system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Where did the Avogadro's number idea come from? And how come a 18th centruy scientist came up with this precise number?

I mean without electronic microscopes how did the man came up with such a precise number? And where did his ideas came from?
gpt
First off: It's named after Avogadro, but Avogadro didn't actually provide an number. He just proposed the idea that the volume and number of atoms in a gas are proportional, all other things (namely, temperature and pressure) are constant.

It was first calculated in 1910 when Robert Millikan, and American Physicist, managed to determine the charge on a single electron; which is doable by reacting very small amounts of things, and looking for the LCD in the charge. Because the charge on a mole of electrons (measurable by arranging a chemical reaction using moles of atoms) was known, it's just a division problem from there.

And it wasn't that precise at the time, but has gotten more precise over the years.