[time-nuts] Standards for units

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Apr 2 21:47:09 EDT 2007


Chuck Harris wrote:
> Chuck Harris 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.)
>>>       
>> 1 meter/39.37 inches = 0.02540 meters/inch = 2.54 cm/inch = 25.4 mm/inch
>>
>> Exactly.
>>     
>
> As long as exact means within a few parts per million:
>
> 1/meter/39.37 inches = 0.025400051 meters/inch ...
>
>   
>> -Chuck Harris
>>     
>
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>   
Chuck

That's a little sloppy for this list surely.

In fact the US currently uses 2 different inches the "survey" inch (plus 
yard, mile etc) which is defined by the relation

1 metre = 39.37 "survey" inches exactly.

This inch is used purely for surveying purposes.

The other inch defined by

1 inch = 25.4mm exactly

is used for everything else.

For confirmation read the appendix C of NIST Handbook 44, where these 
and other units of measurement are described.
Why on earth one has to have different units all called barrels for oil, 
cranberry juice, dried fruits, liquor (this one varies from state to 
state) etc., defies imagination.

Bruce




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