$4$ For the magnetic circuit shown in Fig. $7\mathrm{.}7$ find the current I in the coil needed to produce a flux of $0\mathrm{.}45$ mWb in the air$-$gap. The silicon iron magnetic circuit has a uniform crosssectional area of $3c{m}^2$ and its magnetisation curve is as shown on page $71.$

$[0\mathrm{.}83$ A$]$