Coulomb's
Torsion Balance
Coulomb used a
torsion balance to investigate electrostatic forces. A quantity of
charge q
1
is rubbed on a fixed sphere. A second charge q
2
is rubbed on a sphere on the end of a suspended rod that is free to
rotate. The force exerted by q
1
on q
2
twists the rod and suspending fiber. Turning the suspension head twists
the fiber back so that the two spheres stay the same distance apart. The
magnitude of the force is indicated by the angle through which the
suspension head is turned. Coulomb found that the force exerted by one
charge on another was directly proportional to the magnitudes of both
charges (q
1
q
2
). The bigger the magnitudes, the bigger the force. He also found that
force was inversely proportional to the square of the distance, r,
between the charged spheres. The smaller the distance, the bigger the
force. This is known as Coulomb's Law: F = q
1
q
2
/r2
