Czasopismo | ELEKTRONIKA - KONSTRUKCJE, TECHNOLOGIE, ZASTOSOWANIA | Rocznik 2013 - zeszyt 6

Long-term stability of the national standard of length

Dariusz CZUŁEK

nr katalogowy: 77240

The current definition of length unit, accepted by 17th General Conference on Weights and Measures (CGPM) in 1983, is related to the unit of time . second. According to this definition, the unit of length . 1 m . is the length of the path travelled by light in vacuum in 1 / 299 792 458 seconds. The International Committee of Weights and Measures (CIP M) has established an extended list of quantum radiation sources, recommended to the practical . .mise en pratique?h . realization of the new definition. This list includes both radiation of various types of lasers and of spectral lamps. It is successively expanded. As the new definition of the meter does not provide the way of length measurement, the CIP M added three practical methods for its realization [1.4]: 1) by means of the length l of the path travelled in vacuum by a plane electromagnetic wave in a time t; this length is obtained from the measured time t, using the relation: (1) where: t . measured time, co . speed of light in vacuum 299792458 m/s. This method is possible to apply only in measurement of large distances, e.g. distance from the earth to the moon. 2) by means of the wavelength in vacuum ?É of a plane electromagnetic wave of frequency f; this wavelength is obtained from the measured frequency f using the relation: (2) where: f . measured frequency, co . speed of light in vacuum 299792458 m/s. This method is based on the measurement of the a light source frequency by comparing it with the frequency of cesium clock used in the definition of the unit of second and then determining the wavelength according to the mathematical formula. The wavelength is used as a reference in length measurement. 3) by means of one of the radiations from the list mentioned above (CIP M recommendation), whose stated wavelength in vacuum or whose stated frequency can be used with the uncertainty shown, provided that the given specifications and accepted good practice are follow[...]

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