The kilogram is one of seven base units in the International System (SI) used in science, commerce and everyday life.
However, it is the only one still defined by a physical object – a lump of metal, known as the International Prototype, sitting in a vault in France. All the others have moved with the scientific times and are defined in terms of a fundamental constant of nature so anyone anywhere can reproduce them and they do not change over time.
Under the auspices of the International Bureau of Weights and Measures (BIPM), near Paris, the decision has been made that international effort will focus on two ways of re-defining the kilogram: one of which involves making a perfect sphere from a single crystal of exceptionally pure silicon.
The work will be done with the close cooperation of Australia’s National Measurement Institute (NMI) and CSIRO’s Australian Centre for Precision Optics (ACPO), which share the same site in the Sydney suburb of Lindfield.
While a physical object will still be necessary for calibrating scales and balances, the silicon atoms in the sphere will always remain the same. It is for this reason that the scientists working on what’s known as the Avogadro Project are collaborating to determine what is effectively the number of atoms in a sphere. Once the number of atoms is known, the definition of the kilogram can be based on it from then on.
The best sphere the ACPO team has made had a total out-of-roundness of 35 nanometres. That is, the diameter varies by an average of only 35 millionths of a millimetre, making it probably the roundest object in the world.
“The only people who can make what is likely to be the roundest object in the world are our colleagues at CSIRO’s ACPO,” Dr Barry Inglis, Chief Executive of NMI says.
Mr Alain Picard of the BIPM, who is bringing the silicon to Australia, says: "My laboratory has maintained the International Prototype kilogram since 1889. We are really pleased that this international collaboration will finally let us improve the definition of the unit of mass by basing it on a physical constant."
Ms Katie Green is part of ACPO’s team of expert optical engineers who will fabricate the spheres and Mr Walter Giardini of NMI lends skills in precision measurement to the project.
It has taken three years to produce the 20 cm long cylinder of silicon. The special silicon, known as monoisotopic silicon, was made in Russia and grown into a near perfect crystal in Germany. It will take something like twelve weeks to make one sphere 93 mm in diameter (the team will make two).
Source: CSIRO Australia
Related stories:
Sandia Lab weighs in on new definition for kilogram
The kilogram is losing weight and many international scientists, including some at Sandia National Laboratories, agree that it’s time to redefine it. Scientists are hoping to redefine the kilogram by basing it on standards of universal constants rather than on an artifact standard.
A Better Definition for the Kilogram? Scientists Propose a Precise Integer Number of Carbon Atoms
How much is a kilogram? It turns out that nobody can say for sure, at least not in a way that won’t change ever so slightly over time. The official kilogram – a cylinder cast 118 years ago from platinum and iridium and known as the International Prototype Kilogram or “Le Gran K” – has been losing mass, about 50 micrograms at last check. The change is occurring despite careful storage at a facility near Paris.
Combating Friction and Stiction
Micro-electro-mechanical systems, popularly referred to as MEMS, in small electronic devices often fail because of adhesion and stiction – the attractive force between the surfaces of interacting parts. University of Arkansas researchers have developed a surface-topography engineering method that reduces these forces and will help microscopic parts interact and function smoothly.
New standard mass made with ISTC help
This program began three years ago and it involves scientists from eight countries. The task is immense. A new standard mass will be created with maximum possible precision on a modern level of development for world science and technology.
Of the multitude of measurable values, the most important and the most basic, are time, length and mass. Standards of mass and length were made over one hundred years ago, in the form of the standard kilogram and the standard meter; they are held at the International Bureau of Weights and Measures in Paris. The standard hour was determined through the period of the Earth’s rotation.
Scientists grow 'nanonets' able to snare added energy transfer
Using two abundant and relatively inexpensive elements, Boston College chemists have produced nanonets, a flexible webbing of nano-scale wires that multiplies surface area critical to improving the performance of the wires in electronics and energy applications.
Study shows dual-career academic couple hires on the rise
(PhysOrg.com) -- He has a fancy-sounding title, but Robert Weisberg gives himself a blunt job description when explaining what he does as "special assistant to the provost for faculty recruitment and retention."
An Unconventional Metal
The semiconductor silicon and the ferromagnet iron are the basis for much of mankind's technology, used in everything from computers to electric motors. In this week's issue of the journal
Nature (August 21st) an international group of scientists, including academic and industrial researchers from the UK, USA and Lesotho, report that they have combined these elements with a small amount of another common metal, manganese, to create a new material which is neither a magnet nor an ordinary semiconductor.
Large Hadron Collider set to unveil a new world of particle physics
(PhysOrg.com) -- The field of particle physics is poised to enter unknown territory with the startup of a massive new accelerator--the Large Hadron Collider (LHC)--in Europe this summer. On September 10, LHC scientists will attempt to send the first beam of protons speeding around the accelerator.
