GALVANOVETERS. —The two most reliable evidences of the strength of the galvanic current are its power to deflect the magnetic needle, and to effect chemical decompo sition. To measure one or other of these, is the object of a galvanometer or voltameter. A. magnetic galvanometer shows the stlPength of the current by the amount of the deflec tion of the needle, and shows its direction by the way in which it deflects. The manner in which a needle should turn when influenced by a current is easily kept in mind by Ampere's rule: Suppose the diminutive figure of a man to be placed in the circuit, so that the current shall enter by his feet, and leave by his head; when be looks with his face to the needle, its north, pole almays turns to. his left. The deflecting wire is supposed always to lie in the magnetic meridian. The astatic olvanometer, or galvanometer, is used either simply as a ,galvanoseope, to discover the existence of a current, or as a measurer of the strengths of weak currents. When a needle is placed under a straight wire, through whieli a current passes, it deflects to a certain extent, and when the wire is bent, so as also to pass below the needle, it deflects still more. This is easily understood from the above rule. The supposed figure has to look down to the needle when in the upper wire, and to look up to it in the lower wire, so that his left hand is turned in different ways in the two positions. The current in the upper and the lower wire moves in • opposite directions, thus changing in the same way as the figure; and the deflection caused by both wires is in the same direction. By thus doubling the wire, we double , the deflecting force. If the wire. instead of making only one such circuit round time needle, were to make two. the force would be again doubled, and if several, the force (leaving out of account the weakening of the current caused by the additional wire) would be increased in proportion. If the circuits of the wire be so multiplied as to form a coil, this force would be enormously increased. Two needles, as nearly the same as possible, placed parallel to each other, with their poles in opposite ways and suspended, so as to move freely, by a thread without twist, have little tendency to place themselves in the magnetic meridian, for the one would move in a contrary direction to the other. If they were exactly of the same power, they would remain indifferently
in any position. They cannot, however, be so accurately paired as this, so that they always-take up a fixed position. arising from the one being somewhat stronger than the other. This position is sometimes in the magnetic meridian, sometimes not, according as the needles are less or more perfectly matched. Such a compound needle is called astatic, as it stands apart from the directing magnetic influence of the earth. If an astatic needle be placed in a coil, so that the lower needle be within the coil, and the upper one above it, its deflections will be more considerable than a simple needle, for two reasons: in the first place, the power which keeps the needle in its fixed position is small, and the needle is consequently more easily influenced; in the second place, the • force of the coil is exerted in the same direction on two needles instead of one, for the upper needle being much nearer the upper part of the coil than the lower, is deflected alone by it, and the deflection is in tho same direction as that of the lower needle. An astatic needle so placed in a coil constitutes an astatic galvanometer. Round an ivory bobbin, in one of these instruments, a coil of fine copper wire, carefully insulated with silk, is wound, its ends being connected with binding screws. The astatic needle is placed in the bobbin. which is provided with a vertical slit, to admit the lower needle, and a lateral slit, to allow of its oscillations, and is suspended by a cocoon thread to a hook supported by a brass frame. The upper needle moves on a graduated circle; time compound needle hangs freely, without touching the bobbin. The whole is included in a glass case, and rests on a stand, supported by three leveling screws. When used, the bobbin is turned round by a screw until the needle stands at the zero point, and the wires through which the current iS sent are fixed to the binding screws. The number of degrees that the needle deflects may then be read off.