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(krənŏm`ətər), instrument for keeping highly accurate time, used especially in navigation. Before the advent of radio time signals it was the only device that provided the time accurately enough for a ship at sea to determine its longitude. A mechanical chronometer is a spring-driven escapement timekeeper, like a watch, but its parts are more massively built. Changes in the tension of the spring caused by variations in temperature are compensated for by devices built into it. Modern chronometers are electronic, using the vibrations of a quartz crystal to regulate the rate at which a time-indicating display moves.



a highly accurate portable clock that has been certified by a testing laboratory, such as an astronomical observatory, and which is used to provide the correct time reference (usually Greenwich mean time), essential in determining geographic longitude in navigation and geodesy. The chronometer and the sextant are the basic navigation instruments.

The first chronometers were marine chronometers, the need for which arose in the 16th and 17th centuries with the development of seafaring and navigation. Pendulum clocks, which are highly accurate in stationary installations, proved unsuitable for navigation because the shaking and rolling of a ship at sea disturbed their operation. Numerous attempts by C. Huygens and other scientists to adapt pendulum clocks to marine work failed to produce the desired results. M. V. Lomonosov was one of the first to establish the unsuitability of the pendulum in a marine chronometer. He recommended the use of a compensation balance and developed a clock mechanism driven by four springs to even out the torque transmitted to the balance. The first marine chronometer suitable for practical use was devised by the English horologist J. Harrison in the mid-18th century; it was designed around a clock equipped with a compensation balance. Harrison thus proved the feasibility of producing a marine chronometer, but the design he proposed did not come into wide use.

In the late 18th and early 19th centuries, the mechanical marine chronometer took on the specific design features (distinct from those of conventional clocks) that have been retained without essential change into the 1970’s. They include a chronometer escapement, which, unlike the anchor escapement, transmits not two but one impulse to the balance for each oscillation period, which makes the oscillations of the balance equal and ensures greater precision of operation. The balance is connected to a cylindrical spiral spring and has a bimetallic split rim, which makes it possible to keep the period of oscillations of the balance constant with changes in temperature. A special device called a fusee evens out the torque of the mainspring as it unwinds. Marine chronometers are suspended in gimbals, which keeps the instruments horizontal when the ship rolls.

Chronometers have been used in Russia to determine geographic longitude for cartography since the 1840’s. The Russian astronomers V. Ia. Struve, O. V. Struve, and P. M. Smyslov made significant improvements in the methods used to regulate the running of chronometers and provide temperature compensations.

In the 1940’s, as a result of the appearance of new materials and improvements in the design of clock mechanisms and the technology of manufacturing them, and because of the great sensitivity of chronometer escapements to shock, anchor escapements came to be used in mechanical chronometers, especially small ones, with no loss of accuracy. Pocket and, particularly, wrist chronometers came into wide use. They differ from ordinary clocks only in their superior accuracy, which is the result of high quality in manufacturing and the regulation of the chronometer mechanism. Good mechanical wrist chronometers have a daily variation of ±3 seconds; the change in their daily operation with a 1°C change in temperature is ±0.2 second. Such chronometers are used by pilots, railroad engineers, physicians, engineers, and other specialists whose work requires precise timekeeping.

Mechanical chronometers are sometimes mounted on shock absorbers for use on expeditions in motor vehicles. Such installations are not used under stationary conditions, for example, in a laboratory or astronomical observatory. Some chronometers have a contact device to transmit electrical pulses, for example, at 1-second intervals. Chronometers are regulated according to mean solar time (marine chronometers) or astronomical time (chronometers used for astronomical observations). The diameter of the face of a large, modern mechanical chronometer is approximately 100 mm; in small chronometers the face is no more than 80 mm in diameter. The mechanism of the chronometer is installed on 15 jewel bearings; large chronometers have one diamond balance bearing. Chronometers are wound daily. The mean deviation of daily accuracy is no more than 0.15 second, and the change in daily accuracy with a 1°C change in temperature is ±0.05 second. When used in conjunction with accurate time signals transmitted by radio, mechanical chronometers meet the requirements of modern transportation as well as expeditionary and research work in which time must be kept with an accuracy of as much as tenths of a second per day.

During the 1970’s both large (including marine) and small (including wrist-type) electromechanical and electronic quartz chronometers have become widely used. The basic design of the most common quartz chronometers is the same as that of quartz clocks. Such chronometers do not need gimbal suspensions and shock absorbers because the absence of moving elements in the mechanism makes them resistant to various types of shock. Quartz chronometers do not require winding: one primary cell, such as a mercury oxide or silver oxide cell, will power the mechanism for a year. Electromechanical chronometers usually have hands, whereas electronic chronometers have digital displays with light-emitting diodes or liquid crystals. Quartz chronometers are characterized by highly stable performance: the average daily variation is approximately ±0.01 second for large chronometers and ±0.3 second for wrist chronometers. Wrist chronometers exhibit an error of not more than 5 seconds per month. In the temperature range from 0°C to 40°C, the daily variation of a wrist chronometer does not exceed ± 1 second.


Any extremely accurate watch.
A large, strongly built timepiece that beats half seconds and is especially designed for precise timekeeping on ships at sea.