Part 1

Transcriber's notes:

(1) Numbers following letters (without space) like C2 were originally printed in subscript. Letter subscripts are preceded by an underscore, like C_n.

(2) Characters following a carat (^) were printed in superscript.

(3) Side-notes were relocated to function as titles of their respective paragraphs.

(4) Macrons and breves above letters and dots below letters were not inserted.

(5) [root] stands for the root symbol; [alpha], [beta], etc. for greek letters.

(6) The following typographical errors have been corrected:

ARTICLE MAGNETOMETER: "... the determination of the magnetic elements on board ship is a matter of very considerable importance." 'determination' amended from 'determinaton'.

ARTICLE MAGNETO-OPTICS: "The relation of the magnetic rotation to chemical constitution has been studied in great detail by Perkin, Wachsmuth, Jahn and Schönrock." 'constitution' amended from 'consitution'.

ARTICLE MAGNUS, HEINRICH GUSTAV: "... ('Magnus's green salt' is PtCl2, 2NH3), of sulphovinic ..." 'PtCl2' amended from 'Ptll2'.

ARTICLE MAHOMET: "With this change we may perhaps couple the adoption of the name Allah for the Deity ..." 'Deity' amended from 'Diety'.

ARTICLE MAHOMMEDAN LAW: "It was rather the Moslem leaders who were compelled to abandon their ideas and for the sake of the spread of Islam to accept and incorporate much that was diametrically opposed to the original legislation either of the Koran or of Mahomet's recorded decisions." 'decisions' amended from 'decisons'.

ARTICLE MALAY ARCHIPELAGO: "In 1611 the headquarters of the Dutch was changed from Bantam to Jakarta, which in 1619 was renamed Batavia, and was thenceforward the Dutch capital." 'Jakarta' amended from 'Jakatra'.

ARTICLE MALAYS: "When the first Europeans visited the Malay Archipelago the Malays had already acquired the art of manufacturing gunpowder and forging cannon." 'cannon' amended from 'canon'.

ARTICLE MALAY STATES: "The country is mountainous except close to the coast. The principal rivers are the Patani and the Teloban, long, winding and shallow, and navigable for small boats only." 'the' amended from 'tle'.

ARTICLE MALOCELLO, LANCILOTO: "This was a Genoese expedition, which about 1270 seems to have sailed into the Atlantic, re-discovered the 'Fortunate Islands' or Canaries" 'Atlantic' amended from 'Alantic'.

ENCYCLOPAEDIA BRITANNICA

A DICTIONARY OF ARTS, SCIENCES, LITERATURE AND GENERAL INFORMATION

ELEVENTH EDITION

VOLUME XVII, SLICE IV

Magnetite to Malt

ARTICLES IN THIS SLICE:

MAGNETITE MAJOR, JOHN MAGNETOGRAPH MAJOR MAGNETOMETER MAJORCA MAGNETO-OPTICS MAJORIAN MAGNOLIA MAJORITY MAGNUS, HEINRICH GUSTAV MAJUBA MAGNY, CLAUDE DRIGON MAKALAKA MAGO MAKARAKA MAGPIE MAKART, HANS MAGWE MAKING-UP PRICE MAGYARS MAKÓ MAHABALESHWAR MAKRAN MAHAFFY, JOHN PENTLAND MAKSOORA MAHALLAT MALABAR MAHAN, ALFRED THAYER MALABARI, BEHRAMJI MAHANADI MALABON MAHANOY CITY MALACCA MAHAR MALACHI MAHARAJPUR MALACHITE MAHAVAMSA MALACHOWSKI, STANISLAW MAHAYANA MALACHY, ST MAHDI MALACOSTRACA MAHDIA MALAGA (province of Spain) MAHÉ MALAGA (city of Spain) MAHESHWAR MALAKAND PASS MAHI MALALAS, JOHN MAHI KANTHA MALAN, SOLOMON CAESAR MAHMUD I. MÄLAR MAHMUD II. MALARIA MAHMUD NEDIM PASHA MALATIA MAHMUD OF GHAZNI MALAYALAM MAHOBA MALAY ARCHIPELAGO MAHOGANY MALAIR MAHOMET MALAY PENINSULA MAHOMMED AHMED IBN ABDULLAH MALAYS MAHOMMEDAN INSTITUTIONS MALAY STATES (British) MAHOMMEDAN LAW MALAY STATES (Siamese) MAHOMMEDAN RELIGION MALCHIN MAHONY, FRANCIS SYLVESTER MALCOLM MAHOUT MALCOLM, SIR JOHN MAHRATTAS MALDA MAHSEER MALDEN MAI, ANGELO MALDIVE ISLANDS MAIA MALDON MAIDA MALEBRANCHE, NICOLAS MAIDAN MALER KOTLA MAIDEN MALESHERBES, CHRÉTIEN DE LAMOIGNON DE MAIDENHAIR MALET, LUCAS MAIDENHEAD MALHERBE, FRANÇOIS DE MAID MARIAN MALIBRAN, MARIE FÉLICITÉ MAIDSTONE MALIC ACID MAIHAR MALIGNANT MAIL MALIK IBN ANAS MAILLY, LOUISE JULIE MALINES MAIMANA MALLANWAN MAIMAND MALLARMÉ, FRANÇOIS RENÉ AUGUSTE MAIMBOURG, LOUIS MALLARMÉ, STÉPHANE MAIMING MALLECO MAIMON, SALOMON MALLEMUCK MAIMONIDES MALLESON, GEORGE BRUCE MAIN (river of Germany) MALLET, DAVID MAIN (power or strength) MALLET, PAUL HENRI MAINA and MAINOTES MALLET, ROBERT MAINE, ANNE LOUISE DE BOURBON MALLET DU PAN, JACQUES MAINE, SIR HENRY JAMES SUMNER MALLING, EAST and WEST MAINE (French province) MALLOCK, WILLIAM HURRELL MAINE (U.S. state) MALLOW (town of Ireland) MAINE DE BIRAN, FRANÇOIS-GONTHIER MALLOW (botanical genus) MAINE-ET-LOIRE MALMEDY MAINPURI MALMESBURY, JAMES HARRIS MAINTENANCE MALMESBURY, JAMES HOWARD HARRIS MAINTENON, FRANÇOISE D'AUBIGNÉ MALMESBURY MAINZ MALMÖ MAIRET, JEAN DE MALMSEY MAISTRE, JOSEPH DE MALOCELLO, LANCILOTO MAISTRE, XAVIER DE MALOLOS MAITLAND, EDWARD MALONE, EDMOND MAITLAND, FREDERIC WILLIAM MALONE MAITLAND, SIR RICHARD MALONIC ACID MAITLAND, WILLIAM MALORY, SIR THOMAS MAITLAND, EAST and WEST MALOT, HECTOR HENRI MAITREYA MALOU, JULES ÉDOUARD XAVIER MAIWAND MALOUET, PIERRE VICTOR MAIZE MALPIGHI, MARCELLO MAJESTY MALPLAQUET MAJLÁTH, JÁNOS MALSTATT-BURBACH MAJOLICA MALT

MAGNETITE, a mineral forming the natural magnet (see MAGNETISM), and important also as an iron-ore. It is an iron-black, opaque mineral, with metallic lustre; hardness about 6, sp. gr. 4.9 to 5.2. When scratched, it yields a black streak. It is an oxide of iron having the formula Fe3O4, corresponding with 72.4% of metal, whence its great value as an ore. It may be regarded as a ferroso-ferric oxide, FeO·Fe2O3, or as iron ferrate, Fe´´Fe2´´´O4. Titanium is often present, and occasionally the mineral contains magnesium, nickel, &c. It is always strongly magnetic. Magnetite crystallizes in the cubic system, usually in octahedra, less commonly in rhombic dodecahedra, and not infrequently in twins of the "spinel type" (fig. 1). The rhombic faces of the dodecahedron are often striated parallel to the longer diagonal. There is no distinct cleavage, but imperfect parting may be obtained along octahedral planes.

[Illustration: FIG. 1.]

Magnetite is a mineral of wide distribution, occurring as grains in many massive and volcanic rocks, like granite, diorite and dolerite. It appears to have crystallized from the magma at a very early period of consolidation. Its presence contributes to the dark colour of many basalts and other basic rocks, and may cause them to disturb the compass. Large ore-bodies of granular and compact magnetite occur as beds and lenticular masses in Archean gneiss and crystalline schists, in various parts of Norway, Sweden, Finland and the Urals; as also in the states of New York, New Jersey, Pennsylvania and Michigan, as well as in Canada. In some cases it appears to have segregated from a basic eruptive magma, and in other cases to have resulted from metamorphic action. Certain deposits appear to have been formed, directly or indirectly, by wet processes. Iron rust sometimes contains magnetite. An interesting deposit of oolitic magnetic ore occurs in the Dogger (Inferior Oolite) of Rosedale Abbey, in Yorkshire; and a somewhat similar pisolitic ore, of Jurassic age, is known on the continent as chamoisite, having been named from Chamoison (or Chamoson) in the Valais, Switzerland. Grains of magnetite occur in serpentine, as an alteration-product of the olivine. In emery, magnetite in a granular form is largely associated with the corundum; and in certain kinds of mica magnetite occurs as thin dendritic enclosures. Haematite is sometimes magnetic, and A. Liversidge has shown that magnetite is probably present. By deoxidation, haematite may be converted into magnetite, as proved by certain pseudomorphs; but on the other hand magnetite is sometimes altered to haematite. On weathering, magnetite commonly passes into limonite, the ferrous oxide having probably been removed by carbonated waters. Closely related to magnetite is the rare volcanic mineral from Vesuvius, called magnoferrite, or magnesioferrite, with the formula MgFe2O4; and with this may be mentioned a mineral from Jakobsberg, in Vermland, Sweden, called jakobsite, containing MnFe2O4. (F. W. R.*)

MAGNETOGRAPH, an instrument for continuously recording the values of the magnetic elements, the three universally chosen being the declination, the horizontal component and the vertical component (see TERRESTRIAL MAGNETISM). In each case the magnetograph only records the variation of the element, the absolute values being determined by making observations in the neighbourhood with the unifilar magnetometer (q.v.) and inclinometer (q.v.).

_Declination._--The changes in declination are obtained by means of a magnet which is suspended by a long fibre and carries a mirror, immediately below which a fixed mirror is attached to the base of the instrument. Both mirrors are usually concave; if plane, a concave lens is placed immediately before them. Light passing through a vertical slit falls upon the mirrors, from which it is reflected, and two images of the slit are produced, one by the movable mirror attached to the magnet and the other by the fixed mirror. These images would be short lines of light; but a piano-cylindrical lens is placed with its axis horizontal just in front of the recording surface. In this way a spot of light is obtained from each mirror. The recording surface is a sheet of photographic paper wrapped round a drum which is rotated at a constant speed by clockwork about a horizontal axis. The light reflected from the fixed mirror traces a straight line on the paper, serving as a base line from which the variations in declination are measured. As the declination changes the spot of light reflected from the magnet mirror moves parallel to the axis of the recording drum, and hence the distance between the line traced by this spot and the base line gives, for any instant, on an arbitrary scale the difference between the declination and a constant angle, namely, the declination corresponding to the base line. The value of this constant angle is obtained by comparing the record with the value for the declination as measured with a magnetometer. The value in terms of arc of the scale of the record can be obtained by measuring the distance between the magnet mirror and the recording drum, and in most observations it is such that a millimetre on the record represents one minute of arc. The time scale ordinarily employed is 15 mm. per hour, but in modern instruments provision is generally made for the time scale to be increased at will to 180 mm. per hour, so that the more rapid variations of the declination can be followed. The advantages of using small magnets, so that their moment of inertia may be small and hence they may be able to respond to rapid changes in the earth's field, were first insisted upon by E. Mascart,[1] while M. Eschenhagen[2] first designed a set of magnetographs in which this idea of small moment of inertia was carried to its useful limit, the magnets only weighing 1.5 gram each, and the suspension consisting of a very fine quartz fibre.

_Horizontal Force._--The variation of the horizontal force is obtained by the motion of a magnet which is carried either by a bifilar suspension or by a fairly stiff metal wire or quartz fibre. The upper end of the suspension is turned till the axis of the magnet is at right angles to the magnetic meridian. In this position the magnet is in equilibrium under the action of the torsion of the suspension and the couple exerted by the horizontal component, H, of the earth's field, this couple depending on the product of H into the magnetic moment, M, of the magnet. Hence if H varies the magnet will rotate in such a way that the couple due to torsion is equal to the new value of H multiplied by M. Since the movements of the magnet are always small, the rotation of the magnet is proportional to the change in H, so long as M and the couple, [theta], corresponding to unit twist of the suspension system remain constant. When the temperature changes, however, both M and [theta] in general change. With rise of temperature M decreases, and this alone will produce the same effect as would a decrease in H. To allow for this effect of temperature a compensating system of metal bars is attached to the upper end of the bifilar suspension, so arranged that with rise of temperature the fibres are brought nearer together and hence the value of [theta] decreases. Since such a decrease in [theta] would by itself cause the magnet to turn in the same direction as if H had increased, it is possible in a great measure to neutralize the effects of temperature on the reading of the instrument. In the case of the unifilar suspension, the provision of a temperature compensation is not so easy, so that what is generally done is to protect the instrument from temperature variation as much as possible and then to correct the indications so as to allow for the residual changes, a continuous record of the temperature being kept by a recording thermograph attached to the instrument. In the Eschenhagen pattern instrument, in which a single quartz fibre is used for the suspension, two magnets are placed in the vicinity of the suspended magnet and are so arranged that their field partly neutralizes the earth's field; thus the torsion required to hold the magnet with its axis perpendicular to the earth's field is reduced, and the arrangement permits of the sensitiveness being altered by changing the position of the deflecting magnets. Further, by suitably choosing the positions of the deflectors and the coefficient of torsion of the fibre, it is possible to make the temperature coefficient vanish. (See Adolf Schmidt, _Zeits. für Instrumentenkunde_, 1907, 27, 145.) The method of recording the variations in H is exactly the same as that adopted in the case of the declination, and the sensitiveness generally adopted is such that 1 mm. on the record represents a change in H of .00005 C.G.S., the time scale being the same as that employed in the case of the declination.

_Vertical Component._--To record the variations of the vertical component use is made of a magnet mounted on knife edges so that it can turn freely about a horizontal axis at right angles to its length (H. Lloyd, _Proc. Roy. Irish Acad._, 1839, 1, 334). The magnet is so weighted that its axis is approximately horizontal, and any change in the inclination of the axis is observed by means of an attached mirror, a second mirror fixed to the stand serving to give a base line for the records, which are obtained in the same way as in the case of the declination. The magnet is in equilibrium under the influence of the couple VM due to the vertical component V, and the couple due to the fact that the centre of gravity is slightly on one side of the knife-edge. Hence when, say, V decreases the couple VM decreases, and hence the north end of the balanced magnet rises, and vice versa. The chief difficulty with this form of instrument is that it is very sensitive to changes of temperature, for such changes not only alter M but also in general cause the centre of gravity of the system to be displaced with reference to the knife-edge. To reduce these effects the magnet is fitted with compensating bars, generally of zinc, so adjusted by trial that as far as possible they neutralize the effect of changes of temperature. In the Eschenhagen form of vertical force balance two deflecting magnets are used to partly neutralize the vertical component, so that the centre of gravity is almost exactly over the support. By varying the positions of these deflecting magnets it is possible to compensate for the effects of changes of temperature (A. Schmidt, loc. cit.). In order to eliminate the irregularity which is apt to be introduced by dust, &c., interfering with the working of the knife-edge, W. Watson (_Phil. Mag._, 1904 [6], 7, 393) designed a form of vertical force balance in which the magnet with its mirror is attached to the mid point of a horizontal stretched quartz fibre. The temperature compensation is obtained by attaching a small weight to the magnet, and then bringing it back to the horizontal position by twisting the fibre.

The scale values of the records given by the horizontal and vertical force magnetographs are determined by deflecting the respective needles, either by means of a magnet placed at a known distance or by passing an electric current through circular coils of large diameter surrounding the instruments.

The width of the photographic sheet which receives the spot of light reflected from the mirrors in the above instruments is generally so great that in the case of ordinary changes the curve does not go off the paper. Occasionally, however, during a disturbance such is not the case, and hence a portion of the trace would be lost. To overcome this difficulty Eschenhagen in his earlier type of instruments attached to each magnet two mirrors, their planes being inclined at a small angle so that when the spot reflected from one mirror goes off the paper, that corresponding to the other comes on. In the later pattern a third mirror is added of which the plane is inclined at about 30° to the horizontal. The light from the slit is reflected on to this mirror by an inclined fixed mirror, and after reflection at the movable mirror is again reflected at the fixed mirror and so reaches the recording drum. By this arrangement the angular rotation of the reflected beam is less than that of the magnet, and hence the spot of light reflected from this mirror yields a trace on a much smaller scale than that given by the ordinary mirror and serves to give a complete record of even the most energetic disturbance.

See also Balfour Stewart, _Report of the British Association_, Aberdeen, 1859, 200, a description of the type of instrument used in the older observatories; E. Mascart, _Traité de magnétisme terrestre_, p. 191; W. Watson, _Terrestrial Magnetism_, 1901, 6, 187, describing magnetographs used in India; M. Eschenhagen, _Verhandlungen der deutschen physikalischen Gesellschaft_, 1899, 1, 147; _Terrestrial Magnetism_, 1900, 5, 59; and 1901, 6, 59; _Zeits. für Instrumentenkunde_, 1907, 27, 137; W. G. Cady, _Terrestrial Magnetism_, 1904, 9, 69, describing a declination magnetograph in which the record is obtained by means of a pen acting on a moving strip of paper, so that the curve can be consulted at all times to see whether a disturbance is in progress.

The effects of temperature being so marked on the readings of the horizontal and vertical force magnetographs, it is usual to place the instruments either in an underground room or in a room which, by means of double walls and similar devices, is protected as much as possible from temperature changes. For descriptions of the arrangements adopted in some observatories see the following: U.S. observatories, _Terrestrial Magnetism_, 1903, 8, 11; Utrecht, _Terrestrial Magnetism_, 1900, 5, 49; St Maur, _Terrestrial Magnetism_, 1898, 3, 1; Potsdam, _Veröffentlichungen des k. preuss. meteorol. Instituts_, "Ergebnisse der magnetischen Beobachtungen in Potsdam in den Jahren 1890 und 1891;" Pavlovsk, "Das Konstantinow'sche meteorologische und magnetische Observatorium in Pavlovsk," _Ausgabe der kaiserl. Akad. der Wissenschaften zu St Petersburg_, 1895. (W. Wn.)

FOOTNOTES:

[1] _Report British Association_, Bristol, 1898, p. 741.

[2] _Verhandlungen der deutschen physikalischen Gesellschaft_, 1899, 1, 147; or _Terrestrial Magnetism_, 1900, 5, 59.

MAGNETOMETER, a name, in its most general sense, for any instrument used to measure the strength of any magnetic field; it is, however, often used in the restricted sense of an instrument for measuring a particular magnetic field, namely, that due to the earth's magnetism, and in this article the instruments used for measuring the value of the earth's magnetic field will alone be considered.

The elements which are actually measured when determining the value of the earth's field are usually the declination, the dip and the horizontal component (see MAGNETISM, TERRESTRIAL). For the instruments and methods used in measuring the dip see INCLINOMETER. It remains to consider the measurement of the declination and the horizontal component, these two elements being generally measured with the same instrument, which is called a unifilar magnetometer.

[Illustration: FIG. 1.--Unifilar Magnetometer, arranged to indicate declination.]

_Measurement of Declination._--The measurement of the declination involves two separate observations, namely, the determination of (a) the magnetic meridian and (b) the geographical meridian, the angle between the two being the declination. In order to determine the magnetic meridian the orientation of the magnetic axis of a freely suspended magnet is observed; while, in the absence of a distant mark of which the azimuth is known, the geographical meridian is obtained from observations of the transit of the sun or a star. The geometrical axis of the magnet is sometimes defined by means of a mirror rigidly attached to the magnet and having the normal to the mirror as nearly as may be parallel to the magnetic axis. This arrangement is not very convenient, as it is difficult to protect the mirror from accidental displacement, so that the angle between the geometrical and magnetic axes may vary. For this reason the end of the magnet is sometimes polished and acts as the mirror, in which case no displacement of the reflecting surface with reference to the magnet is possible. A different arrangement, used in the instrument described below, consists in having the magnet hollow, with a small scale engraved on glass firmly attached at one end, while to the other end is attached a lens, so chosen that the scale is at its principal focus. In this case the geometrical axis is the line joining the central division of the scale to the optical centre of the lens. The position of the magnet is observed by means of a small telescope, and since the scale is at the principal focus of the lens, the scale will be in focus when the telescope is adjusted to observe a distant object. Thus no alteration in the focus of the telescope is necessary whether we are observing the magnet, a distant fixed mark, or the sun.

The Kew Observatory pattern unifilar magnetometer is shown in figs. 1 and 2. The magnet consists of a hollow steel cylinder fitted with a scale and lens as described above, and is suspended by a long thread of unspun silk, which is attached at the upper end to the torsion head H. The magnet is protected from draughts by the box A, which is closed at the sides by two shutters when an observation is being taken. The telescope B serves to observe the scale attached to the magnet when determining the magnetic meridian, and to observe the sun or star when determining the geographical meridian.

[Illustration: FIG. 2.--Unifilar Magnetometer, arranged to show deflexion.]

When making a determination of declination a brass plummet having the same weight as the magnet is first suspended in its place, and the torsion of the fibre is taken out. The magnet having been attached, the instrument is rotated about its vertical axis till the centre division of the scale appears to coincide with the vertical cross-wire of the telescope. The two verniers on the azimuth circle having been read, the magnet is then inverted, i.e. turned through 180° about its axis, and the setting is repeated. A second setting with the magnet inverted is generally made, and then another setting with the magnet in its original position. The mean of all the readings of the verniers gives the reading on the azimuth circle corresponding to the magnetic meridian. To obtain the geographical meridian the box A is removed, and an image of the sun or a star is reflected into the telescope B by means of a small transit mirror N. This mirror can rotate about a horizontal axis which is at right angles to the line of collimation of the telescope, and is parallel to the surface of the mirror. The time of transit of the sun or star across the vertical wire of the telescope having been observed by means of a chronometer of which the error is known, it is possible to calculate the azimuth of the sun or star, if the latitude and longitude of the place of observation are given. Hence if the readings of the verniers on the azimuth circle are made when the transit is observed we can deduce the reading corresponding to the geographical meridian.