1. This rule was developed for one very specific purpose - to convert the price of sugar priced in cents per pound (weight) to its price in shillings and pence per hundredweight. Although this calculation could have been made using a "standard" slide rule it would be cumbersome. It first would have have been necessary to multiply by 112, to convert from pounds to hundredweights (abbreviated as cwt.), and then divide by the exchange rate between the US dollar and pound sterling to get the price in shillings. The decimal part of the price in shillings would have had to be multiplied by 12 to get the price in shillings and pence.
2. I have put "Unique" in brackets as the maker, although their name does not appear anywhere. This is very typical of the specialised rules they made. Their construction technique, paper scales covered by clear plastic and wooden stock and slide, meant that specalised rules could be produced very easily with no need for special scale engraving to be set up.
3. An interesting feature of this rule is the "X" mark on both the slide and the stock. This is shown on the detailed image below. Its position is such that the answer will always available on either the stock or slide; a useful feature for a specialised rule whose user would be not be as familiar with switching ends of the rule as a regular slide rule user would. In the example shown below the value of 12/3d appears next to both X marks.
4. The rules is described as the Wilkinson Sugar Rule. This particular rule was given to me by someone who worked for a jam making company, Chivers. The way I obtained the rule is quite interesting. On my way to the airport to go the to the interntional slide rule meeting in Munich in September 2001 - I mentioned to the taxi driver why I was travelling. He said that he had a slide rule in a drawer somewhere. When I came back it was the same taxi driver and he had the rule in his taxi for me.
5. The rule has the instructions on the back and these are given below.
THE WILKINSON SUGAR RULE
INSTRUCTIONS FOR USE
This simple Slide Rule provides a quick means of converting the price of sugar from cents per lb. into shillings and pence per cwt. at any rate of exchange $-£. It will deal with all other commodities whose prices fall within the same range.
Example: If the price is 3.50 cents per lb. and the rate of exchange is 2.80 dollars to the pound sterling find 3.50 in the lowest scale and set the cursor index to this value. Now move the slide to the right to bring the value 2.80 on the rate scale into coincidence with the cursor index. Opposite the arrow X on the slide will be found the price 28s. per cwt. Conversion in the opposite sense is similarly made.
Example: A commodity which costs 37s. per cwt. is to be quoted in cents per lb. Rate of exchange 3.20. Set the slide so that the arrow X on the slide is coincident with 37s. on the top scale. Then opposite 3.20 in scale C will be found 5.28 in scale D. This is the price in cents per lb.
All the common calculations involving multiplication, divisions, percentages, proportion. etc.. may be quickly made with the aid of scales C and D. Instructions for the use of scales C and D will be found in pp. 1 to 3 of the Half Hour instruction on Slide Rules which is supplied with this Wilkinson Sugar Rule.
Currently perforce — as indeed at any time pre-war — New York Terminal market is the guide to the "World Price" of Sugar. Quotations for spot and forward positions are always available if you wish to observe the trend of the market, should the rate of exchange fluctuate — as it may — then the Wilkinson Sugar Rule makes awkward calculations easy and accurate — instantly.