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Kilogram to be overhauled?


October 28, 2010

A rendering of the platinum-iridium cylinder, that currently defines the weight of a kilogram (Image: Greg L)

A rendering of the platinum-iridium cylinder, that currently defines the weight of a kilogram (Image: Greg L)

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It’s one of those things where if you think about it too much, your head might explode. We know there are 1,000 grams in a kilogram, and 1,000 kilograms in a metric ton, but how was it ever decided what any of these units actually physically weighed? Well... the modern metric system is part of the Système International d'Unités (International System of Units) or SI. It states that a kilogram is the weight of one specific 130 year-old platinum-iridium cylinder, which is kept in a vault at the International Bureau of Weights and Measures in France... and no, don’t ask how they knew if they’d got its weight right, when they were making it. The problem is, that cylinder’s mass changes slightly over time. Now, a worldwide effort is under way to change the definitive weight of a kilogram to something more permanent.

The kilogram is the only “base unit” in the SI that is still defined by a physical artifact – the other base units are the second (time), the meter (length), the ampere (electric current), the kelvin (thermodynamic temperature), the mole (amount of substance) and the candela (luminous intensity). That said, the ampere, mole and candela are all defined by their relationship to the kilogram – for instance, a mole is defined as the number of carbon-12 atoms whose total mass equals 12 grams. Therefore, these units are also ultimately linked back to the ever-so-slightly-changing French block of metal.

Researchers at the National Institute of Standards and Technology (NIST) are proposing that the kilogram now be defined in terms of the Planck value h, which is a constant in quantum physics. In order to establish an accurate value for h, they have been performing experiments with a watt balance (an electromechanical instrument that measures weight by the strength of an electric current) and trying to determine the mass of one mole of silicon atoms.

A resolution for the new SI will be submitted for consideration at the General Conference on Weights and Measures, which takes place next October. If it passes, the new system could be in use later in the decade.

About the Author
Ben Coxworth An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away. All articles by Ben Coxworth

Just what we need, yet another system of measurement.


I could swear that I read a National Geographic article within the last decade that was talking about the exact same thing, but with respect to the meter and how scientists were trying to define it as a fraction of the speed of light in a vacuum because the physical artifact is changing ever so slightly over time.

Facebook User

Back in middle-school the teacher told us that 1 kilogram is the mass of 1 liter of water at 4 degrees Celsius. It would of-course bind it to the meter, but seems easier than what they are trying to do right now.

Facebook User

Ill have the kilo (give or take) of platinum/iridium once they dont need it anymore!


Here in the United States, we don\'t care a hell of a lot about this. We\'re sticking with those other major superpowers--Liberia and Myanmar--and staying off the metric system. Ralph L. Seifer, Long Beach, California.


Lang, you are right, the Kg was defined as 1 cubic decimeter of water at 4C... The only reason that the platinum replica was made was to calibrate the scales.


So, will one cubic centimeter of water still weigh one gram? How about we change the value of 3 to equal 1/3 of the circumference of a circle? ;)

Facebook User

@rseifer totally agree with you, I think it\'s time you smart citizens start a revolutions and push Obama to change, it\'s really absurd that you still not use the metric system

Francesco Baldacchini

rseifer, scientist in the US use SI metric units for research and publication don\'t they? I thought it\'s only in common usage where imperial measurements are still the norm.

Earl Leonard

Here in progressive England we moved to the French system, even dragging shopkeepers to court for selling goods in pounds weight. Along the motorways are coloured posts spaced at 100 metres [meters for our American cousins], but raise your eyes, and what do you see? \"London, Manchester, or whatever.........so many miles\"! America, stay as you are, what can the frog eaters teach you?


rseifer, \"The United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard. Since 1 July 1959, the international avoirdupois pound has been defined as exactly 0.45359237 kilogram\"

So you are in effect using the kilogram as your unit of mass.

Maurice Childs

Maurice Childs, the USA are NOT in effect using the kilogram as their unit of mass, they use the pound. Your 0.453...whatever is merely the equivalent using a foreign system.


...or they are in fact using the pound for their unit of mass! Ie. 1 Kilo = 2.204623 pounds.

I love the metric system for so many things but still like the Mile and the Foot. Because I'm used to them my mind processes them when comparing things.

I can visualize someone who is 6 Feet tall or how far away someone who lives a mile from me is. If you tell me that someone is 182.88 centimetres my mind just runs a constant band of ????? through it.

Yes, I understand with time we will all get used to it. If we change that let's go ahead and change a bunch of spelling to conform as well and why do we have countries who drive on the left and others on the right. Let's get together and all be the same!

just kidding, I like the differences and nobody stops an individual from using the metric system in their personal life. If there was a pressing reason to change (money, money, money) I suspect people would on their own. I would hate to see us standardize our measurement system before we standardize our language!

Dr. Veritas
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