Alarm

Alarm, or ALARUM (from Ital. all' arme, 'to arms'), is originally a call to arms, or the signal for this purpose, as the loud and hurried peal of an alarm-bell. Now it is commonly used of an instrument or apparatus for awakening sleepers—generally attached to a clock. It usually consists of a hammer and bell, with an escapement that lets it free at any hour arranged, when a spring or descending weight brings the hammer to bear on the bell. The alarm-clock is set beforehand by a separate disc on the face of the clock, which revolves with the hour hand. If the clock is wanted to sound at six o'clock, the figure six on the disc is put beneath the hour hand, and when the revolution of the hour hand brings it opposite six on the clock face, the alarm goes off. There are many more or less ingenious and effective burglar alarms. Some are electric; in others, a string stretched behind the door fires a pistol when disturbed by any one entering. An alarm-whistle may be attached to a boiler, so as to warn those in charge when the water sinks below its proper level.