Potentials for Electrons in Metallic Vapours. 67 



passing through the superheated ionization-chamber, con- 

 densed in the upper half of the tube. The source of elec- 

 trons was a hot platinum, tungsten, or molybdenum wire 

 cathode A, of low resistance, coated with lime. Surrounding 

 the cathode was a cylinder of nickel net B, and outside and 

 coaxial with this a cylinder of sheet nickel 0. The apparatus 

 was evacuated by means of a Langmuir condensation- pump 

 or by a Stimson* two-stage condensation-pump. The latter 

 pump was very kindly made for us by Dr. Stimson of this 

 Bureau. In general the pressure employed, as registered by 

 a McLeod gauge, was about 10 ~ 5 cm. Hg. The experimental 

 procedure consisted in applying a constant retarding potential, 

 usually from 1 to 3 volts, depending upon the metal used, 

 between the net and the outside cylinder and measuring both 

 the total current from the hot wire and that portion of it 

 which reached the outside cylinder, against the retarding 

 field, as functions of a varying accelerating potential applied 

 between the hot wire and net. 



The results obtained with the four vapours are shown 

 graphically in the curves of figs. 2 to 10, and the analyses 

 of the curves are given in Tables I. to IV. 



The curves showing the variation of the partial current to the 

 outside cylinder with the accelerating potential show maxima, 

 or at least pronounced changes in curvature, at successive 

 points which differ in potential by a constant amount. This 

 constant difference gives the resonance potential directly, 

 eliminating any consideration of initial velocities. 



The curves showing the variation with accelerating potential 

 of the total current from the hot wire show r a rapid increase 

 in slope at a point for which the potential, corrected for the 

 initial velocity of the electrons as obtained from the partial 

 current curves, gives the ionization potential. The inelastic 

 nature of the encounters producing ionization is shown in the 

 case of cadmium, fig. 2 curve 7, and sodium, fig. 5 curves 5 

 and 6, by the occurrence of secondary maxima at potentials 

 differing from the ionization potential by multiples of the 

 resonance potential "j". 



The accelerating potential was applied at one end or the 

 other of the hot wire, and the values given in the Tables for 

 the initial velocities therefore include the drop in potential 

 between the point of application of the potential and the 



* Stimson, J. Wash. Acad. Sci. vii. p. 477 (1917). 

 f The method of interpreting the curves is discussed in more detail by 

 the writers in J. Wash. Acad. Sci. vii. p. 517 (1917). 



F2 



