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Concept
Wheatstone bridge
Summary
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of the circuit is its ability to provide extremely accurate measurements (in contrast with something like a simple voltage divider). Its operation is similar to the original potentiometer.
The Wheatstone bridge was invented by Samuel Hunter Christie (sometimes spelled "Christy") in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. One of the Wheatstone bridge's initial uses was for soil analysis and comparison.
In the figure, Rx is the fixed, yet unknown, resistance to be measured.
R1, R2, and R3 are resistors of known resistance and the resistance of R2 is adjustable. The resistance R2 is adjusted until the bridge is "balanced" and no current flows through the galvanometer Vg. At this point, the potential difference between the two midpoints (B and D) will be zero. Therefore the ratio of the two resistances in the known leg (R2 / R1) is equal to the ratio of the two resistances in the unknown leg (Rx / R3). If the bridge is unbalanced, the direction of the current indicates whether R2 is too high or too low.
At the point of balance,
Detecting zero current with a galvanometer can be done to extremely high precision. Therefore, if R1, R2, and R3 are known to high precision, then Rx can be measured to high precision. Very small changes in Rx disrupt the balance and are readily detected.
Alternatively, if R1, R2, and R3 are known, but R2 is not adjustable, the voltage difference across or current flow through the meter can be used to calculate the value of Rx, using Kirchhoff's circuit laws. This setup is frequently used in strain gauge and resistance thermometer measurements, as it is usually faster to read a voltage level off a meter than to adjust a resistance to zero the voltage.
At the point of balance, both the voltage and the current between the two midpoints (B and D) are zero. Therefore, , , .
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