[time-nuts] Standards for units

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Apr 2 01:38:19 EDT 2007

David Dameron wrote:
> Hi all, 
> I just realized that a meter is defined by the speed of light., see
> http://physics.nist.gov/cuu/Units/meter.html
> It is only to 9 significant digits, so if the speed of light (in some
> controlled environment) is measured more precisely than this, the meter and
> all other derived length units will change?
> (I was taught that 1 meter was 39.37 inches, to define the inch
> , but now I see more of 1 inch = 2.54 cm, as someone just referred to.)
> I find the standard for the Ampere, mentioned in the nist  pages above more
> difficult, as 2 infinite wires to measure the force between cannot be
> found! Was the coulomb the standard before? Does anyone have other web
> pages to recommend?
> (Am still learning about the 1948 changes to electrical units,
> international and absolute volts, etc. Before finding this list, did not 
> think much about the differences, about 500 ppm., with a 3 1/2 digit dvm.)
> David D.

No, the speed of light in vacuum has in effect been fixed by this 
All that will happen is the meter will be able to be realised more 
The 1 meter = 39.37 inches is a relatively inaccurate (2ppm) approximation.
At one time the US, UK and Canada all had slightly different length inches.
The fractional differences weren't very large several orders of 
magnitude lower than the fractional differences between the imperial 
gallon and the US gallon for example.


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