Page:Radio-activity.djvu/531

 has been observed by Sarasin, Tommasina and Micheli for a wire made active by exposure to the open air. This difference in the rates of discharge for positive and negative electricity is probably connected with the presence of particles of dust or small water globules suspended in the gas. The experiments of Miss Brooks (section 181) have shown that the particles of dust present in the air containing the thorium emanation become radio-active. A large proportion of these dust particles acquire a positive charge and are carried to the negative electrode in an electric field. This effect would increase the rate of discharge of the electroscope when charged negatively. In later experiments, Ebert and Ewers noticed that, in some cases, when the air had been kept in the vessel for several days, the effect was reversed, and the electroscope showed a great rate of discharge when charged positively.

Fig. 103.

J. J. Thomson has observed that the magnitude of the ionization current depends on the direction of the electric field, if fine water globules are suspended in the ionized gas.