discovery of the electron

The Discovery Of The Electron

The electron was discovered in 1895 by J.J. Thomson in the
form of cathode rays, and was the first elementary particle to be
identified. The electron is the lightest known particle which
possesses an electric charge. Its rest mass is Me <approximately
equal> 9.1 x 10 -28 g, about 1/1836 of the mass of the proton or
neutron.

The charge of the electron is -e = -4.8 x 10^-10 esu <elec
trostatic unit). The sign of the electron's charge is negative by
convention, and that of the equally charged proton is positive.
This is somewhat a unfortunate convention, because the flow of
electrons in a conductor is opposite to the conventional direc
tion of the current.

The most accurate direct measurement of e is the oil drop
experiment conducted by R.A. Milikan in 1909. In this experiment,
the charges of droplets of oil in air are measured by finding the
electric field which balances each drop against its weight. The
weight of each drop is determined by observing its rate of free
fall through the air, and using Stokes' formula for the viscous
drag on a slowly moving sphere. The charges thus measured are
integral multiples of e.

Electrons are emitted in radioactivity <as beta rays> and in
many other decay processes. The electron itself is completely
stable. Electrons contribute the bulk to ordinary matter; the
volume of an atom is nearly all occupied by the cloud of elec
trons surrounding the nucleus, which occupies only about 10^-13
of the atom's volume. The chemical properties of ordinary matter are
determined by the electron cloud.

The electron obeys the Fermi-Dirac statistics, and for this
reason is often called a fermion. One of the primary attributes
of matter, impenetrability, results from the fact that the elec
tron, being a fermion, obeys the Pauli exclusion principle.

The electron is the lightest of a family of elementary
particles, the leptons. The other known charged leptons are the
muon and the tau. These three particles differ only in mass;
they have the same spin, charge, strong interactions, and weak
interactions. In a weak interaction a charged lepton is either
unchanged or changed into and uncharged lepton, that is a neutri
no. In the latter case, each charged lepton is seen to change
only into the corresponding neutrino.

The electron has magnetic properties by virtue of (1) its
orbital motion about the nucleus of its parent atom and (2) its
rotation about its own axis. The magnetic properties are best
described through the magnetic dipole movement associated with 1
and 2. The classical analog of the orbital magnetic dipole moment
of a small current-carrying circuit. The electron spin magnetic
dipole moment may be thought of as arising from the circulation
of charge, that is, a current, about the electron axis; but a
classical analog to this moment has much less meaning than that
to the orbital magnetic dipole moment. The magnetic moments of
the electrons in the atoms that make up a solid give rise to the
bulk magnetism of the solid.