At
first Thomson could only produce a single line on his
photographs. From its position Thomson calculated it
was the hydrogen ion that Wien had found, as its mass to charge
ratio was two thousand times greater than the electron.
Even if the discharge tube contained other gases, only
this hydrogen line was produced.
It
looked like ions of hydrogen were carrying positive
charge in the same way that electrons carried negative
charge, but Thomson wanted to be sure. By using a very
large discharge tube at very low pressure, he managed
to produce a second line, with twice the specific charge
of the hydrogen ion.
Thomson
identified this ion as an ionised hydrogen molecule.
Two atoms of hydrogen bond together to form a molecule.
When it becomes ionised one electron is lost, producing
one positive charge on the ion, but its mass is twice
that of the hydrogen atom. This means that the specific
charge of the ionised molecule is half the specific
charge of an ionised hydrogen atom.
