854 



SCIENCE. 



[N. S. Vol. IV. No. 102. 



ments of Hertz upon electric waves have 

 given to the electro-magnetic theory of light 

 a standing almost equal to that of the most 

 firmly established physical theories. 



In 1887 still another class of electro- 

 optical effects was discovered. It was then 

 found that the passage of a spark between 

 the terminals of an induction coil takes 

 place more readily when the negative elec- 

 trode is illuminated by ultra-violet light. 

 The original observation, which was made 

 by Hertz in the course of his experiments 

 on electric waves, has since been abundantly 

 confirmed. It appears that ultra-violet 

 rays possess the power of assisting the dis- 

 charge of negatively charged bodies, the 

 magnitude of the effect being dependent 

 upon the surrounding medium as well as 

 upon the nature of the charged surface. 

 Within the last few years Elster and Geitel 

 have shown that the same effect is in some 

 cases produced by the visible rays. The phe- 

 nomenon has gained additional interest from 

 the discovery by Lenard that a similar dis- 

 charging power is possessed by the cathode 

 rays, and this similiarity has been cited by 

 Jaumann in an attempt to develop a theory 

 of cathodic radiation. Still more recently it 

 has been found that the effect is produced 

 by the X-rays in yet more marked degree. 



In spite of the comparatively short time 

 that has elapsed since Hertz's original dis- 

 covery, the literature of the subject is quite 

 extensive. Strangely enough, only a few 

 of the more important papers have been 

 publshed in English, so that it is a matter 

 of some inconvenience to learn just how far 

 our knowledge of the phenomenon has pro- 

 gressed. For this reason, and in view of 

 the evident importance of the subject, the 

 writer has prepared the following account 

 of the investigations which have thus far 

 been published. ^^ 



* For a very brief account, see J. J. Thomson, Re- 

 cent Researches in Electricity and Magnetism, p. 

 58-62. 



DISCOVERY BY HERTZ. 



1 . As already mentioned, the phenome- 

 non was first observed by Hertz ^ in 1887. 

 The discovery was made during the prog- 

 ress of some of his earlier experiments 

 upon electro-magnetic waves, and was 

 purely accidental. In investigating the 

 field near a Hertzian oscillator by means 

 of an ordinary resonator some difiiculty 

 was met with in observing the faint sparks 

 which occurred at the resonator. With 

 the thought that observations could be 

 more readily made in the dark, the re- 

 sonator spark gap was surrounded by a 

 box of non-conducting material; but it was 

 found that the sparks were now fainter 

 than before, and that the maximum spark- 

 ing distance was noticeably diminished. 

 On removing successively different parts 

 of the box it appeared that only those por- 

 tions lying between the primary and sec- 

 ondary spark were effective. The distance 

 of these portions from the spark gap was 

 immaterial ; if the secondary spark was ad- 

 justed to its maximum length the interposi- 

 tion of a screen at any point between the 

 primary and secondary spark caused the 

 latter to cease. 



2. At first thought it doubtless appeared 

 natural to ascribe the effect to some electro- 

 static or electrodynamic action. Hertz was 

 quickly convinced, however, that this could 

 not be the true explanation, for the screen- 

 ing effect was produced as well by insula- 

 tors, such as rubber, glass and parafi&n, as 

 by conductors. Again, although the phe- 

 nomenon was first noticed when the sparks 

 used were those of an oscillator and resona- 

 tor, yet equality in the vibration periods of 

 the two sparks was not essential. When 

 two sparks were simultaneously produced 

 by any other method the effect was still 

 observed. 



3. The most convenient means of study- 



* Berliner Akademie, June 9, 1887. Wied. Ann. 

 31, 983. 



