The Slide Rule and Logarithmic Numbers (lig)
The slide rule, one of the last vestiges of geek nerddom in the fifties and sixties, is nevertheless a remarkable invention. Unfortunately, this tool is not used in math or science education any more even though the slide rule provides excellent conceptual examples for how logarithmic computations simplify complex numerical computations. Don't worry! You wiil not be required to buy a calculator for this course, but a brief description of how to use a slide rule may help you better conceptualize how logarithmic algebra works.
The slide rule is design according to the additivity of logarithmic numbers. It consists of two sliding logarithmic scales. By knowing how to position one scale over the other scale, it is possible to multiply or divide two numbers, usually more quickly than is possible with a digial calculator. As illustrated by the "slide rules" on the left or right below, either type of logarithmic scale shown for the acetic acid titration profiles can used to make a slide rule.
| Raw Number Scale | Logarithm Scale |
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To understand how a slide rule works for multiplication and division, it may be easier to start out with a description of how to use two "sliding rulers," as shown below, for addition and subtration. By positioning the top ruler over the bottom ruler and moving the blue hairline, two numbers can be added. See how to use this addtion/subtration slide rule to add 7 + 3.
The concepts behind using two rulers to add or subtract and using two logarithmic rulers to multiply or divide are animated with other images like the one above. Click here to go to see other ruler animations .
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© Duane W.
Sears
Revised: August 10, 1998