Coulomb Balance

   Coulomb’s law states that the electrical force between two point charges  depends on the magnitude and sign of each of the charges, and the inverse square of their separation. Charles Coulomb (1736-1806) used the torsion balance for electrostatic force measurements. However, the British physicist John Mitchell earlier suggested the use of the balance, and it was used to investigate the inverse square relationship for magnetic action about 1760 by Tobias Mayer of Göttingen.

   Eight examples of Coulomb Balances are shown below. One of the charges is placed on the small stationary conducting sphere on the lower end of the vertical insulating rod. The other charge (of the same sign) is on a second small conducting sphere placed on the end of the counterbalanced crossbar, suspended by a torsion fiber. The top of the fiber is attached to a rotating cap, marked off in degrees. A paper scale pasted around the outside of the lower glass cylinder indicates the angular position of the rotating system.

    St. Patrick's College: Elliott Brothers, London, mid-to late 19th century
    Amherst College: Ducretet, Paris -- after 1875
    Colby College: no maker's name
    Glasgow: Pixii of Paris
    Middlebury: no maker's name
    Yale: no maker's name
    Cincinnati: Elliott Brothers, London 
     Case Western Reserve University: no maker's name; 60 cm high  

                   St. Patrick's College, Ireland               Amherst College                             Colby College
 
                          Glasgow University                     Middlebury College                       Yale University




                                           University of Cincinnati                       Case Western Reserve University

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