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After the discovery that an electric current flowingthrough a conductor creates a magnetic field around theconductor, there was considerable scientific speculationabout whether a magnetic field could create a currentflow in a conductor. In 1831, it was demonstrated thiscould be accomplished.To show how an electric current can be created by amagnetic field, a demonstration similar to that illustratedin Figure 10-98 can be used. Several turns of a conductor are wrapped around a cylindrical form,and the ends of the conductor are connected togetherto form a complete circuit, which includes a galvanometer.If a simple bar magnet is plunged into thecylinder, the galvanometer can be observed to deflectin one direction from its zero (center) position (Figure10-98A).When the magnet is at rest inside the cylinder, thegalvanometer shows a reading of zero, indicating thatno current is flowing (Figure 10-98B).In Figure 10-98C, the galvanometer indicates a currentflow in the opposite direction when the magnet ispulled from the cylinder.The same results may be obtained by holding the magnetstationary and moving the cylinder over the magnet,indicating that a current flows when there is relativemotion between the wire coil and the magnetic field.These results obey a law first stated by the Germanscientist, Heinrich Lenz. Lenz’s law states:The induced current caused by the relative motion of aconductor and a magnetic field always flows in such adirection that its magnetic field opposes the motion.
When a conductor is moved through a magnetic field,
an electromotive force (emf) is induced in the conductor.
[Figure 10-99] The direction (polarity) of the
induced emf is determined by the magnetic lines of
force and the direction the conductor is moved through
the magnetic field. The generator left-hand rule (not to
be confused with the left-hand rules used with a coil)
can be used to determine the direction of the induced
emf. [Figure 10-100] The left-hand rule is summed
up as follows:
The first finger of the left hand is pointed in the direction
of the magnetic lines of force (north to south), the
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