Important Topic of Physics: Wheatstone Bridge

# Important Topic of Physics: Wheatstone Bridge

## Introduction

A Wheatstone bridge, also known as a resistance bridge, is used to calculate an unknown resistance by balancing two bridge legs, one of which contains an unknown resistance component. This method was invented by Samuel Hunter Christie in 1833 and popularized by Sir Charles Wheatstone in 1843. In this circuit, two known resistors, one unknown resistor, and one variable resistor are bridged. These bridges are very reliable as they provide accurate measurements. This is being used as a means of calibrating measuring instruments such as voltmeters, ammeters, etc. Although digital multimeters are the easiest way to measure resistance today, Wheatstone bridges are still used to measure resistance in the very low milliohm range.

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The information about Wheatstone Bridge from various physics-related articles are available here. Wheatstone Bridge and its general concepts are important topics in physics. Students who want to flourish in physics need to be well known about Wheatstone Bridge to get deep knowledge about it to do well on their exams. The definition, derivation, formula, working principle, limitations and applications are provided here to assist students in effectively understanding the respective topic. Continue to visit our website for additional physics help.

## Overview

Wheatstone bridges, also known as resistance bridges, are used to calculate the resistance of an unknown. This is done by balancing the two different branches of the circuit bridge. One leg has an unknown resistance component. This bridge has two resistors, one with an unknown resistor and the other with a variable resistor. Bridges are considered reliable because they provide accurate measurements. The bridge consists of four arms and two of them are known and the other two have one variable and unknown resistance. Also, the bridge circuit includes a galvanometer and a source of electromotive force. A force is applied between points A and B, and a galvanometer is connected between the other two points, C and D. Due to the potential difference, the current flow in the galvanometer will vary accordingly. Resistance temperature sensors are used to detect temperature changes in metal wires. We know that all metals have their own resistance constant. All metals have a resistance to resist the flow of electrons. Resistive power can be measured as a change in temperature using a resistive temperature sensor. This change is directly dependent on temperature and has a coefficient. This coefficient is known as the temperature coefficient of electrical resistance. Platinum is widely used in sensors. On the other hand, gold and silver are used as excellent electrical conductors.

## Construction of the Wheatstone Bridge

The Wheatstone bridge circuit consists of four arms, of which two arms are made up of known resistors and the other two arms are made up of unknown and variable resistors. The circuit also consists of a galvanometer and an electromotive force source. The EMF source is connected between point b and point and the galvanometer is connected between point c and d. The current flowing through the galvanometer depends on the potential difference across the galvanometer.

## Wheatstone Bridge Principle

Wheatstone bridges operate on the principle of zero deflection. The ratio of resistance is the same and no current flows through the circuit. Under normal conditions, the bridge is unbalanced when current flows through the galvanometer. A bridge is said to be in balance when no current flows through the galvanometer. This condition can be achieved by adjusting the known and variable resistors.

## Derivation of Wheatstone Bridge

The current reaches the galvanometer and divides into two same magnitude currents as I1 and I2.

If the current that flowing through the galvanometer is zero,

We have, I1P=I2R…(1)

The currents flowing in the bridge in a balanced condition is,

I1=I3=EP+QI2=I4=ER+S

E is considered as the emf of the battery.
When substituting the value of I1 and I2 in equation (1), we get

PEP+Q=RER+SPP+Q=RR+SPR+S=RP+QPR+PS=RP+RQPS=RQ…(2)

R=PQS(3)

Equation (2) represents the balanced condition of the bridge while (3) calculates the value of the unknown resistance.
In the figure, R is the unknown resistance, S is the standard arm of the bridge, and P and Q are the ratio arm of the bridge

## Formula for Wheatstone Bridge

R=PSQ

Here, R is said to be the unknown resistance

S is said to be the standard arm of the bridge

P and Q are considered as the ratio of the arm of the bridge

## Applications of Wheatstone Bridge

• Wheatstone Bridge is used for accurate measurement of low resistance. The
• Wheatstone bridge is used in conjunction with operational amplifiers to measure physical parameters such as temperature, light, and strain.
• Quantities such as impedance, inductance, and capacitance can be measured using the change in a Wheatstone bridge.

## Limitations of Wheatstone Bridge

• In the case of low resistance measurement, the resistance of the leads and contacts becomes significant and develops an error.
• In the case of high resistance measurement, the measurement presented by the bridge is so large that the galvanometer is insensitive to imbalance.
• The change in the resistance is because of the heating effect of the current through the resistance. The additional current may be accountable for a permanent change in the value of resistance.

Question 1: What do you mean by a Wheatstone Bridge?

Answer: A Wheatstone bridge being known as a resistance bridge, determines an unknown resistance by balancing the two legs of a bridge circuit. One contains an unknown resistance component.

Question 2: What are the limitations of the Wheatstone Bridge?

• In the case of low resistance measurement, the resistance of the leads and contacts becomes significant and develops an error.
• In the case of high resistance measurement, the measurement presented by the bridge is so large that the galvanometer is insensitive to imbalance.

Question 3: What is the working principle of the Wheatstone Bridge?

Answer: The basic principle of a Wheatstone bridge is zero deviation, which means that the resistance ratios are the same and no current flows through the circuit. Under normal conditions, the bridge is unbalanced when current flows through the galvanometer. The bridge is said to be balanced when no current flows through the galvanometer.

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