The most common type barometer used in homes is the aneroid barometer. Aneroid barometers contain a flexible metal capsule known as an aneroid cell. The cell is made from Beryllium and Copper, and is designed so that small variations in external air pressure make the capsule to expand or shrink. The size of the aneroid cell is calibrated so that the springs and levers attached read off the correct pressure readings.

An aneroid barometer

Setting an Aneroid Barometer

On the face of a barometer there are two pointers. One of them can be moved by yourself, by turning the screw on the face. The other one moves  itself and this one indicates the actual atmospheric pressure. Some barometers allow you to set the barometer to the correct pressure by adjusting a milled screw on the back of the barometer. This screw is set adjusted to 'mean sea level'. Pressure decreases with altitude, and an adjustment of 34 millibars is required for every 1000 feet above sea level.