[time-nuts] OT Prototype Boards

[Chris Albertson]

On Tue, Jun 25, 2013 at 9:17 AM, Bob Stewart <bob at evoria.net> 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?


Some years ago in 1959 the inch was re-defined to be exactly 25.4 mm.
Before that time the inch was only very close to 24.5 mm  But for the last
50+ years 24.5 has been an exact conversion.

Likely people who are now 65+ years old where taught something different in
school if they were in school befoe 1959 and did not keep up with this.
-- 

Chris Albertson
Redondo Beach, California