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LXIX. On the Velocities with which Plioto-Electrons are 

 emitted from Matter. By A. Ll. Hughes, D.Sc, B.A., 

 Mackinnon Student of the Royal Society; Emmanuel College, 

 Cambridge *. 



INVESTIGATIONS by Professor 0. W. Richardson and 

 Mr. Compton f and by the writer { have shown that 

 metallic surfaces, when illuminated by ultra-violet light, emit 

 electrons whose maximum energy is a linear function of the 

 frequency. It was shown that the maximum energy, and 

 not the maximum velocity, was proportional to the frequency. 

 Most of the photo-electrons emerge from the surface with 

 velocities appreciably less than the maximum, on account of 

 loss .of energy by collisions in their journey from the interior. 

 Those which possess the maximum velocities have, presum- 

 ably, not lost any energy in this way. If V be the potential 

 required to stop the fastest electrons, and e the charge, then 

 the experimental relation between the maximum energy Ye 

 and the frequency n is 



Ye = h'n-Y e, (1) 



where h' and V are experimental constants. This is also 

 the type of formula suggested by the quantum theory. If h 

 be Planck's constant, then hn is the unit of energy associated 

 with the frequency n. Since, on several grounds, this unit 

 seems to be the least energy which is involved during 

 emission and absorption of light of this frequency, we may 

 assume, in this particular case, that it is the amount of energy 

 acquired by the photo-electron. Y e represents the loss of 

 energy in getting away from the parent molecule, or more 

 generally, from the surface. Hence the theoretical relation 

 would be 



Ye = hn-Y Q e (2) 



The experimental values for A' are all less than Planck's h by 

 amounts ranging from 10 to 25 per cent. When one 

 considers the fundamental way in which Planck's constant 

 appears in many branches of physics, combined with the 

 straightforward interpretation of the photo-electric effect on 

 Planck's theory of radiation, one is almost forced to the 

 conclusion that h' is ultimately identical with the universal 

 constant h and that the difference is the result of some 

 secondary effect. Richardson and Compton were inclined 

 to attribute the difference to experimental errors, all of which 



* Communicated by Professor Sir J. J. Thomson, O.M., F.R.S. 

 t Richardson <fc Compton, Phil. Mag. p. 576 (1912). 

 t Hughes, Phil. Trans, vol. ccxii. (A) p. 205 (1932).' 



