When a strong electric current is passed through the coil by closing the key the cardboard is gently taped we find that the iron filing arrange themselves in a definite pattern representing the magnetic line of force produced by the current carrying coil.
The magnetic field lines are circular near the current carrying loop as we move away the concentric circles representing magnetic field lines become bigger and bigger at the centre of the circular loop the magnetic field lines are Straight.
Direction of magnetic field is determined by applying the right hand thumb rule to each section of the coil.
We find that the concentric line of force pass through the coil in the same direction.
The strength of magnetic field produced by a circular coil carrying current is
(i) By increasing the number of turns of wire in the coil.
(ii) By increasing the current flowing through the coil.
(iii) By decreasing the radius of the coil.
B = N u I/2r
Clock Face Rule :
A current carrying circular wire or loop behaves like a thin dicks magnet whose one phase is a North pole and the other faces South pole the polarity North or South of the two phase of a current carrying circular coil can be determined by using the clock face rule :
If the current around the face of circular wire flows in the clockwise direction then that phase of circular wire will be South pole
If the current around the face of circular wire flows in the anticlockwise direction then that phase of circular wire will be North pole.