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.)