the 34 dark 34 lamp s become illuminated at a low level by random ignitions and the 34 light 34 lamp s lose a little brightness when two passages of drawing are used the error wavelength in the feed material produced by the first machine is sometimes of the same nominal wavelength as the error produced by the back roll of the second machine also in the textile application of a draw frame the sliver condenser produces a considerable frequency dispersion as shown in fig7 and therefore the sliver produced by the first machine contains a spectrum of frequencies rather than a single one hence such sliver passing through the back roll is unlikely to produce a well defined pattern of illumination in the indicator furthermore there is likely to be some difference in frequency and exact synchronism will occur only with the rolls in the set being measured another way to achieve the same object is to have interrupters fitted to the rolls or shafts rotating in synchronism with the rolls the interrupters allow beams from light sources activated by the error signals from the lvdts or equivalent to pass only at specified positions of the roll the intensity of the light source varies with the error amplitude and no light is emitted until the error exceeds the tolerance level in one example disks rotating synchronously with the affected rolls each have.