Encouraged,
Thomson tried putting helium in the tube, and managed
to obtain a third line, with a charge to mass ratio one quarter
that of the ionised hydrogen atom. We now know that
the helium nucleus contains two protons and two neutrons,
which are uncharged particles of the same mass as protons.
The nucleus of helium is four times more massive than that
of hydrogen, so the ratio of a singly ionised
atom is one quarter the size.
This
gave Thomson his first evidence that positive charge
could be carried by something other than hydrogen. Thomson
created a better vacuum in the tube and found he could
get parabolae for other gases, including nitrogen, oxygen
and carbon.
