magnetic circuit A closed loop of magnetic lines going through elements with different values of permeability.

Consider for example an electromagnet shown in fig. m6 The magnetizing current is i through N turns of the coil. The line integral of H around the circuit is given by,

ò Hdl = H₁l₁ + H₂l₂ + ....... = N I

where l₁, l₂ etc. are the widths of different elements.

Using F = BA = m HA

where A is the area of cross section.

N i = F [( l₁/m ₁A) + ( l₂/m ₂A) + )

The terms l₁ /(m ₁A ) etc.. are called magnetic reluctance, R_m and Ni is called the magneto motive force (mmf). We write,

F = mmf/R_m

Since F = BA is a constant, as we reduce area of cross section, A in a part of the circuit, the flux density B would increase. The principle is used to produce high value of B by tapering the poles at the air gap of an electromagnet.