Abstract: An apertured disk which is mounted on a shaft in a loom causes a first photocell to transmit a first signal once during each revolution of the shaft in a preselected angular position of the shaft, and a second photocell to transmit a second signal in response to each incremental angular displacement of the shaft through one degree. A counter is reset to zero in response to reception of each first signal, and the counter thereupon counts the second signals until the shuttle of the loom causes the generation of a third signal which is generated when the shuttle reaches a predetermined portion of its path. The counter displays or otherwise indicates the number of second signals which are generated between the generation of a first signal and the generation of the next following third signal.