Jrom Faraday to J . J . Thomson* 399 



the opposite direction to the ijathode. Thus, in the neighbour- 

 hood 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 

 vessel ; 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,f 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 



* Proc. Roy. Soc. xlii (1887), p. 371. Hittorf had discovered that very small 

 electromotive forces are sufficient to cause a discharge across a space through which 

 the cathode radiation is passing. 



t Berlin Ber., 1887, p. 487 ; Ann. d. Phys. xxxi (1887), p. 983 ; Electric Waves 

 (English ed.), p. 63. 



I Cf.p. 357. 



By E. Wiedemann and Ebert, Ann. d. Phys. xxxiii (1888), p. 241. 



|| Ann. d. Phys. xxxiii (1888), p. 301. 



