Charges exert forces on one another.
The size of these forces, called Coulomb forces, depend on the magnitude of the charges and the distance between them. Charges with the same sign repel while charges with opposite sign attract. The relationship between the force, the magnitudes of the charges, and the distance is defined by Coulomb's Law. Coulomb's Law states that the force is directly proportional to the product of the two charges and inversely proprotional to the square of the distance between them. A torsion balance is a device that makes use of this relationship to find the magnitude of an unknown charge.