is adjusted for null deflection on the galvanometer.
The temperature
deviation from nominal can then be read on the dial engraving. The galva-
nometer should have a current sensitivity of approximately 2000 mm/A.
(2) When an electronic detector is used instead of a galvanometer,
some 60 Hz pickup may be noticed at the GALV terminals. If this pickup is
excessive the detector ground terminal should be connected to the GND
terminal on the back of the instrument.
(3) The BRIDGE is switched off after use to conserve life of the
mercury cell.
c. State of charge indicator. The state of charge of the batteries may
be checked at any time by briefly pressing down the BATT TEST spring-return
switch and noting the indication on the front panel meter. When the battery
is fully charged, the pointer will swing to the right end of the GREEN band
on the meter. When the pointer reaches the RED band, the batteries must be
recharged.
When the BATT TEST switch is operated during or shortly after
charging, the meter pointer will not settle at one reading but will drift
slightly toward the RED band.
The switch should not be held until a
constant reading is obtained.
CAUTION
When the BATT test switch is depressed, a heavy
current is drawn from the batteries and for this
reason the test should be brief to avoid damage.
d. Overtemperature protection.
An auxiliary thermostat is set to
operate a few degrees above the nominal temperature.
This provides
protection against damage to the standard cells in the event of a failure of
the temperature control system.
(1) Excessive temperature rise is prevented by closing of the
thermostat which blows a fuse to disconnect the heater circuit and at the
same time causes the front panel meter to indicate a fixed deflection at
approximately half scale.
(2) In the event of a failure the cause should be corrected before
the fuse is replaced.
Failure during transit may be caused by a high
ambient temperature, and the standard cell voltages should be carefully
monitored for possible permanent damage.
5.
Theory of operation.
a. Temperature control.
The temperature control system utilizes a
selected thermistor as the detection element.
It is connected into a
patented bistable-oscillator transistor switching system that is sensitive
to changes of a few 10,000th of a degree (figs 2-14 and 2-15 at the end of
the lesson).
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