LINES OF MAGNETIC FORCE — MAGNETIC POLARITY. 
53 
magnetic body (2807.) • Elsewhere, the lines of force go with the same direction 
across the magnetic field where the iron is, as where it is not ; and it is to me a 
proved fact, proved by the numerous results given, that a section of the lines of force 
taken across the magnetic field through the air, where there is no iron, is exactly 
equal in amount of force to a section taken across parallel to and through the iron disc 
(3163.). All iron under induction must have just as much force, i.e. lines offeree 
in its internal parts, as is equivalent to the lines which fall on to, and are continued 
through and out of it ; and the same is true, as it appears to me, of any other para- 
magnetic or diamagnetic substance whatever. The same is true for the magnet itself; 
for a section through the magnet has been shown to be exactly equal to a section 
anywhere through the outer lines of force (3121.), and these sections may be taken 
at the surface of the magnet, where they may be considered as either in the air or in 
the magnet indifferently; and therefore alike in size, shape, power, polarity, and 
every other point. 
3166. 1 have used the phrase conduction polarity on a former occasion (2818. 2835.), 
but so limited, that it could lead to no mistake of my meaning, either then or now. 
It requires no words to show how it is included in the higher and general expression 
of the direction or polarity of the lines of force. 
316/. Some other results with the disc apparatus (3159.) were obtained, which it 
may be useful to describe here. Tin was formed into a disc of OT in thickness, and 
2'5 inches in diameter. The effect of the friction of the copper conductor at its edge, 
was a feeble current, the reverse of that produced in the cases of copper and iron 
(3161.) ; but the current produced by the revolution, and dependent on the polarity 
of the lines of force, was tiie same as before. It produced a swing deflection of 14°'9 
for two revolutions of the disc. 
3168. A disc of hismuth produced far too strong a current by friction against the 
copper conductor, to allow of any useful result in its simple state. A ring of copper 
foil was therefore formed, and being placed tightly on the bismuth disc, was wedged 
up by plates of clean copper foil, so as to produce a clean hard contact; imperfect, 
no doubt, but as general as could be made under the circumstances. When this disc 
was rotated in the one direction, it gave a deflection in the same direction as if a 
copper or iron disc had been used ; when rotated the other way, the deflection was 
little or nothing. This difference is due to the united influence of the rotation effect 
and the friction effect in the one case, and their opposition in the other ; but the 
results show that the lines of force are in the same direction through bismuth, when 
between the magnetic poles, as they are through copper and iron. The induced 
current is small, both because of the bad conducting power of the bismuth and the 
imperfect contact at the edge. When the same copper rim was placed on the copper 
disc, it reduced the deflection of the needle from 26°'5 to 9°'34. 
