An alarm clock is a clock

Clock

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

 that is designed to make a loud sound at a specific time

Time

Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

. The primary use of these clocks is to awake

Awake

Wakefulness is a daily recurring brain state and state of consciousness in which an individual is conscious and engages in a coherent cognitive and behavior responses to the external world such as communication, ambulation, nutritional ingestion and procreation...

n people from their night's sleep

Sleep

Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

 or short nap

Nap

A nap is a short period of sleep.Nap may also refer to:-Politics:* National Action Party , a political movement* National Action Plan on the Elimination of Child Labour...

s; they are sometimes used for other reminders as well. To stop the sound, a button or handle on the clock is pressed; but most clocks automatically stop the alarm sound 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 bell

Bell (instrument)

A bell is a simple sound-making device. The bell is a percussion instrument and an idiophone. Its form is usually a hollow, cup-shaped object, which resonates upon being struck...

s that ring by means of a mainspring

Mainspring

A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...

 that drives a gear that propels a hammer back and forth between the two bells or between the interior sides of a single bell.