A closed magnetic circuit made from silicon steel consists of two sections, connected in series. One is of effective length 42 mm and csa 85 mm2, and the other of length 17 mm and csa 65 mm2. A 50 turn coil is wound on to the second section and carries a current of 0.4 A. Determine the flux density in the 17 mm length section if the relative permeability of the silicon iron under this condition is 3000.