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.
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.