One of a pair of electrodes is a wire located centrally within a
cylinder. The other electrode is the wall of the chamber. Radiation
ionizes the gas within the chamber, permitting the passage of current
between the electrodes. The thickness of a window in the chamber wall
determines the type of radiation it can measure. Only gamma rays will
pass through a heavy metal wall, glass windows will admit all gammas and
most betas, and plastic (Mylar) windows are necessary to admit alpha
particles. Counters of this type, when properly calibrated, will measure
the total amount of radiation received by the body of the wearer.