Unlike a digital computer, which performs calculations strictly upon numbers or symbols, an analog computer translates continuously varying 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, finally directing the result to an output device such as a cathode-ray tube or pen plotter on a roll of paper. Common devices like 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. Built in Greece during the first century b.c., the device used pointers and scales on its face and a complex arrangement of bronze gears to predict the motions of the sun, planets, and stars. Other early measuring devices were also analog computers. Sundials traced a shadow's path to show the time of day. Springweight scales, which have been used for centuries, convert the pull on a stretched spring to avoirdupois. The slide rule was invented about 1620 and is still used, although it has been almost completely superseded by the electronic calculator. In 1905 Rollin Harris and E. G. Fisher of the U.S.
Coast and Geodetic Survey started work on a calculating device that would forecast tides. Dubbed the "Great Brass Brain," it was 11 feet long, 7 feet high, and weighed 2,500 pounds. It contained a maze of cams, gears, and rotating shafts. Completed in 1910, the machine worked as follows: an operator set 37 dials (each representing a particular geological or astronomical variable), turned a crank, and the computer drew up tidal charts for as far into the future as the operator wished. It made accurate predictions and was used for 56 years before being retired in 1966. Vannevar Bush, an electrical engineer at the Massachusetts Institute of Technology, created what is considered to be the first modern computer in the 1930s. He and a team from MIT's electrical engineering staff, discouraged by the time-consuming mathematical computations, called differential equations, required to solve certain engineering problems, began work on a device to solve these equations automatically. In 1935, the incredible second version of their device, dubbed the "differential analyzer," was unveiled: it weighed 100 tons, contained 150 motors, and hundreds of miles of wires connecting relays and vacuum tubes. Three copies of the machine were built for military and research use. Over the next 15 years, MIT built several new versions of the computer. 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, but by this time, 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.
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