Skip Content

Mercury Barometer (Kew Pattern)

Mercury Barometer (Kew Pattern)

Weather family -- CumulusThe barometer is a fixed-cistern barometer, often called the Kew-pattern barometer, which consists of a column of mercury supported in a vertical glass tube (capillary).   The lower end of the column is sealed by immersion in a cistern (storage receptacle) containing a reservoir of mercury.   The space above the mercury in the tube is evacuated and sealed, while the surface of the mercury in the cistern is exposed to atmospheric pressure.  An increase in atmospheric pressure causes the mercury level in the cistern to fall and that in the tube to rise, whilst falling pressure has the opposite effect.

Kew-pattern Barometer

The opening of the mercury-filled capillary dips into the storage receptacle, which is likewise filled with mercury.  The air pressure at the observation point acts on the open mercury surface and maintains the equilibrium of the mercury column in the capillary.   The barometer reading is indicated by the difference in height "H" between the two liquid levels.

Pressure is proportional to the difference between the two levels, but because the relevant parts of the cistern and the tube have uniform cross-sectional areas, the ratio of the level-changes resulting from a given change of pressure is a constant, and it is not necessary to measure both levels.  Only the mercury level in the tube is read off, against a direct-reading scale, which is not precisely linear but slightly contracted to allow for the change of level in the cistern below.

This reading must then be corrected for any deviation from the index error and the standard conditions of temperature (zero degree Celsius) and gravity (9.80665 metres per square second).

Close this window