Gilson

Gilson was known for building inexpensive circular rules with two cursors. The longer arm can be placed on 1 and held in place while the shorter one - which rides atop it - is rotated to, say, 2. The angle between the cursors now corresponds to the length from 1 to 2, i.e. log(2). Then the first cursor can be repositioned to say, 5 and the friction between the two will move the shorter one as well, preserving the 1-to-2 angle and thus pointing to 10, the product of 5 and 2 and the sum of the angles corresponding to log 5 and log 2.

In some ways, I find this the most intuitive style of slide rule. However, I think types with two actual rules (whether linear or circular) are faster.

Gilson made small and big circular rules, and other companies (notably Dietzgen) sold Gilson rules under their own brand.

Gilson Midget

With a diameter of 4", this is the quivalent of a 12" linear rule for multiplication and division. However, scales closer to the center get shorter quickly!

Gilson Atlas

At 8.25" wide, this is a really whopping big slide rule! The 30-revolution spiral scale is 39 feet long, geometrically speaking. But if you actually measure the size of the gradations in the widest turn - which corresponds to the right end of a straight scale - you find that a straight scale would have to be about 60 feet long to match the Atlas's resolution.