494 



Dr. H. S. Allen on Electronic 



by other investigators f. Special interest attaches to an 

 accurate determination of the ionization potential of helium 

 recently carried out by Bazzoni. He concludes that the 

 value 20*5 volts is slightly too large and that the true value 

 is nearer 20 volts. If we assume that NV = nVE is an exact 

 relation, and that N=2, n = 3 for helium, the calculated value 

 of V is 20-33 volts. 



In the case of mercury, ionization of a second type occurs 

 for a potential of 10 volts J, which is almost exactly double 

 the value recorded in the table, so that the corresponding- 

 frequency number, n, would be 60. So far as the verification 

 of the proposed relation is concerned, no great importance 

 can be attached to cases involving such large integers. 



§ 5. Thermionic Potentials. 



Intimately connected with the potentials discussed in the 

 present paper are the potentials observed in dealing with the 

 emission of electrons from glowing solids and the contact 

 potentials between different metals. In these cases the 

 results obtained depend to such an extent on surface con- 

 ditions and the presence of gaseous films, that as yet it is 

 hardly possible to assign to the various elements reliable 

 values that shall be characteristic of the elements themselves. 

 The work that an electron would have to do to escape from 

 the substance may be measured by the equivalent potential dif- 

 ference, <f). The values quoted in Table VI. for <£, the " electron 



Table VI. 



Element. 



N. 



(p (volts). 



Authority. 



N>. 



Carbon 



6 



20 

 22 



26 



28 

 42 

 73 



74 

 78 



90 



4-14 

 4-51 

 3-04 



24* 



3-2* 



2-9 



4-31 



4-31 



4-52 



5-02 



5-1 



3-36 



Langmuir. 



Deininger. 



Horton. 



Langmuir. 



Langmuir. 



Schlichter. 



Langmuir. 



Langmuir. 



Langmuir. 



Deininger. 



Horton. 



Langmuir. 



2X12-4 



2x13-5 



5x12-2 



4X13-2 



6x13-9 



6X13-5 



14x130 



23x13-7 



25x13-4 



30x131 



30x13-3 



22x13-7 



Calcium 



Titanium 



Nickel 



Molybdenum. . 



Tantalum 



Tungsten 



Platinum 



Thorium 



* Preliminary measurements by Dr. Dushman. 



t Pawlow, Proc. Eoy. Son. A, vol. xc. p. 398 (1914) ; McLennan and 

 Henderson, Proc. Roy, Soc. A, vol. xci. p. 485 (1915) ; Goucher, Phys. Rev. 

 vol. viii. p. 561 (1916); Bazzoni, Phil. Mag. vol. xxxii. p. 566 (1916). 



t Tate, Phys. Rev. vol. vii. p. 686 (1916). 



