Explain the experiment to determine the unknown resistance of a wire using a metre bridge under the following point: Precautions (any two).

The precautions that need to be kept in mind while measuring the resistance of an unknown wire: 1. To avoid the error of parallax, the set square should be used to note the null point. 2. The movement of the jockey should be gentle, and it shouldn’t be rubbed. 3. Plugs should be tightly connected to the resistance box. 4. The key K should be inserted only when the observations are to be taken. 5. The connections should be neat, tight and clean.

Derive the principle of Wheatstone bridge by Kirchoff's law.

A Wheatstone bridge consists of four resistances P, Q, R and S connected along four sides of a square (say ABCD) shaped circuit. A galvanometer G is also connected between points B and D. It is based on the principle of null deflection i.e. the ratio of their resistances are equal, and no current flows through the galvanometer and is used to measure unknown resistance. Applying Kirchoff's loop rule to closed-loop A D B A - I 1 R 1 + 0 + I 2 R 2 = 0 . . . . . . . . . . ( i ) Similarly, applying the Kirchoff's loop rule for for loop C B D C - I 2 R 4 + 0 + I 2 R 3 = 0 . . . . . . . . . . ( i i ) From equation (i) I 1 I 2 = R 1 R 2 -----(iii) From equation (ii) I 1 I 2 = R 4 R 3 ------(iv) From the above equations (iii) and (iv), we get: ⇒ R 1 R 2 = R 4 R 3 This is the condition for a balanced wheatstone bridge.

Explain the experiment to determine the unknown resistance of a wire using metre bridge under the following point: Principle.

Meter bridge works on the principle of a balanced wheatstone bridge. Wheatstone bridge is an electrical circuit that consists of four resistors R 1 , R 2 , R 3 , R 4 , galvanometer and external source in a following way: Assuming the above bridge is balanced. Then, at balanced condition of Wheatstone bridge is: R 2 R 1 = R 4 R 3 . Using the above principle we can measure the value of unknown resistance. Here, R is an unknown resistance and S is known resistance and both are not equal. A jockey is connected from terminal B via galvanometer G . As we switch the key K ON, and drive the jockey on the metallic wire of the meter bridge the galvanometer shows deflection. Suppose we attain the balanced point and measure it as l 1 . Thus, at balanced condition: R l 1 = S 100 - l 1 ⇒ R = S l 1 100 - l 1