screwed to a bronze gear plate. The gear plate is fastened inside the
top of the barometer case by brackets and screws.
(2) The aneroid element consists of two pressure sensitive cells
approximately 2 inches in diameter. The cells have been almost
completely evacuated (about 95 per cent) through a tube which is
pinched off and soldered at the end.
(3) Changes in the atmospheric pressure cause the cells to expand
and contract, and tins motion is transmitted by a connecting link to
an adjustable lever mounted on a bimetal temperature compensation
shaft. The link is connected at one end to the aneroid cells and at
the other to the adjustable lever by small pivots and is held in place
by antifriction washers and locking springs. The adjustable lever con-
verts the motion of the link into a rotation of the bimetal temperature
compensation shaft. A small calibrating block near the center of the
bimetal shaft serves as the mounting for the adjustable lever. A counter-
balance rod with a counterbalance weight at the end also is attached
to the calibrating block. The purpose of this balance weight is to
balance the linkage system so that the pointer will not change in indi-
cation when the position of the barometer is changed. A fine coil
spring, 1 inches long, is attached at one end to a rod on the calibrat-
ing block and anchored at the other to the casting. Its purpose is to
prevent backlash by keeping the linkage system under tension.
Note. Both Barometers ML-102-E and ML-103-F are calibrated for a vertical posi-
tion and should be used in that position for accurate readings.
(4) The bimetal temperature compensation shaft is made of a strip
of brass and a strip of invar welded together lengthwise. It is set in
jeweled bearings at the top and bottom. Since the coefficient of expan-
sion of invar is much less than that of brass, changes in temperature
cause the shaft to bow slightly. The bowing of the shaft displaces the
adjustable lever which is mounted near the center of the shaft, and
thereby changes the indication of the pointer slightly. This change
in indication offsets the change resulting from slight movement of the
pressure sensitive cells caused by variations in temperature.
(5) A brass sector is fastened to the bimetal shaft by a hub contain-
ing two setscrews. This sector transmits the motion of the bimetal shaft
to the pointer shaft. Fine gear teeth cut into the arc of the sector mesh
with a small pinion on the pointer shaft. At the opposite end of the
sector is a counterweight to balance the sector on the shaft. A U-shaped
stop is mounted on the casting to limit the movement of the sector.
(6) The pointer shaft is set in jeweled bearings. The pointer, which
fits over the end of the pointer shaft, is made of aluminum, painted
dull black, and has a counterweight on the non-indicating end. The
indicating end, which projects over the scales on the dial, is approxi-
mately 0.011 inch wide.
(7) A hairspring is attached to the pointer shaft on the forward
side of the casting, immediately below the dial. Its purpose is to take
up any backlash between the sector and the pinion of the pointer shaft.
(8) A horseshoe-shaped brass stop is fastened to the brass casting
to prevent expansion of the pressure element to a point below the
lower pressure limit of the instrument. This could happen only when
the instrument is transported by air.
(9) Teeth, cut around the periphery of the gear plate, mesh with a
worm which rotates the mechanism and the pointer so that the aneroid