I'm trying to learn and understand how Coulomb derived his formula. This is from Wikipedia:
He used a torsion balance to study the repulsion and attraction forces of charged particles, and determined that the magnitude of the electric force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.
Now the question and I didn't find anything about it. How did he charge and measure the charge on the balls? It was before batteries were invented (and before voltages were known) and even if he created charges on the ball, how did he know its exact number.
In other words: How did he use the torsion balance along with the little information known at that time to come up with his equation?