Before Hall-effect devices....

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.