35. Difference between the positive and negative ions.
In the course of experiments to determine the charge carried by an ion, J. J. Thomson[1] 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.
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Fig. 7.
This difference in behaviour of the positive and negative ions was investigated in detail by C. T. R. Wilson[2] 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.