Planck's constant

Nov 1, 2007 16:12 GMT  ·  By

The gram was decreed in France on 7 April 1795, to be equal to the absolute weight of a volume pure water to a cube of one hundredth of a meter, at the temperature of melting ice. The kilogram represents the multiple of a gram. The kilogram is the SI base unit for measuring mass.

In 1889, the SI redefines the kilogram, to be equal to the mass of the International Prototype Kilogram, made of a platinum-iridium alloy, machined into a right circular cylinder, of equal height and diameter of 39.17 mm. The kilogram is an essential unit in the SI, multiple other measuring units being defined with the help of the kilogram, such as the newton representing the force necessary to accelerate the kilogram by one meter per second?.

Scientists at the National Physical Laboratory, released results that may affect the stability of the SI, in such a way that it might be the last SI based on a human manufactured object. The next meeting of the General Conference on Weights and Measurement, programmed to take place this month, will probably include a proposal to prepare the redefinition of some of the base units.

Though, by definition, the error of the measured value of the IPK's mass is exactly zero, any change in its mass can be observed over time by comparing it with its official copies stored all over the world, through a process called periodic verification.

Using a watt balance machine, researchers at NPL managed to measure Planck's constant, and recorded results with uncertainties of 36 parts in a billion. The watt balance is an ampere balance, with an extra calibration step, that eliminates the effect of geometry. To eliminate the uncertainties, NPL started work on Mark II watt balance, to reveal the possible source of this discrepancy.

Further researches might establish a definition for the kilogram, based on Planck's constant, that could replace the current International Prototype for the kilogram.