Understanding the Slide Rule

Logarithmic slide rules are to calculators like analog is to digital. Analog can have distortion, while digital is clear. Digital is simply more practical. For the same reason, the slide rule diminished as the faster calculators quickly became affordable. Logarithmic slide rules were mechanical or rather, used by hand, which made them subject to human error while calculators are less subject to human error and can process difficult equations quickly.

Logarithms, or logs, are simply exponents. Exponents are numbers that tell how many times a number should be multiplied onto itself. The exponent is located to the above and right of the base number, which is the number to be multiplied. Exponents are also called powers. For instance, the base number 2 with an exponent of three is said to be "two to the third power." This simply means that two is to be multiplied onto itself three times: 2 times 2 times 2; where 2 times 2 is four, times two, equals eight.

Another way to write two to the third power is eight is by using logarithmic notation or logarithmic form. Logarithmic notation essentially says that two, the base number, should be multiplied 'x' amount of times onto itself to equal the power of 8. The term 'x' is called an unknown making this definition also an equation. In elementary common logs, this means if you know two of those numbers, you can figure out the third, or the unknown. The unknown 'x' we know from the above example is the exponent three. Three is the logarithm.

It is written this way: 3=log₂8 which is the same as saying 2³=8 (exponential form).

The slide rule is based on common logarithms with all the markings based on factors of 10. Later slide rules had many more scales, one being the uniform, linear L scale, to calculate logarithms. Reading the slide rule is like reading any other ruler as the markings are uniform dependent on which scale is being utilized, and the scale being utilized, is dependent on the calculation and its usage.

Slide rules were much like today's calculators in their heyday, creating ways for students and professionals to solve complicated math problems by utilizing basic maneuvers with common sense. By "sliding the rule" over the indicator or movable runner, which looks like a complicated ruler, the user could find solutions to calculations by using various scales on the body of the slide rule. This did not eliminate the need for pencil and paper, but simply allowed for quicker computation, in a basic way.

To find how a slide rule works and what they were used specifically for, check out these links:

Slide Rule Pictorial illustration of the basic slide rule depicts how to manipulate it to solve an equation.

Brief History Timeline site shows picture of an actual slide rule and talks about who invented it.

Basic Slide Rule Instructions Step-by-step instructions on how to use the slide rule.

How the Slide Rule Works An MIT student explains how the most basic slide rule works and how to identify its parts.