Astatic Needle.
A combination of two magnetic needles so adjusted as to
have as slight directive tendency as possible. Such a pair of needles
when poised or suspended will hardly tend to turn more to one point of
the compass than another. The combination is generally made up of two
needles arranged one above the other, with their poles in opposite
directions. This combination is usually called Nobili's pair. If of
equal strength and with parallel magnetic axes of equal length they
would be astatic. In practice this is very rarely the case. A resultant
axis is generally to be found which may even be at right angles to the
long axis of the magnets, causing them to point east and west. Such a
compound needle requires very little force to turn it one way or the
other. If one of the needles is placed within a coil of insulated wire a
feeble current will act almost as strongly to deflect the system as if
the other was absent, and the deflection will only be resisted by the
slight directive tendency of the pair of needles. This is the basis of
construction of the astatic galvanometer. Sometimes coils wound in
opposite directions and connected in series, or one following the other,
surround both needles, thus producing a still greater effect of
deflection.