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A 'marine chronometer' is a timekeeper precise enough to be used as a portable time standard, used to determine longitude by means of celestial navigation.
The term ''chronometer'' is also used to describe watches tested and certified to meet certain precision standards. In Switzerland, only timepieces certified by the COSC may use the word 'Chronometer' on them.

Contents

History

Mechanical chronometers

Today

References

See also

External links

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. To find their longitude, however, they needed a portable time standard that would work aboard a ship. Observation of celestial, "clockwork" motions such as Galileo's method based on observing Jupiter's natural satellites was usually not possible aboard due to the ship's motion. The purpose of a chronometer is to keep the time of a known fixed location, which can then serve as a reference point for determining the ship's position. By comparing local high noon to the chronometer's time, a navigator could use the time difference to determine the ship's present longitude. Since the Earth rotates 360 degrees every day (that is, 24 hours or 1,440 minutes), the time difference between the two points reveals how many degrees separate them. With the degrees of difference in hand, locating the position on a map was a relatively simple matter of spherical trigonometry. (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 creation of a seaworthy timepiece was difficult. Until the 20th century, the best timekeepers were pendulum clocks, and the rolling of a ship at sea rendered the ordinary, gravity-based 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 sea timekeepers 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. Construction of his third machine, designated H3, included novel circular balances and the invention of the bi-metallic strip and caged roller bearings (both inventions are still widely used today). H3's circular balances proved too inaccurate and he eventually abandoned the large machines. Harrison solved the precision problems with his H4 chronometer design. H4 appeared much like a large five-inch (12 cm) diameter pocket watch. H4 was submitted for a £20,000 prize offered by the British government in the early 18th century. His design used a fast-beating balance 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 work proved the possibility of portable precision timekeepers, making them practical by perfecting simpler and more affordable designs was the next problem. Pierre Le Roy and Ferdinand Berthoud in France, and Thomas Mudge in England successfully produced marine timekeepers. Although none of these makers discovered a path to simplicity, they did encourage others by proving that Harrison's design was not the only answer to the problem. The greatest strides toward practicality came at the hands of Thomas Earnshaw and John Arnold, who developed simplified, detached, "spring detent" escapements, moved the temperature compensation to the balance, and improved the design and manufacturing of balance springs. This combination of innovations served as the basis of marine chronometers until the electronic era.
By 1825, the British Navy had begun routinely supplying its vessels with chronometers.^[1]
Although industrial production methods began revolutionizing watchmaking in the middle of the 19th century, chronometer manufacture remained craft-based much longer. Around the turn of the 20th century, Swiss makers like Ulysse Nardin made great strides toward incorporating modern production methods, like fully interchangeable parts, but it was only with the onset of World War II that the American Hamilton Watch Company succeeded in mass production of chronometers in quantity for the US Navy. Despite Hamilton's success, chronometers made in the old way never disappeared from the marketplace during the era of mechanical timekeepers. Mercer, in St. Albans, England, for instance, continued to produce high-quality chronometers by traditional production methods well into the 1970s.
The most complete international collection of marine chronometers, including Harrison's H1 to H4, is at the National Maritime Museum, Greenwich, England.

Mechanical chronometers

The crucial problem was to find a resonator that remained unaffected by the changing conditions met by a ship at sea. The balance wheel harnessed to a spring solved most of the problems associated with the ship's motion. Unfortunately, the elasticity of most balance spring materials changes relative to temperature. To compensate for ever-changing spring strength, the majority of chronometer balances used bi-metallic strips to move small weights toward and away from the center of oscillation, thus altering the period of the balance to match the changing force of the spring. The balance spring problem was solved with a nickel-steel named Elinvar for its invariable elasticity at normal temperatures. The inventor was Charles Edouard Guillaume, who won the Nobel Prize for physics in recognition for his metallurgical work (the only Nobel that has been granted for work related to horology).
The escapement serves two purposes. First, it allows the train to advance fractionally and record the balance's oscillations. At the same time, it supplies minute amounts of energy to counter tiny losses from friction, thus maintaining the equilibrium of the oscillating balance. The escapement is the part that ticks. Since the natural resonance of an oscillating balance serves as the heart of a chronometer, chronometer escapements are designed to interfere with the balance as little as possible. There are many constant force and detached escapement designs, but the most common are the spring detent and pivoted detent. In both of these, a small detent locks the escape wheel and allows the balance to swing completely free of interference except for a brief moment at the center of oscillation, when it is least susceptible to outside influences. At the center of oscillation, a roller on the balance staff momentarily displaces the detent, allowing one tooth of the escape wheel to pass. The escape wheel tooth then imparts its energy on a second roller on the balance staff. Since the escape wheel turns in only one direction, the balance receives impulse in only one direction. On the return oscillation, a passing spring on the tip of the detent allows the unlocking roller on the staff to move by without displacing the detent.
Chronometers often included other innovations to increase their efficiency and precision. Hard stones such as ruby and sapphire were often used as jewel bearings to decrease friction and wear of the pivots and escapement. Until the end of mechanical chronometer production in the third quarter of the 20th century, makers continued to experiment with things like ball bearings and chrome-plated pivots.
Marine chronometers always contain a maintaining power which keeps the chronometer going while it is being wound, and a power reserve to indicate how long the chronometer will continue to run without being wound. Marine chronometers are the most accurate portable mechanical clocks ever made, achieving a precision of around a tenth of a second per day. This is accurate enough to locate a ship's position within 4600 feet after a month's sea voyage.

Today

Quartz clocks and atomic clocks have made mechanical clock-chronometers obsolete for time standards used scientifically and/or industrially.

References

1.
Former Clock & Watchmakers and Their Work, , Frederick James, Britten, Spon & Chamberlain, ,

See also

★ Balance wheel

★ Clock

★ Clockmaker

★ Chronometer watch

★ Horology

★ Jewel bearing

★ John Harrison, who built the first usable marine chronometer

★ Railroad chronometers

★ Rupert Gould, who wrote the definitive history of the marine chronometer

★ Watch

★ Watchmaker

External links

★ National Maritime Museum, Greenwich