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current
flowing in a loop of wire producing the same
magnetism. The units are amperes/metre (A/m) although
milliamperes/metre (mA/m) is commonly used in geophys-
ics. In the cgs measurement system, strength of magnetism
is measured in gauss (G), where 1 G is equal to 1000 A/m.
The intensity or strength of a magnetic
field (B) in the SI
system is measured in tesla (T), but it is too large for
specifying the very weak magnetic
fields measured in geo-
physics, so the nanotesla (nT) is used. The equivalent in
the cgs system is the gamma (
Magnetic field lines
a)
S
N
Bar magnet
b)
γ
), and conveniently 1
γ
is
equal to 1 nT.
S
N
S
N
3.2.3.3
Susceptibility and induced magnetism
For very weak magnetic fields like the Earth
s field (see
Section 3.5.1
)
, the strength of the induced magnetism
(J
induced
) is approximately proportional to the strength of
the externally applied
'
Figure 3.5
Bar magnet. (a) Field lines depicting the magnetic field
of a bar magnet. The field is strongest at the poles where the lines
converge. (b) Breaking the magnet into two pieces produces two
smaller magnets.
field (B) and given by:
B
μ
0
J
induced
¼ κ
ð
3
:
7
Þ
μ
0
is the magnetic permeability of a vacuum and has
a value of 4
where
repel. Magnetic poles of equal strength but opposite polar-
ity always occur as a pair and are known as a magnetic
dipole. The
field of a magnet (a magnetic dipole) interacts
with that of the Earth and the north pole of the magnet is
that which is attracted towards geographic north (and
similarly the south pole is attracted to geographic south).
When a bar magnet is broken into pieces, each forms a
pieces are broken into smaller pieces, and so on.
Magnetism is a vector quantity (see
Section 2.2.2
). The
direction of the magnetism of a dipole is taken as directed
away from the south pole and towards the north pole, the
north pole being the positive pole. The magnetic field of a
dipole is the combined effect of the fields from the two
magnetic poles. By convention, the direction of the mag-
netic
10
-
7
henry/m, and the constant of propor-
π
tionality (
), i.e. the degree to which a body is magnetised
by the external field, is known as the magnetic suscepti-
bility of the body and given by:
κ ¼
μ
0
κ
B
J
induced
ð
3
:
8
Þ
Susceptibility is the ratio of the strength of the induced
magnetism to the strength of the
field that caused it. Since
it is a ratio it has no units. In the cgs system B
=μ
0
has units
of oersted (Oe) and
may be quoted in terms of G/Oe.
Owing to the different way that magnetism is de
ned in the
SI and cgs systems, a susceptibility speci
ed in both differs
by a factor of 4
κ
π
. The conversion from cgs to SI units is:
κ
SI
¼
4
πκ
cgs
ð
3
:
9
Þ
field is taken as directed away from the north (posi-
tive) pole and towards the south (negative) pole. The
field
lines are closed paths and the strength and orientation of
the
field varies with location relative to the poles. It is
strongest at the poles. The magnetic
field of a bar magnet
is like that produced by a constant electrical current
owing
in a loop of wire (see
Section 5.2.2
). This electrical model of
magnetism is the basis for measurements in the SI system.
Since susceptibility values for rocks and minerals are com-
monly small, they are normally stated as
10
-
3
10
-
5
,
i.e. 1000 or 100,000 times their actual value. Whether in SI
or cgs units, volume susceptibility (
'
'
or
'
'
κ
) is usually quoted; the
alternative is mass susceptibility (
). To convert from mass
to volume susceptibility, simply multiply by the density of
the substance.
Magnetic susceptibility depends upon the magnetic per-
meability (
χ
μ
) of the material in the following way:
κ ¼
μ μ
0
3.2.3.2
Magnetic measurement units
In the SI measurement system, the intensity or strength of
an object
μ
0
¼
μ
μ
0
1
ð
3
:
10
Þ
'
is magnetism is defined in terms of an electric
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