Importance

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Abstract

A metrological apparatus has a movable arm 15 which moves up and down between two bearing members 13, both of which define datum positions for the arm 15. The arm 15 has air bearings on the bearing members 13, so that it adopts a position equidistant between the bearing members 13. In this way, the position of the arm 15 contains the average of the errors in the two datum positions defined by the bearing members 13. Where the bearing members 13 have symmetric errors, these will be cancelled. Since the path of maximum straightness of movement of the arm 15 is spaced from the bearing members 13, it is possible to put a workpiece 3 in this path and a turntable 5 for supporting the workpiece 3 is arranged to have its rotational axis in line with the path of maximum straightness. The vertical position of each end of the arm 15 is measured using a respective grating 25 and the average of the measurements is taken to provide an accurate measurement of the height of the centre point of the arm 15 over the workpiece 3, substantially free of Abbe errors.

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First Claim