ALARM. A self-acting contrivance employed to call attention to danger or accidents, or to arouse persons from sleep. The common alarm clock is a familiar example of such a device, and the electric burglar-alarm is another. The -simplest and most common arrangement of bur glar-alarm consists of an electric hell with wires leading to all parts of the windows, doors, and other parts of the building to he protected. The terminals of these wires are set in the framing of the windows and doors, so that if they are opened the action presses springs together and rings the bell in precisely the same way as by pressing the ordinary push-button. All special kinds of alarms for house protection consist of modifications in the method of making the con tact suitable for special purposes, such as laying sheets of tin under the carpet to make contact with the wires when the carpet is stepped upon. Means are also generally introduced for indicat ing which window the signal comes from. This is done by leading the wires from each window separately through an annunciator, which shows through which wire, and consequently from which window, the signal came. The alarm will also sound if a window is carelessly left open. The en tire wiring of houses is also frequently connected with the police station by wire, so that it is notified of any tampering with the house in the absence of its occupant. Bank vaults and safes are also protected by numerous complicated mechanical and electrical devices which in stantly give an alarm to watchmen or police officers of any disturbance due to tampering or attempted burglary. Automatic fire alarms are
made in a variety of forms. A frequent arrange ment consists of a string supporting a weight whose fall sets in operation it train of mechan ism which sounds a bell alarm. The weight is caused to fall by the burning of the supporting string. (See FIRE ALARMS.) In steam boilers an alarm check valve, operating under the pres sure of steam, is employed to give the alarm when the injector ceases to work, or when the water falls below the point of safety. In locomotive boilers a fusible plug is set into the crown sheet over the firebox; this plug remains intact as long as water covers the crown sheet, but melts should it become dry, allowing the steam to es cape into the firebox and warn the engineer of the danger. Telegraph and telephone lines usu ally have some arrangement by which a break in the wires is indicated by a hell alarm. Fog bells, fog whistles, and whistling buoys are forms of alarms, and there are a great variety of other forms, such as alarm compasses, which are con trived to sound an alarm when the vessel de viates from its course; alarm funnels contrived to ring a bell when the liquid has reached a cer tain height in a cask which is being filled, and typewriter alarm bells which ring as the end of the line being written is approached.