Analog computer

An analog computer is a form of computer using electronic or mechanical
phenomena to model the problem being solved by using one kind of physical
quantity to represent another.

The term is used in distinction to digital computers, in which physical or
mechanical phenomena are used to construct a finite-state machine which is
then used to model the problem being solved. There is an intermediate group,
Hybrid computers, in which a digital computer is used to control and
organize inputs and outputs to and from attached analogue devices; for
instance analogue devices might be used to help generate initial values for
iterations.

Computations are often performed, in analog computers, by using properties
of electrical resistance, voltages and so on. For example, a simple two
variable adder can be created by two potentiometers in series. The first
value is set by the first pot (say x ohms), and the second value is set to
the second pot (say y ohms). Measuring the resistance across the two pots
will give the sum in resistance x+y ohms. Other calculations are performed
similarly, using operational amplifiers and other circuits for other tasks.

Idealised analog computers operate on real numbers and are differential,
whereas digital computers are limited to computable numbers and are
algebraic. This means that analog computers have a larger information
dimension rate (see Information Theory), or potential computing domain, than
do digital computers (see Gšdel's incompleteness theorem). This in theory
enables analog computers to solve problems that are inextricable on digital
computers.

Computer theorists often refer to the analog computer as a real computer (so
called because it operates on the set of real numbers), in order to
circumvent popular misconceptions of analog computers.

An example:

   * the abacus is a hand-operated digital computer
   * the slide rule is a hand-operated analog computer

Examples of analog computers:

   * operational amplifier
   * Tide predictors
   * Water integrator
   * Target Data Computer
   * Hydraulic model of UK economy
   * the Antikythera mechanism
   * and (trivially) the slide rule

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