At 12:23 PM 3/6/01 -0500, Steve Smith wrote:

>I would never assume any dimension to be dead accurate
>as you must allow for rounding off and tolerances.

This is true for measured or specified physical dimensions.

However, we should also know that 1 inch is *exactly* 2.54 cm. because that 
is the definition of an inch. So no error is introduced by using that 
number to convert from inches to centimeters or millimeters or vice-versa. 
Any error comes from rounding off the results, or from roundoff in the 
original figures.

So .1 inch is really .1 +/- .05 inch, unless it is an abstract number, and 
it is thus 2.54 mm +/- 1.27 mm.

As a printed circuit designer, I am very accustomed to inch units and don't 
like changing. I can tell you immediately the inch thickness of copper 
which is 1 oz. per square foot, but I'd have to think a while to come up 
with the figure in mm, and I don't even want to think of g/cm^2. But if I 
could start from scratch, I'd definitely go metric. All my science 
training, thirty years ago, was in metric.

[EMAIL PROTECTED]
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433


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