### William Oughtred

Vicar and mathematician, inventor of the slide rule, b. **5 March 1574** (Eton, England), d. **30 June 1660** (Albury).

William Oughtred was vicar of Shalford, Surrey and later rector of Albury. He is known for his achievements as a mathematician.

His work *Clavis Mathematicae* published in 1631 described the notation of numbers in Indian position-value numerals, including decimal fractions, and its use in algebra.

Oughtred invented the slide rule, which moves two logarithmic scales against each other to perform multiplications and divisions. His slide rule was in use by 1633.

A slide rule used in the 1960s and early 1970s

In this example the slide rule is set to solve the example given in the discussion of logarithms:

How much is 12.5 times 5.75?

The correct answer is 71.875. The slide rule gives the answer accurate to 71.9: You place the beginning of scale C at 1.25 on scale D and read the result on scale D at the position of 5.75 on scale C. This achieves multiplication of the two numbers by addition of the length *L*_{1} (from 1 to 1.25) on scale D and the length *L*_{2} (from 1 to 5.75) on scale C.

Like all calculations that use logarithms it does not give the position of the decimal point, which has to be found by estimation of the result. (10 times 6 is 60, and 12.5 times 5.75 has to be close, so it cannot be 7.19 or 719.)

#### Reference

Dutch Circle of Slide Rule Collectors (2003)

http://www.rekenkring.nl/rknlnln6/Rekenvoorbeeld/SLG640.gif

(accessed 2 September 2004)

home