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.

If the first method be adopted two needles are employed. They are mounted, parallel to each other, a short distance apart, so that they can rotate together as a rigid system about their common axis. Their north poles are in opposite directions, and their magnetic moments are made to be as nearly equal as possible. If the magnetic moments of the two be exactly the same, and the magnetic axes in exactly opposite directions, such a combination when placed in a uniform magnetic field will have no tendency to take up a definite position. In practice this condition of absolute equality is hardly ever realised, and the combination, if free to move, will be urged to a position of equilibrium by a force which will be very small compared with that which would compel either magnet separately to point north and south. It will take, therefore, a smaller force to disturb the combination from that position than would be required for either magnet singly. Such a combination is said to be astatic.

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.