(a) A magnetic compass needle pointing North and South shows deflection when a bar magnet or a current carrying loop is brought near it. This happens because the magnetic fields of the compass needle and the bar magnet (or current carrying loop) interact with each other.
(b) Salient features of magnetic field lines:- 
(i) Magnetic field lines follow the direction from the North Pole to the South Pole.
(ii) Magnetic field lines always show concentric pattern.
(iii) Magnetic field lines do not cross one another.
(iv) Closer the field lines; stronger is the magnetic field and vice-versa is also true.
(v) Magnetic field lines are closer near the poles; which shows greater strength of magnetic field near the poles.

The following diagram depicts the pattern and direction of magnetic field lines around a straight current-carrying conductor.
Right Hand Thumb Rule: If a current carrying conductor is held by right hand, keeping the thumb straight and if the direction of electric current is in the direction of thumb, then the direction of wrapping of other fingers will show the direction of magnetic field.

Magnetic Field Due to Circular Loop Current-Carrying Conductor:

In case of a circular current carrying conductor, the magnetic field lines would be in the form of concentric circles around every part of the periphery of the conductor. Since, magnetic field lines tend to remain closer when near the conductor, so the magnetic field would be stronger near the periphery of the loop. On the other hand, the magnetic field lines would be distant from each other when we move towards the centre of the current carrying loop. Finally, at the centre, the arcs of big circles would appear as a straight lines.
Magnetic field and number of turns of coil: Magnitude of magnetic field gets summed up with increase in the number of turns of coil. If there are 'n' turns of coil, magnitude of magnetic field will be 'n' times of magnetic field in case of a single turn of coil.

Activity:
To show the effect of magnetic field on current -carrying conductor

Materials Required:
A small aluminium rod, a horse-shoe magnet, battery, plug key, wires and a stand.

Procedure:
The aluminium rod is suspended horizontally from the stand and tied to two wires at its ends. The wires are attached to rheostat, battery and a plug key to make the circuit.

The horse-shoe magnet is positioned in a way that the aluminium rod lies between the two poles of the magnet. If the South Pole is above the aluminium rod and the North Pole is below it. The plug key is inserted to initiate current supply to the rod.

It is observed that the aluminium rod deflects towards left.
When the direction of the current is reversed the aluminium rod deflects towards right.

Conclusion:
When a current carrying conductor is placed within a magnetic field, the conductor experiences deflection.Fleming's Left Hand Rule explains the direction of displacement in this case. Let us assume that the current is moving in anti-clockwise direction in the loop. In that case, the magnetic field would be in clockwise direction; at the top of the loop. Moreover, it would be in anticlockwise direction at the bottom of the loop.

Electromagnetic Induction: When a conductor is set to move inside a magnetic field or a magnetic field is set to be changing around a conductor, electric current is induced in the conductor. This is just opposite to the exertion of force by a current carrying conductor inside a magnetic field. In other words, when a conductor is brought in relative motion vis-a-vis a magnetic field, a potential difference is induced in it. This is known as electromagnetic induction.

Activity:
- To demonstrate electromagnetic induction Materials Required:
- A galvanometer, coil, bar magnet and some wires.
Procedure:
- The coil is inserted over a hollow tube of cardboard.
- With the help of wires, the two ends of the coil are attached to the galvanometer.
- The north pole of the bar magnet is moved towards the end 'B' of the coil.
- It is observed that the galvanometer needle shows deflection to right.
- When the magnet is moved away from the coil, the galvanometer needle shows deflection towards left.
- When the magnet is in static position, no deflection is seen in galvanometer needle.
- Induction of electric current in the coil is the cause of deflection in galvanometer needle.
- If the magnet is kept stationary and coil is moved, then also the galvanometer needle shows deflection.
Conclusion:
- When the coil and the bar magnet are in relative motion, a current is induced in the coil.