1889-90.] Prof. Knott on Interactions of Magnetisations. 401 
The Interactions of Circular and Longitudinal Magnetisa- 
tions. By Prof. C. Gr. Knott, D.Sc. 
(Read July 21, 1890.) 
Preliminary Note. 
In the course of an extended series of investigations into the 
relations of magnetism and stress, part of which is already pub- 
lished in the Transactions (vol. xxxv.), I was led to consider the 
effect of a current passing along a wire upon its longitudinal mag- 
netic intensity. Wiedemann, Buff, and Villari* have discussed 
this problem in some of its aspects. It does not seem possible, how- 
ever, to deduce from their results completely satisfactory conclusions 
as to the effect of the current upon the apparent longitudinal per- 
meability. The question, as it presented itself to my mind, was not 
so much as to the effect of a varying current along the wire upon 
the apparent longitudinal moment, but rather as to the behaviour 
of the wire in various longitudinal fields according as it was carry- 
ing a current or not. It was to be expected, in accordance with the 
results of previous investigators, that a diminution of longitudinal 
intensity would be an evident accompaniment of a steadily sustained 
circular magnetisation. In other words, the susceptibility to a 
longitudinal magnetising force would, in all probability, be smaller 
when a current was passing along the wire. This has been fully 
established in the experiments, of which this forms a short pre- 
liminary note. Other effects, however, have been observed, which 
are (so far as I know) novel, and which may lead to clearer views 
regarding the internal structure of magnets and magnetised matter. 
In the experimental work I have been assisted by Mr Tsuruta, a 
graduating student of physics in the Imperial University. 
The wire to be experimented with was carefully annealed, and 
then inserted as the core of a solenoid of copper wire of 138 coils 
per centimetre length. The wire and solenoid lay magnetically east 
and west, and at a suitable distance from the one end a delicately 
suspended mirror with small magnets attached to its back served in 
the usual way as the magnetometer. The deflections were measured 
by the familiar reflected beam method, the spot of light moving 
* For references, see Wiedemann’s Die Lehre von der Eledricitat , vol. iii. 
pp. 456-462 ; for theoretical discussion, see p. 476. 
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