A thin insulated wire forms a plane spiral of N = 100 tight turns carrying a current

Question:

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

A thin insulated wire forms a plane spiral of N