Short Answer

A Wheatstone bridge is an electrical circuit used to measure an unknown resistance by comparing it with a known resistance. It consists of four resistors arranged in a diamond shape, with a galvanometer connected between two opposite points. When the bridge is balanced, no current flows through the galvanometer, allowing the unknown resistance to be calculated accurately.

The Wheatstone bridge is widely used in laboratories, sensors, and measuring instruments because it gives very precise results. It works on the principle of balancing voltage drops in different branches of the circuit.

Detailed Explanation

Wheatstone bridge

A Wheatstone bridge is a special type of electrical circuit designed to measure an unknown resistance by comparing it with known resistances. It is one of the most accurate and reliable methods for resistance measurement and has been used for more than a century in laboratories and engineering applications. The bridge is named after Sir Charles Wheatstone, who popularized the arrangement.

The Wheatstone bridge works based on the principle of null deflection, which means adjusting the circuit until no current flows through the galvanometer. At this balanced point, a mathematical relationship forms between the resistances, allowing us to calculate the unknown value easily.

Structure of a Wheatstone bridge

The Wheatstone bridge consists of:

• Four resistors (R₁, R₂, R₃, and Rₓ) arranged in a diamond or bridge shape
• A galvanometer (G) connected between two junctions
• A battery or power source connected across the other two junctions

Here:

• R₁ and R₂ form one arm of the bridge
• R₃ and Rₓ (unknown resistor) form the other arm

The galvanometer detects whether there is any current flowing between the arms.

Working principle of Wheatstone bridge

The Wheatstone bridge works on the idea of balancing potential differences. When the bridge is balanced, the ratio of resistances in one arm is equal to the ratio of resistances in the other arm.

The condition for a balanced bridge is:

R₁ / R₂ = R₃ / Rₓ

At this balanced condition, no current flows through the galvanometer, and it shows zero deflection. This allows the unknown resistance Rₓ to be calculated as:

Rₓ = (R₃ × R₂) / R₁

This equation is accurate and simple, which makes the Wheatstone bridge very useful in measurements.

How the Wheatstone bridge works

1. A power source is connected to the bridge.
2. Resistances R₁, R₂, and R₃ are chosen or adjusted.
3. The unknown resistance Rₓ is connected in its place.
4. The galvanometer detects the difference in potential between the two midpoints.
5. R₁, R₂, or R₃ is adjusted until the galvanometer reads zero.
6. When no current flows through the galvanometer, the bridge is balanced.
7. The unknown resistance is calculated using the balanced condition formula.

This method is extremely precise because it does not rely on measuring current directly; instead, it relies on achieving a balance.

Why the Wheatstone bridge is accurate

• It avoids measurement errors caused by instrument resistance.
• It uses ratio comparison, which is more precise than direct measurement.
• The method works even with very small or very large resistances.
• The null method (zero deflection) eliminates human and instrument error.

Because of these advantages, the Wheatstone bridge is used whenever high accuracy is required.

Applications of Wheatstone bridge

The Wheatstone bridge has many practical uses:

1. Measuring unknown resistance
   Used in laboratories to find precise resistance values.
2. Strain gauges
   Used to measure small changes in physical forces by detecting resistance changes.
3. Temperature sensors
   Thermistors and RTDs are often connected in Wheatstone bridge circuits to measure temperature changes.
4. Light sensors
   LDRs (Light Dependent Resistors) are used in bridges to measure light intensity.
5. Electrical testing equipment
   Used in calibration and testing of circuit components.
6. Precision instruments
   Used in scientific research and industrial monitoring.

Advantages of Wheatstone bridge

• Very high accuracy
• Simple design and easy to use
• Works for a wide range of resistances
• Eliminates errors through balance method
• Useful in sensor-based circuits

Limitations of Wheatstone bridge

• Cannot measure rapid changes because balancing takes time
• Accuracy decreases if the galvanometer is not sensitive
• Not suitable for very low resistance measurements unless modified (Kelvin bridge)

Even with these limitations, the Wheatstone bridge remains one of the most dependable methods for resistance measurement.

Conclusion

A Wheatstone bridge is an electrical circuit used to find unknown resistance by comparing it with known resistances. It works on the principle of balancing voltage in a closed loop, where no current flows through the galvanometer at balance. Because of its high accuracy, it is widely used in measuring instruments, sensors, and laboratory experiments. Understanding the Wheatstone bridge is essential for studying advanced electrical measurements and instrumentation.