By combination of this equation with (2) obtained by measurement of the magnetic deflexion, both u and e/m can be determined.
Simon found by this method that
e
-- = 1·865 × 107.
m
It will be seen later ([section 82]) that a similar value was deduced by Kaufmann for the electrons projected from radium.
These results, which have been based on the effect of a magnetic and electric field on a moving ion, were confirmed by Weichert, who determined by a direct method the time required for the particle to traverse a known distance.
The particles which make up the cathode stream were termed “corpuscles” by J. J. Thomson. The name “electron,” first employed by Johnstone Stoney, has also been applied to them and has come into general use[[93]].
The methods above described do not give the mass of the electron, but only the ratio of the charge to the mass. A direct comparison can, however, be made between the ratio e/m for the electron and the corresponding value for the hydrogen atoms set free in the electrolysis of water. Each of the hydrogen atoms is supposed to carry a charge e, and it is known that 96,000 coulombs of electricity, or, in round numbers, 104 electromagnetic units of quantity are required to liberate one gram of hydrogen. If N is the number of atoms in one gram of hydrogen, then Ne = 104. But if m is the mass of a hydrogen atom, then Nm = 1. Dividing one by the other e/m = 104. We have seen already that a gaseous ion carries the same charge as a hydrogen atom, while indirect evidence shows that the electron carries the same charge as an ion, and consequently the same charge as the atom of hydrogen. Hence we may conclude that the apparent mass of the electron is only about ¹⁄₁₀₀₀ of the mass of the hydrogen atom. The electron thus behaves as the smallest body known to science.
In later experiments J. J. Thomson showed that the negative ions set free at low pressures by an incandescent carbon filament, and also the negative ions liberated from a zinc plate exposed to the action of ultra-violet light, had the same value for e/m as the electrons produced in a vacuum tube. It thus seemed probable that the electron was a constituent of all matter. This view received strong support from measurements of quite a different character. Zeeman in 1897 found that the lines of the spectrum from a source of light exposed in a strong magnetic field were displaced and doubled. Later work has shown that the lines in some cases are trebled, in others sextupled, while, in a few cases, the multiplication is still greater. These results received a general explanation on the radiation theories previously advanced by Lorenz and Larmor. The radiation, emitted from any source, was supposed to result from the orbital or oscillatory motion of the charged parts constituting the atom. Since a moving ion is acted on by an external magnetic field, the motion of the charged ions is disturbed when the source of light is exposed between the poles of a strong magnet. There results a small change in the period of the emitted light, and a bright line in the spectrum is, in consequence, displaced by the action of the magnetic field. According to theory, the small change in the wave-length of the emitted light depends upon the strength of the magnetic field and on the ratio e/m of the charge carried by the ion to its mass. By comparison of the theory with the experimental results, it was deduced that the moving ion carried a negative charge, and that the value of e/m was about 107. The charged ion, responsible for the radiation from a luminous body, is thus identical with the electron set free in a vacuum tube.