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The Sensitiveness of a Galvanometer
The sensitiveness of a galvanometer will depend on the couple which tends to bring the needle back to its position of equilibrium, and is increased by making that couple small. The couple is proportional to the magnetic moment of the needle and to the strength of the field in which the magnet hangs. Two methods are employed to diminish its value.
When used for a galvanometer the coils are made to surround one needle only; the other is placed outside them, either above or below as the case may be. The magnetic action of the current affects mainly the enclosed magnet; the force on this is the same as if the other magnet were not present, and hence, since the controlling force is much less, the deflexion produced by a given current is much greater. This deflexion is still further increased by the slight magnetic action between the current and the second magnet. In some cases this second magnet is also surrounded by a coil, in which the current is made to flow in a direction opposite to that in the first coil, and the deflexion is thereby still further augmented. In the second method the strength of the field in which the needle hangs is reduced by the help of other magnets; if this method be adopted, the advantages of an astatic combination may be partly realised with an ordinary galvanometer by the use of control magnets placed so as to produce a field of force opposite and nearly equal to that of the earth at the- point where the galvanometer needle hangs. The magnetic force tending to bring the needle back to its equilibrium position can thus be made as small as we please - neglecting for the moment the effect of the torsion of the fibre which carries the mirror - and the deflexion produced by a given current will be correspondingly increased. The increase in sensitiveness is most easily determined, as in §69, by observations of the time of swing, for if H represent the strength of the field in which the magnet hangs, we have seen (§69) that H=4π2K/M T2, M being the magnetic moment, K the moment of inertia, and T the time of a complete period. But, being small, the deflexion produced by a given current, on which, of course, the sensitiveness depends, is inversely proportional to H; that is, it is directly proportional to the square of T. The method of securing sensitiveness thus by the use of a control magnet is open to the objection that the small variations in the direction and intensity of the earth's magnetic force, which are continually occurring, become very appreciable when compared with the whole strength of the field in which the magnet hangs. The sensitiveness, and, at the same time, the equilibrium position of the magnet, are, therefore, continually changing.
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