Science IJrogres 



No. 15. May, 1895. Vol. III. 



LIGHT AND ELECTRIFICATION. 



IF an ordinary gold-leaf electroscope with a freshly 

 scraped zinc cap or knob is charged with negative 

 electricity, and is then exposed to the unfiltered rays of 

 a source of ultra-violet light, its charge will leak away 

 rapidly. 



If, further, the electroscope so treated be extremely 

 sensitive, it will be found not only to lose its negative 

 charge, but to acquire a slight trace of positive charge 

 under the action of lio-ht. 



These are the simple and fundamental facts of what is 

 virtually a new subject or branch of physics, the subject 

 of photo-electric action, the influence of illumination on 

 electrified surfaces. 



SUMMARY HISTORICAL SKETCH. 



The above facts were discovered by Hallwachs, but their 

 discovery originated in an observation made by Hertz, that 

 the sparking of an induction coil was facilitated when the 

 light of another spark fell upon its secondary terminal. 

 Hertz proved conclusively that this was a real action of 

 light, and not due to some more direct electric action, nor 

 yet to any convection of chemical or dissociated products 

 from one spark to the other, such as Schuster and others 

 have found to be likewise a real and genuine effect having 

 influence on some forms of electric discharge. Hertz also 

 tried other sources of light, and by passing the rays through 



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