Wheatstone bridge principle and resistance?

Question

Why 4 resistance is used in Wheatstone bridge? Why not 3 or 2
resistance?Tell me reasons.

Answer 1

ll strain-gauge configurations are based on the concept of a Wheatstone bridge. A Wheatstone bridge is a network of four resistive legs. One or more of these legs can be active sensing elements.

The Wheatstone bridge is the electrical equivalent of two parallel voltage divider circuits. R1 and R2 compose one voltage divider circuit, and R4 and R3 compose the second voltage divider circuit. The output of a Wheatstone bridge is measured between the middle nodes of the two voltage dividers.

A physical phenomena, such as a change in strain applied to a specimen or a temperature shift, changes the resistance of the sensing elements in the Wheatstone bridge. The Wheatstone bridge configuration is used to help measure the small variations in resistance that the sensing elements produce corresponding to a physical change in the specimen.

Strain-gauge configurations are arranged as Wheatstone bridges. The gauge is the collection of all of the active elements of the Wheatstone bridge. There are three types of strain-gauge configurations: quarter-bridge, half-bridge, and full-bridge. The number of active element legs in the Wheatstone bridge determines the kind of bridge configuration.

Each of these three configurations is subdivided into multiple configuration types. The orientation of the active elements and the kind of strain measured determines the configuration type.

Answer 2

The whole point of the bridge is that each of two pairs of series-connected resistances forms a voltage divider such that the two center nodes are at equal voltage. Obviously, to have two such pairs requires four resistors. Typically, in devices using this, the center nodes are connected by a galvanometer of some type, and one of the resistors is actually a device under test.