A coil (9) is wound on a tube (8) which generates a homogenous magnetic field. A second coil (11), which also generates a homogenous magnetic field, is wound on a cylinder which is moved inside the tube (8). Alternative the outer or inner coil can be replaced by a permanent magnet. The coils ar powered by direct current. The magnetic fields generated resist movements of the cylinder relative to the tube. The amount of resistance can be steplessly controlled by adjusting the level of the current.
