To see how this is used in practice, consider a coil of N turns wound onto a closed ring shaped former with a very high
(note: r (steel) = 2000, r (silicon iron) = 7000, r (nickel) = 600).
A current I amps is passed through the coil, creating a magnetic field H within the former. The flux density is
webers/m2 and the flux is B.A ,where A i the area of the core. The flux density is mainly confined within the core because the core has a much larger than air. ( r>>1). Provided this is the case, then the flux density outside the core may usually be neglected in comparison.Because of its high , the former effectively 'guides' the flux around the loop in a similar way to current being guided around an electrical circuit by a high conductance (low resistance) wire.