640 
their sign of magnetisation, intermediate positions of equilibrium are 
taken up by the erystals. The bending-back of the rod gives rise to the 
same phenomenon. 
Next cobalt was investigated. This material was at our disposal 
only in the form of cubi with edges of 9 mm. Ten of these cubi 
placed in a row and kept together by thin paper, formed a rod which 
could be lifted as a whole by our magnet. The total induction was 
therefore of the same order of magnitude as for the iron notwith- 
standing the thin air-layers between the separate cubi. Result: 
cobalt produces a sound of the same nature as that of iron but of 
smaller intensity. Without an amplifier the phenomenon is well detectable 
with iron, but hardly with cobalt. 
Nickel behaved differently. Even without amplifier separate ticks could 
be heard distinctly. With a three-stage triode-amplifier these ticks 
‘are very loud and remind one of explosions. The degree of purity of 
the nickel used was not known accurately. Perhaps small impurities 
influence this phenomenon. 
Electrolytic iron gives the characteristic soft-iron sound, but con- 
siderably weaker. 
The maximum intensity of sound was obtained by a 1.98 mm. 
thick nickel-steel wire. With the three-stage triode-amplifier the sound 
that is heard in the telephones during the magnetisation of nickel- 
steel can hardly be endured by the ear and, when the telephones 
are laid on the table, it was very well audible everywhere in a 
room of 7 7 m. and even outside. The sound of this material 
consists of a very great uumber of explosions quickly succeeding 
each other; this makes this nickel steel wire especially suitable for 
a further investigation of the discontinuities. 
In the experiments described above the continuous change of the 
tield was obtained in the same way as by BARKHAUSEN viz. by 
bringing a permanent magnet gradually nearer with the hand. The 
distinctness with which the phenomenon was observed in the case 
of nickel-steel allows a more accurate working-method. Hence we 
tried to produce a continuous change of the field by means of a 
gradually increasing current. 
Experimentally, however, it is not a simple matter to alter the 
intensity of the field by means of a current so gradually that, without 
iron, no sound is heard with a triode amplifier. Some experiments 
were made with a second solenoid placed inside the first and 
through which a current was sent that was also caused to flow through 
a very great selfinduction (a Rumkorj-magnet with short-circuited 
iron-core). When now a rod of nickel-steel was placed in the inner 
