INDEXED 



T H E 

 LONDON, EDINBURGH!, and DUBLIN" 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[SIXTH SERIES.] 



APRIL 1911. 



XLVII. On the Ultra-Violet Light from the Mercury Arc. 

 By A. Ll. Hughes, M.Sc, B.A., Research Scholar of 

 Emmanuel College, Cambridge *. 



1. 



THE velocities of electrons emitted from surfaces illu- 

 minated by ultra-violet light have been investigated 

 by Lenard f , Ladenburg J, and Hull§. Ladenburg dis- 

 covered that the maximum velocity of the electrons was 

 proportional to the frequency of the light used. The range 

 over which he worked was from X 26*00 to A, 2010. Hull 

 was able to show that the law held down to X 1230. 



In all these experiments, the source of light and the illu- 

 minated plate were separated by a window of quartz or 

 fluorite, so that the shortest wave-length which could be 

 employed was determined by the transparency of the window. 

 The maximum velocity of the electrons can only be obtained 

 when the illuminated plate is in a very good vacuum. As 

 the sources of light usually employed (the spark, or the 

 discharge in hydrogen) require the presence of some gas, the 

 vessel containing the illuminated plate must be separated 

 from the source of light by a window of quartz or other 

 transparent material. The shortest wave-length transmitted 



* Communicated by Professor Sir J. J. Thomson. 

 t Lenard, Ann. der Pkys. viii. p. 149 (1902). 

 % Ladenburg, Phys. Zeits. ix. p. 821 (1908). 

 § Hull, Phys, Zeits. x. p. 537 (1909). 



Phil Mag, S. 0. Vol. 21. No. 124. April 1911. 2 D 



