Consider a solenoid of length l, n windings, and radius b (l is much longer than b). a current i is flowing through the wire. if the radius of the solenoid were doubled (becoming 2b), and all other quantities remained the same, the magnetic field consider a solenoid of length , windings, and radius ( is much longer than ). a current is flowing through the wire. if the radius of the solenoid were doubled (becoming 2), and all other quantities remained the same, the magnetic field would become one half as strong. would remain the same. would become twice as strong.