An astatic needle is a combination of magnetised needles with their poles turned opposite ways.
At [Fig. 85] a diagram of such an instrument is shown. Two magnets D and C are joined, with the north pole of one over the south pole of the other, forming one suspended system. In the ordinary form of astatic galvanometer the needles D and C are about two inches long, and are each covered by a coil, these latter being so joined that the current must circulate in opposite directions round the two so as to deflect both magnets similarly. The deflection of the needles D and C is observed by means of a pointer or glass needle A, B, rigidly connected with the astatic system by a prolongation of the brass rod connecting the needles D and C. The coils are flat and of the shape indicated in [Fig. 85], and are also made in two halves, placed side by side with just sufficient space between them to allow the rod to hang freely.
This form of galvanometer, though less delicate than the preceding one, is still a very sensitive one, and should only be applied in the case of fine and delicate tests.
Differential Galvanometer.—A differential galvanometer consists of a magnetic needle surrounded by two separate coils of equal length and material carefully insulated from each other and wound in opposite directions. In using it one circuit acts against the other. If a current of equal strength were passing through each there would be no deflection of the needle, because the influence in both directions is equal. If one current were stronger than the other, the needle would be deflected by the stronger.
This form of galvanometer will be found extremely useful in connection with a system of electrical tests.
Latimer Clark's double shunt differential galvanometer is the instrument best adapted for submarine mine tests.
Detector Galvanometer.—A detector galvanometer is usually made with a vertical needle, and is employed to detect and roughly estimate the strength of a current where no particular accuracy is required.
It consists of a magnetic needle pivoted in the centre of a coil of insulated wire, and having an index needle attached to move with it, the latter appearing on a dial, divided into 360 equal arcs or portions: a diagram of such an instrument is shown at [Fig. 86].
This instrument should be of small size and portable form, and as sensitive as it is possible to make it, under such conditions.
Three Coil Galvanometer.—The three coil galvanometer is provided with a vertical needle, and is in other respects very similar in appearance to the detector galvanometer before described. It is formed with three coils of 2, 10, and 1000 ohms resistance; each coil is connected with a brass plate on the top of the box which encloses the whole, and may be switched into circuit by means of a plug at will. The object of the three resistances is to suit the different resistances that may occur, with a perfect, or imperfect state of the electrical combination in connection with each mine. A diagram of this instrument is shown at [Fig. 87], the dotted portions are inside the case.