A special metal S conducts electricity without any resistance. A closed wire loop made of S does not allow any change in flux through itself by inducting a suitable current to generate a compensating flux. This current gives rise to a magnetic moment which in turn repels the source of magnetic field or flux. Consider such a loop, of radius a, with its center at the origin. A magnetic dipole of moment m is brought along the axis of this loop from infinity to a point at distance r(>> a) from the center of the loop with its north pole always facing the loop, as shown in the figure below. The magnitude of magnetic field of a dipole m at a point on its axis at distance r is `(mu_0 m)/(2 pi r^3)`. The magnitude of the force between two magnetic dipoles with moments `m_1` and `m_2` separated by a distance r on common axis with their north pole facing each other is `(k m_1 m_2)/r^4` where k is a constant of appropriate dimensions. The direction of this force is along line joining two dipoles. When the dipole m is placed at a distance r from center of the loop the current induced in the loop will be proportional to