## William Oughtred (1574 - 1660)

William Oughtred was one of the world’s great mathematicians. However, today we do not recognise his name and neither is his truly remarkable invention still in use. His invention - the slide rule - is a victim of the calculator and the computer.

He attended Eton School and then King’s College in Cambridge, receiving his M.A. in 1600.

In 1603 he was ordained as an Anglican priest. In the 17th century, becoming a clergyman was a common career option for an educated man.

Like most mathematicians of the time Oughtred was probably not taught mathematics at school or University - he taught himself through his own investigations and by reading the work of others.

He reputedly had a burning curiosity for mathematics and devised symbols for mathematics that are still in use today. Examples of Oughtred’s symbols are ‘x’ for multiplication, and ‘:’ for ratio.

## Oughtred’s Slide Rule

A slide rule is simply a way of doing sums using a pair of rulers - one ruler sliding along the other. Here is a very simple slide rule for addition, but it may also be used for subtraction. It shows how to obtain the answer to the sum, 5 + 3. How might it be used to perform the subtraction, 9 - 6?

The scales on such a simple slide rule are themselves simple - they advance by equal amounts. But the real breakthrough that Oughtred made was to devise a slide rule that could be used for multiplication and division - the scale he needed was a logarithmic scale.

Edmund Gunter created a logarithmic rule in 1620, which could be used to multiply and divide using a pair of dividers to mark off one scale on another. Oughtred was the first to see that a simpler and more sophisticated method of multiplication and division could be achieved by placing two logarithmic rules side by side and using the position of the numbers relative to each other to calculate the desired results.

He also developed the circular slide rule, which operated in the same fashion as a straight slide rule, except that it makes use of an inner and an outer ring. Oughtred published his work in a book *Circles of Proportion and the Horizontal Instrument* in 1632.

Oughtred also constructed sun dials and was interested in surveying and maps.