


376 RADIO-ACTIVITY OF THE ATMOSPHERE [CER 
charge at the rate of 4°1 volts per hour for a potential of 210 volts, 
and 4:0 volts per hour for a potential of 120 volts. Taking the 
charge on an ion as 3'4 x 10~ electrostatic units, this corresponds 
to a production of 26 ions per second. 
Rutherford and Allan’ repeated the results of Geitel and 
Wilson, using an electrometer method. The saturation current 
was observed between two concentric zine cylinders of diameter 
25°5 and 7°5 cms. respectively and length 154 cms. It was found 
that the saturation current could practically be obtained with a 
potential of a few volts. Saturation was however obtained with 
a lower voltage after the air had remained undisturbed in the 
cylinders for several days. This was probably due to the gradual 
settling of the dust originally present in the air. 
Later observations of the number of ions produced in air in 
sealed vessels have been made by Patterson?, by Harms’, and by 
Cooke’, The results obtained by different observers are shown 
in the following table. The value of the charge on an ion is taken 
as 3:4 x 10-” electrostatic units : 






| Number of ions 
| Material of vessel produced per c.c. | Observer 
| per second 
Silvered glass... 36 C. T. R. Wilson 
BYSSs)* vas Ape 26 | 3 ‘ 
Zine M Wee 27 Rutherford and Allan 
Glass... «ds ee 53 to 63 Harms 
Tron $c ae 61 | Patterson 
| Cleaned brass... 10 | Cooke 
| | 

It will be shown later that the differences in these results are 
probably due to differences in the radio-activity of the containing 
vessel. 
219. Effect of pressure and nature of gas. C. T. R. Wilson 
(loc. cit.) found that the rate of leakage of a charged conductor 
varied approximately as the pressure of the air between the pres- 
sures examined, viz. 43 mms. and 743 mms. of mercury. These 
results point to the conclusion that, in a good vacuum, a charged 
1 Phil, Mag. Dec. 1902. 2 Phil. Mag. August, 1903. 
3 Phys. Zeit. 4, No. 1, p. 11, 1902 4 Phil. Mag. Oct. 1903. 
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