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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.
This is good, and very important. But exciting?
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.
How many times do you need a very precise absolute measurement in drug development or nanotech ?
https://xkcd.com/2073/
I'm amazed at your constant ability to out-think all of our scientists. Your value is wasted as a Slashdot troll. If only they'd have seen you for your genius we'd have cured cancer by now.
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.
In this case, the balance works by applying a magnetic field on one side, not a weight, so it's not independent of gravity.
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.
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.
Although if you’re attempting this on Jupiter, you’ll likely be distracted due to asphyxiation and crushing pressure - so work fast.
#DeleteChrome
Basically what they did was they defined Planck's constant to be a fixed value pretty close the the calculated number previously used. So instead of calculating Planck's constant from an arbitrarily defined kilogram they define the kilogram (and a few other constants) from an arbitrarily defined Planck's constant. This takes the error bars away from Planck's constant and the other fundamental measurements fall out naturally as a result to precisely defined and fixed numbers.
I know it's pedantic to have this discussion outside of a classroom environment, but Kilogram is a unit of mass and not a unit of weight.
I see your pedantry and raise you one "I guess that you meant force".
Custom Solvers 2.0 = Alvaro Carballo Garcia = varocarbas.
Any individual Kibble balance will also deteriorate and be out of range of the others. There will also be differences in construction since no system is perfect. You might as well use the current method.
I have a hard time telling if you are a troll or just ignorant. What they did was they redefined Planck's constant to be a fixed number. So instead of measuring Planck's constant from an arbitrarily chosen value for the kilogram they measure the kilogram from an (sort of) arbitrarily chosen Planck's constant.
I thought it was a joke.
Our friend at Veritasium does an excellent job breaking this down:
https://www.youtube.com/watch?...
I tend to rant.
But current definition of current (pun not intended) is defined trough force
This isn't true. Electric current is the flow of a electric charge and isn't composed by force or any other magnitude. The Ampere is one of the few SI units which isn't formed by other units. Force/energy is relevant to understand how a current is created, but it doesn't define its essence.
Custom Solvers 2.0 = Alvaro Carballo Garcia = varocarbas.
Not OP, but I'm pretty sure the answer is 'yes': the whole point is that it is not 299,792,458.00, or 299,792,458.000, but *exactly* 299,792,458, so with as many zeroes after it as you wish that are actually significant.
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.
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...no system is perfect.
The new method is not purported to be perfect. It does, however, allow people to measure a kilogram with as much accuracy as the current system without shipping reference weights around the world for comparison.
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.
Note to ACs: I usually delete AC replies without reading them. If you want to talk to me, log in.
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.
Tea you have to measure locally gravity to calibrate the kibble balance. Apparently that can be done to better than 1e-8 accuracy by measuring the acceleration of a free falling body.
I understand: newer is always better.
Yeah I know. Defining the Kg to something fixed is right up there with replacing sysv with systemd.
unlike the old method, which was 0.000005%.
Plus and Minus a random change that makes it impossible to know what it was or will be for sure.
Sorry disregard that, logic error, off for coffee.
A pound is a weight, not a mass which is not the same thing. While its definition may be based on the kilogram it must also be based on a value for the gravitational field since it is a force.
The international pound has been defined as exactly 0.45359237 kg.
That's interesting. Since the pound is a weight and the kilogram a mass unless they also fixed the gravitational field in that definition it's open to abuse.
By it's very definition a balance in independent of gravity.
No it is not. Even a standard balance relies on the gravitational field for both sides being equal and, if you get precise enough, this may not be true. However, the watt balance balances the force of gravity with an electromagnetic force. Part of the measurement also requires determining the local gravitational field but this is something that you can measure accurately which is why this is still a far better definition than using a lump of metal outside Paris.
First thanks for the explanation, but are you sure your meant 'infinite'?
Yes, it is a definition, not a measurement. Essentially we define the fundmental, unchanging constants of the universe as so many SI units and then use measurements to precisely define the individual units. In fact, in particle physics, we actually start by defining these fundamental constants as '1' unit each and then derive units for energy, momentum, mass etc. from them creating a system known as "natural units". It's not very practical for everyday quantities but very useful if you are dealing with fundamental physics.
I know it's pendantic, but you can use a small object hanging from a string to measure the local ratio between mass and weight.
Escher was the first MC and Giger invented the HR department.
No, just strikes me as kinda funny...and also my usual observation that the US doesn't use the metric system for anything in most citizens' real daily lives.
That no one really uses it here.
I know. I was just saying the same thing the other day when I picked up a 2 liter of soda and a bottle of 500mg aspirin at the store. While I was out, my daughter asked me to pick up a new set of headphones with a 3.5mm jack. Since I was at the store I picked up some new LED bulbs to lower my monthly kilowatt usage at home. That reminded me that I needed to order some new 80 and 120mm case fans for my desktop too. I'll probably add 16 more GB of RAM while I'm at it. On the way home some idiot in a 5.0 liter Ford Mustang cut in front of me and got out of his car carrying a baseball bat. Fortunately when he saw my Glock 9mm, he got back in his car and left 2 seconds later. I don't know how he knew I had my wife's 3 carat emerald ring in the car.
Except that since the pound is defined as a fraction of a kilogram, it too is implicitly a measure of mass, not weight.
Please stand clear of the doors, por favor mantenganse alejado de las puertas
The second is out of date. Caesium is a horribly outdated method of measuring time. Modern atomic clocks, using strontium quantum gasses, are roughly ten orders of magnitude better.
But because of how the second is defined, you can't use a more accurate clock. The errors in caesium clocks are part of the definition. Remove the error and you're not measuring seconds even if you're measuring more accurately.
It's probably better to use fundamental units as the starting point, or at least something close, rather than arbitrary objects in nature.
Ideally, it shouldn't matter if things get measured more accurately, you won't break anything.
If you can't do that, then the definitions should be aiming at the ten orders more accurate results that can be obtained.
As for constants, they should be justified geometrically, kept simple, or defined in terms of underlying physics.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
Yes, your statement is pedantically correct. A "pound" can be expressed as a mass, disproving the OPs statement that a pound is a strictly a weight. But your post is also incomplete, in that it tends to imply that a pound is ONLY a mass. My intent was to point out that you were both correct.
Frankly, they're not the the pedantic one here. You are. If a person says one unit of measurement is based off of another unit of measurement, it would be proper to assume that the definition used for one of the terms of measurement was the one valid for the relational statement. It would be improper to assume the person was incorrectly using the wrong definition for one of the terms.
PSI uses "pound" as a measure of force, where one pound-force is equal to the force exerted by one pound (mass) under the acceleration of 1g. One pound-force is what is meant when most people speak of "pounds", but in common usage the same is true of the kilogram - most people mean "kilogram-force" when they say "kilogram". The fact remains that "pound" is an SI-derived unit of mass, not weight.
Please stand clear of the doors, por favor mantenganse alejado de las puertas
This. In theory the non-metric unit of mass is supposed to be the "slug" but pound gets overloaded just like kilogram does, to be both mass and force.
After re-reading my posts, I have to further apologise because of my behaviour in general. I didn't understand your point, didn't read your comments properly, didn't care much (not even doing a basic research, reading what I was linking!!), reacted in unnecessarily aggressive ways and wasn't even able to keep the discussion reasonably simple/focused what is particularly important when dealing with so complex issues. This is even more reprehensible by bearing in mind that I was the one spontaneously answering to your question. I am really sorry. The reasons/excuses? Being a bit too tired and some other issues (some nonsensically weird interactions with pretty ignorant and aggressive individuals here lately), but then why posting at all? Actually, my usual reaction when seeing certain behaviours in a website is simply stop using it, at least, temporarily. Completely my fault and sorry again. In any case, I think that I have shared quite a few worthy ideas which hopefully will be useful for someone.
Custom Solvers 2.0 = Alvaro Carballo Garcia = varocarbas.
I think if there's such a calamity we won't give a shit about having accurate measurements, we'll be trying to avoid being eaten.
We got from length of the king's arm to the distance light travels in so many vibrations of a something atom. When the need arises, we can do it again.
Confucius say, "Find worm in apple - bad. Find half a worm - worse."