with an expansion apparatus similar to that described in section 34. On expanding the air, the presence of the ions is shown by the appearance of a slight cloud between the plates. These condensation nuclei carry an electric charge, and are apparently similar in all respects to the ions produced in gases by X rays, or by the rays from active substances.
Wilson found that the loss of charge of the insulated system was independent of the locality. The rate of discharge was unaltered when the apparatus was placed in a deep tunnel, so that it did not appear that the loss of charge was due to an external radiation. From experiments already described, however (section 279), it is probable that about 30 per cent. of the rate of discharge observed was due to a very penetrating radiation. This experiment of Wilson’s indicates that the intensity of the penetrating radiation was the same in the tunnel as at the earth’s surface. Wilson found that the ionization of the air was about the same in a brass vessel as in one of glass, and came to the conclusion that the air was spontaneously ionized.
Using a brass vessel of volume about 471 c.c., Wilson determined the number of ions that must be produced in air per unit volume per second, in order to account for the loss of charge of the insulated system. The leakage system was found to have a capacity of about 1·1 electrostatic units, and lost its 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 Allan57 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 Patterson58, Harms59, and Cooke60. 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:
| Material of vessel | Number of ions produced per c.c. per second | Observer |
|---|---|---|
| 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 varied approximately as the pressure of the air between the pressures examined, viz. 43 mms. and 743 mms. of mercury. These results point to the conclusion that, in a good vacuum, a charged body would lose its charge extremely slowly. This is in agreement with an observation of Crookes, who found that a pair of gold-leaves retained their charge for several months in a high vacuum.