The Wheatstone bridge or the resistance bridge is the special arrangement of resistors that is used to calculate the unknown resistance by balancing of bridge circuit. It was discovered by Charles Wheatstone.

## Wheatstone bridge circuit

### Construction

The bridge contains four resistances out of which the value of two resistances are known. The third one is of variable resistance and the forth one is unknown. We find the value of forth resistance with the help of other three. The circuit also have a galvanometer and a voltage source.

The voltage source is connected to point a and c the galvanometer is connected to point b and d.

Resistor R1 and R2 are known.

The resistor R3 has variable resistance. ( A resistor whose resistance can be adjusted.

The resistor R4 is unknown.

### Working principle

The Wheatstone bridge is operated on the principle of null defection.

When the ratio of R1 and R2 is equal to R3 and R4, no current flows through the galvanometer and it shows null deflection.

Normally the bridge remains in unbalanced condition and current flows through galvanometer.

We need to obtain balanced condition for the calculation of value of 4^{th} resistor. The balanced condition is achieved by adjusting the variable resistor, when R1 and R2 are fixed and R4 is unknown.

The null deflection that is no current through galvanometer indicates the balanced condition.

Now we can easily calculate the value of R4 using formula.

### Wheatstone bridge formula

The formula used under balanced condition is

R1/R2 = R3/R4

R4 = ( R2 R3 ) / R1

### Wheatstone bridge applications

- It is used to measure low value resistance accurately.
- It can measure the quantities like impedance, capacitance and inductance.
- The Wheatstone bridge is used to measure physical parameters such as strain, temperature and light.

#### Frequently asked questions

**What is Wheatstone bridge ?**

- The Wheatstone or the resistance bridge is basically a device that is used to calculate an unknown resistor by balancing two arms of the bridge arrangement.

**What is the Wheatstone bridge principle ?**

- The Wheatstone bridge is operated on the principle of null defection. When the ratio of ( resistances ) R1 and R2 is equal to R3 and R4, no current flows through the galvanometer and it shows null deflection.

**When the Wheatstone bridge is balanced ?**

- The bridge is recognised as balanced, when no current flows through the galvanometer. This condition is obtained by adjusting the variable resistor.

**When is the Wheatstone bridge said to be unbalanced ?**

- The bridge is said to be in unbalanced condition, when current flows through the galvanometer. This is the normal condition.

**Author**

Akash Sharma

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