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An Ionisation Chamber is used for detecting mainly the α and β rays and to compare the activities of source emitting these rays. 
Basically it consists of a chamber filled with a gas like air or argon at normal pressure it is fitted with a pair of electrodes E1 and E2 between which a cinstant p.d. is maintained. It also carries a window W made of mica, 0.002 mm thick and coated with graphite to make it conducting. 
When the rays to be detected enter the chamber through the window, the ionize the gas between the electrodes. The positive ions & the electron produced in the chamber move in opposite directions towards the electrodes, so that an ionisation current ( ≈ 10^-12 A) flows in the external circuit. This current is measured by measuring the p.d. established by it in high resistance R by means of an electrometre. 
The p.d. across the electrodes is so selected that all the ions collects at that electrode & their is no secondary ionisation. 
Under the condition, the ionisation current is proportional to number of pairs of ions & hence to the number of particles entering the chamber. 
Therefore, the activities of the sources can be compared by comparing their ionisation current.
i.e.  “more ionisation current means more activity”
An ionisation chamber is less sensetive to β-particles in comparison to α-particles because β-particle produce less ionisation.