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http://www.theage.com.au/articles/2007/06/14/1181414466901.html?from=top5

SECURELY tucked away inside a French vault is a lump of metal known as the International Prototype. A mixture of platinum and iridium, it was made in the 1880s to define the mass of a kilogram.

But work by a team of Australians could help pave the way for the retirement of this century- old prototype, as weight and measurement experts across the globe work towards a more scientific definition of the kilogram.

The project requires the development of perfect silicon spheres, and optical engineers at CSIRO's Australian Centre for Precision Optics — considered world leaders in the craft — are doing their part.

Scientists will use the spheres to determine how many silicon atoms make up a kilogram, and this will be used as the new definition — bringing the kilogram into line with other base units such as the metre and the second, which are all defined by physical constants.

"It's really an atom-counting exercise … and we'll come up with a new definition of the kilogram based on atoms, rather than based on the thing in Paris," explained Walter Giardini, of Australia's National Measurement Institute.

CSIRO's optical engineers will form two perfect spheres from a 20-centimetre cylinder of exceptionally pure silicon that arrived in Australia last night. The silicon, which has taken three years to produce, was made in Russia and grown into a near-perfect crystal in Germany.

The precision optics centre, located in the Sydney suburb of Lindfield, has already made about a dozen spheres for what is known as the Avogadro Project — with the most perfect sphere so far just 35 nanometres away from being perfectly round.

This means the diameter of the sphere varied by an average of only 35 millionths of a millimetre, making it a top contender for the title of the roundest object in the world.

A spherical shape was chosen for the project because it has no edges that might be damaged, and the volume can be calculated by using its diameter.

Optical engineer Katie Green, who will be involved in the precise cutting, grinding and polishing of the spheres, said it was exciting to be a part of a high-profile international project.

"It's probably going to take around three months' work, start to finish," she said. "It's been a number of years waiting for this material to be completed, so we're definitely looking forward to seeing it in the flesh, so to speak."

After the completion of the spheres, the silicon objects will be sent around the world to be measured and analysed by scientists.

http://www.csiro.au

http://www.bipm.fr
 

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no we don't - we know a kilogram is equal to that platnum-iridium rod. Whilse the metre has been redefined based on the how far light travels in a vacuum in a given period of time, and a second is redefined based on a given number of oscillations of a Caesium-133 atom, a gram is only defined by this rod and has no specific atomic mass.
 

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The weight of the international prototype would be different if you place it in any other places on Earth as there is different pressure due to gravity.

gravity x weight = mass.
F = m x a.
 

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Isn't a gram just the weight of one cubic centimetre of water? So if we have a scientific definition of length then through that also one for a kilogram?
 

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The weight of the international prototype would be different if you place it in any other places on Earth as there is different pressure due to gravity.

gravity x weight = mass.
F = m x a.
Not quite.

F stands for force, which means weight, measured in Newtons.
m stands for mass, measured in kilograms. Mass is what everyone who's not a scientist or engineer actually means when they say weight.
a stands for acceleration, measured in metres per second and for a still object would be the acceleration caused by gravity, or 9.8 m/s^2.

Therefore it is force (or weight) which is proportional to gravity.

So you're right that the weight of the international prototype can vary but its mass stays constant.
 

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Isn't a gram just the weight of one cubic centimetre of water? So if we have a scientific definition of length then through that also one for a kilogram?
Depends for a start on temperature, also on pressure ... and water does funny things ... consider the way it freezes, for instance. Of course, if it isn't pure water, that's even worse ....
 

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The metric systems definitions worked well, I believe, for most of the past. However now science as advanced to the point that we are able to measure quantitites more accurately than what the original definitions specified. We therefore need to redefine the units to a greater level of accuracy and we need to use a specification that is universal and constant.

Water was used to derive a system of weight (mass) measurements from the base unit of length. However water is not a constant and so the weight values can vary depending on the qualities of the water used.



And Miliux, you went a bit wrong with your post because you got the formula the wrong way around. It should be: weight = mass x gravity. This why if an astronaut was stand on a set of scales on the moon, the scales would read less than on Earth. Not because his mass has changed but because the gravity multiplier has, and therefore is weight also has. Mass is measured in kilograms and weight should not be but is done so mainly for convienience as it is really just a laypersons term. Scientific calculations always specify mass and never weight.
 
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