650 Mr. G. Shearer on the 



lines obtained in this way measure the relative ionizations of 

 the gases in the chamber. 



Beatty observed the variation of ionization for pressures 

 up to one atmosphere ; in these experiments, however, pre- 

 liminary observations showed that the ionization pressure 

 curves did not become straight until a pressure over half an 

 atmosphere was reached ; it was, therefore, finally decided 

 to observe from one to two atmospheres. A much longer 

 range of pressures from which to estimate the gradient was 

 thus obtained, and by this means the accuracy possible was 

 increased. In addition, any slight leak of the apparatus at 

 pressures less than atmospheric meant that air was entering 

 the chamber, and the admission of even a very small quantity 

 of air had a very appreciable effect on the resulting measure- 

 ments. On the other hand, by using pressures greater than 

 atmospheric, a small leak was of much smaller importance, 

 as the tendency was for hydrogen to escape and not for air 

 to enter. 



Since the ionization of hydrogen is very small, an ordinary 

 electroscope was too insensitive for its measurement, and in- 

 stead a Dolezalek electrometer was used with a sensitiveness 

 of about 700 divisions to the volt. When using the electro- 

 meter for measurements of the ionization of air, the voltage 

 in the needle was reduced so as to give a convenient rate of 

 leak, and the sensitiveness of the electrometer was compared 

 with that under the previous conditions in order to make 

 the readings of the two ionizations comparable. 



Fig. 3 gives a diagram of the apparatus used. The ioni- 

 zation chamber A was a brass cylinder of length 12 cm. 

 and diameter 10 cm., with a circular window 5 cm. in 

 diameter. The chamber was lined with thick aluminium to 

 absorb the characteristic X-radiations from the brass walls r 

 while the corpuscular emission was reduced and the soft 

 secondary radiation from the aluminium absorbed by covering 

 the whole of the inside with filter-paper. The window 

 through which the rays entered was at first of thin aluminium, 

 but later of mica, thin mica being found more capable of 

 withstanding the effect of the difference of pressure on its sides 

 than thin aluminium. As it had to stand pressures in both 

 directions the mica window was screwed down between two 

 brass rings, the whole being made gas-tight by the use of 

 wax. An electrode of fine aluminium wire entered the 

 chamber through an ebonite plug provided with an earthed 

 guard-ring, while wires in earthed brass tubes connected the 

 electrode with the electrometer. In order to allow for 

 variations in the radiation from the bulb a standardizing 



