The present paper deals with the subject of the magnetization of iron by high-frequency discharges, and the uses of magnetized steel needles for detecting and measuring currents of very great rapidity of alternation. It will be shown that these magnetic detectors offer a very simple means of investigating many of the phenomena connected with high-frequency discharges, and may be used over a wide range of periods of alternation. Not only may these detectors be used in ordinary Leyden jar circuits, but they also offer a sensitive means of investigating waves along wires and free vibrating circuits of short wavelengths.