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VIL On the Emission Velocities of Photo-Electiwis. 
By A. Ll. Hughes, M.Sc., B.A., Research Scholar of Emmanuel College, Cambridge. 
{Communicated by Prof Sir J. J. Thomson, O.M., F.R.S.) 
Eeceived May 21,—Eeacl June 13, 1912. 
Contents. 
Page 
1. Introduction. 
2. Previous work. 
3. Experiments with surfaces formed in vacuo . . 
4. The source of light. 
5. The apparatus. 
6. Velocity distribution curves. 
7. Experimental results with monochromatic light 
8. Discussion of results. 
9. Variations in k and Vq with atomic volume . . 
10. Summary. 
1. Introduction. —Since the discovery of the photo-electric effect by Hertz, many 
experiments have been made on the emission of negative electricity from metallic 
surfaces when illuminated by light. Yet with regard to many important points the 
results are often indefinite and contradictory. Most theories of the photo-electric 
effect indicate definite relations between the velocity of emission of the electrons and 
{a) the nature of the metal from which they are emitted, and {b) the wave-length of 
the incident light. Up to the present, however, the experimental evidence as to 
these two relations must be regarded as quite inadequate to afford anj^ decisive test 
between rival theories. This research was undertaken to obtain, among other things, 
quantitative evidence on these two relations. 
2. Previous Work. — Ladenburg* made some valuable experiments on the velocity 
with which electrons are emitted from metals when illuminated by ultra-violet light. 
He concluded that the maximum emission velocity was inversely proportional to the 
wave-length. The velocity varied from metal to metal; thus, for light of wave¬ 
length X2010, the maximum emission velocity (measured in volts) for platinum was 
* Ladenburg, ‘Phys. Zeits.,’ VIII., p. 590, 1907. 
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VOL. CCXII.-A 490. 
Published separately, August 12, 1912 
