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GALVANOMETER
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GALVANOMETER , an See also:instrument for detecting or measuring electric currents. The See also:term is generally applied to See also:instruments which indicate electric current in See also:scale divisions or arbitrary See also:units, as opposed to instruments called amperemeters (q..v ), which show directly on a See also:dial the value of the current in amperes.
Galvanometers may be divided into See also:direct current and alternating current instruments, according as they are intended to measure one or other of these two classes of currents (see See also:ELECTROKINETICS).
Direct Current Galvanometers.—The principle on which one type of direct current galvanometer, called a movable See also:needle galvanometer, depends for its See also:action is that a small magnet when suspended in the centre of a coil of See also:wire tends to set its magnetic See also:axis in the direction of the magnetic See also: See also:Weber. The magnet had a mirror attached to it, and a telescope having See also:cross wires in the See also:focus was used to observe the scale divisions of a fixed scale seen reflected in the mirror. See also:Lord Mlrror See also:Kelvin (See also:Professor W. See also:Thomson) made the important gatvano- improvement of reducing the See also:size of the needle and attach- meters. See also:ing it to the back of a very small mirror, the two being suspended by a single fibre of cocoon silk. The mirror was made of silvered microscopic See also:glass about s in. in See also:diameter, and the magnetic needle or needles consisted of See also:short fragments of watchspring cemented to its back. A See also:ray of See also:light being thrown on the mirror from a See also:lamp the deflexions of the needle were observed by watching the movements of a spot of light reflected from it upon a fixed scale. This form of mirror galvanometer was first devised in connexion with submarine See also:cable signalling, but soon became an indispensable instrument in the See also:physical laboratory. In course of See also:time both the original form of single needle galvanometer and mirror galvanometer were improved by introducing the See also:Asiatic astatic principle and weakening the See also:external controlling gatvano- magnetic field. If two magnetic needles of equal size and meters. moment are attached rigidly to one See also:stem parallel to each other but with poles placed in opposite directions an astatic system results; that is, if the needles are so suspended as to be See also:free to move in a horizontal plane, and if they are made exactly equal in magnetic strength, the system will have no directive See also:power. If one needle is slightly weaker than the other, the suspended system will set itself with some axis parallel to the lines of force of a field in which it is placed. In a form of astatic needle galvanometer devised by Professor A. See also:Broca of See also:Paris, the pair of magnetized needles are suspended vertically and parallel to each other with poles in opposite directions. The upper poles are included in one coil and the See also:lower poles within another coil, so connected that the current circulates in the right direction in each coil to displace the pairs of poles in the same direction. By this mode of arrangement a greater magnetic moment can be secured, together with more perfect astaticity and freedom from disturbance by external See also:fields. The See also:earth's magnetic field can be weakened by means of a controlling magnet arranged to create in the space in the interior of the galvariometer coils an extremely feeble controlling magnetic field. In instruments having a coil for each needle and designed so that the current in both coils passes so as to turn both needles in the same direction, the controlling magnet is so adjusted that the normal position of the needles is with the maghetic axis parallel to the plane of the coil. An astatic magnetic system used in See also:conjunction with a mirror galvanometer gives a highly sensitive form of instrument (fig. I) ; it is, however, easily disturbed by stray magnetic fields caused by neighbouring magnets or currents through conductors, and therefore is not suitable for use in many places. This fact led to the introduction of the movable coil galvanometer which was first devised by Lord Kelvin as a telegraphic signalling Movable instrument but subsequently modified by A. d'See also:Arsonval Movable others into a laboratory galvanometer (fig. 2). In this coti instrument a permanent magnet, generally of the See also:horse- galvano- See also:shoe shape, is employed to create a strong magnetic field; in See also:meter. which a. light movable coil is suspended. The suspension is bifilar, consisting of two See also:fine wires which are connected to the ends of the coil and serve to See also:lead the current in and out. If such a coil is placed with its plane parallel to the lines of force of the permanent magnet, then when a current is passing through it it displaces itself in the field, so as to set with its axis more nearly parallel. to the lines of force of the field. The movable coil may carry a pointer or a mirror; in the latter form it is well represented by several much used laboratory instruments. The movable coil galvanometer has the See also:great See also:advantage that it is not easily disturbed by the magnetic fields caused by neighbouring magnets or electric currents, and thus is especially useful in the See also:electrical workshop and factory. In the See also:practical construction of the suspended needle fixed coil galvanometer great care must be taken with the insulation of the wire of the coil. This' wire is generally silk-covered, Construe, See also:wound on a See also:frame, the whole being thoroughly saturated See also:don and with See also:paraffin See also:wax. In some cases two wires are wound use. on in parallel, constituting a " See also:differential galvanometer." When properly adjusted this instrument can be used for the exact comparison of electric currents by a null method, because if an electric current is passed en through one wire and creates certain deflexions of the needle, the current which annuls this deflexion . when passed through the other wire must be equal to the first current. In the construction of a movable coil galvanometer, it is usual to intensify the magnetic field by inserting a fixed soft See also:iron core in the interior of the movablvcoil. If the current to be measured is too large to be passed entirely through the galvanometer, a portion is allowed to flow through a See also:circuit connecting the two terminals of the instrument. This circuit is called a shunt and is generally arranged so as to take 0.9, 0.99, or 0.999 of the See also:total current, leaving FIG. i.—Kelvin Astatic Mirror Gal-O•I, o•oI or o•ooi to flow vanometer. See also:Elliott square See also:pattern. through the galvanometer. W. E. See also:Ayrton and T. See also:Mather have designed a universal shunt See also:box or resistance which can be applied to any galvanometer and by which a known fraction of any current can be sent through the galvanometer when we know its resistance (see Jour. Inst. Elec. Eng. Lend., 1894, 23, p. 314). A galvanometer can be calibrated, or the meaning of its deflexion determined, by passing through it an electric current of known value and observing the deflexion of the needle or coil. The known current can be provided in the following manner:—a single secondary See also:cell of any See also:kind can have its electromotive force measured by the See also:potentiometer (q.v.), and compared with that of a See also:standard voltaic cell. If the secondary cell is connected with the galvanometer through a known high resistance R, and if the galvanometer' is shunted, that is, has its terminals connected by another resistance S, then if the resistance of the galvanometer itself is denoted by G, the whole resistance of the shunted galvanometer and high, resistance has a value represented by R+G+S, and therefore the current through the galvanometer produced by an electromotive force E of the cell is represented by SE R (G +S) +GS' Suppose this current produces a deflexion of the needle or coil or spot of light equal to X scale divisions, we can then' alter the value of the resistances R and S, and sp determine the relation between the deflexion and the current. By the sensitiveness of the Movable needle galvanometer. galvanometer is meant the deflexion produced by a known electromotive force put upon its terminals or a known current sent through it. It is usual to specify the sensitiveness of a mirror galvanometer by requiring a certain deflexion, measured in millimetres, bf a spot of light thrown on the scale placed at one See also:metre from the mirror, when an electromotive force of one-millionth of a volt (microvolt) is applied to the terminals of the galvanometer; it may be otherwise expressed by stating the deflexion produced under the same conditions when a current of one microampere is passed through the coil. In See also:modern mirror galvanometers a deflexion of i See also:min. of the spot of light upon a scale at 1 metre distance can be produced by a current as small as one See also:hundred millionth (IO ,) or even one ten thousand millionth (10--10) of an See also:ampere. It is easy to produce considerable sensitiveness in the galvanometer, but for practical purposes it must always be controlled by the See also:condition that the zero remains fixed, that is to say, the galvanometer needle or coil must come back to exactly the same position when no current is . passing through the instrument. Other important qualifications of a galvanometer are its time-See also:period and its dead-beatness. For certain purposes tte needle or coil should return as quickly as possible to the zero position and with either no, or very few, oscillations. If the latter condition is fulfilled the galvanometer is said to be " dead-See also:beat." On the other See also:hand, for some purposes the galvanometer is required with the opposite quality, that is to say, there must be as little retardation as possible to the needle or coil when set in See also:motion under an impulsive See also:blow. Such a galvanometer is called " ballistic." The quality of a galvanometer in this respect is best estimated by taking the logarithmic decrement of the oscillations when the movable system is set swinging. This last term is defined as the See also:logarithm of the ratio of one See also:swing to the next succeeding swing, and a galvanometer of which the logarithmic decrement is large, is said to be highly damped. For many purposes, such as for resistance measurement, it is desirable to have a galvanometer which is highly damped; this result can be obtained by affixing to the needles either light pieces of See also:mica, when it is a movable needle galvanometer, or by winding the coil on a See also:silver frame when it is a movable coil galvanometer. On the other hand, for the comparison of capacities of condensers and for other purposes, a galvanometer is required which is as little damped as possible, and for this purpose the coil must have the smallest possible frictional resistance to its motion through the See also:air. In this See also:case the moment of inertia of the movable system must be decreased or the control strengthened. The Einthoven See also:string galvanometer is another form of sensitive instrument for the measurement of small direct currents. It consists of a fine wire or silvered See also:quartz fibre stretched in a strong magnetic field. When a current passes through the wire it is displaced across the field and the displacement is observed with a See also:microscope. For the measurement of large currents a " tangent galvanometer " is employed (fig. 3). Two fixed circular coils are placed apart at a Tangent distance equal to the See also:radius of either coil, so that a yalvan t current passing through them creates in the central meter. region between them a nearly See also:uniform magnetic field. At the centre of the coils is suspended a small magnetic needle the length of which should not be greater than 13,6 the radius of either coil. The normal position of the needle is at right angles to the See also:line joining the centre of the coils. If a current is passed through the coils, the needle will be deflected, and the tangent of the angle of its deflexion will be nearly proportional to the current passing through the coil, provided that the controlling field is uniform in strength and direction, and that the length of the magnetic needle is so short that the space in which it rotates is a practically uniform magnetic field. Alternating Current Galvanometers.— For the detection of small alternating currents a magnetic needle or movable coil galvanometer is of no utility. We can, however, construct an instrument suitable for the purpose by suspending within a coil of insulated wire a small needle of soft iron placed with its axis at an angle of 45° to the axis of the coil. When an alternating current passes through the coil the soft iron needle tends to set itself in the direction of the axis of the coil, and if it is suspended by a quartz fibre or metallic wire so as to afford a control, it can become a metrical instrument. Another arrangement, devised by J. A. See also:Fleming in 1887, consists of a silver or See also:copper disk suspended within a coil, the plane of the disk being held at 45° to that of the coil. When an alternating current is passed through the coil, induced currents are set up in the disk and the mutual action causes the disk to endeavour to set itself so that these currents are a minimum. This See also:metal disk galvano- meter has been made sufficiently sensitive to detect the feeble oscillatory electric currents set up in the receiving wire of a wireless See also:telegraph apparatus. The Duddell thermal ammeter is another very sensitive form of alternating current galvanometer. In it the current to.be detected or measured is passed through a high resist- ante wire or See also:strip of metal See also:leaf mounted on glass, over which is suspended a closed See also:loop of See also:bismuth and See also:antimony, forming a thermoelectric couple. This loop is suspended by a quartz fibre in a strong magnetic field, and one junction of the couple is held just over the resistance wire and as near it as possible without touching. When an alternating current passes through the resistance it creates See also:heat which in turn acts on the thermo-junction and generates a continuous current in the loop, thus deflecting it in the magnetic field. The sensitiveness of such a thermal ammeter can be made sufficiently great to detect a current of a few microamperes. Additional information and CommentsThere are no comments yet for this article.
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