MadSci Network: Physics

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

Date: Wed Sep 27 05:27:47 2000
Posted by Tony Le
Grade level: grad (science) School: No school entered.
City: San Jose State/Province: CA Country: No country entered.
Area of science: Physics
ID: 970046867.Ph

This concerns the web page:

This describes the "electronic kilogram" project, whose purpose is to 
develop a viable means of deriving the SI-standard kilogram from 
fundamental laws of physics, instead of simply defining a specific block 
of metal as representing a mass of one kilogram.

The drawing on this page is very sparse on details, and I would like to 
know how this apparatus works.  I'd especially like to know:

* The description says that mechanical force is used to counteract gravity 
on a test mass.  However, acceleration due to gravity is not a fundamental 
law of physics; there must be some way it falls out of the picture, but 

* The apparatus converts electrical energy into mechanical energy.  For 
physicist-quality precision, this conversion must be totally lossless, so 
how is this achieved?

* What is actually being measured?  Since all SI units of electricity are 
derived using the kilogram, I assume the apparatus does not measure 

* What will be the form of the new definition, i.e., how will we complete 
the statement:  "A kilogram is that quantity of mass such that ..."?

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

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