Slide Rule

by Chris Chalfant

The slide rule is a mechanical calculator developed in the 17th century based on the work of John Napier. Slide rules could help people perform all sorts of computations including multiplication, division, trigonometry, and logarithms. They were used by the financial and aeronautical industries until electronic calculators and computers became widely available.

A typical slide rule is made up of three strips arranged so that the middle strip can slide relative to the outer strips and a transparent slider that can travel the length of the strips. It works by aligning numbers printed on the middle strip with numbers printed on one of the outer strips and moving the slider to a certain position and reading off the number there.

Side rules are designed to take advantage of the mathematical property log(xy) = log(x) + log(y). This allows you to convert multiplication into addition. By aligning the beginning of the top rule with x on the bottom rule, you can read along the top rule to position y. The point on the bottom rule that lines up with y is the answer. Other calculations could be performed with more complicated rules.

Slide rules were widely used during World War II by pilots and bombardiers to calculate fuel usage, range and altitude. Engineers used them so much, that they became a symbol of the profession and engineering students frequently carried them in special belt pouches. In fact, you can see one on a statue of a student in one of the engineering buildings at Purdue University (see below). The use of slide rules died out in the 1970s when electronic calculators became available.