The slide rule is an extremely clever device that uses logarithms to simplify difficult calculations.  It was invented in the 17th century by the British mathematician, William Oughtred.  He realized that one could multiply or divide numbers by sliding two logarithmically marked rulers next to one another.  Subsequent refinements added many more sophisticated functions and developed slide rules for highly specialized types of calculations.  Users ranged from NASA engineers working on the Apollo Project to sanitation engineers, to artillerymen.  

The Oughtred Society (an association of slide rule collectors) lists over 200 American slide rule patents on its web site between 1851 and 1964.  The last, interestingly enough, is for an "electronic slide rule," presumably a precursor of the electronic pocket calculator, the device which, in the early 1970s, sent the slide rule from the lab bench to the museum virtually overnight.  The pocket calculator will be the subject of next week's Tidbit.

Links:
= highly recommended

"The Slide Rule Universe," from Walter Shawlee and Sphere Research Corporation of Kelowna, British Columbia, Canada.  The ultimate slide rule site--a vast array of slide rules for sale (many of them old but unused), online manuals and instructions, a huge archive of makes and models, and links to other slide rule sites.  The site has been featured in Wired, The New York Times, and on ABC-TV's "World News Tonight."

Gordon P. Smith's "Slide Rule Home Page."  Describes the functions of the various scales, and includes photos of Smith's collection, as well as a bibliography.

"Slide Rule Clings to Life in the Internet Age," by Karen Kaplan, The Nando Times (July 6, 2001). 

The Oughtred Society, an association of slide rule collectors.  The site has a variety of interesting items, including a list of American slide rule patents from 1851 to 1964.

The full text of Edwin Thatcher's 1881 U.S. patent for a slide rule (No. 249177), on American Artifacts.

The Slide Rule Ring, with links to 9 sites.

Ron Manley's Slide Rule Site. Resources for the slide rule collector, including some manuals in PDF.

Peter Holland's, "Slide Rules / Rechenschieber."  In German and English.  Holland has a very neat  downloadable software version of a slide rule--a small program that puts a working slide rule on your screen.

Andrew Davie's Java Slide Rule.  An on-screen version that does not require downloading.  If your browser is Java-enabled, just click and drag the cursor and slide with your mouse.

Make your own circular slide rule with the downloadable templates and instructions on this site, by Charles Kankelborg of the physics department at the University of Montana.

John Current's Slide Rule Page, with photos of slide rules from his collection.

A user-friendly explanation of how a slide rule works, starting with the concept of logarithms.

Site for the 7th International Meeting for Slide Rule and Calculating Machine Collectors.  It took place at the Deutches Museum in Munich, Germany, September 14-16, 2001. 

"Slide Rules," from the Artis Luminis Museum.  Although the site is "under construction," there are a number of interesting things here, including a page with a few slide rule trivia and instructions on how to use a slide rule.

"The Death of the Slide Rule," by James Redin.  A report that, to paraphrase Mark Twain commenting on a report of his own demise, seems "greatly exaggerated."