2. Permanent magnets

2.1 Context

If we plunge a magnet, of magnetic moment m, shaped like a needle on a pivot or suspended from a wire, into a uniform magnetic field H, it takes on an orientation that results from balancing the torque moment τ = m × μ ₀ H applied by the field with the opposite torque applied by a spring or the twisting wire. By letting the sample oscillate, we have a pendulum whose resonant frequency is directly related to m and the pendulum's moment of inertia. If the sample is...