Page 483 - Instrumentation Reference Book 3E

466 Electrical measurements

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Balance condition: Balance condition:

Rl + RL4=R31 +RL1 Rl+RLl=R3Z+RL4
Thus Ri=- R31 + R32
2

R1 is unknown resistance; R3=R4; RL~, RL~, RL~, RL~ lead resistances
are
Bridge connections for first balance Bridge connections for second balance

Balance condition: Balance cohdition:
RI + RLI =Rzi + RL~ R1 + RL4'R22+ RLl
Thus Ri=- R21+ R22
2
Figure 20.53 (a) Smith bridge for four-lead platinum resistance thermometer measurement; (b) Muller bridge for four-lead
platinum resistance thermometer measurement.

and thus the unknown resistance R1 is given by

Current Potential
terminals terminals

Figure 20.54 Afour-terminal resistance.