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 In the fields
of atomic physics
and nuclear physics,
Wilson's cloud chamber
is the most fundamental
device to observe
the trajectories
of particles.
Its basic principle
was discovered
by
C. T. R. Wilson
(UK, 1869 - 1959)
in 1897,
and
it was put
to the practical use
in 1911.
The schematic drawing
of the cross section
of the cloud chamber
is shown in
the following
figure.
The top and the side
of the chamber
is covered
by glasses,
and of a diameter
of several centimeters.
At the bottom
of the chamber,
a piston is placed.
The air filled
in the chamber
is saturated
with vapor of water.
When pulling down
the piston quickly,
the volume of the chamber
would be expanded
and the temperature
goes down, and then
the air inside
would be
supersaturated
with the vapor.
If a charged particle
enters into such
a supersaturated state
to form ions,
the vapor of water
would condense along
the line of the ions,
which is the path of
the charge.
Thus we can observe
the trace,
and also take a photograph.
To make clear the trace,
a light is sometimes
illuminated from
the side.
When placing
the cloud chamber
in a magnetic field,
we can obtain various
informations about
the charged particle
by measuring the curvature
of the trace
and other data.
The bubble chamber
and the spark chamber
have taken place
of the cloud chamber
which is
nowadays used only
for the educational purposes.
Wilson's cloud chamber
has however played
a very important
role in the history
of physics.
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