An instrument for measuring air pressure
used in weather forecasting and for determining altitude. Most commonly
encountered is the aneroid barometer (see illustration
to the right) in which the effect of the air in compressing an evacuated
thin cylindrical corrugated metal box is amplified mechanically and read
off on a scale or, in the barograph, used to draw a trace on a slowly rotating
drum, thus giving a continuous record of the barometric pressure. The aneroid
instrument is that used for aircraft altimeter.
|The aneroid barometer comprises a partially evacuated
corrugated metal box (a), prevented from collapsing by a spring (b).
The strain in this spring, proportional to the difference in pressures
between the air inside and outside the box, is amplified by a train
of levers (c) that operate a pointer (d) that moves over a calibrated
scale. Aneroid barometers are convenient to use but require regular
calibration against an accurate
The barograph is a recording barometer consisting of an aneroid barometer
that has, instead of a pointer, a pen that continuously records atmospheric
pressure on a paper chart wrapped round a revolving drum, usually driven
The earliest barometers, as invented by Torricelli
in 1643, consisted simply of a glass tube about 800mm long closed at one
end and filled with mercury before
being inverted over a pool of mercury. Air pressure acting on the surface
of pool held up a column of mercury about 760mm tall in the tube, a "Torricellian"
vacuum appearing in the closed end of the tube. The height of the column
was read as a measure of the pressure.
In the Fortin barometer, devised by Jean Fortin (1750–1831)
and still used for accurate scientific work, the lower mercury level can
be finely adjusted and the column height is read off with the aid of a vernier
Experiment: A milk-bottle barometer
A simple barometer can be made from a milk bottle or similar bottle.
Cut the end from a round toy balloon. Then stretch this end smoothly
over the mouth of the bottle and tie it in place. Next glue one end
of a drinking straw to the center of the rubber cap. For a scale,
mark a strip of cardboard with graduations and prop it up beside the
Compare the position of the straw pointer from day to day. When the
atmospheric pressure rises, it pushes the rubber cap in; when it falls,
the greater pressure inside the bottle pushes the cap out. Always
take your readings when the air is at the same temperature; otherwise
they will be unreliable.
Magic by Kenneth M. Swezey, p 53 (Kaye & Ward, London, 1971)