MadSci Network: Physics
Query:

Re: How does NIST's electronic kilogram apparatus work?

Date: Wed Sep 27 18:32:05 2000
Posted By: Allan Harvey, Staff,National Institute of Standards and Technology
Area of science: Physics
ID: 970046867.Ph
Message:

They must have sent this to me because I am at NIST, but I'm far away 
(both geographically and technically) from that particular experiment.  
However, it was easy enough for me to look up the details.  What I'll do 
is partially answer some of your specific questions, but also point you to 
articles where you can find the more detailed information which I could 
not include in this answer without taking an unreasonable amount of my 
time.

You asked how gravity comes in -- it actually does not cancel out but has 
to be measured at the location of the experiment.  However, this can be 
done very precisely; the uncertainty in the gravitational force is one of 
their smaller errors currently.  I note that the gravitational constant 
(proportionality in Newton's law of gravitation) is a fundamental constant 
which is not known very precisely, but you can still measure gravitational 
acceleration on Earth very precisely (what is more uncertain is the mass 
of the Earth).

The references I'm about to give you show apparatus details.

With regard to your last 2 questions, what the apparatus measures is the 
balance between the force of gravity on a mass and the force generated 
electromagnetically between two moving coils.  In some sense what is 
currently being measured is the Watt, though looking at it another way 
they are measuring drift in the mass of their kilogram weights.  As you 
mention, the electrical standards in some sense have "secondary" status.  
However, the idea behind much of the project is that some of the 
electrical aspects (Josephson junctions, etc.) are more precisely 
reproducible than the artifact mass that defines the kilogram.  So the 
idea would be to redefine the mass in terms of these electrical 
measurements, making them in some sense primary (much as the meter was 
redefined in terms of optical measurements).

Here's the references which should (along with references therein) give 
all the information you would ever need (if you don't have access to a 
library, the contact on that Web page might be willing to mail you copies):

P.T. Olsen et al., "Monitoring the mass standard via the comparison of 
mechanical to electrical power," IEEE Transactions on Instrumentation and 
Measurement, 40, 115 (1991).

R.L. Steiner et al., "The NIST watt balance: Progress toward monitoring 
the kilogram," IEEE Transactions on Instrumentation and Measurement, 46, 
601 (1997).

R.L. Steiner et al., "A Result from the NIST Watt Balance and an Analysis 
of Uncertainties," IEEE Transactions on Instrumentation and Measurement, 
48, 205 (1999).

B.N. Taylor and P.J. Mohr, "On the redefinition of the kilogram," 
Metrologia, 36, 63 (1999).

Allan Harvey, Physical & Chemical Properties Div., NIST
"Don't blame the government for what I say, or vice-versa."


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