Physics. — “Discontinuities in the Magnetisation”. By Dr. B. van 
DER Por Jr. (Communicated by Prof. H. A. Lorentz). 
(Communicated at the meeting of June 26, 1920). 
In a recent paper, “Zwei mit Hilfe der neuen Verstürker entdeckte 
Erscheinungen”’, in the Phys. Zeitschr., Sept. 1919, Prof. H. Bark- 
HAUSEN describes some experiments by which discontinuities in the 
magnetisation were made detectable by a telephone. To this end 
‚an iron rod was placed vertically in a small solenoid which was 
connected to a triode-amplifier. When a small permanent magnet 
was brought by hand near the iron rod, so that the latter became 
magnetised, a rustling sound in a telephone connected to the 
amplifier could be heard, which sound was due to the induction 
pulses caused by the discontinuities. 
Repeating and extending these experiments we observed some new 
phenomena, which may be described here briefly. 
At the outset it may be remarked, that the mentioned rustling 
was known already in the technics of wireless telegraphy where it 
was regarded as troublesome in the use of the magnetic detector of 
Marcon! *). 
In our experiments we used a so-called three-stage low-frequency 
amplifier, in which the energy-transport from triode to triode took 
place by means of small transformers. The terminals of the solenoid 
that contained the iron to be magnetised were connected with the 
filament and the grid of the first triode either directly or by means 
of a small transformer of suitable dimensions. 
The rustling in the telephone, due to the induction-pulses in the 
solenoid must primarily be caused by a discontinuous change of the 
total flux through the solenoid which accompanies the sudden changes 
of magnetisation-direction of molecule-groups or of iron-erystals. 
When in this phenomenon the magnetisation of the separate iron- 
crystals is reversed suddenly by the external field, we should expect 
that the change of the number of lines of force that are already 
present in the case of spontaneous magnetisation and which must 
describe in the air small curves near the surface of the iron, will be 
best observed by means of a solenoid fitting narrowly round the iron. 
1) Ecctes. Wireless Telegraphy and Telephony, 2nd Ed. p. 284, 285. 
41 
Proceedings Royal Acad. Amsterdam. Vol. XXIIL. 
