transmission of information by visible, audible, or other detectable means. Since prehistoric times humans have sought and employed ever more effective means of communicating over distance. Signal fires on mountain tops announced awaited events. In Africa a sophisticated system of drum beating was used, where the tone and the rhythm determined the meaning. Some Native Americans used smoke signals to send messages. In the American Revolution the colonists used combinations of three articles for signaling—a barrel, a basket, and a flag placed in different positions atop a post. In the early days of railroading, men were posted at places where accidents might occur, e.g., at stations and tunnels, and signaled to the engineer by flag or lamp. Modern railroad signaling depends upon electrically operated indicators which constitute a semaphore
system. The position of a movable arm swinging from a pivot indicates whether the train can go forward or must wait and whether it should go rapidly or slowly. Most electric trains, including subways, are fitted with automatic devices that turn the power off and the brakes on if the engineer ignores a stop signal. Railroads also use light signals to duplicate the positions of a semaphore arm. In marine signaling, flags have been used for hundreds of years. The International Code of Signals was compiled by the British government in 1857 and, by international agreement, was amended to the present system in 1901. It utilizes a number of flags of various colors and forms; each flag represents a certain letter or number, and the flags are hoisted to convey a message to another vessel or to the shore. Blinker lights are much used by naval units. At night flash lamps using Morse code
or another code may be used for signaling. Fog signals are commonly used by vessels and lighthouses when visibility conditions make visual warnings ineffective. While radio and telephone apparatus have largely superseded many of these devices, they are still used as backup systems of communication. In modern times aerial and marine craft are commonly given directional data through radio beacon signals or from orbiting satellites to get a fix on their position. In the home, signaling takes on numerous forms from the simple doorbell to intercom systems. Some intercom systems use dedicated wires, and others the power lines by means of carrier-current signaling. Various remote-control devices use signaling: infrared signals, sent in coded pulses, are used to control the function of audio and video entertainment devices. See information theory
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