These are instruments for marking, drawing or writing, formed of graphite, colored chalk or materials of similar properties, and having a tapering end, inclosed, generally, in a cylinder of softwood. [Fig. 167] represents a ruling pencil; its point is a parallelogram or of a wedge shape. In ruling, the length view rests against the square; its shape gives considerable strength to the lead and allows the making of a very fine line. [Fig. 168] differs in the point of the pencil shown, as may be observed in the illustration.
A pencil that is hard is best for mechanical drawing; one that will retain a good point for some considerable time. Pencil lines should be made as light as possible; the presence of lead on the surface of the paper tends to prevent the ink passing to the paper, and in rubbing out pencil lines the ink is reduced in blackness, and the surface of paper is roughened, which is a disadvantage. As little erasing or rubbing out as possible should be done.
DIVIDERS AND COMPASSES.
These instruments, while they appear alike, have a separate use: the dividers are used to space off distances and dimensions; especially are they necessary in reading drawings made to scale. Compasses are used for describing circles, curves, etc., dividers are used for marking out spaces.
Two forms of the dividers are shown in [figs. 169 and 170]; the simplest, plainest form is shown in [fig. 169]; these are used for rough spacings; [fig. 170] represents a pair of dividers fitted with an adjustable screw controlled by a steel spring in one leg; by this a very exact measurement can be made. [Fig. 170] is intended to exhibit what is called a “hair-spring divider.”
PROPORTIONAL DIVIDERS.
These dividers differ from the ordinary ones shown in [figs. 169 and 170] in that they are provided with four steel points, one pair of which being set to the full dimension will be reproduced by the other pair, but in a smaller, or reduced size.
[Fig. 171] are “bisecting” dividers, being proportional dividers, which, when open, one end measures double the distance of the other.
[Fig. 172] are proportional dividers; the points at one end are capable of being changed, to measure practically any desired proportion at the other end, by altering the position of the pivot where the legs cross one another. The lower connecting link is a micrometer adjustment, for minute measurements.
[Fig. 173] are proportional dividers which are marked for the proportions of lines and radii of circles, being provided with a rack movement for adjustment.