What is an aneroid barometer? Draw a neat and labelled diagram to explain its construction and working.

Aneroid barometer has no liquid. It is light and portable. It is calibrated to read directly the atmospheric pressure.

Construction —

The figure above shows the main parts of an aneroid barometer. It consists of a metallic box B which is partially evacuated. The top D of box is springy and is corrugated in form of a diaphragm as shown in fig (b).

At the middle of the diaphragm, there is a thin rod, toothed at it's upper end. The teeth of rod fit well into the teeth of a wheel S attached with a pointer P which can move over a circular scale.

The circular scale is graduated and is initially calibrated with a standard barometer so as to read the atmospheric pressure directly in terms of the barometric height.

Working —

When the atmospheric pressure increases, it presses the diaphragm D and the rod L gets depressed. The wheel S rotates clockwise and pointer P moves to the right on the circular scale.

On the other hand, when atmospheric pressure decreases, the diaphragm D bulges out due to which the rod L moves up and the wheel S rotates anticlockwise.

Consequently, the pointer shifts to the left and the pressure is read over the calibrated scale.