The atmospheric pressure is measured with a fortins barometer
Answers
The barometer is an instrument used to measure atmospheric pressure. Fortin’s barometer is a modified form of Torricelli’s simple barometer.
Construction
A Fortin’s barometer consists of a narrow glass tube of length about 90 cm. This tube is closed at one end. The tube is completely filled with mercury and kept inverted in a cistern filled with dry mercury. Usually, the glass tube is protected by enclosing it in a brass tube. The upper part of the brass tube has a slit that enables the level of the mercury in the glass tube to be seen. A scale graduated in millimetres is attached to the brass tube. This functions as the main scale. For accurate measurement, a vernier scale that can slide over the main scale is also fixed to the barometer. The vernier scale can be moved up and down using a screw.
The bottom of the cistern is like a bag made of flexible leather. The mercury level can be adjusted by means of a screw provided underneath. There is an ivory pointer in the cistern, placed at the top. The tip of this pointer coincides with the zero of the main scale. The level of the mercury column in the cistern can be changed with the screw under it. It is so adjusted that the ivory point is exactly at the surface of the mercury in the cistern. The whole apparatus is fixed in a vertical position.
A Torricellian barometer (sometimes called a mercury barometer) is an inverted (upside-down) glass tube standing in a bath of mercury. Air pressure pushes down on the surface of the mercury, making some rise up the tube. The greater the air pressure, the higher the mercury rises