**Fixation**

As mentioned above, the numerical value of the Planck constant depends on the system of units used to describe it. Its value in SI units is known to 50 parts per billion but its value in atomic units is known *exactly*, because of the way the scale of atomic units is defined. The same is true of conventional electrical units, where the Planck constant (denoted *h*_{90} to distinguish it from its value in SI units) is given by

with *K*_{J–90} and *R*_{K–90} being exactly defined constants. Atomic units and conventional electrical units are very useful in their respective fields, because the uncertainty in the final result does not depend on an uncertain conversion factor, only on the uncertainty of the measurement itself.

There are a number of proposals to redefine certain of the SI base units in terms of fundamental physical constants. This has already been done for the metre, which is defined in terms of a fixed value of the speed of light. The most urgent unit on the list for redefinition is the kilogram, whose value has been fixed for all science (since 1889) by the mass of a small cylinder of platinum–iridium alloy kept in a vault just outside Paris. While nobody knows if the mass of the International Prototype Kilogram has changed since 1889 – the value 1 kg of its mass expressed in kilograms is by definition unchanged and therein lies one of the problems – it is known that over such a timescale the many similar Pt–Ir alloy cylinders kept in national laboratories around the world, have changed their relative mass by several tens of parts per million, however carefully they are stored, and the more so the more they have been taken out and used as mass standards. A change of several tens of micrograms in one kilogram is equivalent to the current uncertainty in the value of the Planck constant in SI units.

The legal process to change the definition of the kilogram is already underway, but it was decided that no final decision would be made before the next meeting of the General Conference on Weights and Measures in 2011. The Planck constant is a leading contender to form the basis of the new definition, although not the only one. Possible new definitions include "the mass of a body at rest whose equivalent energy equals the energy of photons whose frequencies sum to 135,639,274×1042 Hz", or simply "the kilogram is defined so that the Planck constant equals 6.62606896×10−34 J·s".

The BIPM provided *Draft Resolution A* in anticipation of the 24th General Conference on Weights and Measures meeting (2011-10-17 through 2011-10-21), detailing the considerations "On the possible future revision of the International System of Units, the SI".

Watt balances already measure mass in terms of the Planck constant: at present, standard mass is taken as fixed and the measurement is performed to determine the Planck constant but, were the Planck constant to be fixed in SI units, the same experiment would be a measurement of the mass. The relative uncertainty in the measurement would remain the same.

Mass standards could also be constructed from silicon crystals or by other atom-counting methods. Such methods require a knowledge of the Avogadro constant, which fixes the proportionality between atomic mass and macroscopic mass but, with a defined value of the Planck constant, *N*_{A} would be known to the same level of uncertainty (if not better) than current methods of comparing macroscopic mass.

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