FIRE-ALARM. A fire-alarm system consists of a telegraph with a network of wires running from a main office to all parts of a district. and provided with signal-boxes conveniently placed and accessible, from which signals may be sent to the fire department by any one without special knowledge of the instruments. simply by turning a crank or pulling a hook. Each signal-box con tains a. transmitter or device to make and break an electric circuit, in a manner more or less closely resembling the following description, which will serve to illustrate the principle in volved: A metallic wheel is provided with teeth, the spaces between which are filled with some non-conducting substance; a contact spring rests against the wheel, and as the wheel is turned. touehes in succession the projecting teeth, closing the circuit at. each tooth. and causing a signal at the central office. or, in small towns where there is no central office, at all the engine-houses with which the wire is connected. If the arrange ment on the wheel should he two teeth, a space, three teeth, a space. and two teeth. followed by long spate. one rot at' of the wheel would give Iwo signals. then three, then two, or the number 232, and this number will be repeated as often as the wheel is rotated. The wheel may be turned by a crank or by a spring. The signal is re eeived upon the common horse recording instru ment, and registered on a, strip of paper; or both its receipt at and transmission from the cen tral station may be recorded by pens on a re volving paper roll. There is often provided :tn inner box, to which the firemen only have keys, w Inch contains telegraph instruments for use in communicating with headquarters, by nietins of which the officer in charge is enabled to give information and call for such extra and special supPlies as are needed. The outer or tire-alarm boxes are generally only to be opened with keys intrusted to citizens, the nearest resident, the etc., but automatic doors for alarm-boxes are also used. These may be opened without a key by simply twisting the door-landle.
In cities having a central otlice in eonnection with the fire-alarm system, the alarm signals are all received here and then repeated to the local engine or hose house, either by hand or by auto matic machinery under the control of the cent rat operator. The signals are in some systems sent
to all the companies, and the fire attended by those that are nearest and assigned to that dis trict. In other systems the signals are only sent by the central operator to sueh companies as should go to the tire. For less important cities an automatic repeater may be used, thus doing away with the necessity of a central office at tendant. In small towns and villages with only a few alarm-boxes and one wire circuit, the alarm may be operated directly from the call-box. In large cities, however, many circuits are neces sary, which, in connection with the apparatus at the central office, may be so arranged that two in dependent signals are sent out to the proper stations for every fire by the use of separate instruments and circuits for each. The first sig nal in the fire station is sounded on a gong and the box number is printed on a tape. It is sent by a single movement of a lever. The second signal is in accordance with a code, the number of sounds indicating the exact response to be made to the call. A gong sounds the box DlltD her, which is also shown on an indicator with large figures. The central stations are equipped with various other devices, such as annunciators to show what companies are sent to a fire, and. therefore, which are still available; automatic circuit-testing machines, reporting breaks in the circuit, if any. at set intervals; switchboards, storage batteries, controlling and testing boards. instruments for measuring the current used to charge' the battery, and also that in the wire circuits. The simpler forms of call-boxes are constructed in various ways, and to secure great er reliability, there are provided lightning ar resters; cut-outs to prevent interference from high potential lighting or power currents; trap keys which cannot be recovered in case of alarms sent by private citizens until a fireman releases them by another key; and the keyless boxes and telegraph instruments already mentioned. The wire system may be strung on poles or placed underground. It must be carefully protected from harm in either case, particularly from ac cidental connection with any other electric wires. When put underground, wires may be placed in tile, wood, or iron conduits. See SUBWAYS FOR PIPES AND WIRES.