ON THE LINES OF MAGNETIC FORCE WITHIN A MAGNET. 
39 
3117 . So, by this test there exist lines of force within the magnet, of the same 
nature as those without. What is more, they are exactly equal in amount to those 
without. They have a relation in direction to those without ; and in fact are conti- 
nuations of them, absolutely unchanged in their nature, so far as the experimental 
test can be applied to them. Every line of force therefore, at whatever distance it 
may be taken from the magnet, must be considered as a closed circuit, passing in 
some part of its course through the magnet, and having an equal amount of force in 
every part of its course. 
3118. When the axial part of the wire is dismissed and the magnet employed in 
its place, so as to be included in the circuit, it is easy to see how it acts the part of 
the conductor. For suppose the wire itself to be continued 
from N to h, fig. 12, by any of the three paths indicated by 
dotted lines, the effect is the same in all the cases, both by 
experiment (3093.) and by principle (3100.). For whatever 
the form of the path, it will in one revolution intersect the 
same amount of lines of force within the magnet, as are intersected in the contrary 
direction by the part of the wire outside the magnet; and when the magnet is em- 
ployed to complete the circuit in place of the internal wire, then its substance pro- 
duces precisely the same result ; for direction and every other circumstance which 
influences the result remains the same: one conductor has simply been substituted 
for another. The great mass of the magnet might be supposed able to do something 
more than the thin wire, but the reason why it only equals it in effect will be seen 
hereafter (313/.). And as the axial wire, in revolving, does nothing but conduct 
(3095.), all the effect being produced by that part which represents a radius between 
the axis and the equator (3098.) ; so the magnet, revolving as a cylinder, is as to 
its mass like the revolving wire ; with the exception of so much of it as represents a 
radius connecting together the two points at the pole or axis and at the equator, 
where communication with the wire is completed. As was shown long ago (220.), 
if a cylinder magnet be revolved, and the ends of the galvanometer wires a c be ap- 
plied to the extremities of its axis, no current is evolved ; but if a be applied to one 
end, it matters not which, and c be applied at the equator or any other part on the 
surface of the cylinder, a current always in the same direction for the same rotation 
will be produced. 
3119. Further to prove these points, the magnets were cut in half through the 
equatorial plane, and then, either a disc of copper placed there, or a wire radius only, 
or the magnets brought together again: and these three arrangements were used in 
succession to complete the circuit from the axial wire (3095.) to a fixed wire at the 
surface of the equator. Whichever was employed the current produced was the 
same, both in direction and amount. If the cylinder magnet above described (3118.) 
be terminated at the ends by attached discs of silver or copper, the wires applied to 
their surfaces, as they revolve with the magnet, produce precisely the same currents 
as to direction as if applied to the surface of the magnet itself (218. 219.). 
