Theory of Magnetism. 239 
neutral south and north magnetism, and then the sum of the 
magnetic moments of the molecules of iron cut through must 
be equal in magnitude to that of the molecules or space- 
elements cut through outside the iron. 
The order of the electrical phenomena would then be, that 
an electrical difference of potential appearing between two 
bodies situated in an insulating medium would produce upon 
their surfaces an accumulation of electricities at rest of oppo- 
site polarity, the magnitude of which would depend upon the 
resistance opposed by the surrounding insulating material to 
the electric disturbance. This resistance depends upon the 
geometrical conditions, and upon a special coefficient of dis- 
tribution belonging to the particular material. If the sur- 
rounding space is not insulating, but more or less a conductor 
of electricity, then electric currents are produced the strength 
of which is again dependent upon the sum of the resistances 
opposed to the propagation of the electricity. The electric 
current, or the electricity in motion, has, again, the property 
of attracting similarly-directed currents, or the bodies con- 
veying them, and of repelling oppositely-directed currents. 
If we assume, with Ampére, that the magnetic material is filled 
with preexisting molecular currents, then the electric current 
must tend to turn these elementary solenoids out of their 
position of equilibrium, so that their axes fall upon the peri- 
phery of circles which concentrically surround the current- 
earrier. If any particular material (e.g. iron) contains a 
larger number of such circular currents in the unit volume, 
then the work of the current must be greater, since a greater 
number of solenoids in which the current produces rotation is 
contained in each cross-section of the concentric ring. But 
since, moreover, the strengthening effect which the consecu- 
tive cross-sections must produce upon each other, on account 
of the diminished distance apart of the elementary solenoids, 
is now greater, therefore, for both reasons, the sum of the 
moments of a concentric ring consisting of iron must be 
greater than that of a space-ring of like dimensions, filled 
with a less magnetic material. This may also be expressed 
by saying that iron and the other so-called magnetic sub- 
stances oppose a less resistance to magnetic polarization than 
the non-magnetic substances, or that their magnetic conduc- 
tivity is greater. Magnetic action at a distance cannot take 
place with rings of homogeneous material which encircle a 
conductor concentrically, since all the lines of force run within 
the ring. The conditions are different in an iron ring which 
is not complete. Since the resistance offered by iron to mag- 
netic disturbance is only about go of that of air, as appears 
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