Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/419

 vessel; the opposite direction to the cathode. Thus, in the neighbourhood of the positive discharge, the current was flowing in two opposite directions at closely adjoining places; which could scarcely happen unless the current in one direction were carried by particles moving against the lines of force by virtue of their inertia,

Continuing his researches, Schuster showed in 1887 that a steady electric current may be obtained in air between electrodes whose difference of potential is but small, provided that an independent current is maintained in the same that is to say, a continuous discharge produces in the air such a condition that conduction occurs with the smallest electromotive forces. This effect he explained by aid of the hypothesis previously advanced; the ions produced by the main discharge become diffused throughout the vessel, and, coming under the influence of the field set up by the auxiliary electrodes, drift so as to carry a current between the latter.

A discovery related to this was made in the same year by Hertz, in the course of the celebrated researches which have been already mentioned. Happening to notice that the passage of one spark is facilitated by the passage of another spark in its neighbourhood, he followed up the observation, and found the phenomenon to be due to the agency of ultra-violet light emitted by the latter spark. It appeared in fact that the distance across which an electric spark can pass in air is greatly increased when light of very short wave-length is allowed to fall on the spark-gap. It was soon found that the effective light is that which falls on the negative electrode of the gap; and Wilhelm Hallwachs extended the discovery