Chronometer

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

[edit] History

Bréguet twin barrel box chronometer.

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 aboard a ship. 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. Conceptually, 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 24 hours, 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 problem of creating 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. His third machine replaced one headache with a bigger one, and he eventually abandoned the large machines altogether. Harrison finally solved the precision problems with his H4 chronometer, essentially a large five-inch (12 cm) diameter pocket watch, which he 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 pioneering work proved the possibility of portable precision timekeepers, making them practical by perfecting simpler and more affordable designs stood as the next problem. Pierre Le Roy and Ferdinand Berthoud in France, and Thomas Mudge in England successfully produced marine timekeepers of their own designs. Although none of these great makers discovered a path to simplicity, they did encourage others to enter the field by proving that Harrison's design did not represent 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 process of balance springs. This combination of technological innovations served as the basis of marine chronometers until the electronic era.

Although industrial production methods began revolutionizing watchmaking in the middle of the 19th century, chronometer manufacture remained craft-based for 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 finally succeeded in fully harnessing mass production to produce 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.

[edit] 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. Eventually, the balance spring problem was solved by the development of 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 by far 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 many other technological innovations designed to increase their efficiency and precision. Hard stones such as diamond, ruby, and sapphire were often use as jewel bearings to decrease friction and wear of the pivots and escapement parts that made repeated contact. Chronometer makers also took advantage of the physical properties of rare metals such as gold, platinum, and palladium. 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.

[edit] 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:

• Tourbillon
• Perpetual Calendar
• Minute repeater
• Equation of time
• Power reserve
• Moon phases
• Double chronograph

[edit] 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 chronometers are now lost.

[edit] Officially Certified Chronometers

Over 1,000,000 Officially Certified Chronometer certificates, mostly for mechanical wrist-chronometers (wristwatches) with sprung balance oscillators, are being delivered each year, after passing the COSC's most severe tests and being singly identified by an officially recorded individual serial number. According to COSC, a chronometer is a high-precision watch capable of displaying the seconds and housing a movement that has been tested over several days, in different positions, and at different temperatures, by an official, neutral body (COSC). Each movement is individually tested for several consecutive days, in five positions and at three temperatures. Each movement is individually measured. Any watch with the denomination "chronometer" is provided with a certified movement.

[edit] See also

• Clock
• Clockmaker
• COSC
• Horology
• Jewel
• Railroad chronometers
• Watch
• Watchmaker
• Webb C. Ball
• Chronograph

[edit] External links

• American Watchmakers-Clockmakers Institute
• Federation of the Swiss Watch Industry
• Contrôle Officiel Suisse des Chronomètres
• National Maritime Museum, Greenwich