| analog computer
any of a class of devices in which continuously
variable physical quantities such as electrical
potential, fluid pressure, or mechanical motion are
represented in a way analogous to the corresponding
quantities in the problem to be solved. The analog
system is set up according to initial conditions and
then allowed to change freely. Answers to the problem
are obtained by measuring the variables in the analog
model. .
The earliest analog computers were special-purpose
machines, as for example the tide predictor developed in
1873 by William Thomson (later known as Lord Kelvin).
Along the same lines, A.A. Michelson and S.W. Stratton
built in 1898 a harmonic analyzer having 80 components.
Each of these was capable of generating a sinusoidal
motion, which could be multiplied by constant factors by
adjustment of a fulcrum on levers. The components were
added by means of springs to produce a resultant.
Another milestone in the development of the modern
analog computer was the invention of the
so-called differential analyzer in the early 1930s by
Vannevar Bush, an American electrical engineer, and his
colleagues. This machine, which used mechanical
integrators (gears of variable speed) to solve
differential equations, was the first practical and
reliable device of its kind.
Most present-day electronic analog computers operate
by manipulating potential differences (voltages). Their
basic component is an operational amplifier, a device
whose output current is proportional to its input
potential difference. By causing this output current to
flow through appropriate components, further potential
differences are obtained, and a wide variety of
mathematical operations, including inversion, summation,
differentiation, and integration, can be carried out on
them. A typical electronic analog computer
consists of numerous types of amplifiers, which can be
connected so as to build up a mathematical expression,
sometimes of great complexity and with a multitude of
variables.
Analog computers are especially well suited to
simulating dynamic systems; such simulations may be
conducted in real time or at greatly accelerated rates,
thereby allowing experimentation by repeated runs with
altered variables. They have been widely used in
simulations of aircraft, nuclear-power plants, and
industrial chemical processes. Other major uses include
analysis of hydraulic networks (e.g., flow of
liquids through a sewer system) and electronics networks
(e.g., performance of long-distance circuits).
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