SM0486
and accurately converted from resistance to temperature values by the Mueller
bridges.
You use the resistance thermometer to calibrate thermometers,
thermocouples, and thermometer calibrators.
f. An examination of the bridge circuit (figure 5) should help you understand
its operating principle and the operation of the controls you use to null the
bridge and obtain the temperature values. We can simplify the explanation of the
bridge circuit in figure 5 by using simplified circuits of its primary components.
For instance look at figure 6.
g. The diagram in figure 6 is a simplified version of the upper portion of
the Mueller bridge in figure 5.
The resistances labeled "A" and "B" in figure 6
represent the ratio arms of the Mueller bridge in figure 5.
The slidewire in
figure 6 is a resistance balancing device with the same function as the slidewire
where a null is obtained in the galvanometer (G). The null indicates the equality
existing between ratio arms A and B. You check the equality of the ratio arms by
interchanging arms A and B.
h. The bridge rheostat arm shown at A in figure 7 is the same R shown in
figure 5. The shunt type decade shown at B in figure 7 is the type of decade shown
in A of figure 7 and as part of R in figure 5. The inclusion of the rheostat arm
shown in figure 7 (and its shunt decades) permits measurements as small as 0.001
ohm.
Special shunt type decades are used so that small resistance changes which
occur in devices such as the resistance thermometer are not masked (hidden) by
contact resistance.
i. The last division of the Mueller bridge discussed is shown in figure 8.
The resistor R represents the adjustable rheostat arm labeled "F" in figure 5. The
letter X represents a four-terminal resistance such as the one in the resistance
thermometer.
L1, L2, L3, and L4 are the leads which connect the resistor
(resistance thermometer) to the bridge for a measurement.
j. If the galvanometer is connected to binding post 2, as shown in figure 9,
the resistance X and the lead L4 (with its resistance) is connected in the
adjustable arm R.
k. If the ratio arms A and B are equal resistances and the resistance of L1
is equal to that of L4, the bridge is balanced by adjusting the rheostat arm until
its resistance equals the resistance of the arm in which X is located.
Usually,
leads L1 and L4 are interchanged and the successive readings averaged so that you
can record the reading presented when L1 is equal to L4 with the galvanometer at
its NULL position.
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