What Is a Slide Rule Used For?
The slide rule is an analog precursor to the digital scientific calculator, developed in the 17th century, and used widely up until the recent past. With a slide rule, a person can perform complex or difficult mathematical calculations much more quickly than is possible with paper and pencil.
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Multiplication
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Multiplication is performed by adjusting the movable scale so that its marking for the number '1' on scale C is lined up with the number desired to multiply by on scale D. Any number along C, multiplied by the target number, equals the number directly across from it on D.
Division
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Division is just as easy to perform. To divide, line up the divisor on scale D with the dividend on scale C. The number on D that is aligned with C's number '1' is the quotient.
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Powers and Roots
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More complicated operations are possible on rules that have scales made for them, though everything uses the same process of alignment. Raising a number to any power is accomplished by aligning the number on the LL scale with the 1 on the C scale. The power to which it is to be raised, on the C scale, will align with the answer on the LL scale. Powers of 2 are even simpler to calculate: any number on the fixed A scale is the square of the number beneath it on the D scale. Reversing the process gives a number's root.
Trigonometry
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Trigonomic functions can also be calculated on a slide rule. Sines are found by comparing the C and S scales (no adjustment necessary, since they are both found on the same rule). Cosines also make use of C and S, though the scale of numbers on S going in reverse (sometimes in a different color ink than the left-to-right scale) are used instead. The scales T and C are compared to calculate tangents.
Logarithms
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Logarithms are calculated with the various LL scales, each of which is used in different applications. Basic logarithmic functions can be figured by comparing the C and LL3 scales. Set the '1' on the C scale to the base number you wish to use, on LL3. The log of each number on the LL3 scale will equal its counterpart on C, according to your base.
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