174 Mr. D. C. H. Florance : A Study of the Ionization 



A balance method, similar to that described bj Townsend 

 (Phil. Mag. May 1903), was used in these experiments for 

 measuring the ionization currents. This method was found 

 to be far more accurate and certain than the ordinary rate 

 method. A Dolazalek electrometer of sensibility about 

 400 mm. divisions per volt was used. The experimental 

 arrangement is shown in fig. 1. 



Variation of Current ivith Voltage. 



It is important that at each given pressure the saturation 

 current should be measured ; but it was found quite impos- 

 sible to obtain complete saturation, even at low pressures, with 

 a voltage of 2200. In fig. 2 (PI. I.) the saturation curves for 

 the ionization due to the /3 rays from Ur X are shown when 

 the pressure in the ionization-chamber was raised from 5 to 

 70 atmospheres, and the voltage was increased from 80 ta 

 1800 volts. Although for no pressure was complete satura- 

 tion obtained, it should be noticed that all the curves are of the 

 same character, and that there is a constant ratio between 

 the ionization currents at any two given pressures when the 

 voltage is constant. It would therefore appear that the true 

 saturation current at any pressure is a definite multiple of 

 the ionization current measured for a particular voltage* 

 The shape of the saturation curves due to ft rays at high 

 pressures resembles those obtained for a rays* at ordinary 

 pressure. There appears to be the same difficulty in 

 obtaining complete saturation. The saturation curves ob- 

 tained when 7 rays were used as the source of ionization 

 were similar to those obtained for the /3 rays of UrX. 



An experiment was then carried out to see whether the 

 saturation curves depended on the activity of the source. 

 Two definite pressures, 5 and 25 atmospheres, were chosen. 

 The saturation curves were plotted when the voltage was 

 increased from 80 to 1800 volts. By a suitable adjustment 

 of the lead screens in front of the radium, the ionization 

 current for a given voltage and pressure was doubled. It 

 was found that the ionization current was also doubled for 

 all voltages at both pressures. 



The numbers quoted by Laby and Kaye (Phil. Mag. Dec 

 1908) seem to support the above results, except that they 

 got complete saturation at 1500 volts for a pressure of 

 8 atmospheres and at 3500 volts for a pressure of 15 atmo- 

 spheres. The writer had not available higher voltages than 

 2200, and was unable in any case to obtain evidence of com- 

 plete saturation. In the experiments to be described later,. 



