Batteries not required

The throw-away society does not appeal to Merlin, who tends to hang on to things long after they become obsolete. Thus it is that, in addition to a modern scientific calculator, Merlin has in his desk drawer a Hewlett Packard HP41 calculator, purchased over 30 years ago. It still works, but the batteries, an odd size, are not easy to come by.

Beside it resides a calculating device that requires no batteries — a slide rule. A relic of Merlin’s 1950s schooldays, it still occasionally sees action and is good for certain complex computations.

What a slide rule will not do, or at least not easily, is the simple addition of a column of figures — an operation important in Merlin’s role as treasurer of a local history society.

Slide rules were the order of the day for many years, and only supplanted in the early 1970s when electronic calculators became readily available. Slide rules were of many types, often designed for particular types of calculation.

The scales are logarithmic and to multiply two numbers, for instance, one simply adds together the logarithms of the numbers by moving the slide over the main part of the slide rule, called the stock. The precision of a slide rule’s calculations depends on the length of the scales, as this affects the accuracy with which the operator can set the numbers.

In Merlin’s first job, as a laboratory technician (before studying pharmacy) he carried out chemical analyses and then calculated the result with an impressive cylindrical slide rule, some 50cm long, made of mahogany and brass.

A recent article in the Bulletin of the Scientific Instrument Society describes this device, the Fuller Calculator, made by W. F. Stanley & Co Ltd in the early 1900s. Instead of two flat scales sliding over each other as in a standard slide rule, the Fuller Calculator has two cylinders which slide over each other, the spiral scale being some 41ft long — rather different from Merlin’s 12in rule. The Fuller Calculator would do calculations to a precision of four places of decimals, which is impressive for such a relatively simple device.

Modern scientific calculators, whose displays often include graphs as well as the results of calculations, may be a big step forward but they are not as satisfying to use as a slide rule.

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