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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 See also:

SHORT, FRANCIS JOB (1857– )

short lines of light; but a piano-cylindrical lens is placed with its See also:

AXIS (Lat. for " axle ")

axis horizontal just in front of the recording See also:

SURFACE

surface. In this way a spot of light is obtained from each mirror. The recording surface is a See also:

SHEET

sheet of photographic See also:

paper wrapped See also:

ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round a See also:

DRUM (early forms drome or dromme, a word common to many Teut. languages, cf. Dan. tromme, Ger. Trommel: the word is ultimately the same as " trumpet," and is probably onomatopoeic in origin; it appears late in Eng. about the middle of the 16th century)

drum which is rotated at a See also:

CONSTANT, BENJAMIN (1845-1902)

constant See also:

SPEED, JOHN (1552–1629)

speed by clockwork about a horizontal axis. The light reflected from the fixed mirror traces a straight See also:

LINE

line on the paper, serving as a base line from which the See also:

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 See also:

scale the difference between the declination and a constant See also:

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle, namely, the declination corresponding to the base line. The value of this constant angle is obtained by comparing the See also:

RECORD (Lat. recordari, to recall to mind, from cor, heart or mind)

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 See also:

minute of arc.

The See also:

time scale ordinarily employed is 15 mm. per See also:

HOUR

hour, but in See also:

MODERN

modern See also:

INSTRUMENTS

instruments See also:

PROVISION (Lat. provisio)

provision is generally made for the time scale to be increased at will to 18o 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 See also:

RESPOND

respond to rapid changes in the See also:

earth's See also:

field, were first insisted upon by E. Mascart,' while M. Eschenhagen2 first designed a set of magnetographs in which this See also:

IDEA (Gr. Ibia, connected with i&eiv, to see; cf. Lat. species from specere, to look at)

idea of small moment of inertia was carried to its useful limit, the magnets only weighing 1.5 See also:

GRAM

gram each, and the suspension consisting of a very See also:

FINE

fine See also:

QUARTZ

quartz fibre. Horizontal Force.—The variation of the horizontal force is obtained by the See also:

motion of a magnet which is carried either by a bifilar See also:

sus-See also:

PENSION (Lat. pensio, a payment, from pendere, to weigh, to pay)

pension or by a fairly stiff See also:

metal See also:

WIRE (A.S. wir, a wire; cf. Swed. vire, to twist, M.H.G. wiere, a gold ornament, Lat. viriae, armlets, ultimately from the root wi, to twist, bind)

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 See also:

meridian. In this position the magnet is in See also:

EQUILIBRIUM (from the Lat. aequus, equal, and libra, a balance)

equilibrium under the See also:

ACTION

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 See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change in H, so long as M and the couple, 9, corresponding to unit twist of the suspension See also:

SYSTEM

system remain constant. When the temperature changes, however, both M and B in See also:

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 See also:

fibres are brought nearer together and hence the value of 0 decreases.

Since such a decrease in B would by itself cause the magnet to turn in the same direction as if H had increased, it is possible in a See also:

GREAT

great measure to neutralize the effects of temperature on the See also:

READING

reading of the instrument. In the case of the unifilar suspension, the provision of a temperature See also:

COMPENSATION (from Lat. compensare, to weigh one thing against another)

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 See also:

PATTERN

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 See also:

Schmidt, Zeits. fiir 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 i 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 See also:

KNIFE (0. E. cuff, a word appearing in different forms in many Teutonic languages, cf. Du. knijf, Ger. Kneif, a shoe-maker's knife, Swed. knif; the ultimate origin is unknown; Skeat finds the origin in the root of " nip," formerly " knip "; Fr. canif is a

knife edges so that it can turn freely about a horizontal axis at right angles to its ' See also:

REPORT (O.Fr. report or raport, modern rapport, from O.Fr. reporter, mod. rap porter, Lat. reportare, to bring back, in poetical use only, of bringing back an account, news, &c.)

Report See also:

BRITISH

British Association, See also:

Bristol, 1898, p. 741. 2 Verhandlungen der deutschen physikalischen Gesellschaft, 1899, I, 147; or Terrestrial Magnetism, 1900, 5, 59. Ii length (H. See also:

Lloyd, Proc. See also:

ROY, WILLIAM (1726-1790)

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 See also:

INFLUENCE (Late Lat. influentia, from influere, to flow in)

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 See also:

side of the knife-edge. Hence when, say, V decreases the couple VM decreases, and hence the See also:

north end of the balanced magnet rises, and See also:

VICE

vice versa. The See also:

CHIEF (from Fr. chef, head, Lat. caput)

chief difficulty with this See also:

FORM (Lat. forma)

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 See also:

ZINC

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 See also:

BALANCE (derived through the Fr. from the Late Lat. bilantia, an apparatus for weighing, from bi, two, and lanx, a dish or scale)

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 See also:

order to eliminate the irregularity which is See also:

apt to be introduced by dust, &c., interfering with the working of the knif e-edge, W. See also:

Watson (Phil. Mag., 1904 [6j, 7, 393) designed a form of vertical force balance in which the magnet with its mirror is attached to the See also:

mid point of a horizontal stretched quartz fibre. The temperature compensation is obtained by attaching a small See also:

WEIGHT

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 See also:

DIAMETER (from the Cr. &a, through, µErpov, measure)

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 See also:

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary changes the See also:

CURVE (Lat. curvus, bent)

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 3o° 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 See also:

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 See also:

COMPLETE

complete record of even the most energetic disturbance. See also See also:

Balfour See also:

Stewart, Report of the British Association, See also:

Aberdeen, 1859, 200, a description of the type of instrument used in the older observatories; E. Mascart, Traiti de magnetisme terrestre, p. 191; W. Watson, Terrestrial Magnetism, 1901, 6, 187, describing magnetographs used in See also:

India; M. Eschenhagen, Verhandlungen der deutschen physikalischen Gesellschaft, 1899, 1, 147; Terrestrial Magnetism, 1900, 5, 59; and 1901, 6, 59; Zeits. far 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 See also:

PEN (Lat. penna, a feather, pen)

pen acting on a moving See also:

STRIP

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 See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place the instruments either in an underground See also:

ROOM

room or in a room which, by means of See also:

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, 1 I ; See also:

Utrecht, Terrestrial Magnetism, 1900, 5, 49; St Maur, Terrestrial Magnetism, 1898, 3, 1; See also:

POTSDAM

Potsdam, VerSffentlichungen See also:

des k. preuss. meteorol. Instituts, " Ergebnisse der rnagnetischen Beobachtungen in Potsdam in den Jahren 1890 and 1891 ;" See also:

PAVLOVSK

Pavlovsk, Das Konstantinow'sche meteorologische and magnetische Observatorium in Pavlovsk," Ausgabe der kaiserl. Akad. der Wissenschaften zu St See also:

PETERSBURG

Petersburg, 1895. (W.

End of Article: MAGNETOGRAPH

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