Slide-rules were widely used and sold into the 1970s and were advertised in competition with the early electronic hand-held calculators, so one is included as a comparison with the electronic calculators with which it competed.
Distinctive features: Slide rule - very widely used, mainly for scientific calculations, before electronic calculators became cheap.
This is a Kingson Four Rule combination pocket slide-rule with an "Addiator" type calculator on the other side.
It is featured here to illustrate a simple, small, pocket, slide rule. Generally, they are at least twice as long, to give greater accuracy, and often have more scales, for more specialised calculations. The Otis King cylindrical slide rule has an extremely long scale for improved accuracy in a small physical size.
The basic slide rule is suitable for multiplication and division, but not for addition and subtraction, which is the purpose of the "Addiator" on the other side of this unusual model. A slide rule has scales both on the fixed body and also on the sliding middle section, while s sliding cursor is a useful aid in determining the alignment of the scales.
The example in the photograph above has scales marked (on the left of the scales) "A", "B", "C1", "C", and "D", and has been set to give a multiplier of 2 by sliding the "1" of scale "C" to align with the "2" of scale "D". The result of muliplying, for example, 3 x 2 can be determined by aligning the cursor with the "3" on scale "C" and reading the answer off scale "D", that is 6.
With practice a slide rule is very quick in use, but the accuracy is dependent on the skill of the user, especially when interpolating answers which lie between the markings on the scales. One disadvantage is that the slide rule is oblivious to the placing of the decimal point, which has to be determined separately, often by a quick mental approximation to the answer. The electronic calculator has the advantages that it provides an answer accurate to 6, 8, or more, decimal places and also puts the decimal point in the correct position.
160 x 40 x 8 mm (6.25" x 1.6" x 0.3").
1960s to early 1970s.
Made in Hong Kong.
The slide rule featured here is unusual since the reverse side has an "Addiator" type calculator for addition and subtraction. This can be used with totals up to 999,999, or, by putting the decimal point two places from the right, currencies up to 9,999.99
© Text & photographs copyright Nigel Tout 2000-2021 except where noted otherwise.