*Macmillan Science Library: Computer Sciences*. Copyright © 2001-2006 by Macmillan Reference USA, an imprint of the Gale Group. All rights reserved.

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## 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*

## See Also

Thompson, J...

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