[time-nuts] Standards for units

Chuck Harris cfharris at erols.com
Mon Apr 2 22:09:49 EDT 2007


>>> 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.

My recollection when I posted the earlier letter was 24.5 mm/inch was
exact by definition, and I recalled that 39.37 inches/meter was also
exact... a quick calculator check to 6 figures showed that it was exact,
then I tried to 9 figures, and saw that pesky little 0.000000051.

> 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.

I think the biggest push for metric is the chance to start fresh.  But
when metric includes piezes, and poises, and sthenes and steres and stokes...
I guess it ain't all that fresh.

-Chuck




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