Before electricity, if you woke up in the middle of night it would have been difficult to tell what time it was unless you owned a clock or watch with a repeater complication. Repeaters sound the time on demand in an audible format, using bells or gongs. Different versions of repeaters indicate the time in increasingly accurate ways. A quarter repeater indicates time accurate to the quarter hour. Minute repeaters indicate time accurate to the minute.
Repeaters consist of four mechanical systems working together. The winding works, gear train and escapement, racks and snails, and hammers and gongs.
In a watch, a repeater is usually activated by a slide on the edge of the watch case. This slide serves two distinct purposes. First, it activates the repeater mechanism. Second, it winds a small mainspring, separate from the mainspring the watch uses to power the movement. A repeater needs a substantial amount of torque to run. If the repeater were powered from the watch’s primary mainspring, it would cause a drop in amplitude in the balance, resulting in a loss of accuracy.
Just like the main movement in a watch, a repeater needs it’s own gear train and escapement to transmit power from its mainspring to the rest of the mechanism. The repeater escapement has the unique requirement of being quiet in order to not disturb the sound the hammers make when striking the gongs. To achieve this, a centrifugal governor is used. The governor can be compared to the spinning of an ice skater. When the ice skater spreads their arms, they slow their spin. A governor has two arms on pivots, with weights near the end. As the gear train turns the governor quickly, its arms spread out to equalize the rate of spin, and act as an escapement.
A series of snails connected to the motion works are used to indicate the correct amount of hammer strikes for hours, quarters and minutes. The snails are specially shaped cams with steps. The hour snail has 12 steps, the quarter snail 4, and the minute snail 14. These steps are used to stop the movement of the rack at the correct place. The rack is activated when the gear train starts running, and it moves until it touches it’s corresponding snail.
The hammers strike the gongs to sound out the time. The racks have teeth that match the maximum amount of hammer strikes that could happen. These teeth trip the hammers as they pass, causing the hammer to strike the gongs. The gongs are thin steel wires, shaped in a circle to encompass the movement. Different lengths of these gongs produce different tones.