Kilogram Gets a New Definition (bbc.com)

Scientists have changed the way the kilogram is defined. Currently, it is defined by the weight of a platinum-based ingot called "Le Grand K" which is locked away in a safe in Paris. On Friday, researchers meeting in Versailles voted to get rid of it in favour of defining a kilogram in terms of an electric current. From a report: The decision was made at the General Conference on Weights and Measures. But some scientists, such as Perdi Williams at the National Physical Laboratory in the UK, have expressed mixed feelings about the change. "I haven't been on this project for too long but I feel a weird attachment to the kilogram," she said. "I think it is such an exciting thing and this is a really big moment. So I'm a little bit sad about [the change]. But it is an important step forward and so the new system is going to work a lot better. It is also a really exciting time, and I can't wait for it to happen."

Le Grand K has been at the forefront of the international system of measuring weights since 1889. Several close replicas were made and distributed around the globe. But the master kilogram and its copies were seen to change -- ever so slightly -- as they deteriorated. In a world where accurate measurement is now critical in many areas, such as in drug development, nanotechnology and precision engineering -- those responsible for maintaining the international system had no option but to move beyond Le Grand K to a more robust definition.

Comments

excitement

[phik]

This is good, and very important. But exciting?

Re:excitement

[orlanz]

Yes. Very much so. Even to those outside the field. To officially, precisely, and accurately say something is a "kilogram" our current method was well over a hundred years old and basically sneaker net.

You took a clone of the official standard and compared it against that. The clone couldn't be directly touched, couldn't overly jostled, had an expiration, sits in a double vacuum, and had to be physically moved in proximity to what was being measured or, more likely, calibrated.

With this method, you can build a simple machine (Kibble balance) [nearly] anywhere in the universe, calibrate it, find its measuring relationship to the universal constants, and you can figure out the "kilogram". THAT IS AWESOME.

Layman terms: Before, you had to say "I want this many rocks worth of your stuff." Followed by handing over the rocks. Now you can say "We both know what 1 and 3 are. I want 15 of what you got."

The measurement of how much of something you have, the kilogram, officially stands at the same level as the Meter, Second, and Temperature. All of which are based on universal constants and not the measurement of some useless fool's biology.

Re:I prefer the pound

[swillden]

Wrong, the pound was defined as that in 1959. nothing changed here

Since the pound is defined in terms of the kilogram, changing the definition of the kilogram implicitly changes the definition of the pound as well.

Re:Gravitational Field Varies

[novakyu]

That's right.

P.S. Replying to say that Wikipedia did list the value of Planck's constant as newly defined: h = 6.62607015 * 10^-34 J*s. What's special about this value is that it's exact (note the lack of specification of experimental uncertainty), so if you were to write it as h = 6.626070150000000000000 * 10^-34 J*s, as annoying as that might be, you are not wrong (all those zeros—and more—are significant).

P.P.S. I guess they didn't update the table since the value is not effective until May.