FIRE ALARM, a system of sigialk for announcing the location of a newly-discovered fire and summoning fire-fighters to extinguish it. In cities, what is known as the municipal type of fire alarm is in use. It is a highly organized telegraphic system, partly automatic The signal boxes are located at convenient points throughout the city, close enough to gether so that no one wishing to send in an alarm has far to go to reach a box. In New York city the farthest distance to be traveled is 400 feet. These signal boxes are connected with wire circuits or loops which pass through fire department headquarters. Here the alarm is received and repeated mechanically or by hand to the fire companies detailed to protect that particular locality.
When an alarm is to be sent, the outer door of the signal box is opened (as provided), and in some boxes a tell-tale bell rings to notify the bystanders or a nearby policeman that the box is being used. This is to prevent the mis chievous use of the box. On the inner door is a hook or handle which, being pulled down, releases a mechanism operated by clock-work setting in motion a disc wheel having notches cut in its rim. The notches operate a lever key which makes and breaks contact as with a telegraph key. The number and spacing of these notches determines the signal reaching headquarters. Here a gong strikes the number of the box several times at the same moment. printing on a recording telegraph instrument dashes in accordance with the notches on the dispatching wheel in the signal box ; thus. — ---- represents box 134 Upon receipt of this signal the operator from its case a disc bearing the same number of notches, slips it into the dispatch box of the fire-house circuit and releases the mechanism; or he may send it by hand with a telegraph key, or, if either of these lines is out of order, he may use the telephone. The signal may go to all the engine houses in the city or it may go only to the companies who are to go to that particular fire. At the engine house the signal sounds twice on a small bell and once on a big gong located on different circuits, so as to pro vide against a chance breakdown of one or the other circuit. The time which elapses between the pull of the box and the last stroke of the big gong in the engine house averages in New York city 50 seconds, and the engine is gen erally out of the house and on its way to the fire before the big gong has ceased sounding.
The signal boxes are connected in series on the circuit, the approved number for one cir cuit being from 7 to 10. In a large city the number of these loops becomes very large and consumes a great deal of wire, which must be kept in repair, and it has been the custom to place as many as 50 boxes on a single circuit.
In one case now remedied there were 94 boxes on one circuit. With the regular type boxes it was quite possible for two boxes to be pulled at the same moment and the signals be confused at headquarters. The more boxes on a circuit, the, more chance of such confusion, so that (succession" boxes have been installed in some cities. These boxes hold their signals by means of a special magnet as long as the circuit is being used to transmit another alarm, the armature dropping and releasing the automatic mechanism when the regular current is restored. By this device alarms from any number of boxes on a given circuit will go in to head quarters consecutively.
In towns where there is no paid fire depart ment, and the fire-fighting force is composed of volunteers, the box alarms are arranged to be struck automatically on a large fire-bell, or the same mechanism may be made to blow a fire-whistle.
Private fire alarms — those protecting large or small private factory plants — are usually arranged in the same way as for towns, with the addition in most cases of a fire-detecting system. The latter consists of a series of thermostats placed in every enclosure, including closets, at suitable distances apart in the build ing to be protected. In the event of fire the thermostat, through making or breaking of electrical contact (according to its pattern), sends its alarm to an alarm box which auto matically rings a fire-bell or blows a fire whistle. Or it may go simply to a watchman's office, whence thg general alarm is given by hand.
The thermostat system is used also in schools and other buildings, and in cities may be made • to connect with a trip-box which sets off the alarm in the nearest street signal box.
The chief improvement in fire alarm systems in recent years have been, besides the succession box, the speeding up of the strokes of the alarm from an interval of two seconds apart to one second. This cuts in two the time required to receive and to transmit any given alarm. An addition has been made to the signal-box mech anism, 'affording telephone communication with headquarters, and also patrol and ambul ance calls. The electric current is now fur nished by storage batteries instead of cell-bat teries, affording a constancy of current formerly unknown. In the larger cities the fire alarm wires have been placed in underground con duits, thus avoiding accidents to the system from falling poles.