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          The enigmatic Spirule

    The calculating device below is a Spirule.

    "Duh...", you say, "Of course it's a Spirule! Everyone knows that!"  Well - I didn't know. This item in my collection has the distinction that I have absolutely no idea how to use it, and this in spite of having a 16-page instruction booklet. Rather humiliating...

    The Spirule is made of clear sheet plastic, and consists of a disc that can rotate on a pivot in a wide rectangular ruler. Both pieces are covered with graphs, scales and other markings. The device is about 30 cm long.

Spirule
Click photo to see a larger scan

    What little material I can find on the web places this device as the invention of the late Walter R. Evans, a brilliant electrical engineer who developed the "Root-Locus Method" for designing automatic control systems. He then invented the Spirule to help "nail down specific points on the locus - to establish the gain to use and the exact natural frequency and damping that will result". This device was manufactured by Evans's company in the 60's, and 100,000 of them were actually shipped before personal computers took over.
    As far as I can tell the Spirule was used in conjunction with some graph on paper; by aligning it over the paper and rotating its parts properly, various results could be read out. Perhaps one day I'll get the book referenced in the instructions and figure it out.
Spirule instruction manual
Click photo to enlarge

Exhibit provenance:
    I got this item courtesy of a friend of my father's, who evidently had it from the days when it was the best method around for computing Root Loci... whatever they are.

More info:
    - Here is a scan of the complete Spirule instruction manual.
    - You can read more about Walter Evans here.

Spirule instructions
Click photo to enlarge
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