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. A 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 S. 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. 
