Slide Rule

The slide rule is an analog device for performing mathematical computations. The first slide rule was created in 1630 by British mathematician William Oughtred (1574–1660). His device was based on the logarithmic scale created by British astronomer Edmund Gunter (1581–1626) in 1620. Gunter's work, in turn, was based on the principle of logarithm set forth by Scottish mathematician John Napier (1550–1617) in 1614. What is known as the modern slide rule took shape during the first half of the 1800s.

The slide rule was used by scientists and engineers from 1640 through the 1960s and 1970s. The most familiar modern slide rule is basically a ruler with a sliding piece. Both parts of the device are marked with a scale of digits. The number of significant digits that a slide rule can contain is limited by the size of the rule. Circular and cylindrical slide rules were created to provide more significant digits. Circular slide rules operate on a set of concentric circles, while cylindrical slide rules use scales that can be manipulated by spinning them around a central rod. Today, slide rules are primarily a collector's item, having been replaced by computers and hand-held electronic calculators.

Logarithms and Slide Rules

In the 1600s, John Napier determined that any real number can be expressed as a power (log) of another number and these values could be published as logarithmic tables. He discovered that adding and subtracting logs is the same as multiplying and dividing real numbers.

The common logarithmic tables express real numbers as powers of ten. For example 2 is 10^0.3 and 2's log is 0.3. Also 2 2 2 8. 0.3 0.3 0.3 0.9, the log of 8. Edmund Gunter's logarithm scale arranged numbers from 1 to 10, spacing the numbers in proportion to their logs. Number 2 was spaced 0.3 from 1; 4 was 0.6 from 1, and so forth. Based on this scale, William Oughtred created the slide rule as an instrument for multiplying and dividing, based on logs. As mathematicians discovered new ways to use the slide rule, more scales were added to determine squares, roots, common logarithms, and trigonometry functions.

Bertha Kugelman Morimoto

Bibliography

Thompson, J. E. The Standard Manual of the Slide Rule, 2nd ed., Princeton, NJ: D. Van Nostrand Co., Inc., 1952.