1. Speed of light (defines the meter): exactly 299792458 m/s
2. Josephson's (defines the 1990 standard volt):
2e/h-bar, angular format—967195.8pi × 109 per
second per volt
2e/h, cyclic format—483597.9 × 109 per second per
volt
3. Frequency definition of kilogram: 135639274×1042 [cycles] per second (proposed by Mohr and Taylor, chair and former chair of CODATA)
4. Logically equivalent exact value of Planck's constant:
h, cyclic format— 2997924582/135639274 ×
10-42 joule second.
h-bar, angular format—2997924582/271278548pi ×
10-42 joule second.
The metric system is in transition towards definitions based solely on the unit of time (atomic clock second) and adopted exact values of certain constants.
A notable step was taken in 1983 when the meter was defined by specifying that the standard speed of light be exactly 299792458 meters per second.
In 1990 the BIPM established its voltage standard by specifying that Brian Josephson's constant (2e/h, the operating characteristic of modern gauges) be exactly 483597.9 billion cycles per second per volt. Although this standard is in use the official definition has not yet been changed to be consistent with practice.
In 1999 the CGPM called for a redefinition of the kilogram along the lines of the 1990 standards, and the following year two leading members, Mohr and Taylor, supplied this proposed redefinition.
The kilogram is the mass of a body at rest whose equivalent energy equals the energy of a collection of photons whose frequencies sum to 135 639 274×1042 [cycles per second].
Mohr and Taylor also suggested that the larger Planck's constant be made exactly equal to 2997924582/135 639 274 × 10-42 joule second. This value follows from their suggested definition of the kilogram.
For more on-line about their proposals, see their August, 2000 article in the Electronic Journal of Differential Equations: Quantum electrodynamics and the fundamental constants, by Peter J. Mohr and Barry N. Taylor.
http://ejde.math.swt.edu/conf-proc/04/m1/mohr.pdf
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