Barometer is an instrument used to measure atmospheric pressure.
How can we measure atmospheric pressure by mercury barometer?
Mercury barometers consist of a glass tube about 80 cm long,closed at one end.The tube is filled with mercury and then placed,open end downward,in a reservoir of mercury.As the atmospheric pressure changes ,the level of the mercury changes and to a lesser degree,so does the level in the reservoir.The pressure is measured by the difference h between the levels.The space at the top of the tube is known as a torricellian vacuum.
In Fortin’s barometer ,the scale for measuring the height is fixed and the level of the mercury in the reservoir is adjusted by moving the flexible bottom of the reservoir to be at zero of the scale.The difference in height can then be read directly.There are also instruments in which the scale is moved so that its zero consides with the lower mercury level.
Which instrument is used to measure atmospheric pressure as well as altitude?
The aneroid barometer is used to measure atmospheric pressure as well as altitude.It consists basically of an evacuated flat cylindrical closed metal box with corrugated flexible faces that are kept from collapsing together by a spring.As the external pressure varies,the distance between the flat faces alters and a pointer is operated.Such an instrument is calibrated by comparison with a mercury barometer.It is also used as an Altimeter.:-P