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The passage of electricity from a metal to a gas, or from a gas to a metal, occurs in a manner not yet well understood. The present investigation was undertaken to determine what conditions of the metal or gas favor the transfer of electricity from one to the other, and, if possible, to throw some light on the mechanism of surface conduction. While the results here presented are crude, they bear directly on the theory of metal-gas conduction, and the method, though very simple, is believed to be new. The work suggested itself during some spectroscopic work with Plücker tubes.

If we use a direct current in studying the electrode drop in potential, we are confronted by the the great difficulty in keeping the condition of the surface, temperature, pressure, etc., constant, and in reproducing these conditions in successive tests. But if we use an alternating current of a maximum voltage just a little above the maximum total drop in potential between electrodes, conditions can not appreciably change between successive alternations, and the excess of current in one direction

may be taken as a rough measure of the relative surface conductivity at the two electrodes. It is hardly conceivable that there can be any rectifying effect due to the gas itself away from the electrodes, for this would require not only molecular or atomic dissymmetry, but a fixed orientation of these particles. Hence we must regard rectification as due to conditions existing at the metal-gas surface and in the immediately adjacent metal and gas. The general form assumed by a single-phase alternating current wave when a conducting gas is included in the circuit is discussed later on in relation to the dissymmetry of the wave caused by partial rectification.

The greater part of the work here described was done with the current from a transformer of 100:2,000 volts and 600 watts capacity. Occasionally some of the work was repeated on smaller 100: 1,000 volt transformers of 100 or 300 watts capacity. To control the current in the secondary a variable noninductive resistance of about 100 ohms was placed in the primary of the transformer. The current in

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