Slide rules are divided into thirds. The top and bottom are fixed in place but the middle section slides back and forth. Each section has scales — numbers and line marks for calculations. The slide rule is able to do many types of calculations, such as: multiplication, division, trigonometry, squares and square roots, cubes and cube roots, and log-log scales. The first one was built by William Oughtred, a cleric teaching math in England in the 1600s. It was based on John Napier's discovery of logarithms. For generations of engineers, technicians and scientists, the slide rule was an essential part of their daily lives. In 1972 Hewlett-Packard came out with the first handheld electronic calculator. Practically overnight, the slide rule had become obsolete.
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- The Slide Rule was used for astronomical work and calculations. In the 19th century, Germany used a slide rule of 2 meters (6 ft 5 in) long in their observatory;
- During the World War II, bombardiers and navigators needed to perform quick calculations. The tool used to calculate the quick math was a slide rule. An office of the U.S. Navy designed a generic slide rule "chassis" with an aluminum body and plastic cursor. It turned into celluloid cards (printed on both sides) that could be placed for special calculations. The process was invented to calculate range, fuel use and altitude for aircraft, and then adapted to many other purposes;
"While studying at Indiana University (IU) in the early 1950’s, I attended a three week Slide Rule course. Engineering students at Purdue carried their slide rule in a holster dangling from their belt as a badge of engineering. The Slide Ruler was considered to be fast and accurate." - Gerald Dix
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