In any magnetic circuit, flux created is given by

θ = m.m.f.   =      AT
    reluctance  l /μ_oμ_ra

or  AT  required = θ . 1  x l = B X l = H X l  ampere-turns.
                          a  μ_oμ_r       μ_oμ_r

Hence for determination of AT for a magnetic circuit.

(i) Fist find field strength H in each part of the magnetic circuit, (ii) find the length of various parts of magnetic circuit (iii) find the number of ampere turns required for the various paris of magnetic circuit from the relation AT= Hl  where l   is the length of the part in metes and lastly, (iv) find total number f ampere-turns for the whole series magnetic circuit by adding ampere-turns determined for various paths in magnetic circuit.

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