856 



SCIENCE. 



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



9. A consideration of the results above 

 described led Hertz to the conclusion that 

 the effect must be due to the ultra-violet 

 rays given out by the active spark. He 

 therefore tried the effect of other sources of 

 light, and found that all sources whose 

 spectra are rich in ultra-violet rays produced 

 an increase in the maximum sparking dis- 

 tance of the passive spark. Noticeable 

 effects could be observed with the magne- 

 sium light even at considerable distances. 

 The lime light was somewhat less active, 

 while flames, such as burning gas, benzine, 

 alcohol, CSj, and others, had only a slight 

 influence. No action could be observed 

 with sunlight. 



10. A photographic study of the spectra 

 of the lights used showed not merely that 

 the active rays are beyond the visible re- 

 gion, but that they lie at the extreme 

 end of the photographic spectrum. By far 

 the best source for these experiments ap- 

 peared to be the electric arc. Most of the 

 experiments already described were re- 

 peated by the use of the arc light with com- 

 plete success. 



EARLIER EXPERIMENTS OF WIEDEMANN AND 

 EBERT, AND HALLWACHS. 



The discovery of Hertz seems to have 

 attracted immediate attention. Early in 

 1888, only a few months after the publica- 

 of Hertz's work, two articles on the subject 

 appeared simultaneously in Wiedemann's 

 Annalen. 



11. One of these articles, by Wiedemann 

 and Ebert,* described experiments similar 

 in character to those of Hertz, but per- 

 formed with somewhat different apparatus. 

 In place of the induction coil a Holtz ma- 

 chine was used. This proved to be more 

 convenient, as well as more certain in its 

 action. Ultra-violet rays were furnished 

 by an arc lamp. The spark gap was en- 



*Wied. Ann. 33, p. 240, 1888. Abstract in 

 Phil. Mag. 25, p. 162. 



closed in a glass tube containing a quartz 

 window, so that experiments could be con- 

 veniently performed with different gases 

 and at different pressures. The terminals 

 were platinum spheres 3 mm. in diameter. 

 A convenient means of measuring the ef- 

 fect was furnished by a second spark gap in 

 parallel with the one studied, but screened 

 from the rays of the arc. This could be 

 adjusted until sparks passed across it rather 

 than at the passive spark gap ; the change 

 required in this adjustment when the pas- 

 sive spark was illuminated then gave a 

 measure of the effect produced by the rays. 

 With this apparatus the more important 

 experiments of Hertz were repeated, and 

 his results confirmed. 



12, Wiedemann and Ebert next investi- 

 gated the following question : Do the ul- 

 tra-violet rays merely assist the commence- 

 ment of sparking, or is the action a contin- 

 uous one ? A Geissler tube placed in series 

 with the active spark showed that the ef- 

 fect was continuous. When the spark was 

 illuminated the Geissler discharge had all 

 the characteristics that are possessed by 

 such discharges when the tube is directly 

 connected with the machine, t. e., the illu- 

 minated spark gap acted much as a contin- 

 uous conductor. As soon as the rays from 

 the arc were cut off, the discharge became 

 discontinuous, such as is usually observed 

 when the connection to the machine con- 

 tains a gap. A telephone placed in series 

 with the spark gap gave a noticeably higher 

 tone when the gap was illuminated. It 

 thus appears that ultra-violet rays reduce 

 the resistance offered by the spark gap to a 

 disruptive discharge, and that this action 

 continues as long as the illumination lasts. 



13. To investigate what part of the arc 

 is most effective in producing the rays 

 needed, an image of the arc was thrown on 

 the passive gap by means of a quartz lens. 

 The effect was found to be greatest when 

 the rays came from the positive carbon. 



