ME. G. GOEE Olr ELECTEOTOESION. 
545 
of the iron, and these are all profoundly affected by rise of temperature, and as it does 
not occur in non-magnetic metals (see Section 4), it is very probable that the torsions 
would be modified and prevented by heating the iron to redness during the passage of 
the currents. In a paper “ On the Magnetism of Electrodynamic Spirals” (see Phil. 
Mag., Oct. 1870) I have shown that with the same electric current passing through two 
successive spirals of iron and copper wire, the tangential or transverse magnetism of 
the iron is much greater than that of the copper at ordinary temperature ; but as the 
temperature of the two spirals is raised to full redness, the tangential magnetism of the 
two becomes equal. 
25. Is the axial-current state uniformly distributed \ 
To examine this I employed an iron wire 2-6 m. long and 1*75 mm. diameter, and 
compared the magnitudes of the torsions produced in it by a current in a helix 61 cen- 
tims. long surrounding the lower end of the wire, with those obtained when the coil 
surrounded its middle portion. The former averaged 31 mm. and the latter 3'2. The 
magnitudes of the movements, compared with those obtained with longer coils, indi- 
cated that the amount of torsion varied directly as the length of the wire within the 
helix. 
I also strewed fine iron filings upon a thin sheet of glass lying upon a horizontal 
annealed iron wire conveying a strong voltaic current in a direction at right angles to 
the terrestrial magnetic meridian, and vibrated the glass ; no unequal distribution of the 
filings could be detected. 
26. Retentive power of iron for axial-current influence. 
As an axial current passed through a recently annealed rod of iron or steel leaves it 
in a different physical state (without twisting it), and one succeeding a coil-current 
leaves the bar in a twisted condition, it is evident that iron possesses a retentive power, 
not only for the influence of a coil-current, but also for that of an axial one, and that 
the residuary axial-current state may exist with or without the condition of twist. 
In consequence of this retentive power of iron and steel for those effects, both the 
direction and the magnitude of the torsions in every case depend not only upon the 
kind and direction of the currents being applied, and upon the condition of the rod 
with regard to previous mechanical strain, but also upon the state of it with regard to 
both those residuary influences. 
27. Will coil-currents remove the residuary effect of axial ones % 
In these experiments a soft iron wire 2‘6 m. long and 3 mm. diameter was used, 
no weight being attached to it, and the battery of twelve cells was arranged for intensity 
of three. 
(1) After passage of a down ward current. — Five coil-currents of alternately opposite 
direction were passed, the first producing a north pole below. The directions and 
MDCCCLXXIV. 4 D 
