32 IONIZATION THEORY OF GASES [CH. 
the above equation. Some of these cases will be considered in 
section 47. 
27. The general shape of the current-voltage curves for gases 
exposed to the radiations from active bodies is shown in Fig. 3. 
This curve was obtained for 45 grams of impure radium 
chloride, of activity 1000 times that of uranium, spread over an 
area of 33 sq. cms. on the lower of two large parallel plates, 
4-5 cms. apart. The maximum value of the current observed, 
which is taken as 100, was 1:2 x 10~* amperes, the current for low 
voltages was nearly proportional to the voltage, and about 600 
volts between the plates was required to ensure approximate 
saturation. 
In dealing with slightly active bodies hke uranium or thorium, 
approximate saturation is obtaimed for much lower voltages. 
Tables I. and II. show the results for the current between two 
parallel plates distant 0°5 cms. and 2°5 cms. apart respectively, when 
one plate was covered with a thin uniform layer of uranium oxide. 
TABLE [. TABLE II. 
0:5 cms. apart 2°5 ems. apart 
Volts Current Volts Current 
STS) 18 of) hace 
“25 36 1 14 
5 55 2 7 
il 67 4 47 
2 72 8 64 
4 79 16 ies 
8 85 37°5 81 
16 88 112 90 
100 94 375 97 
335 100 800 100 
The results are shown graphically in Fig. 4. 
From the above tables it is seen that the current at first in- 
creases nearly in proportion to the voltage. There is no evidence 
of complete saturation, although the current increases very slowly 
for large increases of voltage. For example, in Table I. a change of 
voltage from 125 to 25 volts mereases the current from 18 to 
36°/, of the maximum, while a change of voltage from 100 to 335 
volts increases the current only 6°/,. The variation of the current 
per volt (assumed uniform between the range of voltages con- 
sidered) is thus about 5000 times greater for the former change. 
