Example 6.27. An iron-ring of mean length 30 cm is made up of 3 pieces of cast-iron. Each piece has the same length, but their respective diameters are 4, 3 and 2.5 cm. An air-gap of length 0.5 mm is cut in the 2.5 – cm. Piece. If a coil of 1000 turns is wound on the ring, find the value of the current has to carry to produce a flux density of 0.5 Wb/m2 in the air gap. B-H curve data of cast- iron is as follows : B (Wb/m2 ) : 0.10 0.20 0.30 0.40 0.50 0.60 H (AT/m) : 280 680 990 1400 2000 2800 Permeability of free space = 4π × 10−7 Neglect Leakage and fringing effects.