$($Griffiths ed$\mathrm{.}5,8\mathrm{.}15)$ A very long solenoid of radius a$,$ with n turns per unit length, carries a current Is$.$ Coaxial with the solenoid, at radius b $$ a$,$ is a circular ring of wire with resistance R$.$ When the current in the solenoid is gradually decreased, a current Ir is induced in the ring. $($a$)$ Calculate Ir in terms of dIs$/$dt$.$