FIRE-ALARM, apparatus, mechanical, electric, and telegraphic, used for detecting fires, or for warning the fire department that fires exist. A series of signal-boxes is distributed over a given area, each box having a distinctive number, and being con nected with the central station and with alarm-bells in the several engine-houses. A signal-box contains a transmitter, consisting of a metallic wheel, provided with suitably arranged teeth; the spaces between the teeth are filled with some non-conducting sub stance, as ivory; a contact spring rests against the wheel, and, as the wheel is turned, touches in succession the projecting teeth, at each tooth making the circuit, and taus. ing a signal at all receiving stations. If the arrangement should be, two teeth, a space, three teeth, a space, and two teeth, followed by a long space, one rotation of the wheel would give two 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, acting so long as a detent is held away. - The signal.is received upon the
common Morse iristruthent, and recorded on a strip of paper.
The automatic fire-detecter is a thermometer which has a platinum wire sealed into its bulb, and a second wire inserted at the mark of a certain temperature on the scale. The wires are in a telegraphic circuit, which remains broken until the increasing tem perature causes the mercury to rise in its tube, and complete the connection by contact with the second wire. A signal is instantly communicated to the central station, show ing the danger and the precise building at which it exists; an indicator, usually placed near the door, further shows from which apartment the signal was made, directing the fireman to the precise place where his services are in demand. The thermometer is usually placed in the ceiling. Insurance companies make reduced rates upon buildings provided with this instrument. Other devices have employed the ignition of powder, the expansion of metallic rods, or the breaking of wires, but none are so delicate or so, worthy of reliance as that described.