An alarm clock is a clock that is designed to wake a person at a specific time. The primary use of these clocks is to awaken people from their night's sleep or short naps; they are sometimes used for other reminders as well. Some use sound, some use light, and some use sensors to identify when a person is in a light stage of sleep, in order to avoid waking someone when they're deeply asleep, which causes tiredness, even if the person has had adequate sleep. To stop the sound or light, a button or handle on the clock is pressed; but most clocks automatically stop the alarm if left unattended long enough. A classic analog alarm clock has an extra hand or inset dial that is used to specify the time at which to activate the alarm.
Traditional mechanical alarm clocks have one or two bells that ring by means of a mainspring that drives a gear that propels a hammer back and forth between the two bells or between the interior sides of a single bell. In some models, the back encasement of the clock itself acts as the bell. In an electric bell-style alarm clock, the bell is rung by an electromagnetic circuit and armature that turns the circuit on and off repeatedly.
Digital alarm clocks can make other noises. Simple battery-powered alarm clocks make a loud buzzing or beeping sound to wake a sleeper, while novelty alarm clocks can speak, laugh, sing, or play sounds from nature.
Some alarm clocks have radios that can be set to start playing at specified times, and are known as clock radios. A progressive alarm clock, still new in the market, can have different alarms for different times (see Next-Generation Alarms). Most modern televisions and cell phones have alarm clock functions to turn on or make sounds at specified times.
The ancient Greek philosopher Plato (428–348 BC) was said to possess a large water clock with an unspecified alarm signal similar to the sound of a water organ; he used it at night, possibly for signalling the beginning of his lectures at dawn (Athenaeus 4.174c). The Hellenistic engineer and inventor Ctesibius (fl. 285–222 BC) fitted his clepsydras with dial and pointer for indicating the time, and added elaborate "alarm systems, which could be made to drop pebbles on a gong, or blow trumpets (by forcing bell-jars down into water and taking the compressed air through a beating reed) at pre-set times" (Vitruv 11.11).
The late Roman senator Cassiodorus (c. 485–585) advocated in his rulebook for monastic life the water clock as a useful alarm for the 'soldiers of Christ' (Cassiod. Inst. 30.4 f.). The Christian rhetorician Procopius described in detail prior to 529 a complex public striking clock in his home town Gaza which featured an hourly gong and figures moving mechanically day and night.
In China, a striking clock was devised by the Buddhist monk and inventor Yi Xing (683–727). The Chinese engineers Zhang Sixun and Su Song integrated striking clock mechanisms in astronomical clocks in the 10th and 11th centuries, respectively. A striking clock outside of China was the water-powered clock tower near the Umayyad Mosque in Damascus, Syria, which struck once every hour. It was constructed by the Arab engineer al-Kaysarani in 1154. In 1235, an early monumental water-powered alarm clock that "announced the appointed hours of prayer and the time both by day and by night" was completed in the entrance hall of the Mustansiriya Madrasah in Baghdad.
From the 14th century, some clock towers in Western Europe were also capable of chiming at a fixed time every day; the earliest of these was described by the Florentine writer Dante Alighieri in 1319. The most famous original striking clock tower still standing is possibly the one in St Mark's Clocktower in St Mark's Square, Venice. The St Mark's Clock was assembled in 1493, by the famous clockmaker Gian Carlo Rainieri from Reggio Emilia, where his father Gian Paolo Rainieri had already constructed another famous device in 1481. In 1497, Simone Campanato moulded the great bell (h. 1,56 m., diameter m. 1,27), which was put on the top of the tower where it was alternatively beaten by the Due Mori (Two Moors), two bronze statues (h. 2,60) handling a hammer.
User-settable mechanical alarm clocks date back at least to 15th-century Europe. These early alarm clocks had a ring of holes in the clock dial and were set by placing a pin in the appropriate hole.
Another mechanical alarm clock was created by Levi Hutchins, of New Hampshire in the United States, in 1787. This device he made only for himself however, and it only rang at 4 AM, in order to wake him for his job. The French inventor Antoine Redier was the first to patent an adjustable mechanical alarm clock, in 1847.
Alarm clocks, like almost all other consumer goods in the United States of America, ceased production in the spring of 1942, as the factories which made them were converted over to war work during World War II, but they were one of the first consumer items to resume manufacture for civilian use, in November 1944. By that time, a critical shortage of alarm clocks had developed due to older clocks wearing out or breaking down. Workers were late for, or missed completely, their scheduled shifts in jobs critical to the war effort because "my alarm clock is broken". In a pooling arrangement overseen by the Office of Price Administration, several clock companies were allowed to start producing new clocks, some of which were continuations of pre-war designs, and some of which were new designs, thus becoming among the first "postwar" consumer goods to be made, before the war had even ended. The price of these "emergency" clocks was, however, still strictly regulated by the Office of Price Administration. The first radio alarm clock was invented by James F. Reynolds, in the 1940s and another design was also invented by Paul L Schroth Sr.
Modern digital alarm clocks typically feature a radio alarm function and/or beeping or buzzing alarm, allowing a sleeper to awaken to music or news radio rather than harsh noise. Most also offer a "snooze button", a large button on the top that stops the alarm and sets it to ring again at a short time later, most commonly nine minutes. Some alarm clocks also have a "sleep" button, which turns the radio on for a set amount of time (usually around one hour). This is useful for people who like to fall asleep with the radio on.
Digital clock radios often use a battery backup to maintain the time in the event of a power outage. Without this feature, digital clocks will reset themselves incorrectly (usually to midnight) when power is restored, causing a failure to trigger the alarm. To solve this issue, some digital clock radios trigger the alarm at e.g. 00:01 after a reset to midnight, so that at least the user is able to correct the clock and alarm time. Some other radio clocks (not to be confused with clocks with AM/FM radios) have a feature which sets the time automatically using signals from atomic clock-synced time signal radio stations such as WWV, making the clock ready for use right out of the box.
Other alarm signals 
The deaf and hard of hearing are often unable to perceive auditory alarms when asleep. They may use specialized alarms, including alarms with flashing lights instead of or in addition to noise. Alarms which can connect to vibrating devices (small ones inserted into pillows, or larger ones placed under bedposts to shake the bed) also exist.
Computer alarms 
Alarm clock software programs have been developed for personal computers. A computer acting as an alarm clock may allow a virtually unlimited number of alarm times (i.e. Personal information manager) and personalized tones. Online alarm clocks are also available through the use of different websites.
Cell phone alarms 
Many modern cell phones feature built-in alarms that do not require the phone to be powered on for the alarm to go off. Some of these cell phones feature the ability for the user to set the tone of the alarm, and in some cases music can be downloaded to the phone and then chosen to play for waking.
Next-generation alarms 
Among annoyances caused by alarm clocks is sleep inertia, a feeling of grogginess that results from abrupt awakening. Progressive alarm clocks like ZenAwake, claim to solve this issue. They include sunrise alarm clocks, dawn simulators and progressive auditory alarm clocks.
Sleepers can become accustomed to the sound of their alarm clock if it has been used for a period of time, making it less effective. Because progressive alarm clocks have a complex waking procedure, they can deter this adaptation due to the body needing to adapt to more stimuli than just a simple sound alert.
Scientific studies on sleep having shown that sleep stage at awakening is an important factor in amplifying sleep inertia. Alarm clocks involving sleep stage monitoring appeared on the market in 2005. Using sensing technologies such as EEG electrodes or accelerometers, these alarm clocks are supposed to wake people only from light sleep.
See also 
- Humphrey, Oleson & Sherwood 2003, p. 522; Lewis 2000, p. 363
- Landels 1979, p. 35
- Dohrn-van Rossum, Gerhard, "Clocks", Brill's New Pauly, edited by: Hubert Cancik and Helmuth Schneider, 2009
- Joseph Needham, Volume 4, Part 2, pp. 473–5
- Joseph Needham, Volume 4, Part 2, p. 165
- Donald Routledge Hill (1991), "Arabic Mechanical Engineering: Survey of the Historical Sources", Arabic Sciences and Philosophy: A Historical Journal (Cambridge University Press) 1: 167–186 , doi:10.1017/S0957423900001478
- Joseph Needham, Volume 4, Part 2, p. 445
- p. 249, The Grove encyclopedia of decorative arts, Gordon Campbell, vol. 1, Oxford University Press, 2006, ISBN 0-19-518948-5.
- "Monastic Alarm Clocks, Italian", entry, Clock Dictionary.
- Mary Bellis. "History of Clocks". Retrieved 2006-11-02.
- Cecil Adams (1999-11-26). "Why does the alarm clock snooze button give you nine extra minutes, not ten?". The Straight Dope. Archived from the original on 31 December 2006. Retrieved 2007-01-07.
- Reuven Fenton (2007-08-29). "Bio-alarm clocks set for perfect wake-up". Reuters. Retrieved 2008-06-09.
- Humphrey, John William; Oleson, John Peter; Sherwood, Andrew N. (2003), Greek and Roman Technology: A Sourcebook. Annotated Translations of Greek and Latin Texts and Documents, Taylor & Francis Routledge, ISBN 978-0-203-41325-8
- Landels, John G. (1979), "Water-Clocks and Time Measurement in Classical Antiquity", Endeavour 3 (1): 32–37
- Lewis, Michael (2000), "Theoretical Hydraulics, Automata, and Water Clocks", in Wikander, Örjan, Handbook of Ancient Water Technology, Technology and Change in History 2, Leiden: Brill, pp. 343–369, ISBN 90-04-11123-9
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