18, 4 Perkins: The Structure of the Electron 327 
mass of an electron is electric, this is true. But mass is not 
a constant property of electric charge; it depends on the dis- 
tribution of the charge. Therefore we must consider that an 
electron has a certain distribution of charge within it. Evidently 
the conception of negative charge must be applied to portions 
of the electron. It has been customary to assume that an electron 
at rest has spherical symmetry, and that the charge resides only 
on the surface, as it does on charged conductors of observable size. 
The mass of such an electron is accounted for by the accepted 
electromagnetic laws, if its radius is about 1.8 x 107° centi- 
meters. The mass of a spherical electron would vary with its 
velocity, but not according to the law experimentally verified 
(see above). Lorentz’ has shown that it is only necessary to 
modify the spherical electron by a contraction in the direction 
of the motion so that the dimensions in this direction become 
J — y : 1 as compared to the dimensions of the electron at rest. 
This is certainly only a special case of a quite general principle, 
and may be looked upon as the consequence of the imperfec- 
tions of our ideas of space® or, perhaps, of our measurements 
of mass, rather than an actual contraction of the electron. 
The Lorentz elastic shell electron is a satisfactory model for 
electrons, such as the £-rays, which have been separated from 
positive charges, but not for the electrons in the nucleus or 
those which surround the nucleus of an atom. Such electrons 
either are not of the shape suggested by Lorentz, or do not 
follow the classical electromagnetic laws. The fact that there 
are strong magnetic fields in atoms cannot be explained by 
Lorentz electrons rotating in orbits, because there is no 
accompanying emission of light. Now a continuous circular 
current would give a magnetic field without radiation. On this 
basis Parson® has developed his theory of a “magneton,” or 
ring electron, whose diameter is about 3 < 10” centimeters, and 
which rotates with about the velocity of light. 
Compton” has found that the scattering of X-rays is much 
greater than could be explained by the Lorentz electron, and 
‘Lorentz, H. A., Konink. Akad. Wetensch. Amsterdam, Versl. 12 (1904) 
986; Theory of flectrdns; 21%. 
: Minkowski, H., Raum und Zeit, Phys. Zeit. 10 (1909) 104: ef. Cun- 
ningham, E., Relativity and the Electron Theory. London, Longmans, 
Green & Co. (1915). 
"Parson, A. L., Smithsonian Misc. Coll. 65 (1915-16) No. 11. 
“Compton, A. H., Phys. Rev. 14 (1919) 20, 247. 
