Technology

# The First Nerd Tool

## Slide Rules

By Dan Winters and Eric LevinAug 1, 2003 12:00 AM

No frontier sheriff ever strode more proudly down Main Street than Tom Apostol did across the campus of the University of Utah in 1941, wearing his own trusty side arm—a 10-inch Keuffel & Esser log-log Decitrig slide rule. "We all felt very macho walking around campus wearing these like guns in holsters," remembers Apostol, now professor emeritus of mathematics at Caltech. "Plus, we knew how to use the things, and this made a big impression on the girls."

Fast, powerful, and accurate, the slide rule became the weapon of choice for calculation beginning in the 1630s, when Episcopal minister William Oughtred invented the first one. "The slide rule had a tremendous democratizing effect," says Anthony Tromba, chairman of the mathematics department at the University of California at Santa Cruz. "The old princes of Europe and the church could hire their own arithmeticians. But calculation became critically important as the merchant class arose in Europe, and the slide rule helped dramatically in the advance of engineering and business."

The five-inch Keuffel & Esser Deci-Lon, the quintessential 1960s pocket slide rule, cost \$12.50. Recently one sold on eBay for \$180. Chemist Linus Pauling used to astound freshmen at Caltech by multiplying numbers to six decimal places on his pocket slide rule. "He calculated the last two digits in his head," says Caltech math professor emeritus Tom Apostol.

Carpenters, merchants, navigators, artillerymen, and tax assessors came to rely on specialized rules. How much timber could be extracted from a maple log 30 inches in diameter and 41 feet long? How much wine could be drawn from a barrel 38 inches wide at the middle, 24 inches wide at the ends, and 31 inches high? How much gunpowder should be stuffed into a cannon to fire a 12-pound ball 1,200 feet? For every calculation, there was a slide rule that could do the job. Slide rules helped create the steam engine—James Watt and Matthew Boulton would have been lost without theirs. And a slide rule was present at the creation of the nuclear age—Enrico Fermi used his to calculate exactly how far a cadmium control rod should be pulled from a graphite and uranium pile as he demonstrated the first controlled nuclear chain reaction at the University of Chicago in 1942. Moments after making the final calculation, Fermi put away his slide rule, smiled broadly, and announced, "The reaction is self-sustaining." Fermi didn't live long enough to see the reaction of scientists and engineers to the epochal Hewlett-Packard HP-35 electronic pocket calculator in 1972, the device that foretold the doom of the venerable slide rule. At \$395, the HP-35 was out of reach for students like Dick Lyon at Caltech, but he received one as a present in 1973, a year before his graduation. His slide rule "got consigned to the junk heap quickly," Lyon says, "because it was not a comparable tool." But as Lyon progressed in his career—inventing the optical mouse in 1980, helping create a handwriting-recognition system for the Apple Newton in the mid-1990s, and becoming chief scientist at the digital-imaging company Foveon in 1997—he began to miss the look and feel of his old 10-inch Keuffel & Esser. He remembered fondly the tiny metal screws holding it together, the finely cut markings, the silky feel of the slide. He was not alone. In 1991 a group of self-described slide-rule nerds founded the Oughtred Society. Lyon joined, and he got to know retired engineer Bob Otnes, editor of the Journal of the Oughtred Society. Like hundreds of others who now buy and sell slide rules on eBay, the two are passionate collectors. The instruments on these pages come from their collections.

The Achilles' heel of the slide rule is that for most calculations it cannot indicate where the decimal point should go. A circular rule made in Germany around 1870 remedied this problem. The small cogged wheel counted decimal places, clicking each time the pointer made a revolution. Coincidentally, the 17th-century Scotsman who popularized the decimal point, John Napier, also invented logarithms, the engine of all slide rules. Logarithms make it possible to multiply or divide large numbers in streamlined form.

The Oughtred Society's Web site features photos as well as listings of upcoming events: www.oughtred.org.More about the history of slide rules, along with some photos, from the Museum of Hewlett-Packard Calculators: www.hpmuseum.org/sliderul.htm.

Many slide rule sites have photos and instructions for using the rules. Two good sites: www.sliderules.clara.net and www.sphere.bc.ca/test/sruniverse.html.

Dick Lyon is chief scientist at Foveon, a company whose X3 chip may usher in the next generation of digital cameras. Read more in "The Next Photography Revolution" by Eric Levin, Discover, December 2002. This article is available at www.discover.com.

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