[time-nuts] Standards for units

[Neville Michie]

It is interesting to look in the front of the International Critical  
Tables, an encyclopedic set of books containing detailed scientific  
information about 1920. Every nation, (hundreds of them) had their  
own units which were used in commerce and trade. Unfortunately the  
only access on the net to these tables is blocked by a pay barrier.
Their are now very few nations hanging out for their own units.
The story I heard about the inch/metre relationship was that during  
WW2 there were problems making spare parts for aeroplane engines to  
specification, and it was discovered that the French had supplied a  
replica standard metre to the bureau of standards of each major  
country and that these could be quickly used to standardise  
engineering measurements. My version said that 25.400000000000 mm per  
inch was the accepted relationship.

Clocks were made in Napoleons time with ten hour dials with one  
hundred minutes.

Cheers,
Neville Michie

On 02/04/2007, at 4:08 PM, Dr Bruce Griffiths wrote:

> 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
>
>
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