A chronometer is a clock designed to have sufficient long-term accuracy that it can be used as a portable time standard on a vehicle, usually in order to determine longitude by means of celestial navigation. In Switzerland, only timepieces certified by the COSC may use the word 'Chronometer' on them.
Until the mid 1750s, navigation at sea was an unsolved problem. Navigators could determine their latitude by measuring the sun's angle at noon. However to find their longitude, they needed a portable time standard that would work on a ship. Conceptually, at local high noon they could compare the chronometer's time to determine their longitude (in modern practice, a navigational almanac and trigonometric sight reduction tables permit navigators to measure the Sun, Moon, visible planets or any of 57 navigational stars at any time of day or night).
The problem of the clock was difficult. At the time, the best clocks were pendulum clocks, and the rolling of a ship at sea caused these to be inaccurate. John Harrison, a carpenter, developed a clock based on a pair of counter-oscillating weighted beams connected by springs, whose motion was not influenced by gravity or the motion of a ship. His chronometers H1–H3 were all of this design but were large and heavy, were required to be suspended from a beam in a ship, and suffered from small inaccuracies due to precession that did not become apparent until long-distance sea trials occurred.
He finally solved the accuracy problems with his H4 chronometer, essentially a large 5 inch (12 cm) diameter watch, winning a prize from the British Admiralty. His design used a temperature-compensated balance wheel. This method remained in use until microchips reduced the cost of a quartz clock to the point that electronic chronometers became commonplace.
John Harrison invented the balance wheel and escapement mechanism, and was (eventually) given credit and paid for this. Ownership of the design passed to the Admiralty, who now needed manufacturers. Thomas Earnshaw, John Arnold and others tackled this, with the development of the practical spring detent escapement chronometer.
Aaron Lufkin Dennison was the pioneer in the industrial revolution of watch making as early as 1850 in developing the American System of Watch Manufacturing by Interchangeable Parts at the Waltham Watch Company, which is at the base of today's worldwide manufacturing methods. The American Hamilton Watch Co. harnessed mass production to produce chronometers in quantity for the US Navy during World War II.
The crucial problem was to find a resonator that remained unaffected by the motions of a ship at sea. The balance wheel solved that problem. Balance wheels for chronometers used bi-metallic strips to move small weights toward and away the center of the wheel, in order to adjust the period of the balace wheel for the temperature of the chronometer.
The other crucial problem was that the energy of most spring materials changes with temperature. A special alloy of steel was eventually developed, just to solve this problem. Additionally, this spring had to be given a special oval shape.
The escapement drives the balance wheel, usually from a gear train. It is the part that ticks. Escapements have a locking state, and a drive state. In the locking state, nothing moves. The motion of the balance wheel switches the escapement to drive, and the escapement then pushes on the wheel for a brief part of the wheel's cycle.
The escapement is the part of a clock most prone to wear, because it moves the fastest. The efficiency of an escapement's design, that is, how much energy is converted into resonant motion, directly affects the accuracy of a clock, and how long a clock can operate between windings.
A chronometer's escapement is usually designed to minimize the energy and time required to unlock the escapement, so that it affects the resonant frequency of the oscillator as little as possible.
Another way of making the clock more efficient is to use rubies as jewel bearings for the axes and the parts of the escapement that make contact repeatedly. Ideally some kind of ball bearings would be used but it has not been possible to manufacture ball bearings in the required size. The rubies are hard and do not wear to the same extent as metal. They also introduce accurately predictable amounts of friction that can be compensated for in the design. On the back of chronometers it usually says how many jewels the watch contains. More complicated and accurate movements typically have more jewel bearings. In modern watchmaking the rubies used are artificially produced.
In horology terms, a complication in a mechanical watch is a special feature that causes the design of the watch movement to become more complicated. Examples of complications include:
- Perpetual Calendar
- Minute repeater
- Equation of time
- Power reserve
- Moon phases
- Double chronograph
Quartz clocks and atomic clocks have made mechanical clock-chronometers obsolete for time standards used scientifically and/or industrially, although some custom watchmakers can still produce them. The techniques used to mass-produce mechanical clock-chronometers are now lost.
Nevertheless, in Switzerland nowadays, over 1,000,000 Officially Certified Chronometers certificates, mostly for mechanical wrist-chronometers (wrist-watch) with sprung balance oscillator, are being delivered each year, upon having been submitted to the COSC's most severe tests, each singly identified by an officially recorded individual serial number.
- American Watchmakers-Clockmakers Institute (http://www.awi-net.org/)