In the figures, d is a piece in position to be measured, and between it and the bar c is a feeler consisting of a small flat strip of steel, e e, having parallel sides, which are true planes.

When the pressure of contact upon this piece e e is such that if one end be supported independently the other will just be supported by friction, and yet may be easily moved between d and c by a touch of the finger, the adjustment is complete. At the sides of the frame a are two small brackets, shown at k, in the end view, [Fig. 1362], e e being shown in full lines resting upon them, and in dotted lines with one end suspended. The contact-adjustment may thus be made with much greater delicacy and accuracy than in those machines in which the friction is applied to the graduated wheel-rim, because in the latter case, whatever friction there may be is multiplied by the difference in the amount of movement of the graduated rim and that of the bar touching the work.

All that is necessary in the Whitworth machine is to let e e be easy of movement under a slight touch, though capable of suspending one end by friction, and to note the position of the lines of graduation on c with reference to its pointer. By reason of having two operative bars, b, c, that which can be most readily moved may be operated to admit the piece or to adjust the bars to suit the length of the work, while that having the finer adjustive motion, as c, may be used for the final measuring only, thus preserving it from use, and therefore from wear as much as possible; or coarser measurements may be made with one bar, and more minute ones with the other.

So delicate and accurate are the measurements taken with this machine, that it is stated by C. P. B. Shelley, C.E., in his “Workshop Appliances,” that if well protected from changes of temperature and from dust, a momentary contact of the finger-nail will suffice to produce a measurable expansion by reason of the heat imparted to the metal. In an iron bar 36 inches long, a space equal to half a division on the wheel g having been rendered distinctly measurable by it, this space indicating an amount of expansion in the 36-inch bar equals the one two-millionth part of an inch!

The following figures, which are taken from Mechanics, represent a measuring machine made by the Betts Machine Company, of Wilmington, Delaware.

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Fig. 1363.

[Fig. 1363] shows a vertical section through the length of the machine, which consists of a bed carrying a fixed and an adjustable head, the fixed head carrying the measuring screw and vernier while the adjustable one carries a screw for approximate adjustment in setting the points of the standard bars.