The electrical conductivity of the Earth's mantle

The rapid rise in electrical conductivity at depths of 700 km in the Earth's mantle, inferred by S. Chapman and his collaborators from a study of the transient variations of the geomagnetic field, may be well explained on the hypothesis that the process of intrinsic semi‐conduction is of importance in this region. H. Hughes of the Department of Geodesy and Geophysics, Cambridge University, has demonstrated the various conduction processes occurring in olivine and similar minerals and has shown that the results fit in quantitatively with the observational data referred to above. An indirect method of estimating the effect of pressure on the process of intrinsic semi‐conduction has been attempted and does not appear to alter the view that the rapid rise in conductivity is, in essence, a temperature effect. From this work estimates can be made of the electrical conductivity of the lower part of the mantle, which agree with estimates inferred from the spectrum of the secular variation of the geomagnetic field and the explanation of the irregular fluctuations in the length of the day.