History and Function of Telegraph, Cellular Telephone, and Computer

TEXTO 1 • TELEGRAPH

Telegraph, an electrically operated device or system for distant communication (the first ever invented) by means of visible or audible signals. The method used throughout most of the world, based in large part on the mid-19th-century work of Samuel F.B. Morse, utilizes an electric circuit set up customarily by using a single overhead wire and employing the earth as the other conductor to complete the circuit. In the telegraph's simplest form, an electromagnet in the receiver is activated by alternately making and breaking the circuit. Reception by sound, with the Morse code signals received as audible clicks, is the basis for a low-cost, reliable method of signaling. In addition to wires and cables, telegraph messages are now sent by such means as radio waves, microwaves, and communications satellites. Telex is a telegraphy system that transmits and receives messages in printed form. Today the telegraph is less widely used, having been supplanted by telephones, facsimile machines, and electronic mail.

TEXTO 2 • CELLULAR TELEPHONE OR CELLULAR RADIO

Cellular telephone or cellular radio, a telecommunications system in which a portable or mobile radio transmitter and receiver, or 'telephone', is linked via microwave radio frequencies to base transmitter and receiver stations that connect the user to a conventional telephone network. The geographic region served by a cellular system is subdivided into areas called cells. Each cell has a central base station and two sets of assigned transmission frequencies; one set is used by the base station, and the other by mobile telephones. To prevent radio interference, each cell uses frequencies different from those used by its surrounding cells, but cells sufficiently distant from each other can use the same frequencies. When a mobile telephone leaves one cell and enters another, the telephone call is transferred from one base station and set of transmission frequencies to the next using a computerized switching system. The first cellular telephone system began operation in Tokyo in 1979, and the first U.S. system began operation in 1983 in Chicago.

TEXTO 3 • COMPUTER

Computer, a device capable of performing a series of arithmetic or logical operations. A computer is distinguished from a calculating machine, such as an abacus or electronic calculator, by being able to store a computer program (so that it can repeat its operations and make logical decisions) and to store and retrieve data without human intervention. Computers are classed as analog or digital. An analog computer operates on continuously varying data; a digital computer performs operations on discrete data. An analog computer represents data as physical quantities and operates on the data by manipulating the quantities. In a complex analog computer, continuously varying data are converted into varying electrical quantities and the relationship of the data is determined by establishing an equivalent relationship, or analog, among the electrical quantities. Although analog computers are commonly found in such forms as speedometers and watt-hour meters, they largely have become obsolete for general-purpose mathematical computations and data storage by digital computers. Within a digital computer, data are expressed in binary notation (see numeration), i.e., by a series of 'on-off' conditions that represent the digits '1' and '0'. A series of eight consecutive binary digits, or bits, is called a byte and allows 256 'on-off' combinations. Each byte can thus represent one of up to 256 alphanumeric characters. Arithmetic and comparative operations can be performed on data represented in this way and the result stored for later use. Digital computers are used for reservation systems, scientific investigation, and data-processing applications.