A slide rule is a simple, mechanical, analog computer. It has a single moving part – the central slider (and usually a movable cursor to read results). It has etched or printed scales on one or both sides for logarithms, exponents, roots, trigonometry, and sometimes more. A slide rule uses these scales to readily perform calculations. The basic principle is using logarithms to replace multiplication with addition. They advance calculation from simply counting to exploiting the relationships between numbers.
Most slide rules are linear ‘sticks’ that resemble everyday rulers. There are also circular and even cylindrical slide rules that extend the linear scales, fitting equivalent calculating power into a more compact device. There are also scales on the movable bar in the middle (slider). Scales are slid into position to set up a particular calculation, and the result is obtained using a precise cursor. The resolution of the scales (and the user’s eyesight) determine the precision of calculations. It is reasonable to get several significant digits at least.
Today, electronic calculators have almost completely replaced slide rules. In fact, early such pocket calculators were often called ‘electronic slide rules’.
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