1889 - 90 .] Prof. Knott on Interactions of Magnetisations. 405 
is more pronounced when the wire is acquiring a longitudinal 
polarity in the opposite direction to that in which the linear current 
is flowing. Hence, during any cyclic operation, the wire tends to 
acquire an average polarity in the same direction as that in which 
the current flows. 
These effects are more pronounced the stronger the linear current 
is as compared with the magnetising field. In a moderate cyclic 
field, the effect of the linear current may he so strong as to prevent 
the wire ever acquiring other than one kind of polarity. For 
example, this effect was produced in a field ± 4, with a current of 
nearly 2 amperes. For the linear current in one direction the 
( b ) curve never dipped below the zero intensity line ; and for the 
current in the other direction the curve ( c ) never rose above it. 
On the other hand, in stronger magnetising fields, from 10 and 
upwards, the ( b ) and ( c ) curves for the same linear current of 
2 amperes tend to terminal coincidence, but diverge at the inter- 
mediate stages. It is proposed to study more fully these relations, 
and to extend the investigations to nickel and cobalt. 
In the course of these experiments, an effect was noticed, which 
demonstrates the extraordinary complexity of magnetic distribution 
in a magnetised wire. If, before the wire has been magnetised at 
all, a current is passed along it, no appreciable longitudinal polarity 
is produced, as measured by a magnetometer needle placed as in 
the above experiments. Suppose, however, that the wire has been 
pretty strongly magnetised ; and that, in the manner discussed by 
Auerbach, and used almost universally now, the wire is de- 
magnetised by reversals of gradually diminishing magnetising 
currents until the magnetometer needle stands almost exactly at 
zero. It is, of course, generally recognised that the wire so de- 
magnetised cannot be regarded as being in anything like the same 
condition as it was in its originally unmagnetised state, although 
it appears to have lost polarity. That this view is correct may be 
at once demonstrated by simply passing a pretty strong current 
along the wire, when a very pronounced polarity will be evidenced 
by a comparatively large deflection of the magnetometer needle. 
Reversing the current will reverse this polarity. Heating to a red 
heat can alone truly demagnetise an iron wire. 
