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Slide rules were used for quite complex calculations but they required careful setting and reading.

The slide rule consists of moveable scales of numbers representing their logarithms. By sliding the scales against each other it is possible to look up answers to multiplication and division, but because the scales were quite small, errors are easily made. Small slide rules were very useful to get a quick, rough answer to a complex calculation. Larger and circular slide rules were more accurate.

Many thanks to Brian Davies, Aberystwyth for the detailed explanations and loan of the slide rules.