“…The dynamo theory as proposed by Elsasser (1939Elsasser ( , 1946a and developed by Elsasser (1946bElsasser ( , 1947Elsasser ( , 1950Elsasser ( , 1955Elsasser ( , 1956aElsasser ( , 1956b, Bullard (1948, 1949a, 1949b), Bullard and Gellman (1954), and Parker (1955) ispresently considered to be the most promising explanation of the main geomagnetic field and its variations. Reviews of theories of the earth's magnetism are given by Runcorn (1954Runcorn ( , 1955aRuncorn ( , 1955b, Inglis (1955Inglis ( , 1965, Hide (1956Hide ( , 1966, Hide and Roberts (1961), Jacobs (1963), Kern and Vestine (1963), Braginskiy (1964), Lucke (1965), and DeVuyst (1966), The basic idea of the self-exciting dynamo for the earth is the regeneration of a previously existing magnetic field. This is accomplished by means of complicated induction phenomena.…”