An attempt is made to explain the origin of the observed magnetic field of the earth as being due to a current system circulating in the core, the current system in its turn being maintained by world-wide magnetic disturbances. The mantle behaves as a semiconductor, and the conduction electron density is given by the Boltzmann distribution law. The conductivity changes steeply along the radial direction, and a ‘potential hill’ is produced in the radial direction. Owing to interaction of the induced current with the steady magnetic field, a Hall potential is developed; this modifies the existing ‘potential hill,’ and hence the conductivity becomes to some extent dependent on the induced current vector. The consequent nonlinearity rectifies the induced current, and a net amount of unidirectional current aiding the existing magnetic field is left over at the end of the disturbance. This gradually penetrates down to the core and has a decay time of the order of a million years. The net unidirectional current then grows through successive disturbances. It is shown that, for reasonable values of conductivity, temperature, and electron mobility, the magnetic disturbances maintain sufficiently large current in the core so that the earth's magnetism can be explained as entirely due to the magnetic disturbances.