Fire Alarm means of warning in case of fire. Originally, watchmen provided the only fire-alarm system, but, with theadvent of electric power, boxes wired to fire departments provided a warning system from city streetsand such institutional buildings as schools. While some of the latter remain in use, most modern fire-alarmsystems are automatic, consisting of thermostat-activated devices that at a certain temperature eithersound an alarm or report to a central office, such as a municipal fire station. Some alarms are set to gooff whenever the thermostat shows a rapid temperature rise. The thermostat is usually placed at or nearthe ceiling, where it will be most immediately affected by increase in temperature. Another type of alarmis actuated by a photoelectric cell; when smoke darkens the room slightly, the alarm is activated. Onehighly sensitive device contains a small amount of radioactive material that ionizes the air in a chamber.With this device a continuously applied voltage causes a small electrical current to flow through theionized air, and when products of combustion enter, they reduce the current flow and activate the alarm