In
1897 Wilson experimented with radiation from radioactive
uranium as well as ionisation produced by X-rays. He suggested that the
condensation nuclei were free ions produced by the rays,
whose charge would encourage the cloud formation.
Wilson
tried exposing the air to X-rays for longer, but it
didn't reduce the amount of expansion needed to make
the clouds appear. He also found that both the radiation
from uranium and the ionisation produced by X-rays produced the same sort of
condensation.
To
prove that the condensation nuclei were charged ions,
Wilson put an electric field around the cloud chamber.
Even a weak electric field swept the ions away so quickly
that no clouds formed.
