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Alarm

Alarms used for different purposes include:

• burglar alarms, designed to warn of intrusions; this is often a silent alarm: the police or guards are warned without indication to the burglar, which increases the chances of catching him or her.
• alarm clocks can produce an alarm at a given time
• safety alarms, which go off if a dangerous condition occurs. Common public safety alarms include:
  • tornado sirens
  • fire alarms
  • car alarms
  • air raid sirens
  • tocsins - an historical method of raising an alarm
  • Distributed control manufacturing systems or DCSs, found in nuclear power plantss, refineries and chemical facilities also generate alarms.

With any kind of alarm, the need exists to balance between on the one hand the danger of false alarms (called "false positives") -- the signal going off in the absence of a problem ; and on the other hand failing to signal an actual problem (called a "false negative"). False alarms can waste resources expensively and even dangerous. For example, false alarms of a fire can waste firefighter manpower, making them unavailable for a real fire, and risk injury to firefighters and others as the fire engines race to the alleged fire's location. In addition, false alarms may acclimatise people to ignore alarm signals, and thus possibly to ignore an actual emergency: Aesop's fable of The Boy Who Cried Wolf exemplifies this problem.

See also Alarm management, clock.
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