A spring detent chronometer escapement model made by Thomas Mercer Chronometers. 221116

Thomas Mercer was born in 1822 and apprenticed under his grandfather William Walker who is credited with founding a dynasty of watchmakers in St. Helens, Merseyside, England. In 1858, Thomas formed Thomas Mercer Chronometers (TMC) in Islington, London. This was a highly competitive area for chronometer makers since Dent, Fordsham and others worked nearby. Mercer worked hard and built himself a reputation for making high quality instruments. The business grew and in 1874, it was moved to larger quarters in St. Albans. He entered his chronometers in the accuracy trails at Greenwich and won second prize in 1881. This success elevated is reputation and encouraged business contracts from the private and public sectors. TMC often built clocks that were sold to other retailers who in turned put their own names on the dials. Thomas Mercer died in Paris, France in 1900. The business was carried on by Frank Mercer his son. Over the years Mercer chronometers served of several historical voyages. A Mercer chronometer was used by Sir Ernest Shackleton on his 1914 Imperial Trans-Antarctic Expedition. TMC may be best known for industrializing the British chronometer system of production during the first World War. By the 1980’s, Thomas Mercer Chronometers had manufactured a third of all the chronometers made in England to date.

This demonstration model of a spring detent escapement is thought to have been manufactured by Thomas Mercer Chronometer Makers in the last half of the 1970’s. It is reported that this may be one of a half a dozen examples known and are now thought of as prototypes. This model is oversized and illustrates a mesmerizing motion. The large balance wheel measures 6 inches in diameter. The rotation of this wheel takes approximate 3.5 seconds to make a full turn. The action is slow an deliberate. The wooden platform that the mechanism is mounted to measures 9 inches in diameter. From the table surface to the top of the dome is 9.5 inches tall. The dome has been cracked. Once wound from the bottom of the case, the escapement will operate for a number of hours. It is spring powered and very good quality.

This is an oversized model of a the detent escapement. The detent escapement became the universal standard for marine chronometers and were used on ships for almost two centuries before being replaced by electronic clocks in the 1970s. Similar to the lever escapement, the detent escapement is a detached escapement. The design allows the balance wheel to swing undisturbed during the majority of its cycle, except for the brief impulse period. The more commonly made lever escapement provides two impulses per cycle or vibration as compared to the detent escapement which gives just one impulse per cycle or vibration. In other words it interacts with the rotation of the balance every other swing thus interfering with the balance as little as possible.  This type of escapement is not recommended for civilian use, since during the passive vibration, if the watch is jarred or the balance is otherwise disrupted, the movement may stop.  In addition, this escapement is not self starting.

The detent escapement uses a very thin blade spring with a locking pallet to hold the escape wheel in place. When the impulse roller rotates counter clockwise, the discharging pallet lifts up the spring while the impulse pallet contacts the escape wheel allowing it to move forward. The escape wheel is then locked again as the blade spring falls back into place. Since the driving escape wheel tooth moves almost parallel to the pallet and in one direction only, the escapement has little friction and need no oiling. Detent escapement demonstrated to be the most accurate escapement for balance wheel timepieces keeping time to within 1 or 2 seconds per day, an accuracy which approaches that of a modern quartz watch.

This design was invented by Pierre Le Roy (1717–1785) in 1748. It was modified and and improved in the late 18th century British watchmakers John Arnold (1736-1799) and Thomas Earnshaw (1749-1829). Earnshaw’s design proved to be the simplest possible and demonstrated to be the most efficient and reliable establishing itself as the reference and most deployed detent escapement.


For more information about this clock click  here .