D. Here, the currents of the two legs combine and
Radiometer System
flow through R7 to the positive 28 volts.
The radiometer is a temperature indicator that
Note that restoring coil L2, resistor R3, and
uses two coils in a balanced circuit. In some
upper part of potentiometer R2 forms a parallel
instruments, the coils turn between the poles of
path for current flow from point A to pin 2 of
a permanent magnet. In other instruments, a small
R2. Deflection coil L1 connects between points
permanent-magnet rotor turns between stationary
A and B. Therefore, any difference in potential
coils. Radiometer circuits vary in design, but the
between these two points will cause current to flow
principle of operation is very much the same for
through L1.
all.
The radiometer temperature indicator uses a
Figure 6-49 shows a simplified circuit with a
fixed permanent magnet to pull the pointer to an
permanent-magnet rotor. The two coils are
off position when the indicator is not operating.
stationary in the instrument, and the indicator
Thus, current through restoring coil L2 must
needle fastens to the permanent-magnet rotor.
compensate for the pulloff magnet when the
The needle position is determined by how the
indicator is operating. Variations in the resistance
permanent magnet aligns itself with the resultant
of the bulb, because of temperature changes,
flux of the two coils.
cause a change in voltage at point B. It also causes
For an understanding of how the circuit
the resulting change in current through deflection
operates, let's trace the current through the circuit.
coil L1.
Starting at ground, current flows up through the
bulb, centering potentiometer R5 and R6, to point
D. Current through the left leg of the bridge is
Thermocouple System
from ground through R1 to point A. Current then
goes from point A through the lower part of the
Thermocouple temperature indicators show
expansion and contraction potentiometer R2. It
the air temperatures in the heater duct of
also goes from pin 2 of R2 through R4 to point
anti-icing systems and in the exhaust systems of
jet engines.
A thermocouple is a junction or connection
of two unlike metals; such a circuit has two
junctions. When one of the junctions becomes
hotter than the other, an electromotive force
is produced in the circuit. By including a
galvanometers in the circuit, this electromotive
force can be measured. The hotter the high-
temperature junction (hot junction) becomes, the
greater the electromotive force. By calibrating the
galvanometers dial, in degrees of temperature,
it becomes a thermometer. The galvanometers
contains the cold junction.
The thermocouple thermometer systems used
in naval aircraft consist of a galvanometers
indicator, a thermocouple or thermocouples, and
thermocouple leads. Some thermocouples consist
of a strip of copper and a strip of constantan
pressed tightly together. Constantan is an alloy
of copper and nickel. Other thermocouples consist
of a strip of iron and a strip of constantan. Others
may consist of a strip of Chromel and a strip of
Alumel.
The hot junction of the thermocouple varies
in shape, depending on its application. Two
Figure 6-49.-Radiometer type temperature indicator.
common types--gasket and rivet--are shown in
