An iron ring has a cross-sectional area of 400 mm2 and a mean diameter of 250 mm. An air gap of 1 mm
has been made by a saw-cut across the section of the ring. If a magnetic flux of 0·3 mWb is required in
the air gap, find the current necessary to produce this flux when a coil of 400 turns is wound on the ring.
The iron has a relative permeability of 500.