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A Wheatstone bridge is an electrical circuit invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. ^{[1]} It is used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer.
Contents
Operation
In the figure, R_{x} is the unknown resistance to be measured; R_{1}, R_{2} and R_{3} are resistors of known resistance and the resistance of R_{2} is adjustable. If the ratio of the two resistances in the known leg (R_{2} / R_{1}) is equal to the ratio of the two in the unknown leg (R_{x} / R_{3}), then the voltage between the two midpoints (B and D) will be zero and no current will flow through the galvanometer V_{g}. If the bridge is unbalanced, the direction of the current indicates whether R_{2} is too high or too low. R_{2} is varied until there is no current through the galvanometer, which then reads zero.
Detecting zero current with a galvanometer can be done to extremely high accuracy. Therefore, if R_{1}, R_{2} and R_{3} are known to high precision, then R_{x} can be measured to high precision. Very small changes in R_{x} disrupt the balance and are readily detected.
At the point of balance, the ratio of R_{2} / R_{1} = R_{x} / R_{3}
Therefore,
Alternatively, if R_{1}, R_{2}, and R_{3} are known, but R_{2} is not adjustable, the voltage difference across or current flow through the meter can be used to calculate the value of R_{x}, using Kirchhoff's circuit laws (also known as Kirchhoff's rules). 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.
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