In a mercury barometer,the atmosphereic pressure exerted on the level of mercury in a cistern
is balanced by the column of mercury in a tube.Instead of using a
cistern and mercury in this way ,if a flexible box (over which the atmospheric pressure
is allowed to act) is used ,it can respond to pressure changes by
setting in deformations.This principle is used in the construction of
aneroid barometers,the word 'aneroid' meaning 'without liquid'.Thus the sensitive element in this case is a partially exhausted disc shaped metal box(called aneroid cell made of beryllium and copper) which undergoes changes of shape resulting from variations of pressure of the external air.
The ends of the box are usually corrugated to give more flexibility.A central spring prevents complete collapse of the box under the external pressure.The small movements of the box are transmitted by a system of magnifying levers to the pointer moving over a graduated dial indicating the air pressure.
