ce. We should get this gradual instead of abrupt decay of the
current if some of the particles, instead of all starting from rest,
started with a finite velocity; in that case the first particles
stopped would be those which started from rest. This would be when a =
2Xm/eH^2. Thus if we measure the value of a when the magnetic force
first begins to affect the leak to the electrometer we determine
2Xm/eH^2, and as we can easily measure X and H, we can deduce the value
of m/e.

By these methods Thomson determined the value of e/m for the negative
ions produced when ultra-violet light falls on a metal plate, as well as
for the negative ions produced by an incandescent carbon filament in an
atmosphere of hydrogen (_Phil. Mag._ [5], 48, p. 547) as well as for the
cathode rays. It was found that the value of e/m for the negative ions
was the same in all these cases, and that it was a constant quantity
independent of the nature of the gas from which the ions are produced
and the means used to produce them. It was found, too, that this value
was more than a thousand times the value of e/M, where e is the charge
carried by an atom of hydrogen in the electrolysis of solutions, and M
the mass of an atom of hydrogen. We have seen that this charge is the
same as that carried by the negative ion in gases; thus since e/m is
more than a thousand times e/M, it follows that M must be more than a
thousand times m. Thus the mass of the negative ion is exceedingly small
compared with the mass of the atom of hydrogen, the smallest mass
recognized in chemistry. The production of negative ions thus involves
the splitting up of the atom, as from a collection of atoms something is
detached whose mass is less than that of a single atom. It is important
to notice in connexion with this subject that an entirely different line
of argument, based on the Zeeman effect (see MAGNETO-OPTICS), leads to
the recognition of negatively electrified particles for which e/m is of
the same order as that deduced from the consideration of purely
electrical phenomena. These small negatively electrified particles are
called corpuscles. The latest determinations of e/m for corpuscles
available are the following:--

Observer. e/m.

Classen (_Ber. deut. phys. Ges._ 6, p. 700) 1.7728 X 10^7
Bucherer (_Ann. der Phys._, 28, p. 513) 1.763 X 10^7

It follows from electrical theory that when the corpus