Chapter V: Part 5

“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.

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.

Cylinder 1 did not affect the maximum spark length. Cylinders 2-4 reduced the maximum spark to ⅒th of its former value; 7 and 8 to ¹/₁₀₀th, and No. 9 to ¹/₂₀₀th of its former value (viz., from 9 millimetres to ·05 millimetre). Nos. 10 and 11 had but a feeble action, and reduced the spark from 8 to 7 millimetres.

Tube No. 12, filled with distilled water, scarcely affected the spark length; the period of the secondary increases a little, but the maximum spark is the same as before, once syntony is re-established. Filled, however, with dilute sulphuric acid, containing 10, 20, or 30 per cent., the tube reduced the spark considerably, in each case about the same, viz., from 9 to 1·3 about. (Currents induced by Maxwellian radiation in electrolytes had been already observed by J. J. Thomson.)

While trying to re-establish syntony between primary and secondary, I found that the period of the resonator was considerably increased by the cylinders 2-4, but that the maximum spark length was much diminished. With the cylinders Nos. 5-9 in the spiral, it was no longer possible to establish syntony, “a fact which is certainly due to their considerable absorption of energy. Take, for example, cylinder 9: electromagnetic energy must converge rapidly towards it in order to be transformed, and the space finds itself empty of energy as air is exhausted of vapour in presence of an absorbing substance.”

“This absorption is probably due to hysteresis in the ferruginous cylinders; the development of Joulian heat, so typically shown by cylinder 12, being undoubtedly of the same order in cylinders 3-9 as in Nos. 10, 11.

“It is probably by reason of this absorption that I have not succeeded in establishing stationary magnetic waves in a circuit of ferro-paraffin.”

If one of the cylinders 2-9, is wrapped in tinned paper before introducing it into the spiral A, its action is completely stopped. (These conducting cores _diminish_ the period of the resonator; it is much as if the spiral A were partially shunted out; but the maximum spark returns as soon as syntony is re-established.) To examine this further he enclosed the cylinder in drums of cardboard having fine wires either along generating lines, or along circular parallels. The latter suspended the action of an interior ferruginous cylinder, the former did not.

To find to what depths the magnetism penetrated, Birkeland inserted hollow ferruginous drums into A, measured their effect, and then plunged solid cylinders into them to see whether the effect increased.

He thus found that the magnetisation easily traversed 7 millimetres thickness of the 10 per cent. ferro-paraffin, and 5 millimetres of the 25 per cent.

The substance is comparable to a dieletric on the theory of Poisson-Mossotti.

“The results obtained with our magnetic dielectric invite to new researches”--such as the mechanical force excited by electric waves on a delicately-suspended ferro-paraffin needle, and the rate of propagation of Maxwellian waves through such a substance.