On the Electrical Conductivity in the Earth's Core.

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In order to investigate the electrical conductivity in the earth's core, the writer studied the possibility of the magneto-hydrodynamic waves in the core and the shielding and cancelling effect in the earth together with the behavior of time-dependent dipole situated at the surface of the core. From the observational results in seismology that ScS waves are not likely affected by the magneto-hydrodynamic waves and we find no waves which come through the core as the magnetohydrodynamic waves, it may be roughly concluded that 103>σC>10-8 emu while the intensity of magnetic field should be less than 104 emu. After taking into account the shielding and cancelling effect for geomagnetic secular variation which is produced by the time-dependent electric currents flowing near the surface of the core, σc>10-5 emu is tentatively concluded by adopting the criterion that the electric current density for the primary origin of secular variation will not be much larger than the one for the main dipole field, the study being done for a variation having a 100-year period. A similar conclusion is obtained from the study concerning the modification of the magnetic field of a time-dependent dipole situated at the surface of the core. Although the estimate of σc mentioned in this paper is crude, it is of interest that the possible range of ac, say 10-5>σc>10-8 emu, fairly agrees with those obtained before from a quite different standpoint such as physics of metallic iron at high temperature and pressure. The writer is of opinion that Elsasser-Bullards'consideration that σc amounts to the order of 10-6 emu is fairly right judging from both physical and geophysical viewpoints.
1952-09-20

On the Electrical Conductivity in the Earth's Core.

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