placed under stress. As soon as the stress is removed, they return
almost to their original shape. The retardation or failure of the mate-
rial to assume completely its original form is known as hysteresis. All
aneroid barometers are subject to the effects of hysteresis although an
attempt is made to keep these effects to a minimum by the proper
selection and treatment of materials. Over a long period of time hys-
teresis effects gradually tend to disappear.
(2) Hysteresis occurs in the pressure sensitive cell of the aneroid
barometer and for that reason appreciable errors may be noticeable
immediately after a large and rapid pressure change. These errors can
be greatly reduced by waiting a sufficient length of time before taking
a reading. It is difficult to make a specific recommendation as to the
interval of time necessary to reduce hysteresis errors to a minimum
since hysteresis varies with the amount of change in pressure, with
the rate of change, and with the time involved that is, the length of
time at which the barometer has been subjected to a pressure differing
from current atmospheric values.
(3) For pressure changes occurring at any fixed station, hysteresis
errors are small enough to be neglected. When the barometer is sub-
jected to a- comparatively large pressure change for a short time only,
and then restored to its previous values, hysteresis effects are small and
quickly disappear. Hysteresis errors are probably most pronounced
when a barometer is transported by airplane, especially if the flight is
of several hours duration and the plane flies at high altitude. There-
fore, to be reasonably sure of reducing hysteresis errors to a minimum
under such circumstances, it is advisable to wait at least 24 hours before
taking a reading.
b. Temperature. A second and exceedingly important error which
affects the aneroid is temperature changes. Without compensation for
Changes in temperature the errors of an aneroid would be quite large.
Several methods of temperature compensation are used in the barome-
ters covered by this manual.
(1) Aneroid cell. If a barometer is maintained at a constant pres-
sure, a change in temperature, if not compensated, will cause a move-
ment of the pointer on the dial. This indicated change in pressure is
due to a physical change in the dimensions of the instrument and
also to variations in the stiffness of the pressure sensitive cell. This
latter condition may be compensated, for the most part, by leaving a
small amount of air in the cell. Thus when a change in temperature
occurs, the change in pressure of the air inside the cell partially bal-
ances the change in strength of the metal. This does not entirely com-
pensate for changes in temperature. Therefore, temperature correction
curves are provided for some models of Barometer ML-102-(*) and
M L - 316/TM. These corrections, however, are small, indicating that
the greater part of the error has been compensated.
(2) Bimetal temperature compensation shaft. Barometers ML-
102-B, ML-102-E, and ML-102-F use an additional temperature
compensation in the form of a bimetal pivot shaft made of a strip of
invar and a strip of brass welded together lengthwise. Since invar and
brass have different coefficients of expansion, changes in temperature
cause the bimetal shaft to bow slightly, thus changing the indication
of the pointer.