SOME CONTEMPORARY AinWSCES IS PUYSICS~yn m 



Docs this single line appear suddenly at a precise value of the 

 enerR>' of the iinpinRing electrons? This ()uesli(in suggests itself, 

 when one has already studie<l the excitation of X-rays from solids by 

 electrons and the excitation of electrons from solids by light. Here 

 again we meet that tiresome but ineluctable problem, as to what 

 constitutes a sudden appearance, and how we slundd recognize it if it 

 really occurretl. The only consistent way to meet it (consistent, that 

 is, with the wa>s already employed in the prior cases) would be to 

 measure the intensity of the line for various values of the energy of 

 the electrons, plot the cur\e, and decide whether or not it cuts the 

 axis of abscissae at a sharp angle. This is in principle the same 

 method as is used in determining whether a given X-ray frequency 

 appears suddenly at a given value of the energy of the electrons 

 boml)arding a solid; the curves of Fig. 4 were so obtained. Attempt- 

 ing to apply this same method to such a radiation as 2, .536 of mer- 

 cun.', one has the solitar>" advantage that the frequency of the light 

 is sharp and definite (it is not necessary to cut an arbitrary band of 

 radiations out of a continuous spectrum) and two great counteracting 

 disadvantages: the intensity of the light cannot be measured accur- 

 ately (one has to guess it from the effect upon a photographic plate) 

 and the impinging electrons never all have the same energy. Owing 

 probably to these two difficulties, there is no published curve (that I 

 know of) which cuts down across the axis of abscissae with such a 

 decisive trend as the curves of Figs. 2 and 4. Still it is generally 

 accepted that the advent of the single line is really sudden. The 

 common argument is, that one can detect it on a photographic film 

 exposed for a few hours when the energy of the bombarding electrons 

 is (say) 5 equivalent volts, and not at all on a plate exposed for hun- 

 dreds of hours when the bombarding voltage is (say) 4.5 volts. In 

 this manner the energy of the electrons just sufficient to excite 2.536 

 of mercurj- has been located at 4.9 equivalent volts. Dividing this 

 critical energy (expressed in ergs) by the frequency of the radiation, 

 we get 



(4.9e/300) / (f /.00002536) =6.59 • lO"" (5) 



It agrees with the values of the constant which I designated by h 

 in the two prior cases, and the data obtained with other kinds of 

 atoms are not discordant. Gerlach arrives at 6.56- 10~" as the mean 

 of all values from experiments of this type upon many vapours. The 

 evidence is not quite so strong as in the prior cases, but fortunately 

 it is supplemented and strengthened by testimony of a new kind. 

 When electrons strike solids and excite X-rays, it is impnassible to 



