1852.] OF THE ROYAL INSTITUTION. 219 
and able, therefore, to exist by its own mutual relations. It has the 
dual and antithetic character belonging to both static and dynamic 
electricity ; and this is made manifest by what are called its polarities, 
i. e. by the opposite powers of like kind found at and towards its 
extremities. These powers are found to be absolutely equal to each 
other; one cannot be changed in any degree as to amount without 
an equal change of the other; and this is true when the opposite 
polarities of a magnet are not related to each other, but to the 
polarities of other magnets. The polarities, or the northness and 
southness, of a magnet are not only related to each other, through 
or within the magnet itself, but they are also related externally to 
opposite polarities, (in the manner of static electric induction) 
or they cannot exist; and this external relation involves and necessi- 
tates an exactly equal amount of the new opposite polarities to 
which those of the magnet are related. So that if the force of a 
magnet a is related to that of another magnet 6, it cannot act on a 
third magnet c without being taken off from 6, to an amount pro- 
portional to its action onc. The lines of magnetic force are shewn 
by the moving wire to exist both within and outside of the magnet ; 
also they are shewn to be closed curves passing in one part of their 
course through the magnet; and the amount of those within the 
magnet at its equator is exactly equal in force to the amount in any 
section including the whole of those on the outside. The lines of 
force outside a magnet can be affected in their direction by the use 
of various media placed in their course. A magnet can in no way 
be procured having only one magnetism, or even the smallest excess 
of northness or southness one over the other. When the pola- 
rities of a magnet are not related externally to the forces of other 
magnets, then they are related to each other: 7. e. the northness 
and southness of an isolated magnet are externally dependent on 
and sustained by each other. 
Now all these facts, and many more, point to the existence of 
physical lines of force external to the magnets as well as within. 
They exist in curved as well as in straight lines ; for if we conceive of 
an isolated straight bar magnet, or more especially of a round disc 
of steel magnetised regularly, so that its magnetic axis shall be in 
one diameter, it is evident that the polarities must be related to each 
other externally by curved lines of force ; for no straight line can at 
the same time touch two points having northness and southness. 
Curved lines of force can, as I think, only consist with physical lines 
of force. 
The phenomena exhibited by the moving wire confirm the same 
conclusion. As the wire moves across the lines of force, a current 
of electricity passes or tends to pass through it, there being no such 
current before the wire is moved. The wire when quiescent has no 
such current, and when it moves it need not pass into places where the 
magnetic force is greater or less. It may travel in such a course that 
if a magnetic needle were carried through the same course it would 
