| Name and Model Number | Farmar's Profit-Calculating Rule |
| Country of Origin | England |
| Date of Manufacture | After 1915 |
| Front Scales | |
| Back Scales | |
| Scale Length | |
| Type of Rule | Other |
| Construction | Paper on wood. Endpieces in brass. |
| Area of Use | Financial |
| Comment | This peculiarly-shaped slide rule is the fruit of the imagination of Frank Charles Farmar, a slide rule manufacturer from Waterloo, near Liverpool. The object of this instrument was «to provide a rule for commercial calculations, which, for a given size, shall have large calculating capacity and which may be easily read.
Farmar applied for a patent at least in two countries, the UK and the US (respectively in 1912 and 1913). The UK patent 12717 was granted in 1913 and the US patent 1123507 in 1915 (both available through http://worldwide.espacenet.com/). In both patents, the guiding tongue is centered on the slide but this is not the case on the actual slide rule. |
| Rarity | RR |
| Donor | Photo from a collection in Belgium |
| Match Number | 357 |
| 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. |