Wheatstone Bridge

The measurement of resistance is crucial for any electrical circuit. Wheatstone bridge is used to measure the value of unknown resistance. It uses voltage measurement to know the importance of resistance. This bridge is used in multiple compression and tension-based devices like fluid or air pressure sensors, strain gauges, etc.

A robust electrical circuit can calculate the resistance effectively. The simple formula of the course helps calculate the resistance value without any intricate calculations. This bridge is found in differential amplifiers, high impedance amplifiers, instrumentation amplifiers, etc.

What is the Wheatstone bridge?

It is also known as a resistance bridge as it helps measure unknown resistance values. It consists of two known resistors, one variable resistor and one unknown resistor. The Wheatstone bridge circuit is highly reliable as it offers accurate measurements. It has a significant name as it was made famous by Sir Charles Wheatstone in 1843. However, this famous bridge was invented by Samuel Hunter Christie.

Imagine an electrical circuit formed by the four legs of the resistors. The bridge is balanced if the sum of resistances balances both legs. A galvanometer measures it. It is simple to understand the construction and working of this bridge.

Formula

As this bridge finds the unknown electrical resistance, let the value of resistance be R, then,

R = PS/Q

where,

P: the value of the first known resistance

Q: the value of the second known resistance

S: the standard arm of the bridge

Hence, it is easy to calculate the value of the unknown resistance using this bridge. You need the two known resistances and one variable resistor to estimate the unknown resistance value.

Principle

The working principle of this resistance bridge is the null deflection. When the resistance ratio in both arms is equal, no current flows through the galvanometer. It is denoted by null deflection in the galvanometer. Hence, current flows through this electrical circuit under normal conditions, and the bridge is said to be in an unbalanced state. It is said to be in a balanced state only if there is no current flowing in the galvanometer. Thus, the work of the variable resistance or resistor is to establish the balance of the bridge by offering required resistances in the ratio of the four arm resistances.

Applications

• It can measure multiple electrical quantities like inductance, capacitance, impedance and resistances by varying the amounts defined. Hence, it is a multipurpose electrical circuit bridge.

• It is the perfect solution to find the accurate values of small unknown resistances.

• It can measure multiple physical quantities like strain, temperature and light. This is possible by adding an operational amplifier to the existing bridge circuit.

Limitations

• It may not be the ideal solution to calculate small resistances. This is because the resistance of contacts and leads becomes significant and introduces errors in the calculations. Hence, this bridge is modified to Kelvin’s bridge for calculating small resistances.

• It may not be able to find the values of high resistances. This is because the measurements of the unknown resistance become so large that the galvanometer can’t show sensitivity to the deflections. Hence, modifications to this bridge are made to ensure the galvanometer remains sensitive to the highest values of resistances.

• It may not be possible to accommodate the heating effects of the current. It changes the value of resistance that can’t be calculated with this bridge. It is often observed that excessive winds can cause permanent changes in the resistance values.

• The Wheatstone bridge is a delicate device. In an off-balance situation, measurements may not even be accurate.

• Wheatstone bridges are commonly used to measure the resistance of a few ohms to those few kilo-ohms.

• If the four resistances are not equivalent, the circuit’s sensitivity is reduced.

Conclusion

Thus, Wheatstone Bridge is a device used to measure the unknown resistance of a resistor when the other resistance is known. The device has four arms as four resistors and the current is balanced as a ratio of lengths and resistances.