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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: A conundrum about the AWG (wire gauge) table
Summary: Gauge sizes corresponded to commercially available sizes
Message-ID: <6930@milton.u.washington.edu>
Date: 31 Aug 90 04:25:08 GMT
References: <2588@ryn.esg.dec.com>
Organization: University of Washington, Seattle
Lines: 32


In article <2588@ryn.esg.dec.com>, jroth@allvax.dec.com (Jim Roth) writes:
> Many of the standard values in electronics and elsewhere follow an
> approximate log/exp scale to accomodate a wide range of values.
> Wire gauges are no exception, but here's an odd thing - virtually
> none of the diameters or areas in circular mills are "round numbers".

	This is an example of functional evolution of a standard.
To make wire, a smith starts with a rod; tapers the end (hammer/anvil work)
and forces it through successively smaller holes in a 'drawing
plate' type of die.  The earliest wire-drawing I know of was in
France, in 1270.  It's still done that way today (but the dies
are better material).
	So, the rod size determines '0' gauge size.  Modern wire
typically comes from #5 hot-rolled rod; that '0' size could have
been the nominal rod diameter at some time in the past.  The diminution
of diameter on each successive pull through the die determines the
successive gauge sizes (and the amount of money it will cost
you for a pound of the wire).  The gauge/diameter correspondence
presumably was standardized (and more than once; there's British
and American standards that I know of, with slightly different sizes)
by merchants who wanted interchangeable wire from different
suppliers.
	The magic number for the ratio of two successive gauge diameters
comes from the need for the drawing operation not to result in terribly
many fresh starts (and resultant short pieces of wire to be spliced).
The safe stress of the thin wire coming out of the die has to be
sufficient to deform the thick wire going into the die; the ratio
is not size-dependent, so the gauge table naturally comes out logarithmic
in diameter versus gauge number.

		John Whitmore