Fig. 219. MAGNETIC BRIDGE.
A lamp and scale are provided as in the reflecting galvanometer. When
adjusted the magnetic needle hangs as shown in the cut, Fig. 219,
without any tendency to turn towards either curved pole piece. If all
iron parts are symmetrical and of similar metal, a current through the
coils will make no difference. It will work in magnetic opposition upon
the two arms, or, in other words, will maintain both arms at identical
potential.
Fig. 220. POLE PIECES, MAGNETIC NEEDLE AND MIRROR OF MAGNETIC BRIDGE.
If there is the least difference in permeability, length or thickness
between any of the iron bars the magnetic potential of the two curved
arms will differ, and the magnetic needle will turn one way or the
other. In practical use different samples of iron are substituted for
the unwound members of the fourth side of the parallelogram, and the
needle by its motions indicates the permeability.
In the cut, Fig. 220, D D are the ends of the curved pole pieces; A the
wire carrying the mirror B and magnetic needle N, and E is the index
which shows the larger deflections.