Slide rule in the context of "Scientific calculator"

An analog computer or analogue computer is a type of computation machine (computer) that uses physical phenomena such as electrical, mechanical, or hydraulic quantities behaving according to the mathematical principles in question (analog signals) to model the problem being solved. In contrast, digital computers represent varying quantities symbolically and by discrete values of both time and amplitude (digital signals).

Analog computers can have a very wide range of complexity. Slide rules and nomograms are the simplest, while naval gunfire control computers and large hybrid digital/analog computers were among the most complicated. Complex mechanisms for process control and protective relays used analog computation to perform control and protective functions. The common property of all of them is that they don't use algorithms to determine the fashion of how the computer works. They rather use a structure analogous to the system to be solved (a so called analogon, model or analogy) which is also eponymous to the term analog computer, because they represent a model.

A mechanical computer is a computer built from mechanical components such as levers and gears rather than electronic components. The most common examples are adding machines and mechanical counters, which use the turning of gears to increment output displays. More complex examples could carry out multiplication and division—Friden used a moving head which paused at each column—and even differential analysis. One model, the Ascota 170 accounting machine sold in the 1960s, calculated square roots.

Mechanical computers can be either analog, using continuous or smooth mechanisms such as curved plates or slide rules for computations; or discrete, which use mechanisms like pinwheels and gears.

A mechanical calculator, or calculating machine, is a mechanical device used to perform the basic operations of arithmetic automatically, or a simulation like an analog computer or a slide rule. Most mechanical calculators were comparable in size to small desktop computers and have been rendered obsolete by the advent of the electronic calculator and the digital computer.

In 1642, Blaise Pascal invented the first operational mechanical calculator with better tens-carry. Concerned about his father's exhausting work as tax collector in Rouen, Pascal designed the Pascaline to help with the large amount of tedious arithmetic required.