clock (redirected from Analog Clocks)

clock, instrument for measuring and indicating time. Predecessors of the clock were the sundial, the hourglass, and the clepsydra. See also watch.

The Evolution of Mechanical Clocks

The operation of a clock depends on a stable mechanical oscillator, such as a swinging pendulum or a mass connected to a spring, by means of which the energy stored in a raised weight or coiled spring advances a pointer or other indicating device at a controlled rate. It is not definitely known when the first mechanical clocks were invented. Some authorities attribute the first weight-driven clock to Pacificus, archdeacon of Verona in the 9th cent. Gerbert, a learned monk who became Pope Sylvester II, is often credited with the invention of a mechanical clock, c.996.

Mechanical figures that struck a bell on the hour were installed in St. Paul's Cathedral, London, in 1286; a dial was added to the clock in the 14th cent. Clocks were placed in a clock tower at Westminster Hall, London, in 1288 and in the cathedral at Canterbury in 1292. In France, Rouen was especially noted for the skill of its clockmakers and watchmakers. Probably the early clock closest to the modern ones was that constructed in the 14th cent. for the tower of the palace (later the Palais de Justice) of Charles V of France by the clockmaker Henry de Vick (Vic, Wieck, Wyck) of Württemburg. Until the 17th cent. few mechanical clocks were found outside cathedral towers, monasteries, abbeys, and public squares.

The early clocks driven by hanging weights were bulky and heavy. When the coiled spring came into use (c.1500), it made possible the construction of the smaller and lighter-weight types. By applying Galileo's law of the pendulum, the Dutch scientist Christiaan Huygens invented (1656 or 1657) a pendulum clock, probably the first. Early clocks used in dwellings in the 17th cent. were variously known as lantern clocks, birdcage clocks, and sheep's-head clocks; they were of brass, sometimes ornate, with a gong bell at the top supported by a frame. Before the pendulum was introduced, they were spring-driven or weight-driven; those driven by weights had to be placed on a wall bracket to allow space for the falling weights. These clocks, probably obtained chiefly from England and Holland, were used in the Virginia and New England colonies.

Clocks with long cases to conceal the long pendulums and weights came into use after the mid-17th cent.; these were the forerunners of the grandfather clocks. With the development of the craft of cabinetmaking, more attention was concentrated on the clock case. In France the tall cabinet clocks, or grandfather clocks, were often of oak elaborately ornamented with brass and gilt. Those made in England were at first of oak and later of walnut and mahogany; simpler in style, their chief decoration was inlay work.

Electric and Other Clocks

Electric clocks were made in the second half of the 19th cent. but were not used extensively in homes until after c.1930. In an analog clock the hands of an electric clock are driven by a synchronous electric motor supplied with alternating current of a stable frequency. Digital clocks use LCDs (liquid crystal displays) or LEDs (light emitting diodes) to form the numbers indicating the time. The quartz clock, invented c.1929, uses the vibrations of a quartz crystal to drive a synchronous motor at a very precise rate. Some quartz clocks have an error of less than one thousandth of a second per day. The atomic clock, which is based upon the frequency of an atomic or molecular process, is even more precise; a state of the art atomic clock, such as the NIST-F1 (which is the U.S. time frequency standard clock), neither gains nor loses a second in 20 million years.

Some Famous Clocks

One of the most famous clocks is in the cathedral of Strasbourg; the clock was first placed in the cathedral in 1352, and in the 16th cent. it was reconstructed. In the 19th cent. a new astronomical clock (so called because it shows the current positions of the sun, moon, and other heavenly bodies in addition to the time of day) similar to the original clock was constructed; its elaborate mechanical devices include the Twelve Apostles, a crowing cock, a revolving celestial globe, and an automatic calendar dial. Among other well-known clocks of the world are the clock known as Big Ben in the tower next to Westminster Bridge in the British Houses of Parliament and the tower clock in the Metropolitan Life Insurance Company building, New York City.

Bibliography

See F. J. Britten, Old Clocks and Watches and Their Makers (1976); D. S. Landes, Revolution in Time: Clocks and the Making of the Modern World (1985); J. E. Barnett, Time's Pendulum: The Quest to Capture Time from Sundials to Atomic Clocks (1998); E. Bruton, Collector's Dictionary of Clocks and Watches (1999); J. Jesperson and J. Fitz-Randolph, From Sundials to Atomic Clocks: Understanding Time and Frequency (2d ed. 1999).

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clock

A classic pendulum clock. The power to run the clock comes from a slowly falling weight (other …

(credit: © Merriam-Webster Inc.)

Machine or electronic device that measures and records time. Both simple and elaborate clocks, as well as sundials, candle clocks, and sandglasses, were used for measuring time in ancient Egypt, Greece, and Rome. The first mechanical clocks were weight-driven and perhaps were invented for use in monasteries, where the disciplined life required a strict rendering of time. The first European public clock that struck the hours was erected in Milan in 1335, and the oldest surviving clocks are in England (1386) and France (1389). The first domestic clocks appeared late in the 14th century. About 1500 Peter Henlein, a German locksmith, began to make the first portable timepieces, small clocks driven by a spring. Christiaan Huygens invented pendulum clocks in 1656. Big Ben, the great clock at Westminster in London, was installed in 1859 and is the standard for all accurate tower pendulum clocks. The most accurate mechanical timekeepers (within a few thousandths of a second per day) are clocks with short pendulums (about 39 in. [or 990 mm]). In 1929 the vibration of a quartz crystal was first applied to timekeeping; the maximum error of an observatory quartz-crystal clock is only a few ten-thousandths of a second per day. The first atomic clock went into operation in 1951. Atomic clocks, regulated by the natural periodic behaviour of a system of atoms (such as vibrations or emission of radiation), can have accuracies exceeding one billionth of a second per day, making them the most accurate clocks yet invented.

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clock

An internal timing device. Using a quartz crystal, the CPU clock breathes life into the microprocessor by feeding it a constant flow of pulses. For example, a 200 MHz CPU receives 200 million pulses per second from the clock. A 2 GHz CPU gets two billion pulses per second. Similarly, in a communications device, a clock may be used to synchronize the data pulses between sender and receiver.

A "real-time clock," also called the "system clock," keeps track of the time of day and makes this data available to the software. A "timesharing clock" interrupts the CPU at regular intervals and allows the operating system to divide its time between active users and/or applications. See per clock and clockless computing.

The CPU Clock

The quartz crystal generates continuous waves, which are converted into digital pulses.

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clock

1. a timepiece, usually free-standing, hanging, or built into a tower, having mechanically or electrically driven pointers that move constantly over a dial showing the numbers of the hours Compare digital clock watch

2. any clocklike device for recording or measuring, such as a taximeter or pressure gauge

3. Botany the downy head of a dandelion that has gone to seed

4. an electrical circuit that generates pulses at a predetermined rate

5. Computing an electronic pulse generator that transmits streams of regular pulses to which various parts of the computer and its operations are synchronized

6. an informal word for speedometer, mileometer

7. against the clock (in certain sports, such as show jumping) timed by a stop clock

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clock [kläk]

(electronics)

A source of accurately timed pulses, used for synchronization in a digital computer or as a time base in a transmission system.

(horology)

A device for indicating the passage of time, usually containing a means for producing a regularly recurring action. Also known as timing circuit.

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Clock [kläk]

(astronomy)

Horologium

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(processor)

clock - A circuit in a processor that generates a regular sequence of electronic pulses used to synchronise operations of the processor's components. The time between pulses is the cycle time and the number of pulses per second is the clock rate (or frequency). The execution times of instructions on a computer are usually measured by a number of clock cycles rather than seconds. Clock rates for various models of the computer may increase as technology improves, and it is usually the relative times one is interested in when discussing the instruction set.