Slide rule

The slide rule, also known colloquially in the United States as a slipstick, 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. Though similar in name and appearance to a standard ruler, the slide rule is not ordinarily used for measuring length or drawing straight lines.

Slide rules come in a diverse range of styles and generally appear in a linear or circular form with a standardized set of markings (scales) essential to performing mathematical computations. Slide rules manufactured for specialized fields such as aviation or finance typically feature additional scales that aid in calculations common to that field.

William Oughtred and others developed the slide rule in the 17th century based on the emerging work on logarithms by John Napier. Before the advent of the pocket calculator, it was the most commonly used calculation tool in science and engineering. The use of slide rules continued to grow through the 1950s and 1960s even as digital computing devices were being gradually introduced; but around 1974 the electronic scientific calculator made it largely obsolete and most suppliers left the business.

Slide rule in Supervolcano
Several years after the Yellowstone Supervolcano had erupted, Kelly Ferguson was working on a regression analysis using a scientific calculator due to a power outage preventing her from accessing her university's computer network. However, no mater how she simplified her assumptions, her calculator couldn't handle it. In frustration she swore at it. Her husband across the table asked what was wrong and when she explained, he got up and went to the junk drawer and retrieved a slide rule.

He explained how it worked and Kelly found it easy enough to learn. They went through multiplication and division first, then square and cube roots and finally trig functions. Kelly found it even more rudimentary than her calculator but it didn't crash on her equations since it was only good for two to three significant places and so wasn't overloaded. Given the simplifications Kelly was applying, that wasn't crippling and she would have had to redo them anyway on the computer network when the power came back. As she worked, Kelly found she needed to raise something to a fractional power. Colin had to explain how to use the log-log scale but once again Kelly quickly caught on.