Abstract: Into an opening (11) in the bottom part or wall part (2) of a smelting furnace (1) or of a casting ladle, a hollow cylindrical carrier element (7) is inserted which, for example, is releasably connected by means of screws with this bottom or wall part (2). This carrier element (7) surrounds an outlet casing (6) made of refractory material in which a channel of passage (5) for the melt is provided. At places facing each other with regard to the channel of passage (5), recesses (13, 14) are formed in the carrier element (7) in the manner of annular grooves. In each of these recesses (13, 14), there is a coil (17, 19) which is protected from a direct contact with the outlet casing (6) by sections of wall (15, 16) forming the bottom of the recesses (13, 14). One of the coils (17) may be connected with an AC current source for the production of a magnetic field, while the second, opposite coil (19) is connected to an evaluation circuit in which the signals induced in this second coil (19) are evaluated.