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