Fire alarm is a safety device which gives warning of a fire before it spreads. Most fire alarms work on the principle of detecting heat, smoke or flame. Heat detectors sense the rise in temperature due to fire and trigger an alarm by either melting a low fusing alloy or bending a bimetallic strip. In smoke detectors, a beam of light falling on a photocell is interrupted by smoke particles triggering the alarm. Flame detectors usually detect infrared radiation given off by flames to set off an alarm.