Airy points

Airy Points are used for precision measurement (metrology) to support a length standard in such a way as to minimise bending or droop. The points are symmetrically arranged around the centre of the length standard and are separated by a distance equal to

$1/\sqrt{3}=0.57735...$

Airy points (after George Biddell Airy) are normally identified by inscribed marks or lines. For example, a 1000 mm length gauge would have an Airy point separation of 577 mm. A line or pair of lines would be marked onto the gauge 211.5 mm in from each end. Supporting the artifact at these points ensures that the calibrated length is preserved. If the length gauge is not supported at the Airy points, the measurement uncertainty is increased.

It is not necessary to be absolutely exact; the approximation 4/7 = 0.571428 is commonly used even at the highest levels of precision. For example, the 1927–1960 definition of the metre specified that the International Prototype Metre bar was to be measured while "supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other."^[1]

See also

• History of measurement
• Test method
• Units of measurement
• Weights and measures

References

1. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 143, ISBN 92-822-2213-6

• Smith, S. T.; Chetwynd, D. G. (1994). Developments in Nanotechnology. Foundations of Ultraprecision Mechanism Design 2 (Taylor & Francis). p. 323. ISBN 978-2-88449-001-6.
• Phelps, F. M. III (1966). "Airy Points of a Meter Bar" 34 (5). American Journal of Physics: 419–422. Bibcode:1966AmJPh..34..419P. doi:10.1119/1.1973011.
• Lewis, Andrew John (2002) [1993]. "Appendix C: Flexing of length bars". Absolute length measurement using multiple-wavelength phase-stepping interferometry (2 ed.). Department of Physics, Imperial College of Science, Technology and Medicine, University of London. Retrieved 2015-10-13.