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Alarm clock: digital clock, grandfather clock and wall clocks. A clock is an instrument for measuring and recording time, especially by mechanical means, usually with hands or changing numbers to indicate the hour and minute: not designed to be worn or carried about.

The clock is one of the oldest human inventions, meeting the need to consistently measure intervals of time shorter than the natural units: the day, the lunar month, and the year. The historical record reveals that approximately 5000 to 6000 years ago great civilizations in the Middle East and North Africa began to make clocks to augment their calendars. With their attendant bureaucracies, formal religions, and other burgeoning societal activities, these cultures apparently found a need to organize their time more efficiently.

The Ancient Egyptians divided the day into two 12-hour periods, and used large obelisks to track the movement of the Sun. They also developed water clocks, which were probably first used in the Precinct of Amun-Re, and later outside Egypt as well; they were employed frequently by the Ancient Greeks, who called them clepsydrae. The Cho Dynasty is believed to have used the outflow water clock around the same time, devices which were introduced from Mesopotamia as early as 2000 BC. Other ancient timekeeping devices include the candle clock, used in China, Japan, England and Iraq; the timestick, widely used in India and Tibet, as well as some parts of Europe; and the hourglass, which functioned similarly to a water clock.

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The clock is one of the oldest human inventions, requiring a physical process that will proceed at a known rate and a way to gauge how long that process has run. As the seasons and the phases of the moon can be used to measure the passage of longer periods of time, shorter processes had to be used to measure off hours and minutes.

Clocks by Type

# Alarm clock app. Throughout history, clocks have had a variety of power sources, including gravity, springs, and electricity. Mechanical clocks became widespread in the 14th century, when they were used in medieval monasteries to keep the regulated schedule of prayers. The clock continued to be improved, with the first pendulum clock being designed and built in the 17th century.

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# Alarm app. A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style (a thin rod or a sharp, straight edge) onto a flat surface marked with lines indicating the hours of the day. As the sun moves across the sky, the shadow-edge progressively aligns with different hour-lines on the plate. Such designs rely on the style being aligned with the axis of the Earth’s rotation.

Hence, if such a sundial is to tell the correct time, the style must point towards true north (not the north or south magnetic pole) and the style’s angle with horizontal must equal the sundial’s geographical latitude. Sundials were widely used in ancient times. With the knowledge of latitude, a well-constructed sundial can measure local solar time with reasonable accuracy, within a minute or two.

Sundials continued to be used to monitor the performance of clocks until the modern era. However, practical limitations, such as that sundials work only when the Sun shines, and never during the night, encouraged the use of other techniques for measuring and displaying time.

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A water clock
# Alarm online. A water clock or clepsydra is any timepiece in which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel where the amount is then measured. In the ancient world, there were two forms of water clocks: outflow and inflow. In an outflow water clock, the inside of a container was marked with lines of measurement. The container was filled with water, which was allowed to leak out at a steady pace. Observers were able to tell time by measuring the change in water level. An inflow water clock followed the same principle as an outflow one, i.e. the steady dripping of water. Unlike the latter, the former’s measurements were in a second container instead. Based on the amount of water that dripped from the first container, one was able to tell how much time had passed.

The mechanical clock
# Free alarm clock. The mechanical clock is an invention we all use today. According to historical research, the world’s first clock was invented by Yi Xing, a Buddhist monk and mathematician of the Tang Dynasty (618-907). Yi’s clock operated with water steadily dripping on a wheel that made a full revolution every 24 hours.

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An astronomical clock
# Alarm clock for free. Astronomical clocks are over 1,000 years old — some say the first was the mysterious Antikythera Mechanism. They predict the movements of the stars, sun, moon, and planets. They are also among the most beautifully-designed timepieces in the world. An astronomical clock is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the sun, moon, zodiacal constellations, and sometimes major planets.
Astronomical clocks provided more data than just the time of day; in fact, their key purpose was to provide scientific information about astronomy rather than the time. For instance, they measured solar time, sidereal time, or 24-hour analog time. They taught about eclipses, seasons, equinoxes, and lunar phases. If it was important to know the leap year cycles, or sunrise and sunset data, it was readily available from astronomical timepieces. However, did they tell time? These clocks did measure time, but in a different manner than other clocks; time was measured according to the position and movement of stars and planets.

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In general, most of these clocks track some or all of the following:
• Time
• Position of the Sun
• Zodiac constellations
• Planting aspects
• Moon phases
• Hour lines
• Eclipse predictions
• However, as with any timepiece, each individual clock may furnish other specialized data, depending on its intended purpose.

Spring-driven clocks
# Alarm clock free. Spring-driven clocks brought timekeeping out of the tower and into the home. In contrast to their weight-driven predecessors, spring-driven clocks were small–and portable. Their openness to miniaturization led to the development of the first watches in the late 15th century.Despite their advantages, the new timepieces were still prone to considerable inaccuracy. Because of this, many watches were fitted with a sundial and a compass as a backup. At fault was the mainspring itself, the source of the clock’s power. The force exerted by the spring slackened as it unwound; as a result, the clock ran fast when the spring was fully wound but progressively slower as it released.

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In 1674-75, the Dutch astronomer Christiaan Huygens developed a means of controlling the release of energy from the mainspring–the spiral balance spring. Other means had been devised for this purpose, but the advantage of the balance spring, or hairspring, was that it performed similarly to a pendulum in a pendulum clock. Like a pendulum’s swing, the balance spring’s coiling and uncoiling has a natural periodicity that ensures the even release of energy from the mainspring. In a modern spring-driven watch, the spring is mounted on a balance wheel, which turns back and forth in sync with the spring’s oscillations, simultaneously rocking the pallet from side to side. The pallet controls the turning of the gears connected to the clock’s face and thereby maintains a steady transfer of power from the mainspring to the clock’s counting mechanism. While the mainspring is being wound, a ratchet and click keep the winding action from disturbing the watch’s main gear train.

Pendulum clock
# Free online alarm clock. A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a harmonic oscillator; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates. Pendulum clocks must be stationary to operate; any motion or accelerations will affect the motion of the pendulum, causing inaccuracies, so other mechanisms must be used in portable timepieces. They are now kept mostly for their decorative and antique value.

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The indicating system is almost always the traditional dial with moving hour and minute hands. Many clocks have a small third hand indicating seconds on a subsidiary dial. Pendulum clocks are usually designed to be set by opening the glass face cover and manually pushing the minute hand around the dial to the correct time. The minute hand is mounted on a slipping friction sleeve which allows it to be turned on its arbor. The hour hand is driven not from the wheel train but from the minute hand’s shaft through a small set of gears, so rotating the minute hand manually also sets the hour hand.

# Free alarm. In 1675, Huygens and Robert Hooke invented the spiral balance, or the hairspring, designed to control the oscillating speed of the balance wheel. This crucial advance finally made accurate pocket watches possible. The great English clockmaker, Thomas Tompion, was one of the first to use this mechanism successfully in his pocket watches, and he adopted the minute hand which, after a variety of designs were trialled, eventually stabilised into the modern-day configuration.

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The Rev. Edward Barlow invented the rack and snail striking mechanism for striking clocks, which was a great improvement over the previous mechanism. The repeating clock, that chimes the number of hours (or even minutes) was invented by either Quare or Barlow in 1676. George Graham invented the deadbeat escapement for clocks in 1720.

Marine chronometers
# Alarm clock sound. Marine chronometers are clocks used at sea as time standards, to determine longitude by celestial navigation. A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The position of a ship at sea could be determined with reasonable accuracy if a navigator could refer to a clock that lost or gained less than about 10 seconds per day.
This clock could not contain a pendulum, which would be virtually useless on a rocking ship. In 1714, the British government offered large financial rewards to the value of 20,000 pounds, for anyone who could determine longitude accurately. John Harrison, who dedicated his life to improving the accuracy of his clocks, later received considerable sums under the Longitude Act.

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In 1735, Harrison built his first chronometer, which he steadily improved on over the next thirty years before submitting it for examination. The clock had many innovations, including the use of bearings to reduce friction, weighted balances to compensate for the ship’s pitch and roll in the sea and the use of two different metals to reduce the problem of expansion from heat. The chronometer was tested in 1761 by Harrison’s son and by the end of 10 weeks the clock was in error by less than 5 seconds.

Early electric clocks
# Alarm clock sounds. An electric clock is a clock that is powered by electricity, as opposed to a mechanical clock which is powered by a hanging weight or a mainspring. The term is often applied to the electrically powered mechanical clocks that were used before quartz clocks were introduced in the 1980s. The first experimental electric clocks were constructed around 1840, but they were not widely manufactured until mains electric power became available in the 1890s. In the 1930s the synchronous electric clock replaced mechanical clocks as the most widely used type of clock.

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Types of electronic clock

# Alarm clocks. Electric clocks can operate by several different types of mechanism:
Atomic clocks are the most accurate clocks in existence. They are considerably more accurate than quartz clocks as they can be accurate to within a few seconds over thousands of years.
Electromechanical clocks – These have a traditional mechanical movement, which keeps time with an oscillating pendulum or balance wheel powered through a gear train by a mainspring, but use electricity to rewind the mainspring with an electric motor or electromagnet. This mechanism is found mostly in antique clocks.

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Electric remontoire clocks – In these, the gear train was turned by a small spring or weighted lever, called a remontoire, which was wound up more frequently by an electric motor or electromagnet. This mechanism was more accurate than a mainspring, because the frequent winding averaged out variations in the clock’s rate caused by the varying force of the spring as it unwound.
Electromagnetic clocks keep time with a pendulum or balance wheel, but the pulses to keep it going are not provided by a mechanical movement and escapement linkage, but by magnetic force from an electromagnet (solenoid). This was the mechanism used in the first electric clocks, and is found in antique electric pendulum clocks. It is also found in a few modern decorative mantel and desk clocks.
Synchronous clocks rely on the 50 or 60 Hz utility frequency of the AC electric power grid as a timing source, by driving the clock gears with a synchronous motor. They essentially count cycles of the power supply. While the actual frequency may vary with loading on the grid, the total number of cycles per 24 hours is maintained rigorously constant, so that these clocks can keep time accurately for long periods, barring power cuts; over months they are more accurate than a typical quartz clock.

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Tuning Fork clocks keep time by counting the oscillations of a calibrated tuning fork with a specific frequency. These were only made in battery-powered form. Battery-powered clocks have been made using the schemes above with the obvious exception of a synchronous movement.
Quartz clocks are electric clocks which keep time by counting oscillations of a vibrating quartz crystal. They use modern low-voltage DC-powered circuitry, which may be supplied by a battery or derived from mains electricity. They are the most common type of clock today.
Radio-controlled clocks are quartz clocks which are periodically synchronized with the UTC atomic clock time scale via radio time signals broadcast by dedicated stations around the world. They are distinct from clock radios.

Part of clock or How clocks work

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Power source
# cool alarm clocks. This provides power to keep the clock going.

In mechanical clocks, the power source is typically either a weight suspended from a cord or chain wrapped around a pulley, sprocket or drum; or a spiral spring called a mainspring. Mechanical clocks must be wound periodically, usually by turning a knob or key or by pulling on the free end of the chain, to store energy in the weight or spring to keep the clock running.
In electric clocks, the power source is either a battery or the AC power line. In clocks that use AC power, a small backup battery is often included to keep the clock running if it is unplugged temporarily from the wall or during a power outage. Battery powered analog wall clocks are available that operate over 15 years between battery changes.

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# vintage alarm clock. The timekeeping element in every modern clock is a harmonic oscillator, a physical object (resonator) that vibrates or oscillates repetitively at a precisely constant frequency.

• In mechanical clocks, this is either a pendulum or a balance wheel.
• In some early electronic clocks and watches such as the Accutron, it is a tuning fork.
• In quartz clocks and watches, it is a quartz crystal.
• In atomic clocks, it is the vibration of electrons in atoms as they emit microwaves.
• In early mechanical clocks before 1657, it was a crude balance wheel or foliot which was not a harmonic oscillator because it lacked a balance spring. As a result, they were very inaccurate, with errors of perhaps an hour a day.

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# projection alarm clock. The advantage of a harmonic oscillator over other forms of oscillator is that it employs resonance to vibrate at a precise natural resonant frequency or ‘beat’ dependent only on its physical characteristics, and resists vibrating at other rates. The possible precision achievable by a harmonic oscillator is measured by a parameter called its Q, or quality factor, which increases (other things being equal) with its resonant frequency. This is why there has been a long term trend toward higher frequency oscillators in clocks.

Synchronized or slave clocks
Slave clocks, used in large institutions and schools from the 1860s to the 1970s, kept time with a pendulum, but were wired to a master clock in the building, and periodically received a signal to synchronize them with the master, often on the hour.

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Synchronous electric clocks do not have an internal oscillator, but count cycles of the 50 or 60 Hz oscillation of the AC power line, which is synchronized by the utility to a precision oscillator. The counting may be done electronically, usually in clocks with digital displays, or, in analog clocks, the AC may drive a synchronous motor which rotates an exact fraction of a revolution for every cycle of the line voltage, and drives the gear train. Although changes in the grid line frequency due to load variations may cause the clock to temporarily gain or lose several seconds during the course of a day, the total number of cycles per 24 hours is maintained extremely accurately by the utility company, so that the clock keeps time accurately over long periods.
Computer real time clocks keep time with a quartz crystal, but can be periodically (usually weekly) synchronized over the Internet to atomic clocks (UTC), using the Network Time Protocol (NTP). Sometimes computers on a local area network (LAN) get their time from a single local server which is maintained accurately.

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Radio clocks keep time with a quartz crystal, but are periodically synchronized to time signals transmitted from dedicated standard time radio stations or satellite navigation signals, which are set by atomic clocks.

# light alarm clock. This has the dual function of keeping the oscillator running by giving it ‘pushes’ to replace the energy lost to friction, and converting its vibrations into a series of pulses that serve to measure the time.

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In mechanical clocks, this is the escapement, which gives precise pushes to the swinging pendulum or balance wheel, and releases one gear tooth of the escape wheel at each swing, allowing all the clock’s wheels to move forward a fixed amount with each swing.
In electronic clocks this is an electronic oscillator circuit that gives the vibrating quartz crystal or tuning fork tiny ‘pushes’, and generates a series of electrical pulses, one for each vibration of the crystal, which is called the clock signal.
In atomic clocks the controller is an evacuated microwave cavity attached to a microwave oscillator controlled by a microprocessor. A thin gas of cesium atoms is released into the cavity where they are exposed to microwaves. A laser measures how many atoms have absorbed the microwaves, and an electronic feedback control system called a phase-locked loop tunes the microwave oscillator until it is at the exact frequency that causes the atoms to vibrate and absorb the microwaves. Then the microwave signal is divided by digital counters to become the clock signal.

Counter chain
# loudest alarm clock. This counts the pulses and adds them up to get traditional time units of seconds, minutes, hours, etc. It usually has a provision for setting the clock by manually entering the correct time into the counter.

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In mechanical clocks this is done mechanically by a gear train, known as the wheel train. The gear train also has a second function; to transmit mechanical power from the power source to run the oscillator. There is a friction coupling called the ‘cannon pinion’ between the gears driving the hands and the rest of the clock, allowing the hands to be turned to set the time.
In digital clocks a series of integrated circuit counters or dividers add the pulses up digitally, using binary logic. Often pushbuttons on the case allow the hour and minute counters to be incremented and decremented to set the time.

Time display methods

Analog clocks

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# puzzle alarm clock. Analog clocks usually indicate time using angles. The most common clock face uses a fixed numbered dial or dials and moving hand or hands. It usually has a circular scale of 12 hours, which can also serve as a scale of 60 minutes, and 60 seconds if the clock has a second hand. A clock face or dial is the part of an analog clock (or watch) that displays the time through the use of a fixed-numbered dial or dials and moving hands. In its most basic form, recognized throughout the world, the periphery of the dial is numbered 1 through 12 indicating the hours in a 12-hour cycle, and a short hour hand makes two revolutions in a day. A longer minute hand makes one revolution every hour. The face may also include a second hand which makes one revolution per minute.

Digital clocks
# talking alarm clock. Digital clocks display a numeric representation of time. Two numeric display formats are commonly used on digital clocks:

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• the 24-hour notation with hours ranging 00–23;
• the 12-hour notation with AM/PM indicator, with hours indicated as 12AM, followed by 1AM–11AM, followed by 12PM, followed by 1PM–11PM (a notation mostly used in domestic environments).
Most digital clocks use electronic mechanisms and LCD, LED, or VFD displays; many other display technologies are used as well (cathode ray tubes, nixie tubes, etc.). After a reset, battery change or power failure, these clocks without a backup battery or capacitor either start counting from 12:00, or stay at 12:00, often with blinking digits indicating that the time needs to be set.

Auditory clocks
# radio alarm clock. For convenience, distance, telephony or blindness, auditory clocks present the time as sounds. The sound is either spoken natural language, (e.g. “The time is twelve thirty-five”), or as auditory codes (e.g. number of sequential bell rings on the hour represents the number of the hour like the bell Big Ben). Most telecommunication companies also provide a speaking clock service as well.

Word clocks
# alarm clock radio. Word clocks are clocks that display the time visually using sentences. E.g.: “It’s about three o’clock.” These clocks can be implemented in hardware or software.

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Projection clocks
# best alarm clock. Some clocks, usually digital ones, include an optical projector that shines a magnified image of the time display onto a screen or onto a surface such as an indoor ceiling or wall. The digits are large enough to be easily read, without using glasses, by persons with moderately imperfect vision, so the clocks are convenient for use in their bedrooms.

Tactile clocks
# digital alarm clock. Auditory and projection clocks can be used by people who are blind or have limited vision. There are also clocks for the blind that have displays that can be read by using the sense of touch. Some of these are similar to normal analog displays, but are constructed so the hands can be felt without damaging them.

Multi-display clocks
# travel alarm clock. Some clocks have several displays driven by a single mechanism, and some others have several completely separate mechanisms in a single case. Clocks in public places often have several faces visible from different directions, so that the clock can be read from anywhere in the vicinity. Of course, all the faces show the same time. Other clocks show the current time in several time-zones.

Specific types of popular clock

Wall clock
# loud alarm clock. Wall clock time, or wall time, is the human perception of the passage of time from the start to the completion of a task. In the context of a task being performed on a computer, wall-clock time is a measure of the real time that elapses from start to end, including time that passes due to programmed (artificial) delays or waiting for resources to become available.

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Alarm clock
# computer alarm clock. An alarm clock (or sometimes just an alarm) is a clock that is designed to make a sound, or some other signal, at a specific time. The primary utility of these clocks is to awaken people from their night’s sleep or short naps; they are sometimes used for other reminders as well. Most use sound; some use light or vibration. Some have sensors to identify when a person is in a light stage of sleep, in order to avoid waking someone who is deeply asleep, which causes tiredness, even if the person has had adequate sleep.

A cuckoo clock
# kids alarm clock. A cuckoo clock is a typically pendulum-regulated clock that strikes the hours with a sound like a common cuckoo’s call and has an automaton cuckoo bird that moves with each note. Some move their wings, open/close the beak while leaning forward, whereas others only the bird’s body is leaned forward. The mechanism to produce the cuckoo call has been in use since the middle of the 18th century and has remained almost without variation until the present.

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Grandfather clock
# set alarm clock. A longcase clock, also tall-case clock, floor clock, or grandfather clock, is a tall, freestanding, weight-driven pendulum clock with the pendulum held inside the tower, or waist of the case. Clocks of this style are commonly 1.8–2.4 metres (6–8 feet) tall. The case often features elaborately carved ornamentation on the hood (or bonnet), which surrounds and frames the dial, or clock face.

# alarm clock.

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