All that is essential is that the two be touched with an equal degree of contact, and the most ingenious and delicate method yet devised to accomplish this result is that in the Whitworth machine, whose construction is as follows:—

In a box frame a, is provided a slide-way for two square bars, b, c, which are operated by micrometer screws, one of which is shown at j (the cap over b being removed to expose b and j to view). The bars b, c, are made truly square, and each side a true plane. The groove or slide-way in which they traverse is made with its two sides true planes at a right angle to each other; so that the bars in approaching or receding from each other move with their axes in a straight line. At the two ends of the frame the micrometer screws are afforded journal bearings. The ends of the bars b, c, are true planes at a right angle to the axes of b, c. Bar b is operated as follows: Its operating screw j has a thread of 1⁄20 inch pitch; or in other words, there are twenty threads in an inch of its length. It is rotated by the hand-wheel f, whose rim-face is graduated by 250 equidistant lines of division. Moving f through a distance equal to that between, or from centre to centre of its lines of division, moves b through a distance equal to one five-thousandth part of an inch.

The screw in head i for operating bar c also has a pitch of 1⁄20 inch (or twenty threads in an inch of its length), and is driven by a worm-wheel w, having 200 teeth. This worm-wheel w is driven by a worm or tangent-screw h, having upon its stem a graduated wheel g, having 250 equidistant lines marked upon the face of its rim.

Suppose, then, that wheel g be moved through a distance equal to that between its lines of division, that is 1⁄250th of a rotation, then the worm h will move through 1⁄250th of a rotation, and the worm-wheel on the micrometer screw will be rotated 1⁄250th part of its pitch expressed in inches; because a full rotation of g would move the worm one rotation, and thus would move the worm-wheel on the screw one tooth only, whereas it has 200 teeth in its circumference; hence it is obvious that moving graduated wheel g, through a distance equal to one of its rim divisions will move the bar c the one-millionth of an inch; because:

Pitch of
thread		 Rotation of
worm‑wheel		 Rotation of
graduated wheel
1⁄20 inch × 1⁄200 × 1⁄250 = 1⁄1000000

Fig. 1362.

Fixed pointers, as k, [Fig. 1362], enable the amount of movement or rotation of the respective wheels f, g, to be read.

A peculiarly valuable feature of this machine is the means by which it enables an equal pressure of contact to be had upon the standards, and the duplicates to be tested therewith. This feature is of great importance where fine and accurate measurements are to be taken. The means of accomplishing this end are as follows:—