Thomson's
apparatus deflected the positive rays using an electric field, in the XX' direction,
and a magnetic field, in the YY' direction, before allowing them to fall onto
a photographic plate.
The
action of both fields makes ions of the same mass to
charge ratio, m/e, fall onto the same parabolic curves.
Fast ions are deflected less than slower ions, but this
only affects their position on the curve, not which
curve they'll fall on. If the gas in the discharge tube
contains different sorts of ions with different specific
mass, they will be split into separate parabolae by
the time they hit the photographic plate.
