magnet 7 the magnetic coil 8 and the permanent magnet 7 form a drive for the forward and return motion of the ram 2 the instrument housing 1 is divided by a screw type connection 9 into a front part or section 1a and a back part or section 1b as illustrated the front section 1a contains the ram 2 with the permanent magnet 7 while the back section 1b contains the drive coil 8 the back instrument section 1b is shown as a discrete part in fig2 the drive coil 8 is an integral component of the housing part 1b and is advantageously rigidly connected to a supply hose or line 10 given a change of the front instrument part 1a for example for the purpose of cleaning sterilizing or for some other purpose the electrical lines or lead 11 leading to the drive coil 8 need not be separated and thus only plug type connectors 12 and 13 for connecting lines or leads 17 and 18 to lines or leads 24 and 31 whose purposes shall be explained later need be provided to enable separation at the parting location the means for mounting the ram 2 are two bearings 3 and 4 which are designed as sliding bearings which in accordance with the sectional view of fig3 has a plurality of radially inwardly extending ridges separated by axial openings 14 the axial openings 14 of the bearing 3 allow the movement of air between.