| 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.
| 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.