A reactor comprising an annular iron core constituting a closed magnetic path and a conductor wound on the annular core, said iron core being formed of particles of iron or an iron based magnetic material {HEAD ~ each} covered with {TAIL ~ an insulative oxide film} which contains 0.3 to 0.8% of oxygen by weight based on the particle 
A reactor comprising an annular iron core constituting a closed magnetic path and a conductor wound on the annular core , said iron core being formed of particles of iron or an iron based magnetic material { HEAD ~ each } covered with { TAIL ~ an insulative oxide film } which contains 0.3 to 0.8 % of oxygen by weight based on the particle
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