The earth's magnetism naturally holds the compass needle north and south. The magnetic field encircling the wire, being at right angles to the needle (when the wire itself is parallel therewith), operates to turn it from its normal position, north and south, so as to set it partially east and west. However, on account of the fact that the earth's magnetism does exert some force tending to hold the needle north and south, it is evident that no matter how strong the current, the latter can never succeed in turning the needle entirely east and west. The accomplishment of this is further prevented by the reason of the points of the needle, where the magnetic effect is greatest, quickly passing out of the reach of the magnetic field, where it is now practically operated on only in a slight degree. Thus it would take quite a powerful current to hold the needle deflected any appreciable distance. The use of a shorter needle is, therefore, more desirable. It is evident in this style of instrument that the effect of the current cannot be accurately measured, because it acts in opposition to the earth's magnetism, and as this is constantly varying, some method must be employed which will either destroy the earth's magnetism or else neutralize it. In the astatic galvanometer, the earth's magnetism is neutralized by means of _astatic needles_. These consist of a combination of two magnetic needles of equal size and strength, connected rigidly together with their poles pointing in opposite and parallel directions, as shown in fig. 510. As the north pole of the earth attracts the south pole of one of the needles, it repels with equal strength the north pole of the other needle, hence, the combination is independent of the earth's magnetism and will remain at rest in any position. [Illustration: Fig. 512.--Connections of double coil astatic needles. With this arrangement, the direction of current in both coils will tend to turn the system in the same direction, making the needles more sensitive than with a single coil as in fig. 511.] If one of the needles be surrounded by a coil, as shown in fig. 511, the magnetic effect of the current will be correctly indicated by the deflection of the needle. Sometimes each needle is surrounded by a coil, as in fig. 512, the coils being so connected that the direction of current in each will tend to deflect the needles in the same direction. Ques. For what use is the astatic galvanometer adapted? Ans. For the detection of small currents. It is used in the "nil" or zero methods, in which the current between the points to which the galvanometer is connected is reduced to zero.