The unbalanced bridge, when V OUT is not equal to zero, is used to measure several types of physical quantities such as mechanical strain, temperature, or pressure. In a practical bridge circuit, the positive of the R v adjustment can be calibrated to indicate the actual value of R x on a scale or with some other method of display.

A voltmeter can also be used to indicate when the bridge is balanced.įrom equation the value of R v the balance multiplied by the scale factor R 2 /R 4 is the actual resistance value of R x .If R 2 /R 4 = 1,then R x = R v, if R 2 / R 4 = 0.5, then R x = 0.5 R v , and so on. When the needle is at the midpoint, the bridge is balanced because the voltage from A to B is zero and the current the galvanometer is zero, as illustrated. If the current is in one direction, the needle deflects toward the negative side. So if the current through it is one direction, the needle deflects toward the positive side. It differs from a regular ammeter in that the mid-scale point is zero. The galvanometer is essentially a very sensitive ammeter that senses current in either direction. Then, the unknown resistance is found as:Ĭommonly, a measuring instrument called a galvanometer is connected between the output terminals A and B is detect a balanced condition. When R x is placed in the bridge, R v is adjusted until the bridge is balanced as indicated by a zero output voltage. Therefore, R 3 is a variable resistor, which we will call R x. Since R x can be any value, R3 must be adjusted to make R 2/ R 3 = R 2 / R 4 in order to create a balanced condition. Resistor R 2 and R 4 have fixed values so their ratio,R 2/ R 3 , also has a fixed value.

Using the Balance Wheatstone Bridge to Find unknown ResistanceĪssume that R1 has an unknown value, which we call R x.

Therefore, the voltage ratios can be written as: When the bridge is balanced, the voltages across R 1 and R 2 are equal (V 1 = V 2 ) and the voltages across R 3 and R 4 are equal (V 3 = V 4 ). The Wheatstone bridge is in the balanced bridge condition when the output voltage (V OUT) between terminals A and B is equal to zero.

Wheatstone bridge calculator