced in the selected gas 
