Question

Explain with a neat circuit diagram how you will determine unknown resistance using meter bridge.

Answer 1

Hint:A meter bridge is used to find an unknown resistance using some known resistances. It works on the principle of wheat stone’s bridge. When the circuit is balanced, the condition for balancing of the bridge is used to find the value of unknown resistance.

Complete answer:
A meter bridge is used to find an unknown resistance using some known resistances. The circuit diagram of a meter bridge is shown below.

It consists of a wire of length 1 metre. The resistance of this wire is uniform through its length. In parallel to the resistance wire, there are two slots to connect the resistances. In one slot (say X), the known resistance is connected and in the other slot the unknown resistance (Y) is connected. The circuit is such a way that the resistances X and Y are series and these as a whole are in parallel connection with the resistance wire. Then a battery of some emf is connected across the resistance wire, as shown.

A galvanometer is connected in the circuit, in such a way that its one end is connected in between the resistances X and Y. The other end is moveable and placed on the wire. A meter bridge works on the principle of Wheatstone’s bridge. To find the value of the unknown resistance (Y), the circuit is balanced. When the circuit is balanced, there is zero or minimum current in the galvanometer. The length of the wire (l) is called balancing length. Then we use the condition for balancing, i.e. $\dfrac{X}{l}=\dfrac{Y}{100-l}$. With this, we can calculate the value of unknown resistance, since the values of X and l are known.

Note:While performing the meter bridge experiment, the readings of the balancing length must be taken in centimetres and be sure that length of the resistance wire is exactly 1 metre.Most of the time, the current in the galvanometer will not become zero. This is because a galvanometer consists of a small resistance. Therefore, you have to find a minimum current in the galvanometer.