Telephone

The term telephone (from Greek tele, afar, and phone, sound) in a broad sense means a specific type of Figure 1. Telephone set simplified circuit. Illustration by Hans & Cassidy. Courtesy of Gale Group. telecommunications which permits people to carry direct conversations over almost any distance. The articulate speech is transmitted in a form of either modulated electric current propagating along conductors or radio waves. The apparatus used for converting the verbal sounds into electromagnetic signals and vice versa is called a telephone set and is also commonly referred to as a telephone. Telephone communication is one of the most widespread and expeditious forms of communication. It has proved to be invaluable for an exchange of information in all areas of human life, whether it is business, government, science, public health, personal services, or social contacts. There were about 200,000 telephones in the world in 1887, approximately 30 million in 1928, 118 million in 1958, and more than 330 million in 1975.

The general concept of the telephone as a system involves a number of elements. First comes a telephone apparatus itself. Next, there is a variety of means for conveying the electromagnetic signals over distances (transmission paths). Third, the transmission paths are arranged in multi-leveled structures (networks) in a way that permits to interconnect (switch) any desired telephone sets upon request. And finally, there are signalling facilities for directing the switching operations, alerting the person called, and clearing the circuits upon the completion of the call.

A scientific and technological discipline which studies and develops all of the telephone-system's constituents is called telephony. Many problems in telephony draw on the principles and techniques of the electromagnetic theory, the theory of linear and nonlinear circuits, the probability theory, and the queuing theory.