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 {TAIL ~ an iron based magnetic material} each covered with an insulative oxide film which contains 0.3 to 0.8% of oxygen by weight based on {HEAD ~ 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 { TAIL ~ an iron based magnetic material } each covered with an insulative oxide film which contains 0.3 to 0.8 % of oxygen by weight based on { HEAD ~ the particle }
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