The Slide Rule: The Analog Computer That Reached the Moon

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For three centuries, engineers did their hardest arithmetic on a stick with no batteries and no buttons. The slide rule, nicknamed the slipstick in the United States, works because multiplying two numbers is the same as adding their logarithms: slide one logarithmic scale along another and the product appears under the hairline. This episode explains the mechanics and the history, from Edmund Gunter's fixed scale in the 1620s to the moment William Oughtred set two of them side by side and let them slide.

The instrument never showed a decimal point and gave only about three significant digits, so its user had to carry the scale of the problem in mind. That shared effort is what veterans miss. The episode follows the refinements that made the tool a badge of the engineering profession, its role in the Apollo program, and its sudden death in the 1970s at the hands of the pocket calculator, along with the debate over false precision that followed.

• Oughtred did not publish his design, and former student Richard Delamain claimed priority with a circular slide rule, setting off a bitter dispute over who invented it.
• In 1859 French artillery lieutenant Amédée Mannheim built the layout around single-decade C and D scales and added the sliding cursor. In 1895 the Japanese firm Hemmi introduced bamboo rules clad in celluloid.
• Buzz Aldrin carried a Pickett N600-ES on Apollo 11, and specialized rules existed for weighing cattle, timing photographic exposures, and estimating nuclear blast effects.
• Cylindrical models such as the Thacher and Fuller wound a scale as long as 50 feet around a drum for four or five digits of precision.
• Jack Volder's 1959 CORDIC algorithm made digital trigonometry cheap. The $395 HP-35 arrived in 1972, the TI-30 sold for under $25 in 1976, and Texas canceled its high school slide rule competitions by 1980.