Best way for finding a root locus!
The calculating device above 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.
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.
Here is the envelope the Spirule came in, and its fanfold manual.
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.
I used the spirule in a control system course at City College of New York (CUNY) back in the day. Useful tool before digital computers. I also used the slide rule before calculators were invented. Also, a handy tool. I don’t have the spirule in my posession. I lent it out and never got it back. But I do have my handy dandy slide rule.
One should always retain one’s slide rule! 🙂
I used a Spirule in my feedback and controls electronic engineering classes in about 1971 more or less. I actually knew how to use it and knew what Root Locus was. Althoough the concepts were part of my career and maybe the hours and hours of plotting the locus of the roots of an equation (thuse Root locus), I never used a spirule professionally. Cheap computers and calculators were making the scene about then. It would be cool to see a spirule simulation application just for grins. I still have my Spirule, but it has a broken corner, which has absolutely no affect on its usefulness, it is more a a consmetic blemish. I expect 90% of the people who know about the Spirule have stumbled into this page!