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instrument for measuring and indicating time. Predecessors of the clock were the sundialsundial,
instrument that indicates the time of day by the shadow, cast on a surface marked to show hours or fractions of hours, of an object on which the sun's rays fall.
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, the hourglasshourglass,
glass instrument for measuring time, usually consisting of two bulbs united by a narrow neck. One bulb is filled with fine sand that runs through the neck into the other bulb in an hour's time. The date of its invention is unknown, but it was in use in ancient times.
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, and the clepsydraclepsydra
or water clock,
ancient device for measuring time by means of the flow of water from a container. A simple form of clepsydra was an earthenware vessel with a small opening through which the water dripped; as the water level dropped, it exposed marks on the
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. See also watchwatch,
small, portable timepiece usually designed to be worn on the person. Other kinds of timepieces are generally referred to as clocks. At one time it was generally believed that the first watches were made in Nuremburg, Germany, c.1500.
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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 clockatomic clock,
electric or electronic timekeeping device that is controlled by atomic or molecular oscillations. A timekeeping device must contain or be connected to some apparatus that oscillates at a uniform rate to control the rate of movement of its hands or the rate of
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, 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 NIST-F2 (which is one of two U.S. time frequency standard clocks), is accurate to one second in 300 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.


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).


Any instrument for measuring or indicating time, especially a mechanical device with a numbered dial and moving hands or pointers.

What does it mean when you dream about a clock?

Clocks as symbols often reflect the dreamer’s anxiety about not being on top of things, and thus behind schedule. A clock may also symbolize the biological clock that ticks away for people who want children, or those who feel that time is running out for them.


A source of accurately timed pulses, used for synchronization in a digital computer or as a time base in a transmission system.
A device for indicating the passage of time, usually containing a means for producing a regularly recurring action. Also known as timing circuit.




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


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.


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 is used to set the transmission speed and may also be used to synchronize the 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, clockless computing and MHz.

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


Circular images are one of the most important dream symbols, which represent the psychic center of personality. They are symbolic of wholeness, completeness, and unity of the self. The clock is a mandala that revolves and it may represent immortality. On the lighter side, when you are dreaming about a clock, time is an obvious issue. You may be currently experiencing anxiety in regard to a time-sensitive situation. For example, people worry about their “biological clock” running out, or they are concerned about not being “on top of things.” In general, however, this dream may be a reminder that you need to speed up your actions and that time is an important factor. Old dream interpretations say that if you hear a clock strike, or an alarm go off, positive things will happen to you, and if you are winding a clock, you will fall in love! When interpreting this dream, try to remember the time and then attempt to understand how those numbers are meaningful to you.
References in periodicals archive ?
Embedded clocking provides advantages of source-synchronous clocking without problems related to clock-skew from poor strobe signal integrity and mismatched trace lengths.
The latest version of the Rambus XDR Clock Generator represents a state-of-the-art clocking solution that meets the high performance, high bandwidth requirements of today's most demanding applications such as graphics processing and consumer electronics," said David Nguyen, vice president of engineering at Rambus Inc.
We were the first to supply a complete portfolio of four PLL-based solutions for the desktop, server and notebook markets, as well as the first to supply single-chip clocking solutions for the current generation of Intel Centrino mobile technology-based notebooks.
Clocking Solution Addresses Critical Electromagnetic Interference Issues In Next-Generation Notebook PCs
The MPC96877 is also designed to track spread spectrum clocking for reduced electromagnetic interference (EMI).