Kilogram | Wikipedia audio article
Channel: wikipedia tts
Date: 08/12/2018

This is an audio version of the Wikipedia Article:
Kilogram


00:01:50 1 Definition
00:05:25 2 Name and terminology
00:08:50 3 Mass and weight
00:10:11 4 iKilogramme des Archives/i
00:12:16 5 International Prototype of the Kilogram
00:15:25 5.1 Copies of the international prototype kilogram
00:17:49 5.2 Stability of the international prototype kilogram
00:25:56 5.3 Dependency of the SI on the IPK
00:30:22 6 Redefinition agreed on 16 November 2018
00:31:02 6.1 History of redefinition
00:33:52 6.2 Kibble balance
00:40:51 7 Alternative approaches to redefining the kilogram
00:41:26 7.1 Atom-counting approaches
00:41:35 7.1.1 Avogadro project
00:46:42 7.1.2 Carbon-12
00:49:46 7.1.3 Ion accumulation
00:53:57 7.2 Ampere-based force
00:56:02 8 SI multiples
00:58:38 9 See also



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SUMMARY
=======
The kilogram or kilogramme (symbol: kg) is the base unit of mass in the International System of Units (SI). Until 20 May 2019, it remains defined by a platinum alloy cylinder, the International Prototype Kilogram (informally Le Grand K or IPK), manufactured in 1889, and carefully stored in Saint-Cloud, a suburb of Paris. After 20 May, it will be defined in terms of fundamental physical constants.    
The kilogram was originally defined as the mass of a litre (cubic decimetre) of water. That was an inconvenient quantity to precisely replicate, so in 1799 a platinum artefact was fashioned to define the kilogram. That artefact, and the later IPK, have been the standard of the unit of mass for the metric system ever since.
In spite of best efforts to maintain it, the IPK has diverged from its replicas by approximately 50 micrograms since their manufacture late in the 19th century.  This led to efforts to develop measurement technology precise enough to allow replacing the kilogram artifact with a definition based directly on physical phenomena, a process which is scheduled to finally take place in 2019. 
The new definition is based on invariant constants of nature, in particular the Planck constant which will change to being defined rather than measured, thereby fixing the value of the kilogram in terms of the second and the metre, and  eliminating the need for the IPK.  The new definition was approved by the General Conference on Weights and Measures (CGPM) on 16 November 2018. The Planck constant relates a light particle’s energy, and hence mass, to its frequency. The new definition only became possible when instruments were devised to measure the Planck constant with sufficient accuracy based on the IPK definition of the kilogram.