Wheatstone bridge

A Wheatstone bridge is an electrical schema used to
measure an unknown electrical resistance by balancing two
legs of a bridge schema, one leg of which includes the
unknown component. Its operation is similar to the original
potentiometer. It was invented by Samuel Hunter Christie in
1833 and improved and popularized by Sir Charles
Wheatstone in 1843. One of the Wheatstone bridges initial
uses was for the purpose of soils analysis and
comparison.

Operation

In the figure, is the unknown resistance to be measured; , and are resistors of known resistance and

the resistance of is adjustable. If the ratio of the two resistances in the known leg is equal to the ratio of

the two in the unknown leg , then the voltage between the two midpoints (B and D) will be zero and no

current will flow through the galvanometer . If the bridge is unbalanced, the direction of the current indicates

whether is too high or too low. 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 , and

are known to high precision, then can be measured to high precision. Very small changes in disrupt the

balance and are readily detected.

At the point of balance, the ratio of