Magnetic fields tend to concentrate in high-permeability materials. Ferromagnetic
rings are very useful in suppressing the unwanted CM noise in cables. The wires are
wound through the core in such a way that the fluxes due to the CM currents add in
the core, whereas the fluxes due to differential mode (DM) currents or signal currents
subtract in the core. Because almost all the flux is confined to the core, the self and
mutual inductance of the windings are the same (Figure 5.15).