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Re: [Phys-L] The Ingenious Watt Balance (and why we need it)



On 10/07/2016 09:18 AM, Donald Polvani wrote:

the term "Laplace" force

That was a new one on me. I found it unfamiliar and unhelpful. Apparently
French physicists are fond of the term.

Feynman absolutely hated terminology like that. He would have just called
it "the magnetic force on the wire" or some such descriptive expression.

While we are on the subject, IMHO calling it a "Watt" balance seems like
a bizarre and unhelpful neologism. It's got nothing to do with Watt and
precious little to do with watts. If it needs a name at all, why not call
it a Kibble balance? Or call it an IV balance, which is more descriptive.

Not everything with dimensions of IV is conventionally measured in watts.
In particular, reactive loads are measured in kVA, not kW.

The first part says that the ampere is defined, in part, by the kilogram,
but now, with the watt balance, we can define the kilogram, in part, by the
ampere. Don't we now have to change the definition of the ampere, so it
doesn't include use of the kilogram? Explanation of this point would help.

Yes, the video played fast and loose with this. If we didn't /also/
have a redefinition of the ampere, the claims in the video about getting
rid of the kilogram artifact would not be true.

It would have been better to say that there is a coordinated effort to
redefine the ampere, kilogram, kelvin, and mole. Each of those sounds
like an interesting goal unto itself, but the /combination/ is far more
interesting, because that's what allows you to make sense of what's
really going on.

In more detail: We "could" redefine the kilogram in terms of the mass
of the proton, and then just count protons. Then we "could" define the
ampere in terms of mass (and distance and time) as it is now. However,
there is more to metrology than definitions. We need a /practical/ and
/accurate/ way to carry out the measurements. As it turns out, at present,
it seems to be more practical to define the ampere in terms of elementary
charges, and count charges. Then we take the experiment that formerly
defined the ampere and use it *backwards* to define the kilogram.

To make this work, we need to improve the amp-versus-kg experiment.
Kibble's IV balance (aka watt balance) is an improvement over previous
methods. So it fits in as part of a much more elaborate program.

References:
http://www.bipm.org/en/measurement-units/new-si/
http://www.nist.gov/pml/div684/grp02/counting-down-to-the-new-ampere.cfm

Another strange thing in the video was the use of two lasers. Surely
the top laser was more accurate, because of the optical lever arm, so
why not just rely on that?

Also the video was kinda vague about how they measured velocity.

Also it was odd to see so little attention paid to the field geometry,
including so little control over stray fields. This could seriously
mess with the accuracy. If you're not concerned with accuracy, the
whole exercise is not worth the trouble; you could just measure the
magnetic force on a piece of wire and be done with it.

Wouldn't it be easier to use a couple of off-the-shelf loudspeakers?
One could also use hard-disk-drive positioners, which is fancier but
still easier than making coils and stators from scratch.

Also there was no attempt to analyze the accuracy, which is strange for
a metrology experiment.

More generally: They discussed some things in tremendous detail while
glossing over other things.