Magnetic field due to a circular loop carrying current

Magnetic field due to a circular loop carrying current

A cardboard is fixed in a horizontal plane. A circular loop of wire passes through two holes in the cardboard as shown in Fig 3.9. Iron filings are sprinkled over the cardboard. Current is passed through the loop and the  carrying current card board is gently tapped. It is observed that the iron filings arrange themselves along the resultant magnetic field.

The magnetic lines of force are almost circular around the wire where it passes through the cardboard. At the centre of the loop, the line of force is almost straight and perpendicular to the plane of the circular loop.