[time-nuts] OT Prototype Boards

[Bruce Griffiths]

Chris Albertson wrote:
> 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.
>    
To make it even more interesting there were several "flavours" (US, 
Canadian, UK ...)of the inch which all differed by a very small amount.

Bruce