An alarm clock is a clock that is designed to make a loud sound at a specific date and/or time. (A traditional alternate spelling, now superseded, is "alarum.") The primary use of these clocks is to awaken people from their sleep in order to start their days in the mornings, but they are sometimes used for other reminders as well. To stop the sound, a button or handle on the clock needs to be pressed, and some stop automatically after a few minutes if left unattended. A classical analog alarm clock has an extra hand that is used to specify the time at which to activate the alarm.

Traditional mechanical alarm clocks have a bell on top that rings, but digital alarm clocks can make other noises. Simple battery-powered alarm clocks make a loud buzzing sound, or other similar noise to wake a sleeper, while novelty alarm clocks can speak, laugh, or sing. Some alarm clocks have radios that 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).

In a mechanical bell-style alarm clock, a mainspring drives a gear that propels a clacker back and forth between two bells or between the sides inside a single bell. In an electric bell-style alarm clock, the bell rings with an electromagnetic circuit and armature that turns the circuit on and off again repeatedly.

History

A precursor to the alarm clock was a water clock built by the ancient Greeks circa 250 BC where the raising waters would both keep time and eventually hit a mechanical bird that triggered an alarming whistle.

Another precursor to the alarm clock was the striking clock, which dates back to the Chinese Buddhist monk and inventor Yi Xing (683–727).Joseph Needham, Volume 4, Part 2, pp. 473–5 The use of striking clock mechanisms in clock towers dates back to the Chinese engineers Zhang Sixun and Su Song, in the 10th and 11th centuries, respectively.Joseph Needham, Volume 4, Part 2, p. 165 The earliest striking clock outside of China was the 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.

From the 14th century, some clock towers in Western Europe were also capable of chiming at a fixed time everyday, the earliest of which was described by the Florentine 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's alternatively beaten by the Due Mori (Two Moors), two bronze statues (h. 2,60) handling a hammer.