“Two years ago[43] it was proved by conclusive experiments that Hertzian waves travelling along an iron wire magnetise transversely the very thin layer into which the alternating current penetrates, and whose thickness does not exceed some thousandths of a millimetre. Once proved that alternate magnetisation can be produced with such rapidity, other questions present themselves. One asks, for instance, if it is not possible to demonstrate in magnetic cylinders stationary magnetic waves analogous to the electric stationary waves along metallic wires.” [42] Abstracted from _Comptes Rendus_, June 11, 1894, and communicated by Dr. Oliver Lodge. [43] Why two years ago? It was practically proved by Savart early in the century, and has been observed over and over again since. However, it is true that experiments have been more numerous and conclusive of late, and have been pushed to very high frequencies.--O. J. L. The author finds that the conductivity of massive iron makes it an unsuitable substance, and uses instead a mixture of iron filings, or of chemically-obtained iron powder, with paraffin, to which he sometimes adds powdered quartz. This he moulds into cylinders, and inserts as the core of a spiral in an otherwise ordinary Hertz resonator. Fig. 67 shows emitter and receiver drawn to scale; the magnetic cores are introduced into the spiral A, and their effect on the length of the resonator spark is observed. With this arrangement of exciter the _electric_ effect of the spiral is negligible, since it is well removed from electrostatic disturbance, and subject only to magnetic. The spiral is of 12 well-insulated turns, the spark gap is a micrometer with point and knob, and a pair of adjustable plates to vary the capacity for purposes of tuning. He employed 12 different types of cylinder, all about 20 centimetres long, and 4 centimetres diameter. 1. A massive cylinder of soft iron. 2. A bundle of fine iron wires embedded in paraffin. 3-9. Six cylinders of the agglomerate of chemically-reduced iron in powder and paraffin, containing respectively 5, 10, 15, 20, 25 and 50 per cent. of iron. Then for control experiments:-- 10. A cylinder of agglomerate of zinc powder in paraffin, with 40 per cent. of zinc. 11. A cylinder of brass filings in paraffin, 20 per cent. of metal. 12. A tube of glass, 4·5 centimetres diameter, filled with various electrolytes. [Illustration: FIG. 67.] The manner of observing was as follows (the experiments were done in the laboratory of Hertz):-- The resonator, with its spiral empty, was syntonised with the exciter, and the maximum spark measured. It was between 4 and 9 millimetres long in these experiments. Then one or other of the above cylinders was introduced and the spark length measured afresh.