How to Use a Slide Rule Scale

A slide rule was used to perform basic calculations before calculators came along.

A slide rule is basically a sliding mechanical ruler filled with numbers that is designed to perform simple mathematical equations. According to the Outred Society, in 1851, a French artillery officer named Amadee Mannheim standardized a set of four scales to figure out the most common mathematical problems. Scales A and B represent squares and square roots, while C and D represent multiplication and division. This became the basis of slide rule design for the next 100 years.

Instructions

1. Shift the slide ruler to the right, so a number on the C scale lines up with a number on the D scale. As an example, use two for the C scale and five on the D scale.

2. Search for the number five on the C scale, without moving the slide rule any further. Look at the number on the D scale that corresponds with the five from the C scale. The D scale number should be 10, which is the answer to the five times two question.

3. Line up the number 10 on the C scale and the number five on the D scale to perform division. Without moving the slide rule, find the number five on the C scale, and the corresponding number on the D scale, which will be two.

Tips & Warnings

• If no numbers correspond with one another, shift the slide rule to the left instead.

References

• HP Museum: Basic Slide Rule Instruction
• The Oughtred Society: Slide Rule History
• MIT: Slide Rule