Abstract: Apparatus for scanning the relative position of a preselector counter mechanism which includes a plurality of digital discs having radial cams rotated in accordance with the setting of a preselected numerical value on the counter, with each of the radial cams being provided with a scanning arm adapted to initiate a switching operation upon reaching a preselected stage of the counter mechanism, is provided with a plurality of counter wheels each adapted to be set to a pre-selected value with an analogue metering value cam in the shape of an Archimedean spiral being affixed to each of the counter wheels. Each of the counter wheels and the metering value cams is arranged to represent a decade of a decimal system numerical value. Indexing pinions gear each of the counter wheels and the metering value cams with those representing a next adjacent decade.