Geoscience Reference
In-Depth Information
(a)
(b)
(c)
negative
charges
near rim
Figure 8.24.
The development of a self-exciting dynamo. (a) A metal disc rotating on
an axle in a magnetic field. Charge collects on the rim of the disc but cannot go
anywhere. (b) A wire joining the rim of the disc to the axle enables current to flow.
(c) The wire joining the rim to the axle is modified so that it is a coil looping around
the axle. Now the current flowing reinforces the magnetic field, which will induce
more current, thus sustaining the magnetic field. This is a self-exciting dynamo.
(From Bullard (1972).)
exceed the Curie temperatures for magnetic minerals (see Section 3.1.3). These
two facts rule out the model of a uniformly magnetized Earth. The core is pre-
dominantly composed of iron and could produce the magnetic field. The Earth's
magnetic field is not a constant in time but at present is slowly decreasing in
strength and drifting westwards. It undergoes irregular reversals as discussed and
used in Chapter 3. This changeability indicates that it is unlikely that the core
is uniformly magnetized or that there is a magnetic dipole at the centre of the
Earth. This leaves an electrical-current system as the most plausible model for
producing the magnetic field. The problem with such an electrical-current system
is that it must be constantly maintained. If it were not, it would die out in much
less than a million years;
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yetweknow from palaeomagnetic studies that the
magnetic field has been in existence for at least 3500 Ma.
The model that best explains the magnetic field and what we know of the
core is called the
geomagnetic dynamo
or
geodynamo
.Amechanical model of
a
self-exciting dynamo
wasdeveloped in the 1940s by W. M. Elsasser and Sir
Edward Bullard. Figure 8.24 shows how it works. A simple dynamo is sketched
in Fig. 8.24(a):ametal disc on an axle rotating in a magnetic field. The disc
is constantly cutting the magnetic field, and so a potential difference (voltage)
is generated between the axle and the rim of the disc. However, since there is
nowhere for current to flow, the charge can only build up around the rim. In
Fig. 8.24(b),awire is connected between the rim and the axle so that current
is able to flow, but, if the external magnetic field is removed, the current stops
flowing. In Fig. 8.24(c), the wire connecting the rim to the axle is coiled around
the axle; now the current flowing in the coil gives rise to a magnetic field that
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The ohmic decay time (
R
2
/η
,where
η
is the magnetic diffusivity of the core and
R
its radius) of
the core is about 60 000 yr.
