TRAVERSED BY CATHODE RAYS. 
61 
required a potential of only eight volts to excite it, and with the interruptor working 
slowly, this was sufficient to produce sparks of the maximum length in air at normal 
pressure. In practice, the interruptions were made at the rate of 20 to 25 per 
second. 
7. Explanation of the Method adopted for Comparing Ionizations. 
It is well known that, in conduction in Rontgenised gases, and in gases acted upon 
by uranium radiation, the current of electricity obtained does not increase in propor¬ 
tion to the electromotive force applied. The current, after reaching a certain critical 
value, becomes practically stationary, and increases but very little when very large 
increases are made in the electromotive forces. This maximum, or saturation current, 
was also found to characterise the conductivity produced by the passage of cathode 
rays through a gas. With Rontgen or uranium radiation, a field of 400 or 500 volts 
a centimetre has been found to give saturation in most simple gases; but with 
cathode rays it was necessary to apply fields of much stronger intensity. 
As already stated, the distance between either of the electrodes C and D, fig. 5, 
and the dividing partition was about 1‘6 centims. In order to ascertain the saturating 
electromotive force, the plate b was kept at a very high potential, while that of 
a was gradually increased from zero. At each stage the ratio of the currents 
obtained in the two chambers was noted, and it was not until a potential of about 
900 volts was applied to a that an approximation to the saturation current was 
obtained in the chamber A. With a potential difference of 1200 volts the increase 
in the current was small, and an increase only slightly larger was obtained with a 
potential of 1600 volts, or 1000 volts a centimetre. This small increment in the 
current very probably arose from the influence of the field itself. It may be that in 
certain parts of the receiver the rays, acting in conjunction with the applied differ¬ 
ence of potential, had not quite sufficient intensity to produce dissociation. An- 
increase in the field under these circumstances would produce greater ionization, and 
consequently a larger current would be obtained. As this field of 1000 volts a centi¬ 
metre practically produced saturation currents in both chambers, it was used through¬ 
out in measuring the ionizations. Sparking was prevented by using in the charging 
circuit liquid resistances, such as xylol. 
An explanation of the saturation current is that the number of ions used up by the 
current in a given time is exactly equal to the number produced by the rays in the 
same time, or in other words, the ions are removed so rapidly by the applied field 
that recombination is practically eliminated. The saturation current is then a direct 
measure of the ionization produced, and in order to compare the ionizations in any 
two gases, it suffices to measure their saturation currents. In this investigation the 
saturating electromotive force was applied to the plates a and b, the discharge 
tube was then excited, and the currents obtained were used to charge up the con- 
