APPENDIX VII.

ON MAGNETISATION PRODUCEDBY HERTZIAN CURRENTS;A MAGNETIC DIELECTRIC:[42]

BY M. BIRKELAND.

“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.”

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. 67shows 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 theelectriceffect 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.

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.

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.

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.

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 coresdiminishthe 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.

Footnotes:[1]Phil. Mag., XXVI., pp. 229, 230, August, 1888; or “Lightning Conductors and Lightning Guards,” pp. 104, 105; alsoProc. Roy. Soc., Vol. 50, p. 27.[2]Strictly speaking, in the waves themselves there is no lag or difference of phase between the electric and the magnetic vibrations; the difference exists in emitter or absorber, but not in the transmitting medium. True radiation of energy does not begin till about a quarter wave length from the source, and within that distance the initial quarter period difference of phase is obliterated.[3]SeeNature, Vol. XLI., p. 368, where I first described this experiment; or quotation in J. J. Thomson’s “Recent Researches,” p. 395.[4]While preparing for the lecture it occurred to me to try, if possible during the lecture itself, some new experiments on the effect of light on negatively charged bits of rock and ice, because if the effect is not limited to metals it must be important in connection with atmospheric electricity. When Mr. Branly coated an aluminium plate with an insulating varnish, he found that its charge was able to soak in and out of the varnish during illumination (Comptes Rendus, Vol. CX., p. 898, 1890). Now the mountain tops of a negatively charged earth are exposed to very ultra-violet rays, and the air is a dielectric in which quiet up-carrying and sudden downpour of electricity could go on in a manner not very unlike the well-known behaviour of water vapour; and this perhaps may be the reason, or one of the reasons, why it is not unusual to experience a thunderstorm after a few fine days. I have now tried these experiments on such geological fragments as were handy, and find that many of them discharge negative electricity under the action of a naked arc, especially from the side of the specimens which was somewhat dusty, but that when wet they discharge much less rapidly, and when positively charged hardly at all. Ice and garden soil discharge negative electrification, too, under ultra-violet illumination, but not so quickly as limestone, mica schist, ferruginous quartz, clay, and some other specimens. Granite barely acts; it seems to insulate too well. The ice and soil were tried in their usual moist condition, but, when thoroughly dry, soil discharges quite rapidly. No rock tested was found to discharge as quickly as does a surface of perfectly bright metal, such as iron, but many discharged much more quickly than ordinary dull iron, and rather more quickly than when the bright iron surface was thinly oiled or wetted with water. To-day (June 5, 1894) I find that the leaves of Geranium discharge positive electrification five times as quickly as negative, under the action of an arc light, and that glass cuts the effect off while quartz transmits it. (For Elster and Geitel’s experiments, and those of Righi,seeAppendices, p. 115et seq.)[5]SeeB. A. Report, 1884, pp. 502-519; orPhil. Mag., Vol. XIX., pp. 267-352.[6]J. J. Thomson, “Recent Researches,” 344.[7]Wied. Ann., XLVII., p. 77.[8]FitzGerald,Nature, Vol. XLI., p. 295, and Vol. XLII., p. 172.[9]Wied. Ann., 44, p. 74.[10]Weid. Ann., 40, p. 399.[11]Phil. Mag., Vol. XXXI., p. 223.[12]E. Branly,Comptes Rendus, Vol. CXI., p. 785; and Vol. CXII., p. 90.[13]JournalInstitution of Electrical Engineers, 1890, Vol. XIX., pp. 352-4; or “Lightning Conductors and Lightning Guards,” pp. 382-4.[14]SeePhil. Mag., Jan., 1894, p. 94, where this explanation (whether true or not) was first given, and where the author first published his fuller experience of coherer behaviour.[15]This statement has been absurdly misunderstood, as if it was a prediction of what would always be the limit of sensitiveness for any apparatus and any sized sender. Nothing of the kind was in my mind. Such predictions are always preposterous, and I am not obliged to those who imagined that I had been guilty of one of them.—O. J. L., 1899.[16]FitzGerald tells me that he has succeeded with carbon also. My experience is that the less oxidisable the metal, the more sensitive and also the more troublesome is the detector. Mr. Robinson has now made me a hydrogen vacuum tube of brass filings, which beats the coherer for sensitiveness. July, 1894.[17]Cf.Trouton, inNature, Vol. 39, p. 393; and many optical experiments by Mr. Trouton, Vol. 40, p. 398. Since then the above described and depicted apparatus for electro-optic experiments has been imitated in a neat, compact form by Prof. J. Chunder Bose, of Calcutta, and with it he has obtained many admirable and interesting optical results. SeeProc.Roy. Soc.[18]Proc.Roy. Soc., 1879 and 1882.[19]“Dimensional Properties of Matter,”Phil. Trans., 1879.[20]Evening Lecture on “Dust,” by the writer, seeNature, Vol. 31, p. 265; alsoJournalof the Royal Institution, May, 1886.[21]Apparatus for the purpose is now in the catalogue of Messrs. Ducretet, of Paris, but they supply a pair of combs of points. It makes a more interesting experiment if only one point is used, in a moderate space, and the electric supply regulated so as not to hurry the disappearance of the smoke too quickly, but to exhibit the stages of aggregation which precede the final disappearance by deposition. Any kind of smoke serves, but a bit of magnesium ribbon burnt under a bell jar is cleanly and effective. It should be looked at in a window or other good light, of course.[22]Journalof the Institution of Electrical Engineers for 1890, pp. 352-4.[23]“Modern Views,” second edition, p. 359.[24]Journalof the Institution of Electrical Engineers, 1890, p. 352.Seealso remarks by Mr. Stroh in two microphone discussions,Journalof the Institution of Electrical Engineers, 1883 and 1887.[25]Verbally to Section A at Bath, 1888.SeealsoPhil. Mag., August, 1888, p. 229; andThe Electrician, Vol. 21, pp. 607-8.[26]Nature, Vols. 39 and 40.[27]Nature, Vol. 41, p. 368; or, “Modern Views of Electricity,” second edition, p. 338.Seealso Fig. 4, p. 6.[28]Nature, Vol. 41, p. 295; and Vol. 42, p. 172.[29]Phil. Mag., March, 1891; also January, 1894.[30]Wied. Ann., Vol. 40.[31]Phil. Mag., January, 1894.[32]Phil. Trans., 1897, A., communicated to the Royal Soc., June, 1896.[33]Quoted inThe Electrician“Notes,” October 1, 1897.[34]Phil. Mag., May and July, 1897. He also shows electrical cohesion by an emulsion of oil and water, the two liquids, thoroughly shaken up, at once separating when exposed to strong electrical influence.[35]A fact noticed by Bichat and Blondlot.[36]In this apparatus the mercury amalgams of K and Na are run through a fine funnel, so that the freshly-formed surface of the drops may be illuminated. Under these circumstances, while pure mercury fell from -185 to -175 volts in 30sec., amalgam of zinc fell from -195 to -116 in 15sec., amalgam of sodium fell from -195 to 0 in 10sec., and an amalgam of potassium fell from -195 to 0 in 5sec.[37]SeeFig. 7, page 9.[38]Corresponding to the activity of this gas as found by Wiedemann and Ebert (Wied. Ann., XXXIII., p. 258), in their researches on the influence of light on ease of sparking.[39]See Fig. 13, p. 15.[40]Comptes Rendus, vol. 107, p. 559.[41]Berichte der Naturforschenden Gesellschaft zu Freiburg i. B., Bd. IX. Heft 2, June 21, 1894.[42]Abstracted fromComptes 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.

Footnotes:

[1]Phil. Mag., XXVI., pp. 229, 230, August, 1888; or “Lightning Conductors and Lightning Guards,” pp. 104, 105; alsoProc. Roy. Soc., Vol. 50, p. 27.

[2]Strictly speaking, in the waves themselves there is no lag or difference of phase between the electric and the magnetic vibrations; the difference exists in emitter or absorber, but not in the transmitting medium. True radiation of energy does not begin till about a quarter wave length from the source, and within that distance the initial quarter period difference of phase is obliterated.

[3]SeeNature, Vol. XLI., p. 368, where I first described this experiment; or quotation in J. J. Thomson’s “Recent Researches,” p. 395.

[4]While preparing for the lecture it occurred to me to try, if possible during the lecture itself, some new experiments on the effect of light on negatively charged bits of rock and ice, because if the effect is not limited to metals it must be important in connection with atmospheric electricity. When Mr. Branly coated an aluminium plate with an insulating varnish, he found that its charge was able to soak in and out of the varnish during illumination (Comptes Rendus, Vol. CX., p. 898, 1890). Now the mountain tops of a negatively charged earth are exposed to very ultra-violet rays, and the air is a dielectric in which quiet up-carrying and sudden downpour of electricity could go on in a manner not very unlike the well-known behaviour of water vapour; and this perhaps may be the reason, or one of the reasons, why it is not unusual to experience a thunderstorm after a few fine days. I have now tried these experiments on such geological fragments as were handy, and find that many of them discharge negative electricity under the action of a naked arc, especially from the side of the specimens which was somewhat dusty, but that when wet they discharge much less rapidly, and when positively charged hardly at all. Ice and garden soil discharge negative electrification, too, under ultra-violet illumination, but not so quickly as limestone, mica schist, ferruginous quartz, clay, and some other specimens. Granite barely acts; it seems to insulate too well. The ice and soil were tried in their usual moist condition, but, when thoroughly dry, soil discharges quite rapidly. No rock tested was found to discharge as quickly as does a surface of perfectly bright metal, such as iron, but many discharged much more quickly than ordinary dull iron, and rather more quickly than when the bright iron surface was thinly oiled or wetted with water. To-day (June 5, 1894) I find that the leaves of Geranium discharge positive electrification five times as quickly as negative, under the action of an arc light, and that glass cuts the effect off while quartz transmits it. (For Elster and Geitel’s experiments, and those of Righi,seeAppendices, p. 115et seq.)

[5]SeeB. A. Report, 1884, pp. 502-519; orPhil. Mag., Vol. XIX., pp. 267-352.

[6]J. J. Thomson, “Recent Researches,” 344.

[7]Wied. Ann., XLVII., p. 77.

[8]FitzGerald,Nature, Vol. XLI., p. 295, and Vol. XLII., p. 172.

[9]Wied. Ann., 44, p. 74.

[10]Weid. Ann., 40, p. 399.

[11]Phil. Mag., Vol. XXXI., p. 223.

[12]E. Branly,Comptes Rendus, Vol. CXI., p. 785; and Vol. CXII., p. 90.

[13]JournalInstitution of Electrical Engineers, 1890, Vol. XIX., pp. 352-4; or “Lightning Conductors and Lightning Guards,” pp. 382-4.

[14]SeePhil. Mag., Jan., 1894, p. 94, where this explanation (whether true or not) was first given, and where the author first published his fuller experience of coherer behaviour.

[15]This statement has been absurdly misunderstood, as if it was a prediction of what would always be the limit of sensitiveness for any apparatus and any sized sender. Nothing of the kind was in my mind. Such predictions are always preposterous, and I am not obliged to those who imagined that I had been guilty of one of them.—O. J. L., 1899.

[16]FitzGerald tells me that he has succeeded with carbon also. My experience is that the less oxidisable the metal, the more sensitive and also the more troublesome is the detector. Mr. Robinson has now made me a hydrogen vacuum tube of brass filings, which beats the coherer for sensitiveness. July, 1894.

[17]Cf.Trouton, inNature, Vol. 39, p. 393; and many optical experiments by Mr. Trouton, Vol. 40, p. 398. Since then the above described and depicted apparatus for electro-optic experiments has been imitated in a neat, compact form by Prof. J. Chunder Bose, of Calcutta, and with it he has obtained many admirable and interesting optical results. SeeProc.Roy. Soc.

[18]Proc.Roy. Soc., 1879 and 1882.

[19]“Dimensional Properties of Matter,”Phil. Trans., 1879.

[20]Evening Lecture on “Dust,” by the writer, seeNature, Vol. 31, p. 265; alsoJournalof the Royal Institution, May, 1886.

[21]Apparatus for the purpose is now in the catalogue of Messrs. Ducretet, of Paris, but they supply a pair of combs of points. It makes a more interesting experiment if only one point is used, in a moderate space, and the electric supply regulated so as not to hurry the disappearance of the smoke too quickly, but to exhibit the stages of aggregation which precede the final disappearance by deposition. Any kind of smoke serves, but a bit of magnesium ribbon burnt under a bell jar is cleanly and effective. It should be looked at in a window or other good light, of course.

[22]Journalof the Institution of Electrical Engineers for 1890, pp. 352-4.

[23]“Modern Views,” second edition, p. 359.

[24]Journalof the Institution of Electrical Engineers, 1890, p. 352.Seealso remarks by Mr. Stroh in two microphone discussions,Journalof the Institution of Electrical Engineers, 1883 and 1887.

[25]Verbally to Section A at Bath, 1888.SeealsoPhil. Mag., August, 1888, p. 229; andThe Electrician, Vol. 21, pp. 607-8.

[26]Nature, Vols. 39 and 40.

[27]Nature, Vol. 41, p. 368; or, “Modern Views of Electricity,” second edition, p. 338.Seealso Fig. 4, p. 6.

[28]Nature, Vol. 41, p. 295; and Vol. 42, p. 172.

[29]Phil. Mag., March, 1891; also January, 1894.

[30]Wied. Ann., Vol. 40.

[31]Phil. Mag., January, 1894.

[32]Phil. Trans., 1897, A., communicated to the Royal Soc., June, 1896.

[33]Quoted inThe Electrician“Notes,” October 1, 1897.

[34]Phil. Mag., May and July, 1897. He also shows electrical cohesion by an emulsion of oil and water, the two liquids, thoroughly shaken up, at once separating when exposed to strong electrical influence.

[35]A fact noticed by Bichat and Blondlot.

[36]In this apparatus the mercury amalgams of K and Na are run through a fine funnel, so that the freshly-formed surface of the drops may be illuminated. Under these circumstances, while pure mercury fell from -185 to -175 volts in 30sec., amalgam of zinc fell from -195 to -116 in 15sec., amalgam of sodium fell from -195 to 0 in 10sec., and an amalgam of potassium fell from -195 to 0 in 5sec.

[37]SeeFig. 7, page 9.

[38]Corresponding to the activity of this gas as found by Wiedemann and Ebert (Wied. Ann., XXXIII., p. 258), in their researches on the influence of light on ease of sparking.

[39]See Fig. 13, p. 15.

[40]Comptes Rendus, vol. 107, p. 559.

[41]Berichte der Naturforschenden Gesellschaft zu Freiburg i. B., Bd. IX. Heft 2, June 21, 1894.

[42]Abstracted fromComptes 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.

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