Fire Alarm Telegraph Signals.-- The im portance of electricity as a time saver in an nouncing the existence of a fire can scarcely be overestimated. A special feature of the electric fire-alarm system is that it not only gives the alarm, but also indicates to the firemen the lo cation of the fire, or within a very short dis tance thereof. It may be noted that a simple fire-alarm system is in many respects similar to the systems just described, in that it consists of a central office or station in which alarm ap paratus and battery are located, and of signal boxes in the street and elsewhere by which to transmit alarms to the central office. A wire connects the central station with the various sig nal boxes in the streets and fire-engine stations. In each signal box is placed a break-wheel, practically similar to that used in the call-boxes of the district messenger or emergency service; the main difference being that owing to the • more exposed position of the fire-alarm boxes and their greater relative importance, more sub stantial boxes are employed. In, general these signal boxes are supplied with an inner and outer door to protect the apparatus from the elements. The signal boxes are provided with a crank or a hook which on being operated . causes an alarm to be sent to the central sta tion giving the number of the box from which the call has emanated. The signal box is also provided with a small electric gong, which rings each time the circuit is opened. This in timates to the one sending in the alarm that the alarm is being properly transmitted. It also serves the purpose of intimating to anyone who might open an adjacent box to send in an alarm therefrom, that the alarm is already being sent, thus preventing interference. At the central station and the various fire stations a gong is struck a number of times corresponding with the number of the box from which the alartn emanates. In the same circuit also an ink-re cording register in the central office marks on a paper strip the number of the signaling box, thus giving a permanent record of such alarm. Inasmuch as it is not good practice to put more than 25 or 30 signal boxes on one circuit, but yet is very important that all the fire sta tions in a given district should receive the alarm, a repeating device is utilized at the cen tral station which receives the alarm from any one of the circuits and thereupon repeats it au tomatically or manually to all the other cir cuits. It frequently happens that two or more street boxes on the same circuit are pulled si multaneously by different people. To prevent the confusion of signals that this would ordi narily entail special arrangements have been devised, termed the non-interfering succession signal fire-alarm boxes. These boxes not only prevent interference with the signals already in process of transmission by any given box, but also permit every box that may luiVe been pulled simultaneously to send in its signal in rotation, automatically.
Automatic Fire-Alarm Signals.—In many of the large cities of the United States an au:c iliary to the regular fire-alarm system, consist ing of some form of thermostat included in a circuit in the building to be protected, is em ployed. This system again is somewhat analo gous to the messenger telegraph system de scribed, the chief difference being that in the one case the signal box is manually operated, while in the other case an increase of tempera ture, due to fire, by affecting the thermostat catrses the alarm to be transmitted. Thermo stats are of different types. One type consists of a crescent-shaped spring, made up of two strips of metal, steel and copper, one over the other. One end of the spring or strips is fas tened to a support, the other end is adjacent to a contact point of an electric circuit As the metals named do not expand equally under in creased temperature the spring as a whole bends or buckles when the temperature in creases, and the movable end makes connection with the contact, thereby closing the alarm cir cuit with the result desired. Other types of thermostats are made of easily fusible alloys. Still another forrn of thermostat consists of a drum-shaped box, holding substances which have a high expansion under increased temper ature. The expansion causes the ends of the box to bulge, thereby closing an alarm circuit.
Police Electric Signals.—These may be considered as more or less amplified fire-alarm systems, consisting as they do of signal boxes placed on poles and in booths along the routes of the policemen ; f rom which boxes signals of all kinds may be transmitted to and from police headquarters. The signal boxes are connected by a suitable wire with headquarters; and each box has a break-wheel, carrying the number of the box. A telephone outfit is 'also placed in the
box by means of which the policemen may con verse with the main station. In fact the tele phone is used nearly exclusively, the policeman as he arrives at a signal box sending in a signal which intimates to the attendant at headquarters the number of the box at which he has arrived in the course of his beat, whereupon the at tendant communicates with the policeman and takes his number, thus getting a record of his whereabouts. If the officer desires to send in a special signal of any kind, as for an ambulance or wagon, or for assistance, he can do so by a special arrangrnent within the box. In some cities certain citizens are supplied with keys of the signal boxes. Such citizens are authorized to send in signals for police assistance in cases of emergency, and thus the police force is prac tically augmented by a volunteer service. In many cities also the police signal apparatus is kept in a kiosk or sentry-box on the sidevvalk curb. On the top of these boxes an electric colored lamp is placed and so connected that it may be lighted from police headquarters to call a policeman to the post for instructions.
Railway Electric Block A blodc system in brief consists of a means of showing manually or automatically certain signals which indicate to the engineer of a train that a cer tain portion or section of the track before hien is uclearp or occupied. The road is divided into sections or blocks which are of various lengths depending in a great measure on the topography of the route and the amount of traffic. The length of a block varies from 600 feet to several miles. The signals employed in a ((block* are either (safety,"caution" or 4danger,4 repre sented by a white light or sign, a green light or sign and a red light or sign, respectively. The sign usually consists of the well-known sema phore arm or blade. In automatic electric sys tems the circuits and apparatus are generally so arranged that the entrance of a train into a °block° sets the danger signal and that signal is displayed until the train passes out of that block into the next, when the danger signal is lowered and the caution signal is shown. When the train passes into a second block ahead the cau tion signal is lowered and safety is shown. The part assigned to electricity in the operation of these signals consists in actuating electro-mag nets which are placed in circuits capable of being opened, closed or short-circuited by the car wheels of a train, which electro-magnets are caused directly or indirectly to operate the signals. In what is known as the Hall Block Signal system the blades are operated against gravity by an electric motor placed in a case at the top or foot of the pole supporting the signals; suitable battery being provided for the operation of the motor. Thus when a train enters a given block a circuit is closed which starts the motor which in turning, by a system of cog-wheel gearing, raises the blade to a pre arranged point, at which time the motor cir cuit is automatically opened and the signal is held in position by an electro-magnet until the train moves out of the blocic, when the blade is released and falls bv gravity, means being provided to graduate and safeguard the fall of the blade by means of friction, due to the action of a centrifugal governor. Another electric block system uses a disc enclosed in a drum shaped box on the top of a pole. This disc is operated by 'an electro-magnet the latter being attracted by an electric current which is under control of the engine, the latter operating, as it passes, a heavy circuit closer by the side of the track. There is one of these circuit closers at the beginning and end of a block. The engine thus sets the signal to danger as it comes into a block and sets it at clear as it leaves the block A device named the Miller Cab signal is different from the foregoing systems. It consists of suitable apparatus and battery for operating certain electric lights in the engine cab. Normally a white light is burning, but when there is another train on the block in which the train enters, the circuits are so oper ated that the white light is cut out of the cir cuit and a red light is lit up in its place. This notifies the engineer of the near proximity of another train. See BLOCK SIGNAL SYSTEM.
For a more detailed and illustrated account of the foregoing systems, the reader may be re ferred to the author's 'American Telegraphy and Encyclopaedia of the Telegraph.'