A digital computer performs calculations based solely upon numbers or symbols. An analog computer, on the other hand, translates continuously changing quantities (such as temperature, pressure, weight, or speed) into corresponding voltages or gear movements. It then performs "calculations" by comparing, adding, or subtracting voltages or gear motions in various ways. The final result is sent to an output device such as a cathode-ray tube or pen plotter on a roll of paper. Common devices such as thermostats and bathroom scales are actually simple analog computers: they "compute" one thing by measuring another. They do not count.
The earliest known analog computer is an astrolabe. First built in Greece around the second century B.C. , the device uses gears and scales to predict the motions of the Sun, planets, and stars. Other early measuring devices are also analog computers. Sundials trace a shadow's path to show the time of day. The slide rule (a device used for calculation that consists of two rules with scaled numbers) was invented about 1620 and is still used, although it has been almost completely replaced by the electronic calculator.
Vannevar Bush, an electrical engineer at the Massachusetts Institute of Technology (MIT), created in the 1930s what is considered to be the first modern computer. He and a team from MIT's electrical engineering staff, discouraged by the time-consuming mathematical computations required to solve certain engineering problems, began work on a device to solve these equations automatically. In 1935, they unveiled the second version of their device, dubbed the "differential analyzer." It weighed 100 tons and contained 150 motors and hundreds of miles of wires connecting relays and vacuum tubes. By present standards the machine was slow, only about 100 times faster than a human operator using a desk calculator.
In the 1950s, RCA produced the first reliable design for a fully electronic analog computer. By this time, however, many of the most complex functions of analog computers were being assumed by faster and more accurate digital computers. Analog computers are still used today for some applications, such as scientific calculation, engineering design, industrial process control, and spacecraft navigation.
[ See also Computer, digital ]