How to Use Slide Rules

The window on a slide rule slides to perform calculations.

Slide rules are calculation devices that allow you to perform a variety of mathematical calculations. The most basic calculations that a slide rule is capable of are multiplication and division. Although slide rules are not as commonly used in modern times -- they have been put aside in favor of scientific calculators and built-in calculators on mobile devices -- converting a number into slide-rule-sized numbers, multiplying or dividing, and converting again to find the result is a simple task that can be performed in under a minute with practice.

Instructions

Slide Rule Basics

1. Divide or multiply each value in the calculation by a power of ten so that it can be represented on the rule. Most rules only have numbers up to ten, meaning to convert 97 to fit you would divide by ten, yielding 9.7, for example.

2. Perform the multiplication or division using the slide rule as described in the corresponding section of this article.

3. Convert the result on the slide to account for the changes made to the values before calculation. For any number that was multiplied, or the top number in a division, perform the opposite calculation on the number. In the example, the result would be multiplied by ten to make up for the number multiplied being divided by ten prior to calculation. For numbers on the bottom of a division, reverse the conversion by performing the same calculation. For the example, had 97 been on the bottom of a division, the result would be divided by 10.

Multiplication

1. Slide the rule so that the "1" reading on the C scale is aligned over one of the numbers being multiplied on the D scale.

2. Align the viewing window over the second number being multiplied so that the vertical bar on the window passes through the number.

3. Read the number on the D scale the bar passes through to find the result of the multiplication.

Division

1. Locate the divisor (the number you are dividing by), on the C scale. For example, in 5 ÷ 3, 3 is the divisor.

2. Adjust the rule so the divisor is positioned over the dividend, the number being divided, on the D scale. For example, in 5 ÷ 3, 5 is the dividend.

3. Slide the window so that the vertical bar is passing through the "1" reading on the C scale.

4. Read the number the vertical bar is passing through on the D scale to find the result of the division.

References

• HPMuseum.com: Basic Slide Rule Instructions