is riot useful when the changes occur at a nearly con-
stant altitude due to the width of the steps and the am-
biguity of di'rection of vertical motion. Two pressure
measuring systems have been found satisfactory for use
in constant-level work and are discussed below. For a
discussion of the radio transmitters which have been
used (the standard T-69 and,the NYU AM-1), see Techni-
cal Report No. 2, Balloon Project, New York University
Research Division.
(1) Olland Cycle Pressure Measuring Instrument
This instrument, shown in Figure 31, is used in
balloon flights as the primary pressure measuring
unit, as it will continuously measure pressure
without ambiguity. It modulates the transmitted
r"dio signal at intervals whose timing is determined
by the pressure of the air at the balloon's position.
As presently designed, the modulator contains a
standard Signal Corps ML-310E radiosonde aneroid
unit, a rotating cyclinder of insulating material
with a metal helix wound around the cylinder, and a
6-volt electric motor which rotates the cylinder.
There are two contacting pens which ride on the cy-
linder and conduct electrical current when they
touch the helix. One pen is fixed in position and
makes a contact at the same time in each revolution
of the helix. This contact is used as a reference
point for measuring the speed of rotation of the
cylinder. The time that the second one, which is
linked directly to the aneroid cell, makes contact
with the spiral, is dependent on the cylinder speeed
and on the pen position which is detennined by the
pressure. By an evaluation chart, the atmospheric
pressure can be detennined as a function of the
relative posi~ion of the pressure contact as compared
to the reference thus eliminating ell rotation effect$
but short term motor speed fluctuations.
Preparation of the modulator for flight consists of
the following steps:
(a) Test the motor operation. When a 6-volt battery
is inserted in the motor circuit with the proper
polarity, the motor should run smoothly at one
re~olution per 60 to 80 seconds. Noisy opera-
tion is probably a sign of dirty or corroded
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