In the middle of a long solenoid there is a coaxial ring of
square cross-section, made of conducting material with resistivity
p. The thickness of the ring is equal to h, its inside and outside radii
are equal to a and b respectively. Find the current induced in the
ring if the magnetic induction produced by the solenoid varies with
time as B = pt, where 3 is a constant. The inductance of the ring
is to be neglected.