Abstract: According to the invention, for shrinking and expanding a tool in the bush part (3) of a tool holder, an induction coil unit (1), in which several polar parts (25) made of a soft magnetic material are radially adjusted in the yoke ring made of a soft magnetic material, is provided. Said polar parts have end faces (29) which are inclined in relation to the axis of rotation (7) of the bush part (3) and can be kept in a linear contact with a conical peripheral surface (31) of the bush part (3) which holds a the tool (11) in a press fit manner. In order to avoid possible local overheating of the bush part (3), the area (39) of the end face (29) parallel to the axis is connected on the output side of the tool to an area (33) of the end face (29) inclined in relation to the axis of rotation (7).