A device for scanning the length of a sheet in a sheet-processing machine including suction-air measuring nozzles alignable with the trailing edge of the sheet and connected to a suction-air source via control elements connected in an electrical circuit of the machine drive, and a feeder for aligning the sheets to be processed at stops for the leading edge of the sheet, and for lifting sheets individually off a pile of sheets includes a fall-type sucker formed of at least one of the suction-air measuring nozzles, the fall-type sucker being formed with a scanning bore directed in the feeder towards the trailing edge of the uppermost sheet in the pile of sheets, and a differential-pressure-measuring valve disposed in a suction-air line connected to the fall-type sucker, said differential-pressure measuring valve being cooperatively associated with the control elements.