1/8 inch in thickness. The lead exhaust tube through which the cell is
evacuated is pinched off and sealed at the end.
(2) The movement of the cell, caused by the varying pressure of
the atmosphere, is transmitted to the gear sector lever by thin strips
of beryllium copper which act as hinges. A fixed hinge serves as the
fulcrum of the gear sector lever. The teeth of the gear sector engage
a small pinion on the pointer shaft. A wire safety stop prevents the
teeth on the gear sector from becoming disengaged from the pinion
gear during severe vibrations or rough handling. A small auxiliary coil
spring is attached to the gear sector lever. The tension of the spring
can be varied by an adjusting disk which has a gear at its lower end
which meshes with a brass adjusting sector (fig. 9). The adjusting
disk is reached through a plugged opening in e plastic window. The
range of adjustment is approximately 10 millibars. Backlash in the
gears is removed by means of a small wire spring attached to the lower
side of the gear sector. The tension of this spring is transmitted to
the pointer shaft by a nylon thread looped once around a small lucite
drum at the base of the pointer shaft. A stop is provided to prevent
expansion of the cell below its lower limit during air transportation.
(3) The pinion is fastened to the pointer shaft which revolves in
a jeweled bearing at the bottom and extends through a hole in the
top bearing plate. The pointer, fastened to the end of the shaft, is
made of three sections of fine aluminum tubing and is clamped to
the shaft by a small setscrew.
b. Magnification. Magnification of the cell movement is accomplished
in three stages:
(1) The first stage is a simple lever magnification, being the ratio
of the distance between the fixed hinge (which acts as the fulcrum of
the lever) and the teeth of the gear sector to the distance between the
(2) The second stage is the ratio of the diameter of the gear sector
to the diameter of the pinion.
(3) The third stage is the ratio of the distance between the pointer
shaft and the scale being read to the radius of the pinion.