Means of warning occupant
s of a building that a fire
) 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 alarm
s are automatic
, and there are a few types.
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.
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
alarms are the most sensitive
. They contain a small amount of radioactive material with ionize
s 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 electric
al flow and the alarm
is set off.
Reference: Encyclopædia Britannica