The arrangement includes a sensor (7) having a measuring spool (8,9) closely placed to a rod-shaped material (1) for determining the solid part share in the rod. The rod is inductively heated by a heating coil (4). The sensor (7) is arranged between the heating coil and the rod. The respective measuring spool may be arranged around the rod, or at least a pair of measuring spools may be arranged pref. in an axial distance to each other, having coils wound in a sense opposed to each other. The output signals of this pair of coils is supplied to an arithmetic processing, particularly an addition circuit (21).
