Magnetic induction, also called electromagnetic induction refers to the production of voltage (or EMF) across an electrical conductor placed inside a varying magnetic field. According to Faraday’s law, for a closed circuit, the induced electromotive force is equal to the rate of change of the magnetic flux enclosed by the circuit. 

B2B1= r1r2

B2= B1r2r1

B2= 10−3× 4/12

B2= 3⋅33 × 10-4 T