[time-nuts] Standards for units

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Apr 2 02:08:23 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.

The 1m = 39.37 inch definition applies to the US survey inch, a unit 
only used in surveys.
Since around 1958 or 1959 the US customary inch has been identical to 
the international inch:
1 international inch = 25.4mm.
The 2ppm difference is significant in geodetic survaeys.

In practice realising the ampere used to mean building a current balance.
The abstract definition employing infinite wires can easily be used 
together with a little calculus to calculate the force between non 
infinite wires of wound into ci=oils and other shapes.

Before the advent of the current balance the unit of charge was defined 
electrochemically in terms of the weight of a standard metal 
(platinum??)electroplated from solution on to the cathode of a 
electroplating cell. The unit of current being defined by extension from 
the unit of charge. Note the SI units as we know them today were not 
then in use.


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