## Meter Bridge – Class 12 | Chapter – 3 | Physics Short Notes Series PDF for NEET & JEE

Meter Bridge: The meter bridge is an electrical circuit used to measure resistance, specifically the resistance of a wire or other component. It is based on the Wheatstone bridge principle, but is simpler and more compact.

The meter bridge consists of a long wire or strip of metal that is divided into two parts by a sliding contact. A resistance is connected in series with one part of the wire, and a voltmeter is connected across the other part. The sliding contact is then moved along the wire until the voltmeter reads zero, indicating that the resistance in the two parts of the wire is equal.

At this point, the resistance of the unknown component can be calculated using the formula R = (l/2) * (ρ / A), where l is the length of the wire, ρ is the resistivity of the material, and A is its cross-sectional area.

The meter bridge is a convenient and simple way to measure resistance, and is often used in physics and electrical engineering experiments, as well as in industrial applications where resistance needs to be measured accurately. However, its accuracy can be limited by factors such as temperature drift, measurement error, and resistance changes in the wire due to aging or other factors.

## Construction of Meter Bridge

The construction of a meter bridge involves the following steps:

• Length of Wire: A long wire or strip of metal, such as copper or aluminum, is used as the main component of the meter bridge. The length of the wire should be sufficient to provide a measurable change in resistance when a known resistance is connected in series.
• Sliding Contact: A sliding contact, such as a metal or carbon rod, is used to divide the wire into two parts. The sliding contact can be moved along the wire to change the length of the two parts.
• Connections: One end of the wire is connected to a power source, and the other end is connected to a voltmeter. A known resistance is also connected in series with one part of the wire.
• Measurement: The sliding contact is moved along the wire until the voltmeter reads zero, indicating that the resistance in the two parts of the wire is equal. At this point, the resistance of the unknown component can be calculated. ## Applications of Meter Bridge

It is commonly used in the following applications:

• Physics and Electrical Engineering Experiments: The meter bridge is a popular experimental setup in physics and electrical engineering courses, as it provides a simple and straightforward way to measure resistance.
• Resistance Measurement: The meter bridge is often used to measure the resistance of electrical components such as wires, resistors, and other electrical devices.
• Calibration of Resistance Standards: The meter bridge can be used to calibrate resistance standards, such as resistance boxes and resistance standards, by comparing their resistance values with the resistance values obtained from the meter bridge.
• Determining Resistivity of Conductors: The meter bridge can be used to determine the resistivity of conductors by measuring the resistance of a known length of wire and calculating its resistivity.
• Quality Control: The meter bridge is used in quality control processes to ensure that electrical components meet specified resistance standards.

## Limitations of Meter Bridge

There are several limitations, including:

• Measurement Error: The accuracy of the meter bridge can be affected by measurement error, such as errors in the position of the sliding contact or the readings of the voltmeter.
• Drift: The meter bridge can be affected by temperature drift, which can cause the resistance of the wire to change, affecting the accuracy of the measurement.
• Resistance Changes: The resistance of the wire used in the meter bridge can change over time due to factors such as aging, oxidation, or other factors, affecting the accuracy of the measurement.
• Range Limitations: The meter bridge has limited range, as it can only measure resistance values within a certain range. If the resistance to be measured is outside of this range, a different measurement method must be used.
• Complexity: Although the meter bridge is a simple circuit, it can be challenging to set up and use, particularly for those without electrical engineering experience.
• Interference: The meter bridge can be affected by electromagnetic interference, such as electrical noise from other devices, which can cause measurement errors.

## Procedure For Finding The Unknown Resistance Using Meter Bridge

• Collect the instruments and prepare connections as shown in the above figure.
• Take some suitable kind of resistance ‘R’ from the resistance box.
• Touch jockey at the point A; look that there exists a deflection in galvanometer on one of the sides, then contact the jockey on point C of wire, then the deflection in galvanometer has to be on another side.
• Find the position of the null point having deflection in the galvanometer that becomes zero. Note the length AB (l) BC = (100 – l).
• Continue the above method for some different values of the ‘R’. Note at least some 5 readings.
• Consider the point where galvanometer shows a 0 deflection; this is called balance point.
• Now, Measure the length of given wire by the use of ordinary scale and radius of the wire by the utilization of a screw gauge, (Take at least five readings).
• Calculate Mean Resistance of Single Unknown Resistance = Total Sum of resistances of Unknown resistance from the above five readings)/5.

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