The present invention involves a structure of a contactless ohmic resistance potentiometer. A current path between two exterior electrodes is provided by a photoconductor, and a means is provided for causing an exposure of a portion of the length of the photoconductor between one exterior electrode and a tapping electrode. This permits a voltage division between two exterior electrodes. The usable surface of the photoconductor layer is delimited by an exterior electrode and a tapping electrode having a definite geometric course between the exterior electrodes. The distance ratio from the tapping electrode to the two exterior electrodes is varied by means of displacing the exposure of the photoconductor which causes the active partial surface of the photoconductor to be varied at will.