The slide-rule

The slide rule (Fig. 1) presents one of the quickest and easiest ways of performing the operations of multiplication, division, raising to a power, and extracting a root, when not more than the first three or four digits of the result are necessary. Both straight and circular slide rules are in common use, but it is generally easier to learn to use the straight slide rule first. We represent here the fundamental principles of the straight (10 inches) slide rule as they are used in multiplication, division, reciprocals, proportion, squaring and extracting square roots.

Fig. 1

The straight slide rule is composed of many bodies or "frames", a slide or "slip stick", and an indicator. The slide is the main body. The indicator is the glass containing the hairline and is used to locate numbers in the various scales.

The Scales. The scales on a slide rule are graduated according to the mantissa of the positive real numbers, or, according to the logarithms of numbers from 1 to 10.

The top scale is divided into ten equal lengths (1 inch each for the 10-inch slide rule). The numbers from 1 to 10 on the bottom scale are located so that they correspond to their logarithms on the top scale. Note that the lengths be-tween the numbers on the bottom scale decreases in size as the numbers become larger. The bottom scale is called a logarithm scale.