Hull — Initial Velocities of the Electrons. 251 



Art. XXVI. — The Initial Velocities of the Electrons 

 Produced oy Ultra-Violet Light ; by Albert "W. Hull. 



[Contributions from the Sloane Physical Laboratory of Yale College.] 



Lenard* was the first to investigate the initial velocities 

 with which electrons are shot out by a metal plate illuminated 

 by ultra-violet light. He found that the velocities varied with 

 the nature of the illuminated metal and the source of light, 

 but were independent of the intensity of the light, from 

 which he concluded that the effect must be due to reso- 

 nance, but that the greater part of the energy of the electrons 

 must come from within the atom, " the resonance playing only 

 a releasing role." Ladenburgf investigated more carefully the 

 nature of this resonance effect, and found that, for a given 

 metal, the maximum velocity of the electrons emitted was 

 directly proportional to the frequency of the incident light. 

 More recently Ladenburg and Markau J have shown that, for a 

 particular spectral region, all the electrons liberated have 

 velocities lying between narrow limits, and they conclude that 

 each wave-length liberates electrons whose frequency of vibra- 

 tion has a definite relation to that of the light, and whose 

 initial velocities are, therefore, either all equal or grouped 

 closely about a mean. 



The range of wave-lengths used by these investigators was 

 from X 2000 to X 2700. The importance of the results makes it 

 desirable that they be verified for a wider range. The present 

 paper contains an account of experiments on this subject in 

 the region of short wave-lengths discovered by Schumann. 



The general method is the same as that used by Lenard and 

 by Ladenburg and Markau. The source of light was an inter- 

 nal capillary discharge tube of the type used by Lyman§ in his 

 spectroscopic work, filled to about one millimeter pressure with 

 hydrogen or carbon dioxide, and closed by a fluorite plate. 

 Different wave-lengths were obtained by interposing various 

 absorbing screens between the discharge tube and the photo- 

 electric chamber. 



Before describing the final experiments, a word should be 

 said about some attempts which were unsuccessful. Using the 

 method of auxiliary field employed by Ladenburg and Markau 

 for preventing reflection of electrons, with an apparatus 

 exactly similar to that shown in figure 4 of their paper, || the 



*P. Lenard. Ann. Physik., ii, p. 359, 1900; viii, p. 149, 1902. 

 f E. Ladenburg, Phys. Zeitschr., viii, p. 590, 1907. 

 % Ladenburg and Markau, ibid., ix, p. 821, 1908. 

 §T. Lyman, Astrophys. J., xxiii, p. 189, 1906. 

 || Ladenburg and Markau, loc. cit. 



