In figure, infinite conducting rings each having current i in the direction shown are placed concentrically in the same plane as shown in the figure. The radius of rings are r, 2r, 2²r, 2³r ... $\infty$, The magnetic field at the centre of rings will be

A small coil C with N= 200 turns is mounted on one end of a balance beam and introduced between the poles of an electromagnet as shown in figure. The area of the coil is S = 1cm², the length of the right arm of the balance beam is I = 30 cm. When there is no current in the coil the balance is in equilibrium. On passing a current I = 22 mA through the coil, equilibrium is restored by putting an additional weight of mass m = 60 mg on the balance pan. Find the magnetic induction field (in terms of x 10^-1 T) between the poles of the electromagnet, assuming it to be uniform.