After studying the changes which take place in the conductivity of gases through which ions are passing under various conditions, I was led to propose the theory of ionization by collision to explain the develop ment of currents in gases. The accuracy of the theory has been established by a large number of experiments specially arranged to measure conductivities which could be compared with the values obtained from theoretical considerations. The researches have brought to light the fact that ionization by collision takes place when comparatively small potential differences are established between electrodes in a gas at a suitable pressure, so that considerable multiplication of the ions may be obtained by this process with forty or fifty volts, and consequently with comparatively small velocities of the ions.
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