52 GALVAN by the rolling of the ship, its centre of gravity is carefully adjusted so as to be in the axis of suspension. The mirror is enclosed in a narrow cell which just allows it room to deflect to the required extent, and damps the Oscillation so effectually that the instrument is “dead beat.” In order to destroy the directive action of the earth, the inconveni- ence of which in a galvanometer for use on board ship is obvious, the case of this galvanometer is made of thick soft iron. which completely encloses the whole, leaving only a small window for the ingress and egress of the ray of light by means of which the motions of the mirror are read; a flat horse-shoe magnet placed on the top of the case still farther overpowers the earth's force and directs the mirror. All these galvanometers may, of course, be wound double and used dit'ferentially. When this is the case, a small auxiliary compensating coil is often used to correct the inequality of the magnetic fields due to the two sets of windings. This auxiliary coil is usually mounted on a spindle in the a.xis of the main coil, a.nd can be moved backwards and forwards till a current passing through it and one set of windings in one direction, and through the other set of windings in the other direction, does not sensibly deflect the mirror. The astatic arrangement described above (p. 51, fig. 4) is often adopted. A galvano- meter of this construction by Elliot Brothers is shown in fig. 6. It may be questioned, how- ever, whether for ordinary purposes the additional pensi- bility thus gained compensates for the increased complexity and cost of the instrument.
- S'ta7zdar¢l Gah'anomele7's.—
When galvanometers are in- tended for measuring currents, there must be some lav.’ con- necting the indications of the needle with the strength of the current in the multiplier. It is therefore of great importance that slight variations in the position of the magnet should not introduce large or irregular (incalculable) variations into the indications of the instrument. Accordingly in standard instru- ments the' windings are much farther removed from the mag- net than in sensitive galvanometers, and in the best forms the multiplier is so disposed that it produces a uniform field of magnetic force around the needle. The earliest forms of standard galvanometer were the tangent and sine compasses invented by Pouillet. The first of these consists simply of a single vertical coil of wire, with a magnet suspended at its centre, whose deflexion may be read in any of the various ways already described. If the length of the magnet be very small, the magnetic field in its neighbourhood may be regarded as uniform, and the electromagnetic couple will be propor- tional to cos 0, 0 being the deflexion from the plane of the windings. If the windings be arranged so as to be in the magnetic meridian,‘ the couple due to the earth’s action tending to bring the magnet back to its position of equilibrium will be proportional to sin 0, hence the current strength will be proportional to tan 0. FIG. 6.—Elliot’s Astatic Galvanometer. ‘ This can be done most easily by means of a mirror attached to the multiplier and adjusted so as to be parallel to the windings. OMETER If the multiplier be movable about a vertical axis through angles which can be measured in any way, the instrument may be used as a sine compass. The current is applied and the multiplier turned round after the magnet until the axis of the latter is again parallel to the windings. The current strength is now clearly proportional to sin (7, where 0 is the deflexion of the multiplier from the mag- netic meridian. When the instrument is used in this way, the needle being always brought into the same position relative to the windings, the uniformity of the magnetic field is a matter of indifference, and there is no necessity for the needle to be short. Gaugaiu attempted to improve the tangent galvanometcr by suspending the magnet ex- cent-rically at a point in the ‘ axis of the coil distant from . the centre by half the radius of the coil. This, however, is in reality the reverse of an im- provement.'-’ A real advance, however, was made by Helmholtz, who placed two equal parallel and vertical coils, one on each side of the magnet, each at a distance from it equal to half the common radius. In fig. 19, at the end of his second volume, Maxwell gives a diagram of the lines of force due to two equal parallel circular circuits, from which it will be seen that the magnetic field at the centre of such an arrangement of currents is very approximately uniform. This approximation may be carried Still farther by adding 8. third F 10. 7.—Galvanometerdesigned coil parallel to the two others, by P1'°fe~‘50“ M“-““'9“- and equidistant from} thgnp The wire is vo_nn(l in_ two parallel channels cut in a cylindrical block In some examples of Helm- of hard wood, cach an inch m-0...: he1tz’s the wi,nd- :‘.:*.:‘..*:,':.3.::.°‘.::‘::.:’s. ings are arranged on a Conloal inch, and the distance between surface, so that the ratio of the ,,,re"f.,f,:"cl':f§ “a“.i“S 0‘ ‘ml’ ‘° the ‘1iS‘a“°° .‘.‘.‘.1iJElf.‘.?§§i°$3§..Li. ‘ilflinfii.-‘1li'.? Of 1l3S pla.ne fI‘0Yl1 the centre Of needle to swing freely without the magnet shell be 2 :1- In :.":; reality this is unnecessary, pro- vided the ratio of the depth and breadth of the usual rect- angular channel be properly adjusted (see Maxwell, vol. ii. screwed two caps containing .1 piece of plane parallel glass and :1 pluno-convex lens respectively. thc formcr for subjective, the latter for objective reading. By mcam of a slit and screw in the stem which supports the in.~'trumcnt. a horizontal bar can be fixed pnrall:-l sec_ 7 representg to the axis of the multiplier. On 5‘ . this a (it ficcting magnet can he 11 galVaI10Inel5el' Of the klnd mounted, so that the galvanomctcr described can be used as a magnetometer. Ilcduction of Galvtmomclcr Imlicali0ns.—Vlien the position of every layer of wire in the multiplier is known with suflicicnt accuracy, and the multiplier arranged so as to produce a sq-;1si'oly uniform field, the electromagnetic action per unit of ('urren'tc:d1 he calculatcd for every position of the magnet. In this case tln: galvanometer is an absolute instrument. When we possess one absolute instrument it is easy to evaluate the indications cf any other in absolute measure by means of it; we have only to pass the same current through both galvanometcrs in series and compare the readings. The best way, however, to construct a St1l.lltlfll'll _c:alvanr.. meter is to provide for uniformity of field in the core of the multiplier, and find the resultant electromagnetic force for unit current, or, as it is called, the constant of the instrument, by comparison with a pair of equal standard coils of large diameter (18 in. to 24 in. ). Theec :2.~.'e arranged vertically on the same axis, the distance bct".ec7_1 them being equal to the mean radius, just as in Hclmholtz's ;:al7a.non1et«-r The galvanomcter to be tested is placed symmetrically between the
9 See Maxwell, Electricity] and .l[a_r/7zct:'s7n, vol. ii. sec.-=. 713, 733.