**All about Slide Rules -**

A slide rule is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as roots, logarithms and trigonometry, but is not normally used for addition or subtraction. Slide rules come in a wide range of styles. They are generally linear, circular, or spiral, with a standardized set of markings (scales) essential to performing mathematical computations. Rules made for specialized fields typically feature special scales for making calculations common to that field. William Oughtred and others developed the slide rule in the 17th century based on the logarithms of John Napier. Before the development of pocket calculators slide rules were the most commonly used calculation tool in science and engineering. Their use grew through the 1950s and 1960s while digital computers were being introduced. In the mid 1970s the electronic scientific calculator made slide rules largely obsolete and most companies stopped making them.

Our Slide Rule display at the Main Arlington Library first floor:

Basic concepts

A basic slide rule uses two logarithmic scales for quick multiplication and division of numbers, operations that are hard to do correctly on paper. The scales on slide rules have common names. These two main scales are called the C and D scales. Fancier slide rules allow other calculations like square roots, exponentials, logarithms, and trigonometric functions. The C and D scales are single decade scales - they range from 1 to 10 across the length of the slide rule. Double decade scales known as A and B range from 1 to 100. In general, calculations are performed by aligning a mark on the sliding central strip with a mark on one of the fixed strips, and then observing the relative positions of other marks on the strips. Numbers aligned with the marks give the approximate value of the calculated result. The user determines the location of the decimal point in the result, based on mental estimation. Addition and subtraction steps in a formual are usually done mentally or on paper, not on the slide rule. Most slide rules consist of three linear strips of the same length, aligned in parallel and interlocked so that the central strip can be moved lengthwise relative to the other two. The outer two strips are fixed so that their relative positions do not change. Some slide rules ("duplex" models) have scales on both sides of the rule and the slide strip. Others have scales on one side only ("simplex" rules). A sliding "cursor" is used to find matching points on scales that are not next to each other or are on the other side of the rule in duplex models.

The World's Largest slide rule:

For more details about slide rules see the following sections of the Wikipeda Article on slide rules: