To demonstrate that a magnetic field exists around a current-carrying conductor, the instructor connects a heavy coppers wire in a series with a switch across a dry cell battery. the copper wire is bent to support itself vertically and then inserted through a hole in the lucite sheet, which is held in the horizontal position. with the switch close, iron feeling-which has the property of alleging themselves along the line of the force in a magnetic field is sprinkled on the lucite.

The lucite is tapped lightly to make it easier for the iron feeling to fill into position. you see that the filling arranges themselves in the concentric circle, showing that the magnetic line of force from a circular pattern is actually the result of the magnetic field, the instructor opens the switch and spreads the filling evenly over the cardboard,  then repeat the demonstration.

You see that each time the circuit current flows, the filling arranges themselves to show the magnetic field. to demonstrate the direction of the magnetic field around the current-carrying conductor, a compass needle can be used. 

A compass needle is nothing more than a small bar magnet that will line itself up with the line of force in a magnetic field. you know the previous demonstration that the magnetic field is circular. therefore, the compass needle always will be positioned at the right angle to the current-carrying conductor.  

The iron filing is removed from the lucite and the compass needle is placed on the lucite about 2 inches away from the conductor. with no current flowing, the north pole end of the compass needle will point to the earth's magnetic north poles. when current flows through the conductor. as the compass needle is moved around the conductor, observe that the compass needle always maintains itself at the right angle to right to it.

This proves that the magnetic field around the conductor is circular. using the left-hand rules you can check the magnetic field which was indicated by the compass needle. the direction in which the finger goes around the conductor is the same as that of the north pole of the compass needle. if the current thought of the conductors is reversed, the compass needle will point in the opposite direction indicating that the direction of the magnetic field has been reserved. 

Application of the left-hand rules will be observed in this phenomenon. iron filing indicators circular pattern of the magnetic field. chacking the direction of the magnetic field using compass needle and left-hand rules.