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Some experiments by Prof. J. S. Townsend on the lateral diffusion of a narrow stream of ions moving in an electric field led to the conclusion that negative ions in perfectly dry air are much smaller than those in air containing a small quantity of moisture. It was consequently to be expected that the complete removal of water vapour would cause an increase in the velocity with which negative ions move under the influence of an electric field of force. At his suggestion the following investigation of the velocities of ions in air at low pressures was undertaken, and it was found that, while the complete removal of water vapour had only a small effect on the velocities of positive ions, yet the same cause increased the velocities of the negative ions by a factor varying between 2 and 30 for the range of pressures and electric forces used in the experiments. The method adopted was to let the ions travel between two gauzes under a known electric force for a timet and then to reverse the field. Ifv is the velocity of the ions andd is the distance between the gauzes, then ions starting from one gauze will reach the other ift ≮d /v . Ift is gradually decreased, it is possible to find, by means of an electrometer, when ions cease to reach the second gauze; when this happensv =d /t .

The effect of small traces of water vapour on the velocities of ions produced by Röntgen rays in air