326 y/z-e Philippine Journal of 'Science mi 
THE MASS OF AN ELECTRON 
According to the accepted electromagnetic theory any charged 
body has a certain mass (resistance to change of state of motion) , 
which is due to nothing but the electric charge and depends upon 
its spacial distribution. The more concentrated a charge is, the 
greater is the mass due to it. For a given charge at a definite 
concentration the corresponding mass practically follows New- 
ton’s assumption of constant mass for any velocity up to and 
including those of cathode rays ; that is, about one-tenth of the 
velocity of light. The idea that all the mass of ordinary matter 
is of this electromagnetic character was somewhat revolutionary 
when proposed by Larmor, 5 but the assumption of mass of any 
other kind, at least in the case of electrons, has been since shown 
to be entirely superfluous. 
The mass of the electron has been found to be 9.01 X 10' 28 
grams G for all of the ordinary velocities at which it has been 
measured. According to experiments with the very rapid 
electrons given off by radium, which have been studied at veloc- 
ities from three-tenths to eight-tenths of that of light, the 
apparent mass does not stay constant at this figure, but increases 
inversely with y/c 2 -v 2 (in which c is the velocity of light 
and v that of the electron 6 ) . Thus at one-tenth of the velocity 
of light the apparent mass of an electron is 9.1 X 10' 28 ; 
at three-tenths, 9.5 X 10 -28 ; at five-tenths, 10.4 X 10" 28 ; and at 
eight-tenths, 15.0 X 10' 28 grams. As the experiments referred 
to measure the mass by forces transverse to the direction of 
motion, this is called the transverse mass. The longitudinal mass 
has been measured indirectly by optical experiments designed 
to show the absolute velocity of the earth through the ether. 
The explanation of the failure of such experiments to show any 
such absolute motion implies that the apparent longitudinal mass 
of an electron varies inversely with (c 2 -v 2 )\ 
ELECTRON MODELS WHICH HAVE BEEN PROPOSED 
It would be logical to suppose that, since we find actual nega- 
tive charges to be made up of units, we could consider these 
units, that is, electrons, not as “charged bodies,” but as indivis- 
ible elements of negative charge. In the sense that the total 
6 Larmor, Phil. Trans. Roy. Soc. 186 (1895) 697. 
‘Bucherer, A., Ann. d. Phys. 28 (1908) 513; 29 (1909) 1063; Wolz, R., 
Ann. d. Phys. 30 (1909) 273; Schaefer, C., Phys. Zeit 14 (1913) 1117. 
