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I want to create a measurement circuit for measuring specifically D.C. current using 'old-school' inductive techniques.

Before Hall effect devices became ubiquitous in clampmeters for current measurement there were some current clamps marketed that could measure D.C. currents using inductive principles - obviously a bit more cunning than the more straightforward task of A.C. current measurement. I am intending to undertake a project to accurately measure D.C. current using inductive techniques and I wish to authoritatively employks these (now) little-known methods. Does anyone have any technical information they can provide me? Patent references would also be useful. Thanks.