Computer, electronic device that can receive a set of instructions, or program, and then carry out this program by performing calculations on numerical data or by manipulating other forms of information. The modern world of high technology could not have come about except for the development of the computer. Different types and sizes of computers find uses throughout society in the storage and handling of data, from secret governmental files to banking transactions to private household accounts. Computers have opened up a new era in manufacturing through the techniques of automation, and they have enhanced modern communication systems. They are essential tools in almost every field of research and applied technology, from constructing models of the universe to producing tomorrow’s weather reports, and their use has in itself opened up new areas of conjecture. Database services and computer networks make available a great variety of information sources. The same advanced techniques also make possible invasions of personal and business privacy. Computer crime has become one of the many risks that are part of the price of modern technology.   TYPES OF COMPUTERS 1.   Analogue Computers The simplest analogue calculating device is the slide rule, which employs specially calibrated scales to facilitate multiplication, division, and other functions. The analogue computer is a more sophisticated electronic or hydraulic device that is designed to handle input in terms of, for example, voltage levels or hydraulic pressures, rather than numerical data. In a typical electronic analogue computer, the inputs are converted into voltages that may be added or multiplied using specially designed circuit elements. The answers are continuously generated for display or for conversion to another desired form. 2.   Digital Computers Everything that a digital computer does is based on one operation: the ability to determine whether a switch, or “gate”, is open or closed. That is, the computer can recognize only two states in any of its microscopic circuits: on or off, high voltage or low voltage, or—in the case of numbers—0 or 1. The speed at which the computer performs this simple act, however, is what makes it a marvel of modern technology. Computer speeds are measured in megahertz, or millions of cycles per second. A computer with a “clock speed” of 100 MHz—a fairly representative speed for a microcomputer—is capable of executing 100 million discrete operations each second. Supercomputers used in research and defense applications attain speeds of billions of cycles per second. Digital computer speed and calculating power are further enhanced by the amount of data handled during each cycle. If a computer checks only one switch at a time, that switch can represent only two commands or numbers; thus ON would symbolize one operation or number, and OFF would symbolize another. By checking groups of switches linked as a unit, however, the computer increases the number of operations it can recognize at each cycle. For example, a computer that checks two switches at one time can represent four numbers (0-3) or can execute one of four instructions at each cycle, one for each of the following switch patterns: OFF-OFF (0); OFF-ON (1); ON-OFF (2); or ON-ON (3).   FUTURE DEVELOPMENT One continuing trend in computer development is microminiaturization, the effort to compress more circuit elements into smaller and smaller chip space. For example, in 1999, scientists developed a circuit the size of a single layer of molecules, and in 2000 IBM announced that it had developed new technology to produce computer chips that operate five times faster than the most advanced models to date. Also in 2000, scientists discovered a way to transfer information on an atomic level without relying on traditional wires or circuits. This effect, dubbed the "quantum mirage", describes how an atom of matter placed in an elliptical-shaped structure on a solid surface reflects itself at other points within the ellipse, thereby relaying information. Researchers are also trying to speed up circuitry functions through the use of superconductivity, the phenomenon of decreased electrical resistance observed in certain materials at very low temperatures. As the physical limits of silicon-chip computer processors are being approached, scientists are exploring the potential of the next generation of computer technology, using, for instance, devices based on deoxyribonucleic acid (DNA). The “fifth-generation” computer effort to develop computers that can solve complex problems in ways that might eventually merit the description “creative” is another trend in computer development, the ideal goal being true artificial intelligence. One path actively being explored is parallel processing computing, which uses many chips to perform several different tasks at the same time. Parallel processing may eventually be able to duplicate to some degree the complex feedback, approximating, and assessing functions of human thought. One important parallel processing approach is the neural network, which mimics the architecture of the nervous system. Another ongoing trend is the increase in computer networking, which now employs the worldwide data communications system of satellite and cable links to connect computers globally. There is also a great deal of research into the possibility of “optical” computers—hardware that processes not pulses of electricity but much faster pulses of light.