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# 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 Gödel'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:
Also see signals, set theory, computability theory, differential equation, dynamical systems, chaos theory.

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