350 L. Page — A Century's Progress in Physics. 



discovered that the energy with which a negatively 

 charged particle is ejected is entirely independent of the 

 intensity of the light, and further investigation showed 

 it to depend only on the frequency. Einstein suggested 

 that the electrons appearing in this so-called photo-elec- 

 tric effect start from within the metal with an initial 

 energy hv. In passing through the surface a resistance 

 is encountered, however, so he concluded that the energy 

 with which the fastest moving electrons appear outside 

 the metal should be equal to hv less the work done in 

 overcoming this resistance. Recent experiments not 

 only confirm this relation, but provide a most satisfac- 

 tory method of determining the value of h. Millikan 16 

 finds it to be 6-57 (10) -27 ergs sec, which gives the quan- 

 tum for yellow light a value sixty times as great as the 

 heat energy of a monatomic gas molecule at 0°C. That 

 this large amount of energy can be transferred from the 

 incident light to the ejected electron is quite out of the 

 question; it must come from within the atom. In this 

 way some indication is obtained of how vast intra-atomic 

 energies must be. 



Structure of the Atom. — The generally accepted model 

 of the atom is that due chiefly to Rutherford. 17 He con- 

 siders it to be constituted of electrons revolving about a 

 positive nucleus either singly or grouped in concentric 

 rings, in much the same manner as the planets revolve 

 around the sun. Experiments on the scattering of alpha 

 rays, however, show that the nucleus, while it must have 

 a positive charge sufficient to neutralize the charges of 

 all the electrons moving around it, cannot have a volume 

 of an order of magnitude greater than that of the elec- 

 tron. The number of unit charges residing on it, except 

 in the case of hydrogen, which is supposed to consist of a 

 singly charged nucleus and only one electron, is found to 

 be approximately half the atomic weight. Thus helium, 

 with an atomic weight of about four, has a doubly 

 charged nucleus with two electrons revolving about it, 

 and lithium a triply charged nucleus and three electrons. 

 The number of unit charges on the nucleus is supposed to 

 correspond with the atomic number used by Moseley in 

 interpreting the results of his experiment on the X-ray 

 spectra of the elements. 



Now the electron which is revolving around the posi- 



16 E. A. Millikan, Phys. Kev., 7, 355, 1916. 



17 E. Kutherford, Phil. Mag., 21, 669, 1911. 



