Slide rule

The slide rule is a mechanical precursor of the pocket calculator. It was invented in 1625 by William Oughtred.

Once widely used for rapid, approximate scientific and engineering calculations, a slide rule is an analog computer consisting of three interlocking calibrated strips. The central strip can be moved lengthways relative to the other two. A sliding cursor with an alignment line can record an intermediate result on any of the scales.

One slide rule remains in daily use around the world, the E-6B. This is a circular slide rule first created in the 1930s for pilots to help with dead-reckoning. It is still available in all flight shops, and remains widely used. While GPS has greatly reduced the use of dead-reckoning for aerial navigation, the E-6B remains widely used as a backup device and the majority of flight schools demand its use.

Materials

The best older slide rules were made of bamboo, which is dimensionally stable, strong and naturally self-lubricating. They used scales of celluloid or plastic. Some were made of mahogany. Later slide rules were made of plastic, or aluminum painted with plastic.

All premium slide rules had numbers and scales engraved, and then filled with paint or other resin. Painted or imprinted slide rules are inferior because the markings wear off.

Early cursors were metal frames holding glass. Later cursors were acrylics or polycarbonates sliding on teflon bearings.

Magnifying cursors can both help engineers with bad eyes, and double the accuracy of a slide rule.

Premium slide rules included clever catches so the rule would not fall apart by accident, and bumpers so that tossing the rule on the table would not scratch the scales or cursor.

The recommended cleaning method for engraved markings is light scrubbing with steel-wool. For painted slide rules, and the faint of heart, use diluted commercial window-cleaning fluid and a soft cloth.

History

Slide rules came into wide use in the 1850s, as engineering became a recognized professional activity.

In World War II, bombardiers, navigators, and other warriors who required quick calculations often used specialized slide rules. One office of the U.S. Navy actually designed a generic slide rule "chassis" with an aluminum body and plastic cursor into which celluloid cards (printed on both sides) 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.

Throughout the 1950s and 1960s the slide rule was the symbol of the engineer's profession in the same way that the stethoscope symbolized the medical profession. Some engineering students and engineers actually carried 5 inch pocket slide rules in their belts, in addition to using a 10 or 20 inch rule for precision work at home or at the office. All this came to an end in the 1970s, when the advent of miniaturised scientific calculators made slide rules obsolete.

Most slide rules are now collectors' items. A very popular model is the Keuffel & Esser Deci-Lon, a premium scientific & engineering slide rule, available in ten inch "regular" and five inch "pocket" variants. Another prized model is the eight inch Scientific Instruments circular slide rule.
At the turn of the millennium, the collectors' market still doesn't seem to be drying up, and as recently as 2002, new slide rules were being located in the back-shelves of university book-stores, even though production ended in 1973.

See also: Computing timeline, Abacus, Nomogram, Four Figure tables, Napier's bones, Counting rods