Large-Scale Direct-Current Earth Potentials and the Geomagnetic Dynamo

A proposal was made by Runcorn 35 years ago that measurements of the large-scale zero frequency Earth potential on the surface of the planet could be used to place limits on the toroidal magnetic field strength at the core-mantle boundary. This intriguing idea has been little pursued over the years, largely because of technological difficulties. We review the present status of investigations of the Earth's dynamo using this basic idea. We also discuss briefly some planned future measurement possibilities for pursuing such work, as well as note a number of significant experimental problems which can potentially produce systematic errors in the results.