472 Mr Baker, Influence of Ultra Violet Radiation 



Note on the Influence of Ultra Violet Radiation on the Dis- 

 charge in a Vacuum Tube having a polished Zinc Electrode. By 

 Will C. Baker, 1851 Exhibition Scholar, Queen's University, 

 Kingston, Ont. 



[Read 19 May 1902.] 



Warburg 1 , from his researches upon the influence of radiation 

 on the spark discharge between metal spheres, concluded that 

 although ultra violet light falling on the kathode reduces the 

 "period of delay" (Verzogerungsperiode), it does not sensibly alter 

 the minimum potential difference required to start a spark. In 

 the cases of zinc and brass, however, no satisfactory measurements 

 could be made owing to complications resulting from the Hall- 

 wachs effect. 



Kreussler 2 found that when a kathode of a metal showing this 

 Hallwachs effect, is raised to a potential but slightly below that 

 required to start a spark to an anode a millimeter away, the 

 effect of ultra violet light falling on it is to liberate a current 

 the magnitude of which rapidly increases as the potential differ- 

 ence between kathode and anode approaches the sparking value. 



In view of these results Professor J. J. Thomson suggested 

 the advisability of measuring the "negative drop" in a tube, the 

 polished zinc kathode of which would be illuminated with ultra 

 violet light, in order to determine whether the radiation produced 

 any measurable change. 



A tube was constructed as shown in the figure. Q is a window 

 of quartz, 2 mm. thick, opposite which (at V) was placed the 

 source of ultra violet light. The anode A is of copper, and is 

 set in a side tube so as not to obstruct the radiation that was 

 to fall on the kathode. N is a search needle, cased in glass to 

 within 6 mm. of its point. The tube terminated in a piece of 

 smaller tubing shown at 8. The zinc kathode K was firmly 

 fixed in a second bit of tubing T, that closely fitted inside S, and 

 which could be slid out to permit the zinc to be polished. A small 



1 Wied. Ann. Bd. lix. no. 1. 



2 Ann. der Physik, Bd. v. no. 10, 1901. 



