Means of warning occupants of a building that a fire (smoke) is present, so that they might escape before it reaches them.

In earlier times watchmen who sat in high perches were the only warning. Many of these watch towers were part of the fire hall, and are still standing in many places.

Modern fire alarms are automatic, and there are a few types.

Thermostat alarms detect a rise in the temperature and go off when there is a quick rise in temperature or when the temperature reaches a set point.

Photo-electric cell activated alarms detect the darkening of the room by smoke. (It seems to me this alarm would be the least effective and the most annoying, going off everytime you try to set the mood.)

Radioactive alarms are the most sensitive and reliable. They contain a small amount of radioactive material with ionizes the air in a small compartment. The battery or power source supplies the compartment with a small current which flows through the ionized air. When products produced by combustion enter the compartment they reduce the electrical flow and the alarm is set off.



Reference: Encyclopædia Britannica