A clock with a large dial and centre sweep seconds hand moving over a 0-60 graduated scale, with a subsidiary hand to indicate elapsed time on a second scale graduated 0-60 or 0-100 minutes; expressly designed for photographic darkroom use. Generally, the hands are coated with luminescent paint to allow them to be seen in total darkness. A simple clock movement with a balance wheel is used, often fitted with pull winding but sometimes wound from the rear, which can be set to zero, started and stopped by other cords, pushbuttons or levers. An alarm bell may be fitted to remind the user that the required period has elapsed. These clocks are mounted in circular drums, often of tin plate or plastic; synchronous electric clock movements are also used.