Among other useful applications of the telegraph is the _fire alarm system_. In 1852 Channing and Farmer, of Boston, Mass., devised a system of telegraphic fire alarms, which was adopted in the city of Boston (U. S. Pat. No. 17,355, May 19, 1857), and which in varying modifications has spread through all the cities of the world, introducing that most important element of time economy in the extinguishment of fires. Hundreds of cities and millions of dollars have been thus saved from destruction. Similar applications of local alarms in great numbers have been extended into various departments of life, such as _District Messenger Service_, _Burglar Alarms_, _Railroad-Signal Systems_, _Hotel-Annunciators_, and so on. [Illustration: FIG. 12.--TELEGRAPHING BY INDUCTION.] For furnishing current for telegraphic purposes the dynamo, and especially the storage battery, have in late years found useful application. In fact, in the leading telegraph offices the storage battery has practically superseded the old voltaic cells. _Telegraphing by induction_, _i. e._, without the mechanical connection of a conducting wire, is another of the developments of telegraphy in recent years, and finds application to telegraphing to moving railway trains. When an electric current flows over a telegraph line, objects along its length are charged at the beginning and end of the current impulse with a secondary charge, which flows to the earth if connection is afforded. It is the discharge of this secondary current from the metal car roof to the ground which, on the moving train, is made the means of telegraphing without any mechanical connection with the telegraph lines along the track. As, however, this secondary circuit occurs only at the making and breaking of the telegraphic impulse, the length of the impulse affords no means of differentiation into an alphabet, and so a rapid series of impulses, caused by the vibrator of an induction coil, is made to produce buzzing tones of various duration representing the alphabet, and these tones are received upon a telephone instead of a Morse register. The diagram, Fig. 12,[1] illustrates the operation. [1] From “Electricity in Daily Life,” by courtesy of Charles Scribner’s Sons. To receive messages on a car, electric impulses on the telegraph wire W, sent from the vibrator of an induction coil, cause induced currents as follows: Car roof R, wire _a_, key K, telephone _b c_, car wheel and earth. In sending messages closure of key K works induction coil I C, and vibrator V, through battery B, and primary circuit _d_, _c_, _f_, _g_, and the secondary circuit _a_, _h_, _i_, charges the car roof and influences by induction the telegraph wire W and the telephone at the receiving station.