The slide rule is mechanical device used by mostly engineers and scientists for fast and accurate

multiplication, division, to find roots and powers, and other simple math problems. It consists of a fixed

section with upper and lower parts marked with different logarithmic scales, a center part that slides

between the parts of the fixed section and is also marked with scales, and a clear cursor that can be moved

to help align the scale marks. The slide rule has almost been totally eliminated because of the 1970?s

invention of the hand-held electronic calculator. Today it is mostly used by engineers and collected by

antique collectors. A basic slide rule from the 1940?s that cost thirty dollars at the time would now be

worth over two hundred dollars.

Most people think that the only type of slide rule is the one that is straight and almost resembles a

ruler. There are really three types of slide rules. The first type is the most common one, the Mannheim.

This is basically a long stick with a movable centerpiece as well as a movable window, called the center

piece, to work out math problems.

The second type is the circular slide rule. These are virtually the same as the Mannheim, but bent around in

a circle. The cursor is a pair of radial arms that move around the center or origin. The advantage of a

circular slide rule is that you can fit a longer scale within a certain area.

The third type is the cylindrical slide rule. These are the hardest kind of slide rule to come by and are very

rare today. They are also similar to the Mannheim?s, but the scales are wrapped around a cylinder. These

are the most accurate of slide rules, and also the most expensive, because they are made to be the most

precise and have the longest scales to work with.

The history of the slide rule dates back to 1614 when John Napier discovered the logarithm. This

made it possible to perform multiplication and divisions by addition and subtraction. Even though this was

a great time saver, there was still a lot of work required. The mathematician had to look up two logs, add

them together, and then look for the number whose log was the sum. Edmund Gunter soon reduced the

effort by drawing a number line in which the positions of numbers were proportional to their logs.

The scale started at one because the log of one is zero. Two numbers could be added by measuring the

distance from the beginning of the scale to one factor with a pair of dividers, then moving them to start at

the other factor and reading the number at the combined distance. In 1621, William Oughtred simplified

things further by taking two of Gunter?s lines and sliding them relative to each other which eliminated the

dividers.

In the years that followed, other people refined Oughtred?s design into a sliding bar held in place

between two other bars. The first slide rule in which a sliding scale moved between two fixed sections was

made in 1654. Later on, circular and cylindrical slide rules were developed. The cursor appeared on the

earliest circular models, but appeared much later on straight versions. By the later 17th century, the slide

rule was a common instrument with many different variations. The present form of the slide rule was

developed in 1850 by a French army officer, Amedee Mannheim.

It was the math tool of choice and was used by everyone from tax collectors to Sir Isaac Newton.

By the 20th century, it was especially known as the engineers companion. In college campuses around the

world, engineering students could be recognized by the slide rules they carried in leather cases and strapped

to their belts. Yet, what commonly happens to most inventions happened to the slide rule. The once great

math tool was then almost irrelevant when Hewlett-Packard introduced the HP 35 pocket scientific

calculator in the early 1970?s. Since then you usually only find slide rules in an engineer?s desk or at an

antique shop.

The basic slide rule instructions start out with multiplying numbers on the slide rule. To multiply

two numbers on the typical slide rule, the user sets the left index (start of the scale) on the C scale to line up

with one factor on the D scale. The

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