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 Both Geitel and Wilson found that the leakage of the insulated system in dust-free air was the same for a positive as for a negative charge, and was independent of the potential over a considerable range. The leakage was the same in the dark as in diffuse daylight. The independence of leakage of the potential is strong evidence that the loss of charge is due to a constant ionization of the air. When the electric field acting on the gas exceeds a certain value, all the ions are carried to the electrodes before recombination occurs. A saturation current is reached, and it will be independent of further increase of the electric field, provided, of course, a potential sufficiently high to cause a spark to pass is not applied.

C. T. R. Wilson has recently devised a striking experiment to show the presence of ions in dust-free air which is not exposed to any external ionizing agency. Two large metal plates are placed in a glass vessel connected 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