Page:Radio-activity.djvu/556

 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 × 10^{-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 zinc 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, Harms , and 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 × 10^{-10} electrostatic units:

++-++ ++-++ ++-++
 * | Number of ions |                    |
 * Material of vessel|produced per c.c.|     Observer      |
 * |  per second    |                    |
 * Silvered glass   |       36        |  C. T. R. Wilson   |
 * Brass            |       26        |     "       "      |
 * Zinc             |       27        |Rutherford and Allan|
 * Glass            |    53 to 63     |       Harms        |
 * Iron             |       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.

285. Effect of pressure and nature of gas. C. T. R. Wilson (loc. cit.) found that the rate of leakage of a charged conductor