The
Discovery of the Electron: Electrical Discharges in Gases
Although
standard work continued under 'J J' it was soon eclipsed by
research on the discharge of electricity through gases. The
glass apparatus used, hastily built and continually modified,
present a great contrast to the beautifully finished pieces
of brass and mahogany of the previous era. This was probably
the origin of the 'string and sealing wax' tradition that
was long associated with the Cavendish.
'J J' was
convinced that the key to an understanding of the nature of
matter was the study of discharge phenomena in gases, but
for the first ten years he worked with little success. In
1895 Rontgen announced his discovery of X-rays, and Thomson
very soon found that they made gases conducting. 'This was
a matter of vital importance for the investigations on the
passage of electricity through gases... Until the rays were
discovered the only ways of making electricity pass through
a gas were either to apply very great electric forces to it,
or else to use very hot gases such as flames. In either case
it was exceedingly difficult to get anything like accurate
measurements. The results were apt to be very capricious,
apparently depending upon causes which it was very difficult
to locate... To have come upon a method of producing conductivity
in a gas so controllable and so convenient as that of the
X-rays was like coming into smooth water after long buffeting
by heavy seas.' Rutherford and Thomson soon arrived at the
theory of ionisation of the gas by the X-rays which is accepted
today.
At very
low pressures Crookes discovered that a different type of
radiation, cathode rays, passed through the discharge tube.
'J J' showed that the rays were deflected by a magnetic field
and that a Faraday cylinder placed out of the normal path
of the rays received no charge, but received a large negative
one when the beam was deflected by a magnet into the cylinder.
This seemed to prove that they were negatively charged particles,
but Hertz had found that the rays were not deflected by an
electric field and thought that they were flexible electric
currents flowing through the ether. Eventually Thomson realised
that the absence of electric deflection was due to the residual
gas in the tube being ionised by the rays. The ions were attracted
to the electric plates and cancelled out the applied field.
'The thing to do was to get a much higher Vacuum. This was
more easily said than done... However after running the discharge
through the tube day after day without introducing fresh gas,
the gas on the walls and electrodes got driven off and it
was possible to get a much better vacuum. The deflection of
the cathode rays by electric forces became quite marked...
This result removed the discrepancy between the effect of
magnetic and electric forces on the cathode particles: it
did much more than this, it provided a method of measuring
the velocity of these particles, and also m/e where m is a
mass of a parciel and e its electric charge.' 'J J' found
that m/e for the cathode rays was about 1/1000 times that
of the Hydrogen ion and postulated that they were very light,
negatively charged 'corpuscles' which were a universal constituent
of matter.
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