Blogs

• News
• |
• Art
• |
• History
• |
• Food and Travel
• |
• Science
 Where paleontology meets pop culture Meet the members of the tangled human family tree How human ingenuity is changing the way we live Ideas, news and discoveries from the world of science

November 3, 2010

Replace the Kilogram!

The Denmark-owned replica of the kilogram standard (via wikipedia)

Here’s an easy question: What is a kilogram?

A. 1000 grams
B. a standard unit of mass (often ignored in the United States)
C. a platinum-iridium cylinder kept in a vault in Sèvres, France
D. all of the above

The answer is D, of course. And that’s a problem for the scientists in charge of the science of measurement. You see, that little cylinder hidden in its French vault keeps changing. When scientists compare the mass of the 130-year-old International Prototype Kilogram (as it is officially known) with that of replicas made from the same material and stored in similar conditions around the world, they don’t match up. Those little cylinders are likely undergoing material decay, but since the IPK is officially a kilogram no matter how it compares to its replicas, there’s no way to tell what is going on. And no way to know what a kilogram really is.

It’s a conundrum scientists need to solve because the kilogram is one of a few base units (like the second and the meter) that are used in the definitions of other, more complex units, such as those used to measure temperature, electricity or density. If the kilogram is off, even a little, critical measurements in science, engineering and commerce get messed up.

The kilogram is the only standard that has this problem; years ago scientists replaced other standards based on physical objects, like the meter, with definitions based on fundamental constants in nature, such as the speed of light. And there have been plenty of potential replacements proposed for the kilogram over the years (Wikipedia has a good list; my favorite is the shiny sphere of pure silicon).

The U.S. National Institute of Standards and Technology is backing a definition based on the Planck Constant, a number from quantum mechanics. It’s a more complex definition than a simple cylinder, but it’s unlikely to change over the next century.

NIST submitted its definition last month to the International Committee for Weights and Measures, one of the advisory bodies that oversees such things, and it approved the change. The matter will next be taken up in a resolution by the General Conference on Weights and Measures, the international group that has the power to replace the kilogram, at its next meeting, in October 2011. If the resolution passes, the definition could change by the end of the decade.

“This is actually an advance in the science of measurement,” NIST spokesman Ben Stein [told CBS News]. “This is the next phase. There have been discoveries in measuring the constants of nature, such as the speed of light, the amount of energy in a quantum of energy….we know them pretty well now. These numbers are not changing, so why not define the units in terms of these constants? They’re from the natural world.”

(HT: Geeks Are Sexy)

***

1. [...] This post was mentioned on Twitter by Ron Gilbert, Camilo Nieto, S. Eric Rhoads, Celuloide, Claire Bryce and others. Claire Bryce said: RT @grumpygamer: Apparently the kilogram is not really a kilogram anymore, a fact I plan on bring up at the deli today: http://bit.ly/9DMiqN [...]

2. [...] Link | Image: Clipart [...]

3. WilliamB says:

Why isn’t the International Prototype Kilogram the same as the mass of one liter of water?

4. Stuart says:

The reason why a litre of water is no longer used as the standard for the kilogram is because it’s highly inaccurate by today’s standards (though it was initially huge step forward in the time of regional inches, feet, and pounds). The drift between the kilogram artifact masses (~70 micrograms) is a major source of concern regarding the definition of mass. What they’re trying to do is to define the kilogram in the terms of something that is far more accurate (perhaps a thousand times more accurate). The main reason for this is that the uncertainty of the kilogram propagates through all the units based on it (quite a few). Using a litre of water as a basis for the kilogram would be a significant step backward in terms of accuracy; there is too much variability in that definition to achieve the accuracy of the prototype kilogram (and even the prototype kilogram’s uncertainty is undesirable).

5. _ul_was here says:

answer is quite simple : 1Kg is 1.000ml of pure water: ( density of water=1) .
1 liter of water is a cube which measures 10x10x10 centimeters = 1 decimeter cube .
. i’ve lost the definition of the meter ( calculated fraction of the lightspeed), but it can be found easily.
thus , the definition of a Kilogram ( or a gram) is directly relative to the speed of light , wich is supposed to be constant

Comments are moderated, and will not appear until Smithsonian.com has approved them. Smithsonian reserves the right not to post any comments that are unlawful, threatening, offensive, defamatory, invasive of a person's privacy, inappropriate, confidential or proprietary, political messages, product endorsements, or other content that might otherwise violate any laws or policies.