Dempster RotaRule Model AA Circular Slide Rule
Item History & Price
| Reference Number: Avaluer:4295152 | Modified Item: No |
| Country/Region of Manufacture: United States |
The RotaRule is a very sophisticated circular slide rule designed and manufactured by John R. “Jack” Dempster in Berkeley, California from approximately 1928 to 1948. The original RotaRule was called the Model A, and approximately 400 of those were made. This one is the successor to the Model A, the Model AA, which has 3 additional scales. It seems incredible that a slide rule this sophisticated could be that old. The cursor pivots on ball bearings and has a... ‘brake’ out at the rim, released with push buttons, to keep it from flopping around. The friction on the inner rotating disc is adjustable via three spring-loaded shoes. In addition to circular C and D scales, there is an additional pair of 50-inch long C and D scales, each of which makes 4 spirals. There is a scale for compound interest, a total of three scales for surveying, log log scales (1.01 to 20, 000), and the usual trig, square, and cubing scales. The RotaRule was so intelligently designed that in the 1960’s, long after Dempster’s patents had expired, both SIC and Pickett copied features of the RotaRule’s design. Fewer than 2500 Dempster RotaRules were ever sold, and it was most likely a victim of its own superb mechanical design. It was complicated (ball bearings, brakes, springs, push buttons…on a slide rule!), with many parts, and it was expensive to build. In 1939 it sold for $18 ($323 today) and in 1948 it cost $32 ($343 today). This was a slide rule for the very well-heeled engineer.
The RotaRule was supplied with a 45-page manual written by Dempster himself, and a leather case and magnifying glass (which plugs into the hole in the hub with the ring around it) were available. The RotaRule for sale here has no accessories or the manual, just the rule itself. If you need the instructions, the winning bidder can contact me after the auction and I’ll send you a PDF.
This RotaRule Model AA is in nice condition but not perfect. There has been some yellowing of the white plastic, and someone has put strips of clear tape, now yellowed and peeling, over the nuts on the back of the cursor, where 4 screws hold the outer end together. That said, everything works, and this is still a very scarce and very interesting slide rule.
This slide rule is from the collection of Robert K. (“Bob”) Otnes, PhD, one of the founding members of the Oughtred Society and the original editor of the Journal of the Oughtred Society. It is being sold by the Oughtred Society on his behalf.
The Oughtred Society was founded in 1991 by a group of slide rule collectors and is dedicated to the preservation and history of slide rules and other calculating instruments. Membership is open to anyone. Further information about the Oughtred Society can be found at: http://www.oughtred.org/.
PayPal is the only accepted form of payment. Buyers within California will be charged 7.25% sales tax.
Shipping will be by USPS Priority Mail. Shipping charges will be somewhat higher for destinations outside the USA.
The RotaRule was supplied with a 45-page manual written by Dempster himself, and a leather case and magnifying glass (which plugs into the hole in the hub with the ring around it) were available. The RotaRule for sale here has no accessories or the manual, just the rule itself. If you need the instructions, the winning bidder can contact me after the auction and I’ll send you a PDF.
This RotaRule Model AA is in nice condition but not perfect. There has been some yellowing of the white plastic, and someone has put strips of clear tape, now yellowed and peeling, over the nuts on the back of the cursor, where 4 screws hold the outer end together. That said, everything works, and this is still a very scarce and very interesting slide rule.
This slide rule is from the collection of Robert K. (“Bob”) Otnes, PhD, one of the founding members of the Oughtred Society and the original editor of the Journal of the Oughtred Society. It is being sold by the Oughtred Society on his behalf.
The Oughtred Society was founded in 1991 by a group of slide rule collectors and is dedicated to the preservation and history of slide rules and other calculating instruments. Membership is open to anyone. Further information about the Oughtred Society can be found at: http://www.oughtred.org/.
PayPal is the only accepted form of payment. Buyers within California will be charged 7.25% sales tax.
Shipping will be by USPS Priority Mail. Shipping charges will be somewhat higher for destinations outside the USA.
