above the entire web is loaded with pressure over its width the balls 7 are held in their respective positions by means of a cage 8 in such a manner that the balls remain freely movable with respect to one another perpendicularly to the web w that is radially of the roller 1 the cage 8 is held and disposed so that it holds the balls in their respective positions in the transverse longitudinal direction of the web however also the cage is constructed to be able to raise the balls 7 in their entirety from the web w if necessary to draw in a new web to this end the magnetic force of the magnet 2 may be removed and the cage 8 with the balls 7 may be moved in the direction of the magnet 2 the cage itself may be held in position on a mounting of the apparatus not illustrated the strength or magnitude of the magnetic field may for example through alteration of the coil current of the electromagnet or through alteration of the air gap 4 be arranged as desired so that also the pressure of the balls against the web w may be varied the surface of the balls may be formed smooth grooved rough or in any other desired manner depending upon the treatment to be performed on the web in the embodiment of the invention illustrated in fig2 the balls 7 are again held in a cage 8 within the roller jacket 9 is arranged an electromagnet or a permanent magnet 2 through which the balls 7 are drawn against the web w also the transverse.