Chronometer

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A chronometer is a clock accurate enough to 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.

1 History
2 Mechanical chronometers
- 2.1 Complications
3 Today
4 See also
5 External links

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History

Until the mid 1750s, navigation at sea was an unsolved problem due to the difficulty in calculating longitudinal position. 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 that the horizon is visible).

The problem of creating a sea-worthy timepiece was difficult. At the time, the best clocks were pendulum clocks, and the rolling of a ship at sea rendered the pendulum useless. John Harrison, a Yorkshire carpenter, invented 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 first two chronometers used this system but he became rightly convinced that they had a fundamental sensitivity to centrifugal force which meant that they could never be accurate enough at sea. His third machine replaced one headache with a bigger one and he eventually abandoned the large machines altogether.

He finally solved the accuracy problems with his H4 chronometer, essentially a large 5 inch (12 cm) diameter pocket watch, winning a prize of £20,000 offered by the government in the early 18th century. His design used a fast-beating balance wheel controlled by a temperature-compensated spiral spring. This general layout remained in use until microchips reduced the cost of a quartz clock to the point that electronic chronometers became commonplace.

After Harrison's pioneering work had proved the practicality of the device and defined its basic layout, what the Admiralty (and civilian ship-owners) needed was a large number of simpler and cheaper chronometers. Thomas Earnshaw, John Arnold and others tackled this, moving the temperature compensation into the balance wheel, improving the balance spring by making it helical rather than flat, and by developing the practical and simple spring detent escapement. This combination was the defining technology of marine chronometers until the electronic era.

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 Company harnessed mass production to produce chronometers in quantity for the US Navy during World War II.

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Mechanical chronometers

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 from the center of the wheel, in order to adjust the period of the balance wheel for the temperature of the chronometer. Solid balance-wheels of low-expansion steel alloys such as invar give results nearly as good, but are more susceptible to magnetism.

The other crucial problem was that the energy of most spring materials changes with temperature. A special alloy of low-expansion nickel-steel (elinvar) was eventually developed, just to solve this problem. Additionally, balance-wheel springs had to be given a special oval shape. The recipes for "observatory quality" steel balance-springs have been lost because of low production volumes. The original manufacturers (such as Hamilton Watch) are out of business. Some horologists claim that carbon composite springs have comparable qualities, and are nonmagnetic, as well.

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, when the escapement pushes against the wheel (supplies an impulse) 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 a clock more efficient is to use ruby as jewel bearings for the axes and the parts of the escapement that make repeated contact. It could be thought that ball bearings might be used to good effect at the pivots of clocks and watches, but tests seem to show that they do not perform well under the stop-start conditions typical of mechanical timepieces. Ruby is hard-wearing, can take a high polish, and has a low co-efficient of friction with polished hard steel. Synthetic ruby can also be produced very cheaply nowadays, making it the best material for horological bearing surfaces. Lately, ceramics have been experimented with as a new material for chronometer parts subject to high wear rates.

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Complications

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:

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Today

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.

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