[ Ti ] 



VI. Ionization and Resonance Potentials for Electrons in 



Vapours of Lead and Calcium. By F. L. Mohler, 



Ph.D., Paul D. Foote, Ph.D.] and H. F. Stimson, 

 Ph.D.* 



PREVIOUS papers | have discussed measurements made 

 in this laboratory of ionization and resonance poten- 

 tials of a number of metals. A continuation of this work 

 has led Lo satisfactory results for lead and calcium. 



The method of measurement employed has been described 

 in detail elsewhere. The metal is boiled at low pressure in 

 a vacuum tube containing a Wehnelt cathode surrounded 

 by a cylindrical grid and plate. Between the cathode and 

 grid is placed a variable potential to accelerate the electron 

 current from the cathode, while a small retarding field is 

 fixed between the grid and plate. Measurement of the 

 total current leaving the cathode as the accelerating poten- 

 tial is increased shows a sudden increase in the current 

 when the ionization point is reached on account of the direct 

 effect of ionization and the indirect effect of positive charges 

 on the electric field. The " partial current " reaching the 

 plate against a small retarding field decreases when the 

 electrons near the grid lose nearly all their velocity by 

 inelastic impact with vapour molecules. The curve of 

 V partial current " versus accelerating potential shows a 

 series of drops in the current at equal voltage intervals. 

 This interval is equal to the resonance potential, and the 

 distance of the first drop from the origin is the resonance 

 potential minus the initial velocity in volts of electrons 

 leaving the cathode. The initial velocity correction added 

 to the applied potential at the ionization point gives the 

 ionization potential. 



This interpretation of the partial current curves is based 

 on two assumptions : 



1. That when the velocity of an electron is less than that 



corresponding to the resonance potential collisions 

 are elastic. 



2. That when equal to or greater than the resonance 



potential the velocity lost at collision is equal to the 

 resonance potential. 



* Communicated by the Authors, being abstract of paper to appear in 

 4 Bulletin Bureau of Standards.' 



t Tate & Foote, Phil. Mag. xxxvi. p. 64 (1918) ; Bur. of Standards, 

 S. P. 317. Foote & Mohler, Phil. Mag. xxxvii. p. 33 (1919). Jour. Wash. 

 Acad. Sci. viii. p. 513 (1918). Foote, Rognley & Mohler, Phvs. Rev. xiii. 

 p. 59 (1919). 



