Sturgeon's Galvanometer
   Instruments to measure the passage of an electrical current depend on a serendipitous observation. In the course of a lecture demonstration in April 1820, the Danish natural philosopher, Hans Christian Ørsted (1777-1851), passed a heavy current through a metallic wire, and noticed that a nearby compass needle was affected. In the early summer the experiment was repeated under controlled conditions, and on July 20th he published (in Latin) the first paper on electromagnetism. His discovery of the interaction of the magnetic field produced by the current with the compass needle also provided the mechanism for the measurement of the electric current. Shortly after, Andre Marie Ampere (1775-1836) suggested that this effect could serve as the basis for measuring the electric current. 

   The basic galvanometer, devised by the British physicist William Sturgeon (1783-1850) in 1825, allows all of the various combinations of current and magnetic needle direction to be tried out. By making suitable connections to the screw terminals, current can flow to the right or to the left, both above and below the needle. Current can be made to travel in a loop to double the effect, and, with the aid of two identical external galvanic circuits, the currents in the two wires can be made parallel and in the same direction. Note that the wires are insulated from each other where they cross. From a pedagogical point of view, it would be better if the pivoted magnet were smaller, since we usually talk about the magnetic field at a point rather than the average magnetic field over a region. I often the apparatus at the top when starting the discussion of the magnetic fields set up by steady currents, and, with a certain amount of feigned confusions, can get the students to “explain” the situation to me. 

                             Kenyon College
                  Washington and Jefferson College
         St. Patrick's College, Maynooth, County Kildare, Ireland                         University of Cincinnati
   This unmarked example of Sturgeon's Galvanometer is at Colby College in Waterville, Maine. It looks as if it might have been made by a successor company to Daniel Davis, Jr., perhaps Palmer and Hall. 

    Daniel Davis, Jr. of Boston made similar pieces of apparatus in the eighteen forties.

Return to Electrical Measurements Home Page | Return to Home Page