Problem 3.245

A thin insulated wire forms a plane spiral of $N=100$ tight turns carrying a current $I=8 \mathrm{mA.}$ The radii of inside and outside turns (Fig. 3.67 ) are equal to $a=50 \mathrm{~mm}$ and $b=100 \mathrm{~mm}$. Find: (a) the magnetic induction at the centre of the spiral; (b) the magnetic moment of the spiral with a given current.