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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.
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.
No need to feel left out. The definition of the pound is also updated by this:
The international pound has been defined as exactly 0.45359237 kg.
https://xkcd.com/2073/
By it's very definition a balance in independent of gravity. The balance will remain the same and that 1Kg *of mass* will work just as well if you try this experiment on Jupiter.
Yeah, they should have left out the Kibble balance part. That's more confusing than illuminating. In terms a layman can understand, kilogram is now defined the same way meter is: by defining a related physical constant to be an exact value.
Meter is defined today not by a physical object, but by defining speed of light to be exactly 299,792,458 m/s (that is, in significant figure terms, there are significant zeros following the decimal for-ever). With time defined by the atomic clock standard, this definition of speed of light also defines what a meter is (and many different experimental arrangements can be designed to use this relationship to actually calibrate real object).
With the vote today, kilogram is now defined by defining Planck's constant to be exactly 6.626070040 * 10^-34 kg*m^2/s (um, Wikipedia's not updated yet; the exact value they chose might be different from this number; important thing is that the value they chose now has infinite number of significant figures). Since meter and seconds are already defined, defining this constant defines the kilogram, and clever experimentalists can come up with better methods than Kibble balance for calibrating any local kilogram standards.
P.S. BTW, for scientists working in precision measurement area (the area NIST and NSF funds as they relate to fundamental science), this is an exciting news. It's a validation of accomplishments of their field, on the same (or possibly greater) magnitude was when atomic clock standard was adopted for the definition of second.
Our friend at Veritasium does an excellent job breaking this down:
https://www.youtube.com/watch?...
I tend to rant.
By it's very definition a balance in independent of gravity. The balance will remain the same and that 1Kg *of mass* will work just as well if you try this experiment on Jupiter.
But a Kibble Balance isn't that sort of a balance. It's a "single pan balance" which balances gravitational acceleration against acceleration caused by a magnetic field. So the Kibble Balance is very sensitive to changes in the gravitational field.
Luckily, it's possible to measure the local force of gravity with extreme precision, without reliance on the definition of the kilogram. It's done with dropping-mass gravimeters that measure the deflection of a laser beam, so it only relies on standard units of distance and time, and the speed of light as measured in terms of those units, not on the definition of mass. Obviously this is crucial or else you'd need a definition of a kilogram in order to calibrate your Kibble Balance.
So you can do this on Jupiter just fine, but you first have to measure the local gravitational field and adjust the amount of current you feed the Kibble Balance to balance against your kilogram test mass.
The biggest downside of this new method of defining the kilogram is that turning the definition into a measurement is incredibly precise and difficult work. It's so expensive to do correctly that for the foreseeable future there will probably only be a handful of wealthy countries who bother to do it. This means that for practical work, the definition will just be used to calibrate the exemplars that are used today, and everything else will continue as always. But it does mean that we now have a definition which is independent of those exemplars and guaranteed to be perfectly unchanging as long as the Planck constant remains constant.
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