Farmar's Profit-calculating Rule

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Farmar's profit calculator

Farmar's profit-calculating rule - front

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You see a lovely old tea pot in a market stall, marked at 8 pounds, 13 shillings; after a bit of haggling the seller says you can get it at a 15% discount. How much does that come to? Quick!

Perhaps the British could once do those calculations in their head, but even so they might have appreciated Farmar’s Profit-calculating Rule. With this device a twirl of the cylinder and a movement of the slide would instantly give you the correct 7 pounds, 7 shillings - see photo at the right.

Click photo to enlarge

Farmar’s rule is an unusually shaped slide rule that allows the calculation of a variety of commercial problems involving percentages – cost and profit, gross and net prices, discounts and turnover... simple problems to be sure, but when dealing with a non-decimal money system, the Farmar’s could have been quite handy.

The unusual construction can be seen in this photo. It allows you to bring any of the scales on the cylinder next to the slide, a necessary step in the absence of a cursor. If you’re curious how it was used, read the instructions on the photo of the back side below.

So, who was this Mr. Farmar? All I know is that he was called Theo. -- Theodore? – and that he lived at Ramsgate, on the east coast of England, at 100 Station Approach road. With the wonders of Google mapping we can even see his house, if it still stands; but of Mr. Farmar himself I can find not a clue. If you know more of him, let me know and I will share it here in your name!

Click photo to enlarge

Click photo to enlarge

		Exhibit provenance: eBay. More info: None at the moment...

Farmar's profit calculator