Abstract: In an apparatus for slitting sheet material into narrow strips and winding them onto cores, a winding shaft on which the cores are mounted, an electromagnetic clutch for clutching each core to the winding shaft and a potentiometer controlling operation of each clutch independently of the other, each clutch comprising inner and outer concentrically arranged parts, one of which is fixed to the shaft for rotation therewith and the other of which is rotatable about the one part, said parts providing axially disposed poles between which there is a working gap, a powdered magnetizable material in the gap, a coil associated with the inner part through which the current flow is controlled by the potentiometer associated therewith, and sealing means comprising nonmagnetic sealing rings at the ends of the poles for confining the magnetizable material in the working gap, said rings having opposed surfaces which converge from opposite sides of the working gap between the poles toward each other and sealing strips held en