1894.] 



on the Work of Hertz. 



327 



not always such as we should use now, but himself growing in 

 theoretic insight through the medium of what would have been to 

 most physicists a confusing maze of troublesome facts, and disen- 

 tangling all their main relations most harmoniously. 



While Hertz was observing sparks such as these, the primary or 

 exciting spark and the secondary or excited one, he observed as a bye- 

 issue that the secondary spark occurred more easily if the light from 

 the primary fell upon its knobs. He examined this new influence of 

 light in many ways, and showed that although spark light and electric 

 brush light were peculiarly effective, any source of light that gave 

 very ultra-violet rays produced the same result. 



Fig. 7. 



A 



_2El3X 



Alternative P?' 



Experiment arranged to show effect on one spark of light from another. 

 The B spark occurs more easily when it can see the A spark 

 through the window, unless the window is glazed with glass. A 

 quartz pane transmits the effect. 



The above figure represents my way of showing the experiment. 

 It will be observed that with this arrangement the B knobs are at the 

 same potential up to the instant of the flash, and in that case the 

 ultra-violet portion of the light of the A spark assists the occurrence 

 of the B spark. But it is interesting io note, what Elster and Geitel 

 have found, that if the B knobs were subjected to steady strain instead 

 of to impulsive rush — e. g. if they were connected to the inner coats of 

 the jars instead of to the outer coatings — that then the effect of ultra- 

 violet light on either spark-gap would exert a deterrent influence, so 

 that the spark would probably occur at the other, or non-illuminated 



