THE ELECTRON— COMPTON 213 



be emphasized, however, that this estimate of size is not, hke the 

 charge and mass, a definite measurement, but is simply an estimate 

 based on assumptions, at least one of which is quite uncertain. For 

 while we have both logic and experiment to back up the assumption 

 that all the mass of the electron is of this electromagnetic origin, we 

 must confess to utter ignorance regarding the shape of the electron. 

 Indeed, some facts suggest that it may have different sizes and 

 shapes in different environments, as in the free state or in an orbit of 

 an atom or in the nucleus of an atom. So our estimate of 10"^^ cm 

 for the size of the electron is, at best, very crude. 



The idea of electromagnetic mass was strongly supported by the 

 fact that measurements of the mass of very fast moving electrons, 

 through measurements of the ratio of charge to mass of beta rays 

 from radium or cathode rays in high-voltage discharge tubes, showed 

 that their mass is not really a constant thing but increases with the 

 speed of the electron. The value of electron mass given above 

 applies, strictly speaking, only to an electron at rest. Practically, 

 however, it is accurate enough for practical purposes for electron 

 speeds below about one-tenth the speed of Hght. At this speed the 

 electron's mass is about half of one percent larger than if it were at 

 rest. At still higher speeds, the mass increases more and more rapidly, 

 approaching infinite mass as the speed of light is approached. 



These facts, experimentally determined, were sho^vn by Abraham 

 to be of the type expected if the entire mass of an electron is of electro- 

 magnetic origin, due entirely to its electric charge. It was this 

 argument, wliich has since received confirmation from other directions, 

 which was the basis of the theory that all mass, that is, all matter, is 

 electrical. However, the simple electromagnetic concepts were not 

 quite adequate to give an accurate quantitative interpretation of 

 these experiments, and it required the additional introduction by 

 Lorentz of the concepts of the special theory of relativity to bring 

 about complete interpretation of the experiments. 



THE ELECTRON AND QUANTUM THEORY 



Just two things more do we know accurately about the properties 

 of electrons, in addition to their charge and mass. We Imow that 

 they are also tiny magnets of strength equal to the basic unit of mag- 

 netic moment generally called the Bohr magneton. Once the electron 

 had been discovered, it became natural to seek in it also the explana- 

 tion of magnetic phenomena, since it was only necessary to assume 

 that the electricity of an electron is whirling about an axis, and the 

 electron becomes endowed with the properties of a tiny magnet. 

 Parsons, Webster, and others examined the possibilities inherent in 

 various assumed configurations, with interesting results. But it was 

 only with the introduction of the quantum theory for the interpreta- 



