How was the value of Avogadro's Constant found?

Answer 1

In 1811, Avogadro published an article in Journal de physique that clearly drew the distinction between the molecule and the atom. He pointed out that Dalton had confused the concepts of atoms and molecules. The "atoms" of nitrogen and oxygen are in reality "molecules" containing two atoms each. Thus two molecules of hydrogen can combine with one molecule of oxygen to produce two molecules of water. Avogadro suggested that equal volumes of all gases at the same temperature and pressure contain the same number of molecules
which is now known as Avogadro's Principle. Check out this web-page for a lot more cool info on Avogadro!!

Answer 2

Look up Millikan's Oil Drop Experiment.