[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.
>
David
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.
Bruce
More information about the time-nuts
mailing list