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Calculating Rods
© Museum of the History of Science, Oxford
Click Here for More Information This is a beautiful set of old, English, calculating rods, made in the 17th century. There are twelve rods in total, all made of wood from the box tree (box tree wood was very popular in the 17th century for making mathematical instruments.) The rods have their own custom-built case, which is made out of pasteboard, covered in leather, and very carefully decorated by patterns made with decorated metal punches. All in all, it is a mathematical instrument which has a strong historic feel of Jacobean England about it. Each of the calculating rods is 300 mm long and 5 mm square. They would have been used to help people multiply large numbers together. Simple mechanical aids to arithmetic such as these rods were about the only way that people got help to do complicated sums before calculating machines that used cogs and dials were invented. There were several different types of calculating rods around in the 17th century, but these are quite an unusual and rare type.

Every side of each of the rods is divided with vertical lines into 36 sections. Each of the sections has a number in it, carefully punched into the wood. The numbers range from 1/8 to 90,000,000,000. The large numbers are abbreviated by a system of dots that indicate powers of ten.

Calculating Rods
© Museum of the History of Science, Oxford
These calculating rods are similar to the ones invented by the Scottish mathematician John Napier, also called 'Napier's bones' because they were often made out of ivory and looked like real bones. Like many other calculating aids they allow multiplication sums to be made easier by converting them to addition sums. 'Napier's bones' usually came in sets of ten rods with one rod for each digit from 1 to 10. Each rod is a multiplication table for a single number, a 'times tables' for that number. The rod for 5, for example, would have the numbers 5, 10, 15, 20, 25, 30, 35, 40, 45 and 50 on it.

Calculating rods were very widely used in the 16th and 17th century because they helped people to avoid errors when doing multiplication sums. Lots of different types of people would carry sets of Napier's bones around with them in their pockets in case they needed to do a multiplication sum. These rods however are quite a lot longer than Napier's bones usually were, so are too big to put in a pocket. They were probably used by someone who needed to do multiplication with large numbers, such as a banker.

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