SM0486
(4) Operating Principle.
The operating principle of the resistance
thermometer is based on the fact that the resistance of an electrical conductor
increases when the conductor is heated.
Resistance changes due to changes in
temperature can be accurately measured with the resistance bridges you have used
for other measurements. To calibrate the thermometer calibrator, you use a bridge
similar to the Mueller temperature bridge. Since the bridge is an integral part of
the measurement circuit, we discuss the Mueller temperature bridge as it is used in
a resistance thermometer circuit.
4.
MUELLER TEMPERATURE BRIDGE.
Although you have had some training on the construction and use of the Mueller
temperature bridge, we would like to review its construction and the basis on which
it operates.
a. The Mueller temperature bridge is designed to measure temperatures or
temperature difference to a high degree of accuracy when used in conjunction with
precision resistance thermometers. The range of the bridge is 1 to 111.111 ohms.
b. The bridge is balanced by the use of a convenient set of six step-by-step
dial switches.
Certain coils used with these dial switches must be kept at a
constant temperature while making temperature measurements.
These coils are
mounted in a special thermally insulated block whose temperature is kept constant
operating temperature control are mounted in a separate container, thereby
preventing galvanometer deflections when the relay operates. With this arrangement
there is no need for temperature corrections.
c. The sensitivity of the instrument and the damping of the galvanometer can
be adjusted over a reasonably wide range. It is also possible to perform a quick
check of--and if necessary, to adjust the equality of the ratio arms of--the bridge
to read the current flowing in the resistance thermometer. A special terminal and
an adjustable resistor are used in the measurement so that the potential leads of
the thermometer can be equalized as necessary.
d. A mercury contact commutator permits reversing the connections to a four-
lead potential-terminal-type resistance thermometer so that you can cancel the
effect of lead resistance.
A plug and block arrangement is used to determine
whether zero correction is necessary.
Terminals are included for connections to
the external battery (for supplying the bridge), to the external galvanometer, and
to either a three- or four-lead resistance thermometer.
e. As the Mueller bridge principle is discussed, you should remember the
primary purpose for its discussion.
The resistance thermometer is based on the
fact that for every degree the temperature increases or decreases a proportional
amount. These resistance changes can be detected
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