Tuning a magnetic field near a Feshbach resonance allows one to choose the interaction strength between two colliding atoms.  The resonance occurs when the molecular binding energy of a pair of atoms in one magnetic substate is equivalent to the kinetic collision energy of a pair of atoms in a different magnetic substate.  Since these energies depend on the magnetic field present, due to the Zeeman effect, there can exist a point at which the energies coincide.  The effective scattering length depends on the proximity of the magnetic field to the resonance value and diverges exactly on resonance.  The utility of this resonance is to be able to control the interactions between atoms by tuning a magnetic field.  For potassium-40, there is a Feshbach resonance between the states |F = 9/2, m_F = -9/2> and |F= 9/2, m_F = -7/2> at 202.10 G, with a width of 8.0 G.