Abstract: Oxygen or oxygen-containing gas is blown onto a molten crude iron bath inside a converter. A layer of slag or metal-slag emulsion forms atop the molten crude iron bath and grows in height during the course of the refining reaction. The height of such layer is indirectly determined by detecting an electrical characteristic of the layer. The layer of slag or metal-slag emulsion is made to serve as the dielectric of a capacitive reactive and/or as the flux-permeable part of an inductive reactance, and such reactance, or a plurality of such reactances, form part of a resonant circuit.