Magnetic circuits

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.