FUNCTIONING OF PARTS
OF ANEROID BAROMETER.
a. General. (1) The
is a form of "elastic" barometer, that is, one in which the elastic
deformation of some solid system is used as an indicator of atmospheric
(2) The aneroid type of barometer can be made extremely sensi-
tive and is convenient to use because of its portability. A further advan-
tage is that its readings are not affected by variation in the force of
(3) The aneroid barometer is subject to errors due to irregularities
in the elasticity of the metal (par. 1 5) and for this reason it should
not be relied upon as an instrument for routine meteorological obser-
vations unless compared frequently (at least every 90 days) with a
b. The Vidie barometer. The type of aneroid most generally used is
one invented in 1 843 by Lucien Vidie, which uses a wafer-like cell,
(sometimes called diaphragm or capsule) of thin, flexible metal, usu-
ally brass or German silver, which is very nearly exhausted of air.
The opposite sides of the cell are kept apart by a strong internal metal
spring; some models use a stiff external spring for this purpose. Usually
the elastic properties of the spring determine those of the whole instru-
ment. The residual air in the cell can be adjusted to give a partial
correction for temperature. The movement of the cell, caused by varia-
tions in pressure, is greatly magnified, and is indicated on a dial by
differ from the Vidie type in that the material of the cell itself serves
as a spring, making the use of an internal or external spring unnec-
essary. The aneroid element. of these barometers consists of one or
more cells of beryllium copper which have been almost completely
exhausted of air, only enough being left inside to help compensate.
for changes in the spring properties of the cell as a result of changes
in temperature. Corrugations in the cell increase the flexibility of the
metal so that there is a greater movement with changes in pressure.
Changes in the atmospheric pressure cause the cells to expand and
contract. This movement is magnified and transmitted by a lever
system connected to a pointer which indicates the pressure values on
a dial graduated in inches of mercury or the equivalent pressure
value in millibars.
(2) A detailed description of the different models of the barometer
covered by this manual, with the functioning of the aneroid mechan-
OF ANEROID BAROMETER. Factors which
into the readings of the aneroid barometer are hysteresis, temperature,
scale errors, and drift.
a. Hysteresis. (1) One
of the chief causes
in an aneroid
barometer is hysteresis. All elastic materials tend to change shape when