==[]-- Circular slide rule with spiral scale

Wrapping a long logarithmic scale as a spiral on a disk was a common way to fit a very long scale into a manageble volume to increase calculation precision. Gilson Atlas is probably the most known rule of this type.

This emulator does not represent accuratly any of the existing Gilson rules. Actual Gilson rules had different cursor type, different number of scale windings and different tickmark layout. Rather it is a generic "slide rule with one large logarithmic scale wrapped into a spiral".

It is difficult to keep track of windings count when performing a calculation. The single decade log scale along the edge of the instrument is to assist. First user performs estimated calculation with 2-3 digits using a single decade scale and then repeat same calculation on a spiral scale with 3-4 digits precision. Knowing rough estimate of an answer helps determine which winding of a spiral scale is used for lookup.

To operate the emulator drag with a mouse any of the 2 cursors or dial to rotate.


Estimator scale:
Single decade
Single decade with winding identifier
Triple decade


  1. JOS, Spring 1999. E. J. Chamberlain, "Long-Scale Slide Rules"
    I found ten different makers of circular slide rule with spiral scale. The Gilson Atlas was probably the most widely known make of this type in the United States.
  2. JOS, Fall 2000. H. Aldinger, E. Chamberlain, "Gilson Slide Rules -- Part II -- The Large Rules"

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