Slide Rule

Slide Rule is a mechanical contrivance for performing the operations of mnltiplication and division. It consists of a graduated rule, A B C D, having a dovetailed groove in which a second rule, E F G H, can slide, the faces of the two being flush at the upper side. The corresponding scales on the rule and slide are identical, and are such that the distance from the mark 1 to the mark 2 is proportional to the logarithm of 2 [LOGARITHMS], the distance from 1 to 3 to logarithm 3, and so on up to 10; and the spaces between these marks are further subdivided logarithmically, the fineness of the dividing depending upon the length of the rule. Confining our attention for the moment to the scales A B and E F only, suppose that the mark 1 on E F is made to correspond with some mark on A B - say that corresponding to logarithm 1,545, which is at K. Now take any mark on E F - say L, which corresponds to 174; it is clear that the distance from A to L corresponds with logarithm 1,545 and logarithm 174, and the point on A B whicn is now opposite L will be marked with the number whose logarithm is logarithm 1,545 + logarithm 174 - that is, with the product of 1,545 and 174, for the sum of the logarithms of two numbers is the logarithm of their product. We can perform division by reversing this process; if we set a number on E F opposite a number on A B, the distance from A to mark 1 on the slide will be the difference between the logarithms of the two numbers, and mark 1 on the slide will be opposite their quotient. If we have a small brass slide with a mark on it (called a cursor) which fits over the rule, we can set its mark opposite the result of one operation, and use that point as the basis of further mu1tiplication or division without actually reading the number, and in this way complicated calculations may be made without any use of paper or pencil. It is usual to duplicate the divisions on A B and E F - i.e. make the length from A to the end of the rule correspond to logarithm 100, and to graduate the lower half of both rule and slide (G H and C D) in such a way that the distance from A or E to any number is one-half the distance from G or C to the same number. As the logarithm of the square root of a number is half the logarithm of the nnmber, it is evident that the root of a number is to be found on C D opposite the number on A B, and that squares may be found in the converse manner. Special marks are also made to correspond with constants which are often needed (such as pi ) and considerably facilitate many calculations. No account is taken on a slide rule of the index of the logarithm, so that the position of the decimal point must be determined by inspection of the numbers. Slide rules of circular or spiral form are sometimes used, but the one above described is the most common form.