Source of Avogadro's number

["Scott O. Schwahn" <schwahn@CEBAF.GOV>]

Although this thread is getting quite old, I would like to know what the
SOURCE of Avogadro's number is.  For instance, the second is defined as a
certain number of oscillations (disintegrations?) of something - I forget
what - and the  meter is defined as length of - - well, I've forgotten that,
too.

So what defines the number of atoms in a mole?  Convention?  A physical
measurement?

Scott O. Schwahn, CHP
Accelerator Division, M.S. 12A1
Thomas Jefferson National Accelerator Facility
12000 Jefferson Avenue
Newport News, VA 23606
(757)269-7551 (w)
(757)269-7363 (fax)
schwahn@cebaf.gov