Magnetic field due to a current carrying circular loop

A   cardboard is  fixed in a horizontal  plane.A  circular  loop
of  wire passes through  two  holes in the cardboard.Iron filings  are sprinkled  over  the cardboard. Current is  passed  through the   loop   and   the 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.