Wheatstone Bridge and Metre Bridge

Wheatstone Bridge:

Wheatstone Bridge Principle states that if four resistances P, Q, R and S are arranged to form a bridge as shown in figure, if galvanometer shows no deflection, the bridge is balanced. In that case

Note that in Wheatstone bridge circuit, arms AB and BC having resistances P and Q form ratio arm. The arm AD, having a resistance R, is a known variable resistance arm and arm DC, having a resistance S is unknown resistance arm.

(i) Balanced bridge:

The bridge is said to be balanced when deflection in galvanometer is zero i.e. no current flows through the galvanometer or in other words VB = VD. In the balanced condition

On mutually changing the position of cell and galvanometer this condition will not change.

(ii) Unbalanced bridge:

If the bridge is not balanced current will flow from D to B if

i.e. which gives .

(iii) Applications of wheatstone bridge:

Meter bridge, post office box and Carey Foster Bridge are instruments based on the principle of wheatstone bridge and are used to measure unknown resistance.

Metre Bridge:

A slide wire bridge is a practical form of Wheatstone bridge.

It consists of a wire AC of constantan or manganin of 1 metre length and of uniform area of cross-section.

A meter scale is also fitted on the wooden board parallel to the length of the wire.

Copper strip fitted on the wooden board in order to provide two gaps in strips.

Across one gap, a resistance box R and in another gap the unknown resistance S are connected.

The positive pole of the battery E is connected to terminal A and the negative pole of the battery to terminal C through one way key K.

The circuit is now exactly the same as that of the Wheatstone bridge figure.

Adjust the position of jockey on the wire (say at B) where on pressing, galvanometer shows no deflection. Note the length AB ( = l say) to the wire.

Ø Find the length BC ( = 100 – l) of the wire.

According to Wheatstone bridge principle