Slide rule

A mechanical analog calculator that uses sliding logarithmic scales to perform multiplication, division, and other calculations.

Logarithm: A mathematical function that gives the exponent to which a base must be raised to produce a number. Logarithms convert multiplication into addition, the principle behind the slide rule’s scales.
Common logarithm: A logarithm with base ten, used to represent decimal quantities on many slide rules. Its base matches ordinary decimal notation and makes scale values easy to interpret.
Trigonometric function: A function relating angles to ratios of sides in a right triangle or to coordinates on a circle. Sine, cosine, and tangent scales let slide rules evaluate common angle calculations.
John Napier: A Scottish mathematician who published tables of logarithms in 1614. His logarithms supplied the mathematical foundation for calculating devices based on scaled distances.
C and D scales: The paired logarithmic scales used for multiplication and division on a conventional slide rule. Sliding one scale against the other adds or subtracts logarithmic distances.
Proportional scale: A scale on which equal physical distances represent equal changes in a chosen quantity. A logarithmic scale spaces numbers according to their logarithms, not their arithmetic differences.
Exponential function: A function in which a fixed base is raised to a variable exponent. Exponential scales support powers and roots without repeated multiplication.
Edmund Gunter: An English mathematician who developed logarithmic measuring instruments in the early seventeenth century. His logarithmic line enabled multiplication by measuring distances with dividers.
Cursor (slide rule): The transparent movable indicator that aligns positions across a slide rule’s scales. It transfers a value between scales without disturbing the slide’s setting.
Interpolation: The estimation of a value between known points in a scale or data set. Fine results often depend on estimating positions between printed graduations.

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