mechanisms

Beautiful 18th-century cometarium

Beautiful 18th-century cometarium

This mechanical model of a comet’s orbit, based on the action of elliptical gears, is dated to 1766, and is housed at Harvard’s Putnam Gallery. From which:

This apparatus was designed to demonstrate how the speed of a comet varies in its orbit according to Kepler’s law of equal areas. The comet Benjamin Martin chose for this instrument is Halley’s Comet, which goes around the Sun every 75 1/2 years. Martin began producing cometaria before Halley’s Comet made its predicted return, and so was betting that Halley would prove correct in his theory.

Interestingly, the device turns out to be not an entirely accurate demonstration of Kepler’s second law. Physicist Martin Beech of the University of Regina has studied the history and mechanics of cometaria at great length. His clearinghouse page is an excellent source of detailed information.

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The Mechanical Glory of the IBM Selectric Typewriter

A “whiffletree” is a mechanical digital-to-analog converter. Brilliant science-and-technology documentarian Bill Hammack, professor of Chemical and Biochemical Engineering at the University of Illinois, has produced this fascinating video anatomy of IBM’s classic Selectric typewriter, in which a 7-bit whiffletree is employed to convert keypresses (digital) to precisely coordinated tugs (analog) on the control cables that rotate and tilt the type ball. Doubly awesome is the fact that the video features an appendix (yes, a video appendix) which focuses exclusively on the whiffletree itself, closely illustrating its operation with a simple 2-bit case.

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How-To: Watt-style mechanical governor

How-To: Watt-style mechanical governor

Adam Richard Cooper built this hand-cranked model of a classic mechanical governor–which, as MachinistBlog succinctly put it “regulates the speed of steam engines by acting as a negative feedback system”–and made the dimensional drawings and build notes freely available for download at his site. I like the idea of a hand-cranked governor model, particularly, because it provides tactile feedback of the device’s purpose: You crank it faster, it gets harder to crank.

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