| Name and Model Number | Dempster RotaRule Model A |
| Country of Origin | USA |
| Date of Manufacture | 1928 to ~1930 |
| Front Scales | B Gh Gv CT T S'T' CS S CIF CI C D A K |
| Back Scales | L LL1 LL2 LL3 (unmarked = long C) V1 V2 (unmarked long D) |
| Scale Length | 50" (127cm) |
| Type of Rule | Disc |
| Construction | Plastic 5.25" (13 cm) diameter |
| Area of Use | Other |
| Comment | For precision basic math and stadia calculations. Has unique modified spiral scales. In about 1930 with serial number 426 model A was upgraded to model AA with 3 additional scales - see model E image believed to be experimental transition to model AA. Note #426 transistion image courtesy of Dempster Family. |
| Rarity | RRR |
| Donor | Collection of Richard Davis |
| Match Number | 66 |
| Thumbnail (Click Picture for Details) |  |
| In conjunction with our 'donors' we assign a rarity rating to each item, based on the system devised by Herman van Herwijnen |
| (1929 - 2004), renowned Dutch collector, author, and creator of Herman's Archive, a collection of 5,000 slide rule images on CD. |
| • RR = Rare - 1 of 25 collectors has one; it may take a year to find an example in the marketplace. |
| • RRR = Very Rare - only a few known to collectors; may be several years before it comes up for sale. |
| • RRRR = Extremely Rare - a museum piece; unlikely that an example will come up for sale. |