Re: Gilson Two-Cursor Design

Maynard Wright, P. E., W6PAP

Hi, John

Just in case it might be a point of interest with respect to this topic, the AN 5837-1 "Computer, Altitude Correction" that I described in an earlier email is an example of a two-cursor circular rule with a clamp for one of the cursors.   It was supplied to the USAAF by G. Felsenthal & Sons on a 1943 contract.

Best regards,

Maynard Wright

On 6/12/21 3:02 PM, John Runnels wrote:

I seek “crowdsourcing” help for a research topic.  I am researching the history of Gilson slide rules for a possible presentation at IM 2021 (if I can get it ready in time), as well as for eventual publication in JOS.


Gilson slide rules were the most successful circular slide rules in the United States.  And they were among the most successful circular slide rules worldwide (at least, if one excludes pocket-watch types).


The thesis of the proposed presentation is that among the factors that contributed to the success of Gilson slide rules were the following three:


  1. Gilson rules were economically produced, with a sturdy, durable design.
  2. The Gilson scale layout was efficient.
  3. Gilson rules featured a novel improvement over prior two-cursor designs:  A clutch allows the first cursor to move independently of the second cursor; while moving the second cursor automatically causes the first cursor to rotate in tandem with it, maintaining a fixed angle of separation between the two cursors.  No extra effort is required by the user to do this.


I am seeking the collective wisdom of the Slide Rule Community on point No. 3:  Does anyone know of an earlier circular slide rule, published or manufactured prior to 1921, that had two cursors, in which the movement of the two cursors acted in the same manner as Gilson’s?


The use of two cursors (or dividers) was not new in itself, of course.  That idea goes all the way back to Oughtred’s Circles of Proportion.  After Oughtred and before Gilson, others had described mechanisms in which clamps could be manually set or released, so that the two cursors would then move independently or in tandem, as the user selected.  However, such a manually-operated clamping mechanism requires extra steps as compared to the Gilson design, and it also introduces a source of possible operator error.


The hypothesis is that Gilson’s design was the first circular slide rule that had two cursors that operated in this manner — one cursor rotates freely, while rotating the second cursor automatically causes both cursors to move in tandem with a fixed separation.


Gilson’s original patent almost makes this assertion, but the wording stops short of actually saying that.  And of course there is the possibility that, aside from whatever Gilson’s patent specification said, and aside from whatever was known to Gilson or to the patent examiner, there could still have existed relevant references that were overlooked at the time.


(Gilson probably had an earlier date of invention than 1921.  However, 1921 is when his first patent application was filed, so that date provides a good first cut in reviewing what might be potentially relevant.)


I would be grateful for any insight, one way or the other.  Please send replies to the group or directly to me, as you may prefer.




John Runnels

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