Wheatstone bridge

Wheatstonebridge

Wheatstonebridge current

Kelvin bridge by RFT

Wheatstone Bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is based on the principle that in a balanced bridge, the ratio of the two leg resistances is equal to the ratio of the other two leg resistances. This device was invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843.

Principle

The basic idea behind the Wheatstone bridge is to determine the unknown resistance by balancing two halves of a bridge circuit. The bridge consists of four resistances, resistors \(R_1\), \(R_2\), \(R_3\), and \(R_x\) (the unknown resistance), connected in a diamond shape. A voltage source is connected across the bridge, and a voltmeter is connected across one pair of opposite points in the bridge. When the bridge is balanced, the voltmeter reads zero, and at this point, the ratio of two known resistances in the circuit is equal to the ratio of the unknown resistance and the other known resistance. The formula used to calculate the unknown resistance is \(R_x = R_3 \times \frac{R_2}{R_1}\).

Components

The Wheatstone bridge consists of four main components:

• Resistors: Four resistors are used in the bridge. One of these resistors is the unknown resistor whose value is to be measured.
• Voltage source: A DC voltage source is connected to the bridge to provide the necessary power for the measurement.
• Voltmeter: A voltmeter is connected across the bridge to measure the voltage difference. When the bridge is balanced, the voltmeter reads zero.
• Switch: A switch is often included in the circuit to easily open and close the circuit without disconnecting any components.

Applications

The Wheatstone bridge is widely used in various applications, including:

• Measurement of very small resistance values.
• Calibration of measuring instruments.
• In strain gauge and temperature sensor circuits to measure changes in resistance that correspond to changes in strain or temperature.

Advantages

• High accuracy in measuring resistance.
• Capable of measuring very small changes in resistance.
• Simple and cost-effective.

Limitations

• Requires manual adjustment to balance the bridge, which can be time-consuming.
• Not suitable for measuring very high resistances due to sensitivity issues.
• Accuracy depends on the precision of the known resistances in the bridge.

Variations

Several variations of the Wheatstone bridge exist to measure different types of electrical quantities or to improve measurement accuracy. These include the Kelvin Bridge, which is used for measuring very low resistances, and the Maxwell Bridge, used for measuring inductance.

Conclusion

The Wheatstone bridge is a fundamental tool in electrical engineering and physics for precise measurement of resistance. Its principle of operation and ability to measure small changes in resistance accurately make it indispensable in many scientific and industrial applications.