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Background Information on the Proposal
The new paper proposes that the next General Conference on Weights and Measures adopt either one of two definitions for the kilogram to effectively fix its value by selecting a specific value for either the Planck constant or the Avogadro constant. Two types of experiments are leading the effort to realize either of these definitions. The first one measures a kilogram against the amount of magnetic force required to balance a 1-kilogram mass against the pull of Earth’s gravity. The experimental apparatus used to make the measurement is called a watt balance. A kilogram mass is placed on a balance plate that is surrounded by a coil of copper wire, which in turn is surrounded by a coil of superconducting wire. Magnetic fields produced by sending electricity through the coils push on the balance plate to offset the artifact's weight. The amount of electric current and its voltage then is used in defining a kilogram. Electrical power can be related to the Planck constant, defined as the ratio between the frequency of an electromagnetic particle such as a photon of light and its energy. This experimental method of defining the kilogram relies on selecting a fixed value for the Planck constant, which is currently determined experimentally based on the fixed value of the kilogram artifact.

The second proposed way to re-define the kilogram involves counting the number of atoms of a specific atomic mass that equal the mass of 1 kilogram. This method involves using X-rays to measure the spacing between atoms in a perfect crystal to estimate the volume of each atom, and measuring the density of the crystal and weight of the atoms, to arrive at a number of atoms equivalent to 1 kilogram. This experimental method for defining the kilogram depends on selecting a fixed value for the Avogadro constant, which describes the number of atoms or molecules in a specified amount of a substance.

Either method for redefining the kilogram would have positive ripple effects on many other physical constants such as the charge and mass of the electron. The specifics would depend on which definition is chosen. Either way, the uncertainties of some constants would be reduced to 0, while others would be reduced by factors ranging from about 7 to over 1,300, according to the paper.

Physical constants are used by scientists and engineers to make innumerable types of calculations, and also are used in designing and calibrating quantum-based measurement systems. Such systems are becoming more important in technology development and the growth of trade that relies increasingly on electronic testing, quality control and environmental monitoring. Reduced uncertainty in the values of many quantum-based constants also may stimulate new experimental and theoretical work aimed at testing fundamental theories of physics.

The last time a base unit of the SI was redefined was in 1983, when the meter was formally redefined in terms of a new value for the speed of light. In this case, the scientific community decided to fix the value for a physical constant—the speed of light—at a specific value so that length measurements could be made with greater precision. The speed of light is equal to the wavelength multiplied by its frequency. The meter previously was defined by wavelength only; the new definition was made possible by more precise frequency measurements. By redefining the meter to equal the distance light travels in a vacuum in a specific time, scientists were able to determine distance by measuring time and frequency, the two quantities of nature that can be measured experimentally with the greatest precision.

In a similar way, redefinition of the kilogram by assigning a specific value to the Planck constant or to the Avogadro constant will allow scientists to make mass calculations for a wide range of quantum physics problems much more precisely than previously possible.

The authors stress that regardless of which redefinition of the kilogram is chosen, efforts should continue to reduce the measurement uncertainty of both the watt balance and X-ray crystal experiments. Currently, both methods are 10 to 100 times less precise than the measurement uncertainty produced when comparing the kilogram artifact to national standards. What’s more, the results of the two approaches disagree by nearly 1 part per million. Eventually, when uncertainties of experiments with the watt balance or X-ray crystal density method reach a sufficiently low level, the cylinder artifact would no longer be needed. [...]

http://www.physorg.com/news3178.html

7 to over 1300?!

What I now wonder is, why during all of this time, no scientist has devised her own definition of a kilogram for use in a calculaton only. This would allow her to take advantage of this greater resolution immediately instead of having to wait...for what exactly? The sort of maths that is done by string theorists and that sort is purely theoretical; surely they are already using these natural effect only definitions for mass? Chemists in every school world wide titrate in examinatoins using the mole - in fact, you can do a calculation for a theoritcal titration on paper thanks to the mole (or NO THANKS if you are a student that hasnt revised---har har!)

Its simply odd that a Kilogram...that there is 'The Kilogram' in a jar in France, and that this is what defines a Kilogram. Its rather like the foot of the king defining the length of the foot, as they used to do it in England, only worse, because the nature of the universe is being spelled out to everyone based on measurements and inferences drawn from this example mass.

And this is REAL!