system in that the magnetic flux density can be varied at will simply
by varying the battery voltage or by controlling the current through
the coil of wire.
(5) Mention has been made of the value of induced current. Actually, it is
the voltage that is induced in the wire passing through a magnetic
It is this induced voltage that pushes current around the
b. The magnetic field can be moved by physically moving the magnet or, if it
is a magnetic field from an electromagnet, it can be moved by starting and stopping
the current flow in the electromagnet.
(1) In B of figure 15, when the coil of wire is attached to the battery,
current starts to flow through it. This current, as it starts to flow,
builds up a magnetic field. This magnetic field might be considered to
be expanding (like a balloon, in a sense) and moving out from the
electromagnet. As the field moves outward, its lines of force will cut
through the wire held close to the electromagnet.
This wire will
therefore have voltage induced in it.
(2) If the electromagnet is disconnected from the battery, its magnetic
field will collapse and disappear. As this happens, the lines of force
move inward toward the electromagnet.
Again, the wire held close to
the electromagnet will be cut by moving lines of force and will have a
voltage induced in it. This time, the lines of force are moving in the
opposite direction, and the wire will, therefore, have voltage induced
in it in the opposite direction.
(3) It can thus be seen that voltage can be induced in the wire by three
methods: the wire can be moved through a stationary magnetic field, the
wire can be held stationary and the magnet can be moved so the field is
carried past the wire, or the wire and electromagnet can both be held
stationary and the current through the electromagnet be turned on and
off so that the expanding and collapsing magnetic field moves across
c. The amount of EMF induced in a conductor moving in a magnetic field
depends on four factors:
(1) The strength of the field.
(2) The length of the conductor cutting the field, or the size and number
of turns in a coil when the conductor is wound in the form of a coil.
(3) The speed at which the conductor sweeps through the magnetic field, or
the speed of rotation of a coil in a magnetic field.
(4) The angle at which the conductor passes through the lines of force.
d. Since the armature of an electric generator is composed of many turns of
wire and electromagnets make up its stator, it follows that the output voltage will
depend on the above four factors. The most practical method of