pole of a magnet is called the MAGNETIC
than through the air inside the case, the instru-
FLUX. The number of flux lines per unit area is
ment is effectively shielded, as shown by the watch
known as FLUX DENSITY. The intensity of a
and soft-iron shield in figure 4-2.
magnetic field is directly related to the magnetic
For further information on magnetism, you
force exerted by the field. The force between two
should refer to NEETS, module 1.
poles is directly proportional to the product of
the pole strengths and inversely proportional to
the square of the distance between the two poles.
ELECTRICAL POWER GENERATION
Magnetism can be induced in a magnetic
material by several means. The magnetic material
A generator is a machine that converts
may be placed in the magnetic field, brought
mechanical energy to electrical energy using the
into contact with a magnet, or stroked by a
principle of magnetic induction. This principle is
magnet. Stroking and contact both indicate
based on the fact that
actual contact with the material but are considered
i n magnetic studies as magnetizing by
if a conductor is moved within a magnetic
field in such a way that the conductor cuts
across magnetic lines of force, voltage is
MAGNETIC SHIELDING.--There is no
generated in the conductor.
known INSULATOR for magnetic flux. If a non-
magnetic material is placed in a magnetic field,
The AMOUNT of voltage generated depends
there is no appreciable change in flux-that is,
on (1) the strength of the magnetic field, (2) the
the flux penetrates the nonmagnetic material. If
angle at which the conductor cuts the magnetic
a magnetic material (for example, soft iron) is
field, (3) the speed at which the conductor is
placed in a magnetic field, the flux may be
moved, and (4) the length of the conductor within
redirected to take advantage of the greater
the magnetic field.
permeability of the magnetic material, as shown
The POLARITY of the voltage depends
in figure 4-1. Permeability is the quality of a
on the direction of the magnetic lines of
substance that determines the ease with which it
flux and the direction of movement of the
can be magnetized.
conductor.
The sensitive mechanisms of electric instru-
ments and meters can be influenced by stray
magnetic fields, which will cause errors in their
readings. Because instrument mechanisms cannot
be insulated against magnetic flux, it is necessary
to employ some means of directing the flux
around the instrument. This is accomplished by
placing a soft-iron case, called a MAGNETIC
SCREEN or SHIELD, about the instrument.
Because the flux is established more readily
through the iron (even though the path is longer)
Figure 4-1.--Effects of a magnetic substance in a magnetic
field.
Figure 4-2.--Magnetic shield.
4-3
