Of the seven base SI units, mass is the only one still defined by a material artefact.

Originally, 1 kilogram was defined as the mass of 1000 cm3 of water at 4 °C. Based on this, the international prototype kilogram (IPK) was made in the 1880s from an alloy of 90% platinum and 10% iridium. This alloy is very hard, corrosion resistant and of high density – twice that of lead.

The kilogram is in the shape of a cylinder with a height equal to its diameter (3.9 cm), which allows for a minimum surface area to be exposed to the air. It is stored in a triple-locked vault 8 metres below the offices of the International Bureau of Weights and Measures (BIPM) in Paris.

Problems with the IPK

Although there is a strong historical attachment to the IPK, there are a number of practical problems associated with its continued use as the mass standard:

  • The IPK is used to calibrate six official copies of the kilogram, which are used to calibrate the copies owned by national institutes and the BIPM. In turn, these are used to calibrate stainless steel standards for use by member nations. This long chain of mass measurement and calibration inevitably results in loss of accuracy.
  • Although the standard kilogram is housed under three nested glass bell jars with a carefully controlled atmosphere, there is evidence that there has been a change in mass of at least 50 micrograms over the last 120 years.
  • The stability of mass standards based on the IPK is being overtaken by advances in weighing technology.
  • If the IPK is damaged in any way, by definition, the mass of the universe changes. This is clearly absurd and has prompted a search for a suitable replacement standard based on physical constants.

Several international metrology laboratories, including New Zealand’s Measurement Standards Laboratory (MSL), are investigating alternatives. Two entirely different projects are under way – the Avogadro Project for measuring the Avogadro constant and the Watt Balance Project for measuring the Planck constant.

Avogadro Project

This project involves international collaboration between scientists from Germany, Italy, Belgium, Japan, Australia (CSIRO) and USA. It is based on a physical constant known as the Avogadro constant, which is defined as the number (NA of carbon atoms present in exactly 12 grams of carbon-12. Because atoms are extremely small, this number is huge – 6.022 141 5 x 1023.

Instead of carbon-12, attention has now been focused on silicon-28. Silicon is stable at room temperature, has a well known crystal structure and is machinable to a very high degree of precision. It is now possible to produce very large and very pure crystals of silicon. This technological leap has enabled scientists to produce high-precision 1 kg spheres of silicon. By using well known X-ray crystallographic methods, it is possible to determine the actual number of atoms present in the 1 kg sphere. From these experiments, it is possible to fix a more accurate value for the Avogadro constant.

The Watt Balance Project

A watt balance relates mechanical power to electrical power by comparing the gravitational force on a mass with the electromagnetic force on a current-carrying coil in a magnetic field. Bryan Kibble, a UK metrologist, first proposed this in 1975, and it is one of the favoured approaches to replacing the current mass standard because it will allow the kilogram to be redefined in terms of the Planck constant (h).

In 1900, German physicist Max Planck discovered a relationship between the radiation emitted from a hot object and its temperature. This led Planck to propose that the energy emitted by a hot object comes in very small packets of energy called quanta.

The relationship linking the energy of a given packet and the frequency of its radiation is E = hf, where:

  • E is the energy of the packet
  • f is the frequency of the radiation
  • h is the Planck constant.

Like the speed of light (c), the Planck constant (h) is a fundamental physical constant.

  • c = 3.00 x 108 m/s
  • h = 6.626 x 10-34 Js

Metrologists from MSL are involved with this project, and like the Avogadro Project, it involves international collaboration between Canada, China, France, New Zealand, Switzerland, USA and the BIPM (the home of the SI system).

Nature of science

Science demands and relies upon evidence most often gathered by taking measurements. The system of units used to take these measurements with the required degree of accuracy needs to be robust, reliable and compliant with technical advances in instrument design.

    Published 17 August 2011