| Name and Model Number | E.D. Co. 6 & 10 Inch Proportional Divider |
| Country of Origin | Germany |
| Date of Manufacture | First half of 1900's |
| Front Scales | Small dividers – scale of lines, scale of circles -- large divider has scales for proportions and has a vernier. |
| Back Scales | None |
| Scale Length | 15cm (6 inch) & 25cm (10 inch) |
| Type of Rule | Instrument |
| Construction | Fine machine work on nickel steel, or German silver |
| Area of Use | Nautical |
| Comment | Primarily used for navigation, astronomy, or proportional calculations. There is an article in OS Journal volume 17, No 2, from 2008, by David D McFarland that explains proportional dividers as mathematical calculating tools for finding square roots and solving other math problems. |
| Rarity | RR |
| Donor | Collection of Clay Castleberry |
| Match Number | 355 |
| 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. |