FIRE ALARM, elective signaling equipment connected by wire with a central office for the purpose of notify ing the fire department in case of fire. The instrument is usually reduced to its simplest dimensions so that people not familiar with the system may operate it. Fire-alarm boxes contain devices to make and break electric currents. There is usually a wheel provided with teeth separated by a non-conducting substance and this when turned comes in contact with a spring which opens and closes the circuit at each tooth, thus producing a signal at the central station. The number of the signal box is usually indicated by the arrangements of teeth and spaces and in that way the fire is lo cated. Access to the crank or chain by which the signal is communicated is ob tained either by a key or by twisting the door handle or by breaking the glass— this last being the method most in vogue in large cities. Telegraph instruments
connecting with headquarters for the use of firemen are often attached. The sig nal may be registered on a Morse re cording instrument or by some similar device. In some districts there is an apparatus by which the signal results in the ringing of tower bells or the sounding of steam-whistles. The last method, now supplemented by electricity, was formerly in general use, but the development of the electric telegraph has resulted in greater speed and in many simplifications. From the middle of the 19th century telegraph boxes have been with great velocity, and not infrequently passing unbroken across the sky until lost in the horizon. They differ from ordinary meteors, probably, more in vol ume and brilliancy than in any other dis tinctive characteristic.