Chapter 7-PRINCIPLES OF MEASUREMENT 



Table 7-4. —Selected Values of Mechanical Units in 

 Metric Systems of Measurement. 



standard meter bar was maintained at the 

 National Bureau of Standards in Washington, 

 D.C. 



Note that the standard of length was the 

 meter even for countries that were not on the 

 metric system. The yard was defined in terms 

 of the meter, 1 yard being equal to 0.9144 meter. 



In 1960, the standard of length was changed 

 by international agreement to an atomic con- 

 stant: the wavelength of the orange-red light 

 emitted by individual atoms of krypton-86 in a 

 tube fillecl with krypton gas in which an elec- 

 trical discharge is maintained. By definition, 1 

 meter is equal to 1,650,763.73 wavelengths of 



the orange-red light of krypton-86, and 1 inch 

 is equal to 41,929.399 wavelengths. A device 

 called an optical interferometer is used to de- 

 termine the number of the wavelengths of the 

 orange-red light of krypton-86 in an unknown 

 length. 



Mass 



The standard of mass is the mass of a cylin- 

 der of platinum-iridium alloy defined as having 

 a mass of 1 kilogram. The international standard 

 kilogram mass is kept at the International 

 Bureau of Weights and Measures in France. The 



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