Cox's timepiece is a clock

Clock

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

 developed in the 1760s by James Cox

James Cox (inventor)

James Cox was a British jeweller, goldsmith and entrepreneur best known for creating mechanical clocks, including Cox's timepiece and the Peacock Clock from the State Hermitage Museum in Saint Petersburg....

. It was developed in collaboration with John Joseph Merlin

John Joseph Merlin

John-Joseph Merlin was a Belgian inventor and horologist.He was born Jean-Joseph Merlin in 1735 in the city of Huy, Belgium....

 (with whom Cox also worked on developing automata

Automata

Automata is the plural form of automaton, a self-operating machine. It may also refer to:* "Automata", a short story by E. T. A. Hoffmann* "Automata", a hardboiled science fiction crime series by Penny Arcade...

). Cox claimed that his design was a true perpetual motion machine, but as the device is powered from changes in atmospheric pressure

Atmospheric pressure

Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

 via a mercury

Mercury (element)

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

 barometer

Barometer

A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather...

, this is not the case. The clock still exists today but was deactivated at the time of the clock's relocation to the Victoria and Albert Museum

Victoria and Albert Museum

The Victoria and Albert Museum , set in the Brompton district of The Royal Borough of Kensington and Chelsea, London, England, is the world's largest museum of decorative arts and design, housing a permanent collection of over 4.5 million objects...

 of Great Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

.

Design and history

The clock is similar to other mechanical clocks, except it does not need winding. The change of pressure in the Earth's atmosphere

Earth's atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 acts as an external energy source and causes sufficient movement of the winding mechanism. This keeps the mainspring

Mainspring

A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...

 coiled inside the barrel. The clock is designed to enable the timepiece to run indefinitely and overwinding is prevented by a safety mechanism. The prime mover, encased in a finely detailed clock body, is a Fortin mercury barometer. The barometer contained 68 kilogram

Kilogram

The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

s (150 pound

Pound (mass)

The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...

s) of mercury.

Related to this is Cornelis Drebbel's device of 1610 (though it is unknown whether Cox knew of it). It was a machine that told the time, date, and season. The gold machine was mounted in a globe on pillars and was powered by changes in air pressure (a sealed glass tub with liquid varied in volume through atmospheric pressure changes, rewinding constantly).

The Atmos

Atmos clock

Atmos is the brand name of a mechanical clock manufactured by Jaeger-LeCoultre in Switzerland which does not need to be wound manually. It gets the energy it needs to run from temperature and atmospheric pressure changes in the environment, and can run for years without human intervention.Its...

, manufactured by Jaeger LeCoultre is a modern clock which is similar to Cox's clock although the main driving force is generated from temperature differential, instead of pressure differential.

Journals

• William Nicholson, "Concerning those perpetual motions which are produced in machines by the rise and fall of the barometer or thermometrical variations in the dimensions of bodies". Philosophical Journal.
• William Nicholson, Philosophical Journal, vol I, 1799, p375

Books

• Arthur W. J. G. Ord-Hume: Perpetual Motion: The History of an Obsession. Adventures Unlimited Press 2006, ISBN 1931882517, pp. 110-124
• Arthur W. J. G. Ord-hume, "Clockwork Music", Allen & Unwin, London 1973.
• John Joseph Merlin, "The Ingenious Mechanick". The Greater London Council, The Iveagh Bequest, Kenwood, Hampstead Lane, London, © 1985.