﻿EQUIPMENT AND WORK OF AN AERO-PHYSICAL OBSERVATORY. 15 



influence of the above-described radiation more rapidly than a body 

 charged positively. For example : 



Rate of loss Rate of loss 



when positively when negatively 



electrified. electrified. 



Rusty iron. 16 24 



Polished iron 16 31 



Charcoal 11 25 



Mica 12 26 



The above results show clearly a more rapid loss of the negative 

 charge. 



Professor Oliver Lodge, in a lecture upon the work of Hertz, delivered 

 at the Royal Institution June 1, 1894, used the following language: 



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

 citing 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 pri- 

 mary fell on its knobs. He examined this new influence of light in 

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

 light were peculiarly efl'ective, any source of light that gave very. ultra- 

 violet rays produced the same result. Wiedemann and Ebert and a 

 number of experimenters have repeated and extended this discovery, 

 proving that it is the cathode knob on which illumination takes efl'ect ; 

 and Hallwachs made the important observation, which Righi, Stoletow, 

 Braly, and others have extended, that a freshly polished zinc or other 

 oxidizable surface, if charged negatively, is gradually discharged by 

 ultra-violet light." 



Lodge hints in his lecture of the possible great value of these relations 

 in atmospheric electricity. 



Collecting the observations of the potential fall in the case of nega- 

 tively electrified bodies exposed to sunlight, Elster and Geitel have tab- 

 ulated the values of the mean daily potential, the temperature, and the 

 vapor pressure. These are given on the following page. Would it not 

 be a very happy and profitable research to undertake in the United 

 States a similar series of experiments confirming and extending the re- 

 sults now at hand ? Particularly at Pike's Peak and at Mount Washing- 

 ton could such determinations be efl'ectively undertaken. At the former 

 station the range of vapor pressure is so large that the curves now known 

 could be extended. A physical law expressing the relation of the poten- 

 tial and the intensity of the more refrangible ra37s would perhaps be the 

 outcome. 



