Name the principle on which meter bridge is based.

Hint: A meter bridge is an electrical apparatus that helps in determining the value of unknown resistance of a metal coil.

Complete step by step answer:
Meter Bridge works on the principle of the Wheatstone bridge. A Wheatstone bridge is an electrical network which is used to calculate the value of unknown resistance. It has four resistance connections as shown in the below figure.

By applying Kirchhoff’s voltage law to the above Wheatstone network, we get
$\dfrac{P}{Q} = \dfrac{R}{S} \to \left( 1 \right)$, where P, Q, R and S are the values of the resistances of the four resistors given.G in the above diagram is the galvanometer.
A meter bridge is a way of construction of a Wheatstone network. A simple meter bridge looks as shown in the figure below:

In the above diagram, R and S are variable resistors, G is galvanometer and D is the position of the sliding contact as it is made to move along the wire AC. The sliding contact is moved till the galvanometer shows the reading as zero. This confirms that the Wheatstone bridge is balanced and enables us to apply the equation (1) to determine the unknown resistances. The wire AC is kept against a meter scale 1 metre. The total circuit is charged up by a battery with emf E.
Let us assume R is a known resistance and S is an unknown resistance whose value we have to find
$\dfrac{R}{S} = \dfrac{{{R_{AD}}}}{{{R_{DC}}}}$
If $AD = l$ then $DC = \left( {100 - l} \right)cm$
We know that the resistance of a material is directly proportional to the length of the material.
$\dfrac{R}{S} = \dfrac{l}{{100 - l}}$
By finding the balance point, we can determine the relation between the resistances R and S, and then we can calculate the value of the unknown resistance.

Note:There is another bridge known as Carey Foster Bridge which is a form of Meter Bridge but there is no end resistance in Carey Foster Bridge whereas in meter Bridge end resistance affects our readings. These two bridges almost look similar. Be careful to choose the type of bridge when a diagram is given.