
Redefining the Kilogram - bushido
https://www.scientificamerican.com/article/redefining-the-kilogram/
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jedimastert
There's also an excellent video from Veritasium, a science and education
YouTube channel, about what's being done to define the kilogram

[https://www.youtube.com/watch?v=Oo0jm1PPRuo](https://www.youtube.com/watch?v=Oo0jm1PPRuo)

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aequitas
Also interesting 'Practical Engineering' building a DIY watt scale:
[https://www.youtube.com/watch?v=ewQkE8t0xgQ](https://www.youtube.com/watch?v=ewQkE8t0xgQ)

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petepete
For anyone interested there's an _excellent_ BBC4 documentary on this very
subject by Professor Marcus du Sautoy. It's called Precision: The Measure of
All Things, unfortunately isn't on iPlayer at the moment but it can be found
elsewhere, including DailyMotion.

[https://www.dailymotion.com/video/x2hy5l5](https://www.dailymotion.com/video/x2hy5l5)

~~~
guitarbill
It really is shockingly good. Despite the slightly mundane title, it's
interesting and understandable all the way throught. Highly recommended.

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anarcat
For those actually curious of the details of all those changes, Wikipedia has
a good overview:

[https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_ba...](https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_base_units)

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logicallee
How were these working copies made?

The article makes it seem like there's some "copy" operation you can apply to
the reference artifact, but that seems absurd...

I Googled "working standard kilogram" (no quotes), in case I'd find some
article, and I did find only this:

[https://www.technology.matthey.com/article/17/2/66-68/](https://www.technology.matthey.com/article/17/2/66-68/)

I don't really get a sense after reading, of how these things are used to make
more of them.

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bo0tzz
I would assume you just make an object that will be the working copy, and
align it's weight with that of the standard.

~~~
logicallee
How do they align its weight?

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VLM
The first google term you don't have is "traceable mass calibration NIST" that
will lead you to the NIST price list and terms and conditions.

They do "double substitution procedure" (another google term) for most masses
for about three grand plus or minus incidentials (cleaning, shipping).
Unfortunately the best descriptions I can find of NIST double substitution
procedure is docx unfree format.

However it boils down to a really high quality double arm balance (like a
lever with a pivot in middle and buckets on each end) and its temp controlled
to keep the lengths of the arms identical and they stick your (test) mass on
one side, stack calibration weights on the other side and record what
balances, then remove your mass and stack more calibration masses in its place
optimistically very similar in mass to the other side, but it won't be equal
because one arm is probably a nanometer longer than the other arm or
something. Essentially they're testing it twice in order to factor out a
difference in arm lengths however small. You can get around eight or nine sig
figs of accuracy now a days using this procedure.

I would enjoy seeing an electrostatic force balance like they use for
microgram scale masses.

~~~
logicallee
It's hard to imagine that a physical lever-based mechanical scale is so
accurate, never would have imagined. Thanks for the detailed comment.

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nj65537
NIST has also just released its own article about the redefinition:
[https://www.nist.gov/si-redefinition/kilogram-kibble-
balance](https://www.nist.gov/si-redefinition/kilogram-kibble-balance)

It has quite a few details that give a sense of the engineering challenges
involved.

\- The Kibble balance has a weighing mode, and a calibration mode; the two are
used to cancel out a significant portion of the systematic error introduced by
the mechanical system

\- Measurements are made in a vacuum to eliminate effects of air buoyancy,
which are significant at the desired level of precision.

\- One advantage of the wheel configuration is that it allows the supporting
cables to move the coil exactly vertically, thereby avoiding the undesirable
sideways motion that a coil suspended from a beam balance can experience.

I could go on, but this gives an idea and you can go read it yourself ;)

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jihadjihad
I think this is good. Not a physicist but a lot of the SI definitions seem
contrived and arbitrary [1] rather than based on physical fundamentals.

[1]
[https://en.wikipedia.org/wiki/SI_base_unit](https://en.wikipedia.org/wiki/SI_base_unit)

~~~
Double_a_92
It will be always be arbitrary anyway. Say we used the wavelength of some
special light as unit for length... Why did we choose that light, and not
something else?

The point with the kilogram redifinition is that it's currently tied to some
piece of metal stored in Paris somewhere. We can physically lose the
definition.

~~~
krastanov
We do use resonant frequency of certain atoms to define units, but it is not
arbitrary, rather it was a transition chosen for it's extremely small width
and ease of measurement. It is quite a bit less arbitrary than using the size
of our planet or of an artifact we have made.

And we are getting even better, with definitions based on fundamental
constants now, independent of a particular measurement experiment. This is as
non-arbitrary as it can get (we will continue using a conversion factor to
make the unit human scaled, but that is all).

~~~
Double_a_92
But one meter is still the length we are used to.

So we got the resonant frequency of some atoms, which is arbitrary because
they just happened to be relatively easy to measure for us... Multiplied by
some arbitrary factor so the unit reaches the usual length.

That doesn't seem very fundamental to me. Could you imagine proposing that to
some intergalactical economic union as new standard?

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ElBarto
The resonant frequency of the atom is not arbitrary, it is a physical
constant.

That's the key and the point: It allows to measure exactly 1 second anywhere
in the universe because the resonant frequency of that atom is the same
anywhere in the universe.

Compare with a previous definition of the second as a fraction of the solar
day... A solar day isn't in fact constant and, in any case, once you leave
earth it means nothing at all.

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lanius
>it is roughly five parts in 100 million heavier than all the working
standards, which have been leaving behind a few atoms of metal every time they
are put on scales

Are the working copies really only shedding such a small number of atoms per
use? Just seems like it should be a higher number.

~~~
rjp0008
Well some back of the napkin math, atomic mass of platinum, the heavier
constituent of the standard is 195. So more than 5 mols of these atoms are in
the block, because 1 mol of pt is 195 grams. So there’s around 5 _6_ 10^23
atoms. So 5 parts per 100 million adds up to a lot. Maybe 1.5*10^15 atoms?
It’s been a while since I had chemistry...

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zvrba
Can somebody explain to me why we need both kilogram and mole
([https://en.wikipedia.org/wiki/Mole_(unit)](https://en.wikipedia.org/wiki/Mole_\(unit\)))?
One can be defined in terms of the other.

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Koshkin
Kilogram of any substance weighs the same, whereas the moles of different
substances have different weights (numerically equal to the molecular weight
of the substance).

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Doxin
Sure, but can't the kilogram then be easily defined in e.g. carbon-moles?

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Koshkin
it could (again, based on a particular isotope), but only in theory, because
how would one count the mole worth of atoms?

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Doxin
isn't one mole of carbon-12 by definition 12 grams?

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Koshkin
Unit conversion no longer being an issue, I wish everyone went back to the
imperial units. (Computers are not 10-based anyway.)

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amanaplanacanal
I'm trying to imagine what the benefit would be. Could you explain?

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ggm
Parties of two, three, four, eight, and twelve kids can now look forward to
simple divisions of cake.

You can "spend a penny" again. Although I haven't seen a pay toilet in over 30
years. (I don't mean the ones you put money on a plate at the entry, I mean
one you put money INTO the door)

~~~
grkvlt
What do pennies (or pence) have to do with imperial units? A penny is exactly
1/100 of one pound sterling or GBP 0.01, which is a nice metric style power of
ten division of the larger unit. If you want monetary units more congruent
with imperial, then you may be thinking of l.s.d. or librae, solidae and
denarius - pounds, shillings and pence - with twelve pennies to the shilling.
Note this is UK only, I don't think the USA ever had non decimal currency?

