Alpha particles from
a small amount
of radioactive Americium-241 ionizes the air between
two metal
plates. The plates are connected to a battery
and the voltage
difference causes a weak current to flow between the
plates.
The current flowing through the ionization chamber
drops significantly
when smoke enters the chamber. This is because
the charge
carriers quickly stick to any smoke particles that
enter the ionization
chamber and suffer a large drop in their electrical
mobility. The
drop in current is sensed and used to sound the
alarm.