Page:Radio-activity.djvu/69

 '''35. Difference between the positive and negative ions.''' In the course of experiments to determine the charge carried by an ion, J. J. Thomson observed that the cloud formed under the influence of X rays increased in density when the expansion was about 1·31, and suggested in explanation that the positive and negative ions had different condensation points.

Fig. 7.

This difference in behaviour of the positive and negative ions was investigated in detail by C. T. R. Wilson in the following way. X rays were made to pass in a narrow beam on either side of a plate AB (Fig. 7) dividing the condensation vessel into two equal parts. The opposite poles of a battery of cells were connected with two parallel plates C and D, placed symmetrically with regard to A. The middle point of the battery and the plate A were connected with earth. If the plate C is positively charged, the ions in the space CA at a short distance from A are all negative in sign. Those to the right are all positive. It was found that condensation occurred only for the negative ions in AC when v_{2}/v_{1} = 1·25 but did not occur in AD for the positive ions until v_{2}/v_{1} = 1·31.