4.
Dial
a. The altimeter had a circular-dial (fig. 2) with four scales, two outer scales 6Vs inches in diameter, and two
inner scales 47/8 inches in diameter. All graduations are black and are numbered every 50 meters. Each scale covers
approximately one fourth of the range. Since the pointer (3, fig..2) makes four revolutions to cover the entire range, a
revolution indicator (7) is provided so the observer can determine upon which scale the instrument is operating. The
scales are custom calibrated to intervals of 2 meters, in accordance with table 51 of the Fifth Revised Edition, Smithsonian
Meteorological Tables except that the indicated altitude is 300 meters greater than that given in the tables. In order to
avoid negative values, zero on the scale corresponds to -300 meters of table V. The scale ranges to 4,500 meters of table
V, but is marked 4,800 meters on the dial.
b. An annular mirror (1), located in the dial between the outer and inner scales, is provided so that reading
parallax may be eliminated by superimposing the pointer on its reflected image.
c. A mercury alloy thermometer (5) is mounted in the dial to provide instrument temperature which is required to
obtain maximum accuracy.
d. The pointer may be set to agree with other altimeters or to the correct pressure-altitude by slowly turning the
adjusting screw (6) which is located in the face of the dial. The screw is reached by removing the lid, bezel ring, and
window. This screw should not be turned beyond the adjustment limits marked in red on the dial. A desiccant con-
6
