the contact spring 25 rest against the surface of the cam 19 it should furthermore be mentioned here that the arrangement shown in fig2 can also be modified by way of alternative embodiment in the manner that the cam 19 is arranged on the non visible side of the worm wheel 18 so that the contact spring 25 rests against the worm wheel 18 portions 28 and 29 of the contact spring 25 are bent away from the surface of the cam 19 and bear wiper contacts 30 and 31 which are arranged at different distances from the shaft corresponding to radii of the wiper rings the wiper contacts 30 and 31 contact circular conductive paths with suitable pressure these paths not being shown in fig2 for the sake of greater simplicity but corresponding to the ones shown in fig1 the third embodiment is shown in cross section in fig3 in this embodiment also a motor 1 drives a worm wheel 18 via a worm 17 a shaft 33 of the worm wheel 18 is developed as a crank on one side by a suitable enlargement of the diameter of the shaft 33 and provision of a pin 34 which may be connected to a load such as the aforementioned odometer on the other side of the worm wheel 18 wiper contacts 35 36 are arranged on a correspondingly shaped contact spring 37 in contradistinction to the arrangement shown in fig2 both contacts are arranged on the same side of the shaft 33 this however is immaterial for the operation of the invention since the inner wiper ring covers a full circle the position of the wiper 36 is therefore immaterial fig3 furthermore shows a housing 38 as.