Description
This is a logarithmic slide rule used for converting numbers into percentages. Teachers would have used this to convert marks in tests in the Victorian times. It was found during renovations inside an old cupboard at the staff room of Ermysted’s Grammar School. The initials ‘H.F.L.’ have been scratched into the ruler, presumably by the owner. Slide rules were generally used as the predecessor to calculators.

Developed during the seventeenth century, the modern slide rule is based upon the design by William Oughtred (circa 1630). It is one of many calculation devices that is based on the logarithmic scale, a calculation method invented in 1614 by John Napier.

Before the rise of the pocket electronic calculator in the 1970s, the slide rule was the most common tool for calculation used in science and engineering. It was used for multiplication and division, and in some cases also for ‘scientific’ functions like trigonometry, roots and logs, but not usually for addition and subtraction.

Calculations are determined by aligning a point along the central movable strip with a mark on one of the scales on one of the two fixed strips, and then observing the relative positions of other points along the scales. A logarithm transforms the operations of multiplication and division to addition and subtraction according to the rules log(xy) = log(x) + log(y), and log(x/y) = log(x) - log(y). Moving the top scale to the right by a distance of log(x) aligns each number y, at position log(y) on the top scale, with the number at position log(x) + log(y) on the bottom scale. Because log(x) + log(y) = log(xy), this position on the bottom scale gives xy, the product of x and y.

Created by: Rosanna Evans, year 10 work experience student