In the first place outside calipers are much more used than inside ones, hence the wear on the points are greatest. Again, the pivot is apt to wear, destroying the equality of length of the points from the centre of the pivot; and in the third place the shape of the points of calipers as usually made vitiates the correctness of the measurements.
Fig. 1417.
[Fig. 1417], for example, represents the ordinary form, the points being rounded; hence, when the legs are closed the point of contact between the inside and outside calipers will be at a, while when they are opened out to their fullest the points of contact will be at b. This may, however, be remedied to a great extent by bevelling off the ends from the outside as shown in [Fig. 1416].
Fig. 1418.
The end faces of outside calipers should be curved in their widths, as in [Fig. 1418], so that contact shall occur at the middle, and it will then be known just where to apply the points of the inside calipers when testing them with the outside ones.