How do you measure the electrical charge of something?

Question

Leave out the equation Q=It because it was deprived after 1 C had been found, right? So we are back to when Coulomb first did his experiment-the Torsion Balance (http://en.wikipedia.org/wiki/Torsion_balance#Torsion_balance). According to Wiki, the electrical charge of the ball in that experiment had been already KNOWN? How could he do that?

Answer 1

He didn't have to measure charge with respect to an absolute units.  He just showed that force was proportional to charge on both spheres.

If you rub some fur on amber, you generate a charge.  Touch that to a metal sphere, the sphere becomes charged.  Now, touch three identically sized spheres together -- they all get the same charge.    Then, you can drain off charge from one sphere by touching it to an uncharged sphere, etc.  

Some of the early experiments probably quantified charge as the number of times you rub a glass rod with a piece of fur, etc.  Later (before Coulomb's experiments) they would have used an electroscope.

Also, Leyden jars could be somewhat calibrated to at least give repeatable charge, by using air breakdown.

Also, it should be pointed out that Coulomb was looking for an inverse square law when he began.  He built off the work of Cavendish who showed his for gravity, and others had already suspected such a law.  So if you can show it holds crudely, then you have the basis for a system of units (based on forces).  

Answer 2

He calculated the force needed to move the ball. Then he used his Coulomb's)Law of Electrical Force = kQ1Q2/r^2, k = constant of permittivity, r=distance moved.