like area 30 of the handle 31 or the handles 31 of the pivot bar 12 extends between the two lateral arms 33 in accordance with fig3 a lever 38 which also has a boomerang shape of a lever arrangement 40 is pushed fixed against relative rotation onto the free end of the bar 32 which extends from the bearing 37 and is facing away from the handle 31 the other end of the lever 38 which in contrast to the forwardly pointing lateral arm 33 points downwardly is pivotably connected via a pivot bolt 41 with a straight bar 39 the other end of the straight bar 39 is fastened by means of a cog 42 to a toothed segment 43 which itself is pivotable around its center 44 the cog 42 is disposed on the longitudinal center line extending through the center 44 a tension spring 46 the other end of which is attached to a stationary point 47 can act on the cog 42 the toothed outer are 48 of the toothed segment 42 meshes with a gear wheel 49 which is located fixed against relative rotation on a shaft 51 which is rotationally disposed on the machine frame 36 the end of the shaft 51 away from the gear wheel 49 is provided with an axial slit 52 into which the operating end 53 of a wiper of a rotary potentiometer 56 is form fitted a first pair of cam disks 57 is disposed fixed against relative rotation.