[time-nuts] OT Prototype Boards

[Magnus Danielson]

Hi Bob,

On 06/25/2013 06:17 PM, Bob Stewart wrote:
> Hi Hal,
>
> I had always used 25.4001 or .03937 to do my conversions.  So, I looked online and found the .039370078 and did the reciprocal.  It is, indeed very very close to 25.4.  If you google "25.4001 conversion" you can find lots of tables using that as the conversion factor online.  I don't know where the error came from or why it's quoted so regularly.   But, it appears to be the rounded result of taking the reciprocal of a rounded number.  Don't machinists use this number for conversion?
>
> Thanks for the discussion, everyone.

US Metric Act from 1866:
www.nist.gov/pml/wmd/metric/upload/HR-596-Metric-Law-1866.pdf
http://lamar.colostate.edu/~hillger/laws/metric-act.html

In 1912 C.E. Johansson decided to use 25.4 mm for one inch (at 20 
degrees C), for his gauge-sets. That was later agreed internationallly 
on in 1933, but the US Metric Act remained unchanged, which caused 
confusion ever since.

Interesting article:
http://www.changeover.com/metrology.html

Anyway, unless you need to use a US Survey feet, 1 inch = 25.4 mm 
exactly is what you should be using.

Cheers,
Magnus