The Single Callipers fig. 841, referred to as used for external measurement, are made of two thin plates of sheet steel jointed together at a center, the bows tapering to delicate points the extreme ends of which are slightly rounded; when opened to the required measure, they are fixed by a screw and nut working in a circular mortise.
It is absolutely essential to correct measurement, that all callipers should be applied to the work held exactly square at right angles to its axis, that the entire thickness of their blades may bear upon the object measured. It is evident that if held otherwise than square, the blades can only touch the work by their opposite external corners fig. 343, when the measure varies and is quite untrustworthy. The necessary elasticity of the callipers, if they be unskilfully used, may also be a source of error, as they are capable of being forced over a diameter slightly larger than that at which they have been set and are intended to measure. In testing the cylinder or in copying any object, the callipers are closed upon the work by the hand, so that the points may pass over the diameter measured without difficulty, but with a sensible touch; they are then fixed by the binding screw, and are afterwards retried at the same place, to see that fixing the screw has not shifted the points. They are used held rather lightly in the hand by the joint; the work in the lathe being brought to rest before they are applied to it. Although set so as to pass over just easily, the hand sensibly feels the contact between their points and the objects measured; the comparison of the one diameter with the other depending entirely upon the sense of touch, which unerringly detects whether the contact agrees or differs. As the diameters nearly approach each other in size in course of reduction, the nice appreciation of the touch is materially assisted by constantly referring the callipers to the original object, to again feel the degree of their contact, and then immediately transferring them to the work or copy, to feel for the same degree of contact also there. In the careful measuring required upon small and delicate works, more especially upon those of metal, the binding screw is often dispensed with, as inconvenient; the callipers are then made to open rather stiffly upon the joint, and for minute differences are closed by one bow being gently struck or tapped upon some unyielding surface, like the jaws of the vice or the lathe bearers.
The Double Callipers fig. 342, called by the French "maitre de danse," have the bows made in one piece with straight legs on the opposite side of the center; both ends give the same measure, but externally and internally, so that when the bows are employed to take the measure of a solid object, the legs will just enter and gage the aperture to receive it. The Double side callipers fig. 344, give the same external measure at either end. They are used to measure the thickness of tubular works of a diameter too small to admit the ordinary form of callipers, and also to test the thickness of hollow cylindrical and other work, when from external projections, it is not possible to separate the callipers from the work without opening them and so losing the measure. The one end free of the work, shows the measure taken by the other retained within it.