THE MODERN SLIDE RULE.
The modern form of slide rule, variously styled the Gravêt, the Tavernier-Gravêt, and the Mannheim rule, is frequently made of boxwood, but all the leading instrument makers now supply rules made of boxwood or mahogany, and faced with celluloid, the white surface of which brings out the graduations much more distinctly than lines engraved on a boxwood surface. The celluloid facings should not be polished, as a dull surface is much less fatiguing to the eyes. The most generally used, and on the whole the most convenient size of rule, is about 10½in. long, 1¼in. wide, and about ⅜in. thick; but 5 in., 8 in., 15 in., 20 in., 24 in. and 40 in. rules are also made. In the centre of the stock of the rule a movable slip is fitted, which constitutes the slide, and corresponds to the lower of the two rules of our rudimentary examples.
Fig. 5.
From Fig. 5, which is a representation of the face of a Gravêt or Mannheim slide rule, it will be seen that four series of logarithmic graduations or scale-lines are employed, the upper and lower being engraved on the stock or body of the rule, while the other two are engraved upon the slide. The two upper sets of graduations are exactly alike in every particular, and the lower sets are also similar. It is usual to identify the two upper scale-lines by the letters A and B, and the two lower by the letters C and D, as indicated in the figure at the left-hand extremities of the scales.
Referring to the scales C and D, these will each be seen to be a development of the elementary scales of Fig. 3, but in this case each principal space is subdivided, more or less minutely. The principle, however, is exactly the same, so that by moving the slide (carrying scale C), multiplication and division can be mechanically performed in the manner described.
The upper scale-line A consists of two exactly similar scales, placed end to end, the first lying between Il and Ic, and the second between Ic and Ir. The first of these scales will be designated the left-hand A scale, and the second the right-hand A scale. Similarly the coinciding scales on the slide are the left-hand B scale and the right-hand B scale. Each of these four scales is divided (as finely as convenient) as in the case of the C and D scales, but, of course, they are exactly one half the length of the latter.
The two end graduations of both the C and D scales are known
as the left- and right-hand indices of these scales. Sometimes they
are figured 1 and 10 respectively; sometimes both are marked 1.
Similarly Il and Ir are the left- and right-hand indices of the A
and B lines, while Ic is the centre index of these scales. Other
division lines usually found on the face of the rule are one on the
left-hand A and B scales, indicating the ratio of the circumference
of a circle to its diameter, π = 3·1416; and a line on the right-hand
B scale marking the position of π
4 = 0·7854, used in calculating
the areas of circles. Reference will be made hereafter to the
scales on the under-side of the slide, and we need now only add
that one of the edges of the rule, usually bevelled, is generally
graduated in millimetres, while the other edge has engraved on it
a scale of inches divided into eighths or tenths. On the bottom
face inside the groove of the rule either one or the other of these
scales is continued in such a manner that by drawing the slide out
to the right and using the scale inside the rule, in conjunction
with the corresponding scale on the edge, it is possible to measure
20 inches in the one case, or nearly 500 millimetres in the other.
On the back of the rule there is usually a collection of data, for
which the slips given at the end of this work may often be substituted
with advantage.