3 to be able to detect the furnace current the drain electrode of fet 33 leads to a summation point 46 the signal xi of of the drain electrode serves to regulate the impedance presented by furnace electrode 42 a second measured value the guide value wi of needed for impedance regulation is obtained from the actual electrode voltage value as follows a measuring member 37 produces an electrode voltage value xu which is fed as the dividend to an analog divider 38 from microprocessor 27 a divisor value wz is fed to divider 38 via a d a converter 39 the value wz is inputted in digital form to microprocessor 27 from digital input 26 if fet 33 is rendered conductive the desired value wu also inputted at digital input 26 for the voltage regulation and which is transmitted by microprocessor 27 via d a converter 40 to summation point 46 is made ineffective by microprocessor 27 the guide value desired value wz is determined by the ratio of the furnace voltage uof to the current iof wz uof iof for a certain fixed furnace voltage value uof and a given impedance value wz the furnace current iof is predetermined e g at the beginning of melting with impedance regulation wz and uof respectively are given when the operating diagram melting program is fed into microprocessor 27 the output signal of measuring member 37 xu is also fed to microprocessor 27 via an a d converter 37 39 if at summation point 46 the input to a regulating amplifier 36 is negative i e the magnitude of desired value wi of is greater than.