is the distance between the outer sides of two successive marks so that the detectors are very sensitive to deviation of marks from the position of the detectors if the tape t is fed by means of the larger drum 2 having a circumference slightly longer than the length of a label the rate of feed is slightly higher the tape is slightly overfed eventually the following one of the two successive marks goes beyond the position of the upstream detector r 1 at the time of actuation of the photoelectric detectors as shown in fig6 c the turning on of the detector r 1 breaks the contact r 1 39 in the circuit shown in fig5 then the electromagnetic clutch c 2 associated with the larger drum 2 is de energized to disengage the disc 23 from the drum 2 thereby making the larger drum 2 independent of its drive shaft 21 at this point the larger drum 2 reduces its circumferential speed to that of the smaller drum 3 which is now rotated at a constant revolution by means of its drive shaft 31 the rate of feed of the tape is reduced to the circumferential speed of the smaller drum 3 as a result.