GALVANOMETER, an instrument for de tecting and measuring the intensity, direction or length of duration of an electric current by means of the magnetic force it produces. The many types of indicating instruments such as voltmeters and ammeters, where the pointer is held at zero by some directive force, such as the earth's field, a spring or weight or a perma nent magnet, come under this head. With very few exceptions, all galvanometers may be classed under two heads: (1) Those having a magnetized needle suspended so as to move freely in a horizontal plane, the needle being normally held at zero position by means of the earth's field or an external field produced by auxiliary magnets placed to accomplish this result. The needle is suspended on a pivot or a quartz fibre or fibre of silk or other appro priate material. (2) Those having a coil of wire in place of the needle in the first class. This is the type usually called the d'Arsonnal type, and has the coil suspended by means of a fine wire, which provides a way for leading current to the coil, and has another wire under neath the coil for conducting the current from it. The coil, with the conducting wires, is then suspended between the poles of a magnet, its axis being normally at right angles with the lines of the field. The second class of galvanom eters, those having a moving coil, are to be preferred for most classes of work, all except those requiring the very greatest delicacy, for the following reasons: The readings are but very slightly affected by the presence of an ex ternal field or by magnetic substances in vicinity of the instrument, and are practically independent of the variable influence of the earth's field; this form of instrument may easily be made dead-beat (that is, when the coil is deflected it goes at once to its position with out long vibration), on account of the strong field in which the coil moves; many forms are portable and much less affected by vibrations than those of the first class.
The figure of merit of a galvanometer may be expressed either as the current necessary to cause a deflection of one scale division, or as the resistance through which one volt will cause a deflection of one scale division when such resistance is inserted in the circuit. Such an expression of the delicacy of a galvanometer should be accompanied with the following data: The resistance of the instrument, the scale dis tance and size of one division of the scale. The sensitiveness of a galvanometer is expressed as the difference of potential across the galvanom eter terminals necessary to cause a deflection of one scale division; and to be exact, should be accompanied with the same data as for the figure of merit. Of the moving needle class of galvanometers the tangent galvanometer and the Thomson astatic galvanometers are prob ably in most general use. The tangent galva
nometer is constructed so that the inside diame ter of the coil which surrounds the needle is at least 12 times the length of the needle. Under these conditions, the needle being held at zero by the earth's field, the current strength will vary directly as the tangents of the angle of deflection; hence the name of the instrument. Although at one time much used for the abso lute determination of current strengths, it has of late been replaced by other types, on account of its many correction factors, some of which are of uncertain magnitude, a prolific cause of error being the necessity of knowing the exact value of the horizontal component of the earth's magnetism, which quantity is subject to continual change and is affected by large masses of iron or heavy electric currents in the vicinity. The sine galvanometer is similar but the coil is moved so as to bring the needle back to zero. This makes the sine of the angle through which the coil is moved proportional to the deflecting current.
The Thomson Galvanometer.— This type was designed by Lord Kelvin, and to him we are indebted for the most sensitive instruments as yet made. The moving system consists of a slender quartz rod to the centre of which is fastened a small mirror of glass which reflects a ray of light to a graduated scale. Above and below the mirror at each end of the quartz rod is fastened a complex of carefully selected magnetized needles, minute in size, placed paral lel to the plane of the mirror. In the upper complex the north poles of the needles are all placed in one direction, and in the lower complex the north poles of the needles are placed in the opposite direction, the combina tion forming an astatic system, on which the earth's field exerts a very slight directive force. This directive force exerted by the earth's field would be zero if the two complexes could be made exactly equal in magnetic strength. Each complex of needles is enclosed by two coils, thus making four coils in an instrument. These coils are provided with•binding posts for series or parallel connection, and are connected so that the current flowing through them will cause the magnetized needles to be deflected in the same direction in each complex. An adjustable magnet mounted on top of the in strument provides a directive action on the needles which may be modified to any extent, a weak directive .force increasing the sensitive ness greatly, and also increases the period of oscillation, so that in the best instruments the sensitiveness is limited largely to the patience of the observer. Refinements of this type have eight coils and eight needles or groups of needles.