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An alarm clock or alarm is a clock that is designed to alert an individual or group of people at a specified time. The primary function of these clocks is to awaken people from their night's sleep or short naps; they can sometimes be used for other reminders as well. Most alarm clocks make sounds; some make light or vibration. Some have sensors to identify when a person is in a light stage of sleep, to avoid waking someone who is deeply asleep, which causes tiredness, even if the person has had adequate sleep. To turn off the sound or light, a button or handle on the clock is pressed; most clocks automatically turn off the alarm if left unattended long enough. A classic analog alarm clock has an extra Clock face or inset dial that is used to show the time at which the alarm will ring.
Many alarm clocks have radio receivers that can be set to start playing at specified times, and are known as clock radios. Additionally, some alarm clocks can set multiple alarms. A progressive alarm clock can have different alarms for different times (see next-generation alarms) and play music of the user's choice. Most modern televisions, computers, mobile phones and digital watches have alarm functions that automatically turn on or sound alerts at a specific time.
The Late antiquity statesman 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 City which featured an hourly gong and figures moving mechanically day and night.Dohrn-van Rossum, Gerhard, "Clocks", Brill's New Pauly, edited by: Hubert Cancik and Helmuth Schneider, 2009
In China, a striking clock was devised by the Chinese Buddhism and inventor Yi Xing (683–727).Joseph Needham, Volume 4, Part 2, pp. 473–5 The Chinese engineers Zhang Sixun and Su Song integrated striking clock mechanisms in astronomical clocks in the 10th and 11th centuries, respectively.Joseph Needham, Volume 4, Part 2, p. 165 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 is the subject of a book, On the Construction of Clocks and their Use (1203), by Riḍwān ibn al-Sāʿātī, the son of clockmaker. In 1235, an early monumental water-powered alarm clock that "announced the appointed Salat times 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 Florence writer Dante Alighieri in 1319.Joseph Needham, Volume 4, Part 2, p. 445 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.p. 249, The Grove Encyclopedia of Decorative Arts, Gordon Campbell, vol. 1, Oxford University Press, 2006, ., entry, Clock Dictionary.
The first American alarm clock was created in 1787 by Levi Hutchins in Concord, New Hampshire. This device he made only for himself, however, and it only rang at 4 am, 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, 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. 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.
Some clock radios can accept music sources from an external source, for example, an audio CD or portable music player; the selected music would then be used to awaken the sleeper. Clock models of the 2000s may come with a dock for iPod that also charges the device. They may serve as a typical music player, playing music from aforementioned sources. Another few models offer "nature sounds" like rain, forest, wind, sea, waterfall etc., in place of, or in addition to, the traditional buzzer. Clock radios are powered by AC power from the wall socket. In the event of a power interruption, older electronic digital models used to reset the time to midnight (0:00; 12:00 am) and lose alarm settings. This would cause failure to trigger the alarm even if the power is restored, such as in the event of a power outage. Many newer clock radios feature a battery backup to maintain the time and alarm settings. Some advanced (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 accurate and immune to time reset due to power interruptions.
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. Due to progressive alarm clocks' complex waking procedure, they can deter this adaptation due to the body needing to adapt to more stimuli than just a simple sound alert.
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