2 IV. F. Holman 



metal, and, secondly, that from the ionization of the gas by these 

 carriers. Thus the experiments of Stoletow/ Lenard,- and Var- 

 lev" show that as the gas pressure is reduced indefinitely the pho- 

 to-electric current drops to a definite minimum value, which there- 

 after remains practically constant for all values of the electric 

 field. This is considered as that arising solely from the carriers 

 escaping from the metal. On the other hand, when the discharge 

 takes place in an atmosphere of appreciable density the current 

 may be produced largely by the second factor, the ionization of 

 the gas by the motion of these escaping carriers. For instance, 

 Kreussler* found that in air at atmospheric pressure the current 

 increases very rapidly with the electric intensity, if the latter ap- 

 proaches that required to produce a spark discharge. 



The present investigation was undertaken, therefore, with the 

 intention of studying the current arising from the escaping car- 

 riers alone, as it is affected by changing the store of hydrogen in 

 the surface of the metal. The results were such as to include a 

 study of certain phases of the so-called fatigue and recovery of 

 the metal under the conditions of operation. 



The plan which suggested itself as most likely to yield con- 

 clusive results was based on the discovery of Skinner that as 

 cathode in a glow current the metal gives oft" hydrogen, while as 

 anode it absorbs it. In case, therefore, these conditions can be 

 brought about without otherwise affecting the surface of the 

 metal, this plan furnishes a simple scheme for making a test of 

 the effect of changing the quantity of hydrogen in the metal. As 

 will be seen, however, this condition was attained without ques- 

 tion only in one series of experiments. 



^Jour. de Phys., 9, p. 468. 1890. 



-Avn. d. Physik, 2, p. 359. 1900. 

 'P/ij/. Trans., 202. 1903. 



*Ann. d. Physik, 6, p. 398. 1901. 



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