By using a compass needle describe how can you demonstrate that there is a magnetic field around a current carrying conductor.

Experiment:

In figure below, AB is a wire lying in the north-south direction and connected to a battery through a rheostat and a tapping key. A compass needle is placed just below the wire.

Observation —

(1) When the key is open i.e., no current passes through the wire, the needle points in the N-S direction (i.e., along the earth's magnetic field) with the north pole of needle pointing towards the north direction. In this position, the needle is parallel to the wire as shown in Fig. (a).

(2) When the key is pressed, a current passes in the wire in the direction from A to B (i.e. from south to north) and the north pole (N) of the needle deflects towards the west [Fig.(b)].

(3) When the direction of current in the wire is reversed by reversing the connections at the terminals of the battery, north pole (N) of the needle deflects towards the east [Fig.(c)].

(4) If the compass needle is placed just above the wire, the north pole (N) deflects towards east when the direction of current in wire is from A to B [Fig. (d)], but the needle deflects towards west as in figure (e), if the direction of current in wire is from B to A.

Through the experiment we can say that, when electric current is passed through a conducting wire, a magnetic field is produced around it.