Astatic Needle

A combination of two magnetic needles so adjusted as to have as slight directive tendency as po ible. Such a pair of needles when poised or suspended will hardly tend to turn more to one point of the compa 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. Other astatic needles are shown in the cuts below. [Figures 33 to 35.] 51  STANDARD ELECTRICAL DICTIONARY. IMG:8956969607833025625_051F35.jpg: