An electric current flows through a closed circuit of some conducting
material, such as metal wires. It starts at one pole of a battery, or of
some other electricity generating device, and ends at the other. The two
poles are the positive pole or “anode” and the negative pole or
“cathode”.
If there is a break in the circuit, the current will usually not flow at
all. If, however, the break is not a large one, and the current is under
a high driving force (which is called the “voltage”), then the current
may leap across the break. If two ends of a wire, making up part of a
broken circuit, are brought close to each other with nothing but air
between, a spark may leap across the narrowing gap before they actually
meet and, while it persists, the current will flow despite the break.
The light of the spark, and the crackling sound it makes, are the
results of the electric current interacting with molecules of air and
heating them. Neither the light nor the sound is the electricity itself.
In order to detect the electricity, the current ought to be forced
across a gap containing nothing, not even air.
In order to do that, wires would have to be sealed into a glass tube
from which all (or almost all) the air was withdrawn. This was not easy
to do and it was not until 1854 that Heinrich Geissler (1814-1879), a
German glass-blower and inventor, accomplished this feat. The wires
sealed into such a “Geissler tube” could be attached to the poles of an
electric generator, and if enough voltage was built up, the current
would leap across the vacuum.
[Illustration: _A Geissler tube._ labelled: Current source]
Such experiments were first performed by the German physicist Julius
Plücker (1801-1868). In 1858 he noticed that when the current flowed
across the vacuum there was a greenish glow about the wire that was
attached to the cathode of the generator. Others studied this glow and
finally the German physicist Eugen Goldstein (1850-1931) decided in 1876
that there were rays of some sort beginning at the wire attached to the
negatively charged cathode and ending at the part of the tube opposite
the cathode. He called them “cathode rays”.
These cathode rays, it seemed, might well be the electric current
itself, freed from the metal wires that usually carried it. If so,
determining the nature of the cathode rays might reveal a great deal
about the nature of the electric current. Were cathode rays something
like light and were they made up of tiny waves? Or were they a stream of
particles possessing mass?
There were physicists on each side of the question. By 1885, however,
the English physicist William Crookes (1832-1919) showed that cathode
rays could be made to turn a small wheel when they struck that wheel on
one side. This seemed to show that the cathode rays possessed mass and
were a stream of atom-like particles, rather than a beam of mass-less
light. Furthermore, Crookes showed that the cathode rays could be pushed
sideways in the presence of a magnet. (This effect, when current flows
in a wire, is what makes a motor work.) This meant that, unlike either
light or ordinary atoms, the cathode rays carried an electric charge.
[Illustration: _J. J. Thomson in his laboratory. On his right are early
X-ray pictures._]
This view of the cathode rays as consisting of a stream of electrically
charged particles was confirmed by another English physicist, Joseph
John Thomson (1856-1940). In 1897 he showed that the cathode rays could
also be made to take a curved path in the presence of electrically
charged objects. The particles making up the cathode rays were charged
with negative electricity, judging from the direction in which they were
made to curve by electrically charged objects.
Thomson had no hesitation in maintaining that these particles carried
the units of electricity that Faraday’s work had hinted at. Eventually,
Stoney’s name for the units of electricity was applied to the particles
that carried those units. The cathode rays, in other words, were
considered to be made up of streams of electrons and Thomson is usually
given credit for having discovered the electron.
The extent to which cathode rays curved in the presence of a magnet or
electrically charged objects depended on the size of the electric charge
on the electrons and on the mass of the electrons. Ordinary atoms could
be made to carry an electric charge and by comparing their behavior with
those of electrons, some of the properties of electrons could be
determined.
There were, for instance, good reasons to suppose that the electron
carried a charge of the same size as one that a hydrogen atom could be
made to carry. The electrons, however, were much easier to pull out of
their straight-line path than the charged hydrogen atom was. The
conclusion drawn from this was that the electron had much less mass than
the hydrogen atom.
Thomson was able to show, indeed, that the electron was much lighter
than the hydrogen atom, which was the lightest of all the atoms.
Nowadays we know the relationship quite exactly. We know that it would
take 1837.11 electrons to possess the mass of a single hydrogen atom.
The electron is therefore a “subatomic particle”; the first of this sort
to be discovered.
In 1897, then, two types of mass-containing particles were known. There
were the atoms, which made up ordinary matter, and the electrons, which
made up electric current.