The torsion balance works by isolating the electric charge (using insulators) and converts the resultant force into torque allowing it to be easily and accurately measured.  

 

Coulomb's torsion balance worked by charging two pith balls, one of which was fixed and one of which was attached to end of needle. This needle was attached to a silver wire, with the wire being attached to a torsion micrometer. SEE IMAGE BELOW:

 

This configuration allows the electrical force to be determined by transferring the force into torque.  Whereas Coulomb's predecessors developed tools which merely indicated the presence of charge, the torque balance allowed Coulomb to make quantitative measurements of charge.