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A review is given of the development in the field of cosmical electrodynamics. It is mentioned that the great interest in thermonuclear research has produced a considerable progress in plasma physics. This is of astrophysical interest because it is now possible to check the theories of a plasma by experiment. As an example, a recent experiment in a “homopolar” machine is discussed, and its importance to the theory of the origin of the solar system is emphasized. Conclusions about the origin of the solar system are drawn. In particular, the mechanism by which Saturn's ring has been produced is discussed. It is further pointed out that the moon probably was born as a planet later captured by the earth.

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Definition of the Electro-Magnetic StateThe electro-magnetic state in a certain volume is defined if we know the electric field E and the magnetic field H as functions of space and time. However, because of the first Maxwell equationthe magnetic field variations are defined by the electric current (comprising the current density i and the displacement current aD/Ot). Hence a description in terms of currents is possible, and often physically more interesting than a description in terms of magnetic fields. The displacement current is of importance only for frequencies of the order of the plasma frequency or higher. As in this paper we describe stationary or slowly varying phenomena, we shall neglect it. Measurements of Magnetic Fields and CurrentsIn astrophysics no direct measurements of electric fields have yet been made. All conclusions about electric fields have been reached in an indirect way. The magnetic fields are much better known, because it is possible to measure them by means of the Zeeman effect or the Faraday rotation. However, up to quite recently it was essentially the longitudinal effects which were measured. Of the six electro-magnetic vector components only one, viz. the magnetic component along the line of sight, was measured, so no real understanding of the electro-magnetic state has been possible. Important progress in the knowledge of solar electro-magnetic conditions was recently made by SEVERNY (1964SEVERNY ( , 1965 with systematic measurements of the transverse Zeeman effect. In active regions, including sunspots, he has sometimes found a remarkable rotation of the transverse component H~ and a change of its magnitude from one point to another.If the z-axis of an orthogonal coordinate system points along the line of sight we can calculate the current component iz from = 4rcL ay J

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