the line 66 just as the center between the pair of polar surfaces 62c 1 62c 2 and the center between the pair of polar surfaces 64c 1 64c 2 so that the centers between the pairs of polar surfaces all have the same distance from the bearer bar 42 a pattern like or template like plate 68 see fig4 which is not illustrated in fig2 and 3 which is secured to the housing 30 by two bolts 70 and has perforations 72 for the extremities of the cores 60b and 64b forming the polar surfaces is situated in front of the coils of the electromagnets this plate thus ensures that the polar surfaces always assume one and the same and quite specific position if the electromagnets 60 and 64 are inserted on the recess 36 of the housing 30 and are cast in within the recess the casting resin used for this purpose is not illustrated in the drawing after the electromagnets are inserted and cast in their polar surfaces are ground flat that is to say before the permanent magnet systems 40 and 44 and the bar 42 are installed in the housing 30 the polar surfaces 60c 1 to 60c 2 lying one above another in a row form the so called selection or control station of the magnetic control system the top permanent magnet system 44 equally comprises two pole pieces 74 and 76 whereof the end faces have been marked 78 and 80 the end face 78 of the upper pole piece forms a continuous supporting bar not interrupted at the selection or control station for the selector elements.