Re: Standard Kilogram


"Uncle Al" <UncleAl0@xxxxxxxxxxxxx> wrote in message
news:46F3F319.F33A6368@xxxxxxxxxxxxxxxx
enders@xxxxxxxxxxxxxxxxxxxx wrote:

On Sep 19, 6:31 pm, "Ian Macmillan" <iand...@xxxxxxxxxx> wrote:
The recent report about a 50ug discrepancy in the comparison of the
standard
1 Kg mass with its copies led me to wonder how such precision is
attained,
50ug being 5 x 10^-9 Kg.

Can anyone describe the method of making such a comparison, and how
the
degree of confidence in the result is estimated?

All the best
Ian Macmillan

I assume such a precision is possible by converting a measuring of
mass into a measuring of length.

Measuring a small signal within a big background is not clever. Null
the background to leave the net signal remaining, as with a balance
rather thana scale. The necessary mass differential sensitivity is
not unreasonable to obtain in hardware,

http://prola.aps.org/abstract/PRL/v89/i16/e161102
"Determination of the Gravitational Constant with a Beam Balance "
Phys. Rev. Lett. 89 161102 (2002)

Or, e-mail the folks and ask them how they determined the change.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2


I appreciate the advantage of a balance, but it seems to me that there are
special difficulties measuring a tiny differential between relatively large
masses. Equality of temperatures, volumes and barometric pressure, symmetry
of pivot friction, magnetic and electric fields, convection, possibly even
tidal effects, and so on must give rise to significant uncertainties
difficult to quantify.

I assume that obtaining the required sensitivity is a given. My query is
directed more to the management of the uncertainties and determining the
degree of confidence in the result.

I suppose in the end one has to say that the Standard is correct by
definition and that the copies have drifted off. It does not seem very
satisfactory. Is there a possibility of a standard of mass other than the
mass of a particular lump of metal?

Incidentally the "Determination of the Gravitational Constant with a Beam
Balance " link would have been interesting but seems to require a paid
subscription for access.

I'd be interested to email the folks who determined the change, but as it
was basicly a media report I have no address.

All the best

Ian Macmillan





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