- Wheatstone Bridge
- Principle of Wheatstone Bridge
- Balanced Wheatstone Bridge
- Unbalanced Wheatstone Bridge
- Applications of Wheatstone Bridge
- Kelvin’s Double Bridge

It is an arrangement of four resistances devised by

**Charles Wheatstone**which is used to measure an unknown resistance.

### Principle of Wheatstone Bridge

The Wheatstone bridge principle states that if four resistances P, Q, R, and S are arranged to form a bridge with a cell and key between A and C, and a galvanometer between B and D then the bridge is said to be balanced when the galvanometer shows a zero deflection.In balanced condition$\mathrm{I}_{9}=0 \quad$ so $\mathrm{V}_{\mathrm{B}}=\mathrm{V}_{\mathrm{D}}$ or $\frac{P}{Q}=\frac{R}{s}$. This is called condition of balance.

### Balanced Wheatstone Bridge

The condition of balance depends on resistance P, Q, R, and S. This is independent of the emf of the battery.In-state of balance, the cell and galvanometer can be interchanged. While performing an experiment at start press the cell key $K_{c}$ first and then the galvanometer key $\mathrm{K}_{9}$ and at end remove $\mathrm{K}_{9}$ first and then $\mathrm{K}_{\mathrm{c}}$ to avoid induced effects.

P and Q are called ratio arms, R is the known resistance arm and S is the unknown arm. BD and AC are called

**conjugate arms.**

**The resistance of a balanced Wheatstone bridge between A and C is $\frac{(P+Q)(R+S)}{P+Q+R+S}$.**

The sensitivity of the bridge depends upon the value of resistances. The sensitivity is maximum when P, Q, R, and S are of the same order.

#### Unbalanced Wheatstone bridge

If $\frac{P}{Q}<\frac{R}{s}$ then $V_{B}>V_{D}$ so current flows from $B$ to $D .$

If $\frac{P}{Q}>\frac{R}{s}$ then $V_{B}<V_{D}$ so current flows from $D$ to $B$

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### Applications of Wheatstone Bridge

The**Wheatstone Bridge**has many uses in electronic circuits other than comparing an unknown resistance with a known resistance. When used with Operational Amplifiers, the Wheatstone bridge circuit can be used to measure and amplify small changes in resistance, R

_{X}due.

Wheatstone bridge is not suitable for measurement of very small and very high resistances. Very low resistances are measured by

**Kelvin’s double bridge**while very high resistances by

**leakage method.**

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**Also Read:**

- Types and Effects of Electric Current
- Ohm’s Law and Resistance
- Combination of Resistances
- EMF and Internal Resistances of a Cell
- Cells Connected in Series, parallel and Mixed
- Kirchhoff’s Circuit Law
- Electric Currents in Conductors
- Wheatstone Bridge
- Post office Box
- Wheatstone Meter Bridge
- Moving Coil galvanometer
- Ammeter and Voltmeter
- Potentiometer Working Principle