Part I of this paper is devoted to a description of various experiments performed at wavelengths below 200 cm. Curves are given to show the effect of the earth and various types of inductively excited antennas called wave directors. Part I is concluded with a discussion of beam and horizontally polarized radiation. Part II is devoted chiefly to the magnetron tubes used for the production of very short wavelengths (as low as 12 cm) and the circuit arrangements employed. It is shown that the geometry of the tube and its external connections are of great importance. The effect of variation of plate voltage, magnetic field strength and other factors on the high-frequency output, is described.