3.
Balancing forces
J.J.
Thomson had used an electric field to deflect the cathode
rays, proving that the rays were negatively charged.
A current
in a coil of wire produces a magnetic field. Two coils, arranged
as a 'Helmholtz' pair, will produce a uniform magnetic field. A
beam of charged particles passing through the magnetic field will
be bent at right angles to the field into a circular arc or
complete circle.
Cathode rays
being deflected by a magnetic field, whose direction is into the
paper.
The
magnetic field produces a force which deflects the
cathode rays. In his tube, Thomson positioned the coils so that the
deflection was in the opposite direction to the deflection
produced by the electric field.
By
adjusting the strengths of the fields the rays could
be deflected, in one direction by the electric field,
and back an equal amount by the magnetic field. The
forces were balanced.
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