In this process the coincidence of the vertical lines on the standard and on the piece c are employed to test the end measure on the bar with the line measure on the standard.
Fig. 1360.—General View.
Fig. 1361.—Plan.
[Figs. 1360] and [1361] represent the Whitworth Millionth Measuring Machine, in which the measurement is taken by the readings of an index wheel, and the contact is determined from the sense of touch and the force of gravity.
It is obvious that in measuring very minute fractions of an inch one of the main difficulties that arise is that the pressure of contact between the measuring machine and the surfaces measured must be maintained constant in degree, because any difference in this pressure vitiates the accuracy of the measurement. This pressure should also be as small as is consistent with the assurance that contact actually exists, otherwise the parts will spring, and this would again impair the accuracy of the measurement.
If the degree of contact is regulated by devices connected with the moving mechanism of the machine it is indirect, and may vary from causes acting upon that mechanism. But if it is regulated between the work and the moving piece that measures it, nothing remains but to devise some means of making its degree or amount constant for all measurements; so that if a duplicate requires to be compared with a standard, the latter may first be measured and the duplicate be afterwards measured for comparison.