GAL'VANOM'ETER (from galvanic + Gk. Airpov, metron, measure). An instrument for detecting the presence of an electric current and measuring its magnitude. It consists of a coil of in sulated wire surrounding a magnet, freely hung or pivoted so as to be easily deflected by the passage of a current through the coil. The wire forming the coil is so wound that each turn lies in a plane approximately parallel to the axis of the needle or magnet when at rest. The current in passing through the coil or bobbin of insulated wire pro duces a magnetic field in the space in which the needle hangs and tends to swing the needle around, until it hangs crosswise in the coil. The force tending to deflect the needle is proportion ate to the strength of this field, or, what is the same thing, the strength of the current producing it, and to the length and strength of the needle, while the magnetic force of the earth acts to keep the needle in the direction of the magnetic meridian. Under the influence of these two forces the needle will come to rest in a position where they are in equilibrum. As the shape and strength of the magnetic needle, speaking broad ly, remain the same in a given galvanometer, the instrument affords a means of measuring the strength of any current passed through it, by the amount of motion imparted to the needle.
These conditions can be reversed and the coil suspended and the magnetic field produced by a permanently mounted magnet, as in the case of the D'Arsonval galvanometer described below. Galvanometers are constructed in a great variety of forms, specially suited to various uses, from simple instruments for merely indicating the presence of a current to extremely elaborate ap paratus for making measurements of great ac curacy. The action of the galvanometer depends upon the following principle discovered by Oers ted in 1820: When a magnetic needle is placed under a straight wire, through which a current passes, it is deflected to a certain extent, and when the wire is bent, so as also to pass below the needle, it is deflected still more. The north pole of the needle is deflected to the left if the current is flowing from south to north in a con ductor which is placed above the needle, and vice versa when the conditions are reversed. The di rection of the deflection can be remembered by Ampere's rule which states that supposing a man swimming along the conductor in the direction of the flow of the current and always facing the needle, the latter will be deflected toward his left hand. The current in the upper and the lower wire moves in opposite directions, but as they are on opposite sides of the needle, the de flection caused by both wires is in the same direc tion. By thus doubling the wire, we double the deflecting force. Schweiggers and Poggendorf soon ascertained that if the wire, instead of making only one circuit round the needle, were to make two, the force would be again double, and if several, the force (leaving out of account the weakening of the current caused by the addi tional wire) would be increased in proportion.
If the circuits of the wire are so multiplied as to form a coil, this force would be enormously in creased, and the galvanometer rendered more sensitive. These early galvanometers were called multipliers, and have been much used. The next improvement in the instrument was due to Nobili, who employed two needles, placed parallel to each other as nearly as possible, with their poles turned opposite ways and suspended by a thread without twist. These needles have little tendency to place themselves in the magnetic meridian, for one tends to move in a contrary direction to the other. If they were exactly equivalent, they would remain indifferently in any tion; but they cannot be so accurately paired as this, for they almost always take up a fixed position, arising from the one being what stronger than the other. Such a compound needle is called astatic, as the magnetic influence of the earth does not determine the direction in which it will point. If an astatic needle be placed in a coil, so that the lower needle be in the coil, and the upper one above it, its de flections will be greater than those of a simple needle. for two reasons. In the first place. the power which keeps the needle in its fixed posi tion is small, and the needle is con sequently more easily influenced; in the second place, the force of the coil is ex ,ericu in inc saint. 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 flection is in the same direction as that of the lower needle. An astatic needle so placed in a coil constitutes an astatic galvanometer. The coil is formed of fine copper wire, insulated with silk, and wound on a frame or bobbin. The as tatic needle is placed in this bobbin, which is provided with a vertical slit, to admit the lower needle, and a lateral slit, to allow of its oscilla tions, and is suspended by a cocoon fibre from a hook supported by a brass frame. The upper needle moves over a graduated circle, and the en tire system hangs freely, without touching the bobbin. The instrument is inclosed in a glass case, and rests on a stand, supported by three leveling screws. When used, the bobbin carrying the divided circle with it is turned until the needle stands at the zero point of the scale, and ness. This galvanometer is much used in all kinds of testing work and for reading the delicate signals in ocean telegraphy. Increased sensitive ness may be obtained by using two sets of coils and needles, while there have been Thomson gal vanometers constructed in which there were four such sets of coils.