Resonance Potentials for Electrons in Metallic Vapours. 49 



Summary. 



The ionization and resonance potentials for electrons in 

 vapours of magnesium and thallium have been measured, 

 using the method of Franck and Hertz as modified by Tate. 

 The experimentally determined values for magnesium vapour 

 were 2 '65 volts for resonance potential and 7*75 volts for 

 ionization potential. The corresponding theoretical values 

 on the basis of the quantum relation hv — eV are 2*70 volts 

 and 7*61 volts respectively. The experimentally determined 

 Values for thallium vapour were 1'07 volts for resonance 

 potential and 7*3 volts for ionization potential. The theo- 

 retical value for resonance potential is 1'07 volts. The 

 theoretical value for ionization potential is not known. For 

 magnesium the ionization potential is determined by the 

 limit of the combination series 1*5 S — ntp 2 , and the resonance 

 potential is determined by the first line in this series, m = 2. 

 For thallium the resonance potential is determined by the 

 shorter wave-length member of the first term of the prin- 

 cipal series of doublets. No known series in thallium has 

 a convergence frequency greater than 49263. The observed 

 ionization potential for thallium suggests the presence of an 

 undiscovered series of single lines converging at v=l'5 S 

 = 57000 to 60000. The present work offers evidence that 

 the single line spectra of magnesium and thallium are 

 \ = 4571 and 11513 A. respectively. The general behaviour 

 of the metals as regards ionization and resonance potentials 

 appears to be identical for metals in the same group of the 

 periodic table. In the case of thallium the component of 

 the doublet having the higher frequency determines the 

 value of the energy quantum absorbed by the atom. This 

 fact suggests either the possibility of separate excitation of 

 the components of a doublet by electronic impact or a 

 behaviour of thallium vapour, thus excited, analogous to the 

 emission of characteristic X-rays for which the K group does 

 not appear until the energy of the impacting electrons is 

 greater than that corresponding to K y . From work carried 

 on mainly at the Bureau of Standards the writers have 

 obtained a value of Planck's constant h by the method of 

 ionization and resonance potentials. Using 13 determi- 

 nations on 7 different metals, each determination a mean of 

 several experiments, the final value of A = 6'55 . 10~ 27 erg sec. 

 was obtained in excellent agreement with recent determi- 

 nations by other methods. The question of photoelectric 

 sensibility of metallic vapours has been briefly discussed, and 

 several recent papers upon the subject of ionization and 

 Phil. Mag. S. 6. Vol. 37. No. 217. Jan. 1919. E 



