What is a Metre Bridge?

A metre bridge is a device used to determine the unknown resistance of a coil or any other material that is a conductor. A metre bridge works on the same principle as a Wheatstone bridge, which is used to measure the unknown resistance in a circuit. A metre bridge is made with a one-metre long wire with uniform cross-section areas.

The wire is typically made from manganin, constantan, or nichrome. The wire must be made from these materials because they offer a low-temperature coefficient of resistance and high opposition.

There are several types of metre bridge experiments done in schools and colleges. One of the objectives of the metre bridge experiment is to find the resistance of a wire by metre bridge.

Principle of a Metre Bridge

The basic principle on which a metre bridge works is the same as that of the rule of a Wheatstone connect. A Wheatstone connect is dependent on the rule of invalid avoidance. For example, at the point where the proportion of protections in the two arms is equal, no current will pass through the centre arm of the circuit.

A metre bridge is made up of a one-metre long bridge wire, and has two different sections, one left and one right section. In the left section, we can connect the known resistance, whereas in the right section, we connect the resistance that must be determined. A jockey is also present to detect the balance point. The galvanometer is connected to one side between the known and unknown resistance points. Its connection is moved on the other side along the straight wire to achieve a balance and a reading of zero.

The length is measured from the left-hand side, which is also known as the zero point, to the balance point.

For example:

(X/R) + (L/100 – L)

In this, X is the unknown resistance, and L is the balance point length from the zero ends.

The relative lengths of wire intercepted between the movable galvanometer connection and the two ends are proportional to their resistance, and help in determining the unknown resistance with a known resistance.

How to Find the Resistance of a Wire by Metre Bridge?

You need the following items to find the unknown resistance of a conductor while using a metre bridge.

• Metre bridge
• Galvanometer
• Resistance box
• Unknown Resistance of one-metre length
• Connecting wires
• Screw gauge
• One-way key
• Jockey

Here are the steps to find the unknown resistance using a metre bridge.

1. Collect all the above instruments and prepare your connections.
2. Take a suitable type of resistance ‘R’ from the resistance box.
3. Touch the jockey at one point, and then check if there is a deflection in the galvanometer on any of the sides. Then touch the jockey to another point of the wire and again check for any deflection in the galvanometer. The deflection in the galvanometer has to be on another side.
4. Determine the position of the zero point or null point by seeing the deflection in the galvanometer when it becomes zero. Remember that the length AB (l) BC = (100 – l).
5. Continue with this method for different values of ‘R’. Take at least five readings.
6. Check the point where the galvanometer shows zero deflection. This is your balance point.
7. Now, measure the length of the given wire by using an ordinary scale and the radius of the wire by using the screw gauge. Again, take at least five readings.
8. You can now calculate the mean resistance of the single unknown resistance, which is equal to the total sum of resistances of unknown resistance from the five readings you took, divided by five.

A formula is used to calculate the unknown resistance of a metre bridge. In this, the unknown resistance, S = (100 – l1) R/l1, where R is the known resistance and l1 is the balancing length of the wire.

Precautions during a Metre Bridge Experiment

Some of the precautions you must take while doing a metre bridge experiment include:

• Make sure the connections are tight, clean, and neatly made.
• Plugs must be connected tightly to the resistance box.
• The movement of the jockey should be gentle. Avoid rubbing it across the wire.
• The key should only be inserted when the observations are recorded.
• The null point should ideally be between 45 cm and 55 cm.
• To avoid the error of parallax, use a set square to note the null point.
• The wire should be free from any loops.
• The length of the wire should be measured in two mutually perpendicular directions.

Conclusion 

A metre bridge is a device that is used to find out the unknown resistance of a conductor. It is made with a metre-long wire that has uniform cross-sections. The wire is typically made up of manganin, constantan, or nichrome since these substances have high resistance as well as low-temperature coefficient of resistance. A metre bridge is designed from a Wheatstone bridge, and it is one of the most basic applications of such a bridge.