The warmth of his muscular chest was like a magnet.
Sticking it to the refrigerator with a magnet, she headed for the barn.
Katie looked to the fridge, where a small business card was stuck beneath a cartoon magnet.
She attached the prescriptions to the fridge with another cartoon magnet and smoothed out the paperwork she'd been given from the police station.
Striding back to the kitchen, he deliberately removed the caduceus magnet and centered the picture on the refrigerator door – eye level.
Smiling, he anchored it with the magnet.
He moved the magnet and plucked the picture from the door.
What was it about him that served as a magnet for her eyes?
You're like a magnet lately.
In the second, or " braking off " method, the brake is automatically applied by a spring or weight, and is released either mechanically or, in the case of electric cranes, by the pull of a solenoid or magnet which is energized by the current passing through the motor.
C. Oersted, of the action of the galvanic current on a magnet.
A small permanent magnet is always liable to become demagnetized, or have its polarity reversed by the action of lightning.
This liability is overcome by making such movable parts as require to be magnetic of soft iron, and magnetizing them by the inducing action of a strong permanent magnet.
The magnet ' is wound to a resistance of amperes with accumulators).
circuit-closing apparatus called a relay, which is practi cally an electromagnetic key which has its lever attached to the armature of the magnet and which can be worked by a very weak current.
These tongues are magnetized by the inducing action of a strong horse-shoe permanent magnet, S N, which is made in a curved shape for the sake of compactness.
The electromagnet consists of two coils, each wound on a soft iron core fixed to the poles of a strong permanent horse-shoe magnet.
The armature of the electromagnet is normally attracted by the effect of the permanent magnet, but it is furnished with two antagonistic springs tending to throw it upwards.
The shafts are turned by the pull of the magnet upon the coils, and the motions of the transmitting pencil are thus reproduced.
The magnet between the poles of which the rectangular signal coil moves is built up of a number of thin flat horseshoe-shaped permanent magnets of a special quality of steel, and is provided with adjustable pole pieces.
The recorder coil is connected mechanically to a second similar coil, which is suspended between the poles of a laminated magnet, so that the motions of the two are similar.
This magnet is excited by an alternating current, and the current induced in the second coil is after rectification sent through an ordinary siphon recorder.
The magnet was mounted with its end carrying the coil opposite, and very close to, the centre of the piece of clock spring.
A much better form of electromagnetic ammeter can be constructed on a principle now extensively employed, which consists in pivoting in the strong field of a permanent magnet a small coil through which a part of the current to be measured is sent.
construction of this instrument is as follows: - Within the instrument is a horseshoe magnet having soft-iron pole pieces so arranged as to produce a uniform magnetic field.
It is essential that the permanent magnet should be subjected to a process of ageing so that its field may not be liable to change subsequently with time.
The capillary tube can be raised or lowered at will by running a magnet outside the tube, and the heights of the columns are measured by a cathetometer or micrometer microscope.
In Englishspeaking countries the ore is commonly known as magnetite, and pieces which exhibit attraction as magnets; the cause to which the attractive property is attributed is called magnetism, a name also applied to the important branch of science which has been evolved from the study of phenomena associated with the magnet.
If a magnet is dipped into a mass of iron filings and withdrawn, filings cling to certain parts of the stone in moss-like tufts, other parts remaining bare.
The regions of greatest attraction have received the name of poles, and the line joining them is called the axis of the magnet; the space around a magnet in which magnetic effects are exhibited is called the field of magnetic force, or the magnetic field.
Upon one of these is based the principle of the mariner's compass, which is said to have been known to the Chinese as early as I ioo B.C., though it was not introduced into Europe until more than 2000 years later; a magnet supported so that its axis is free to turn in a horizontal plane will come to rest with its poles pointing approximately north and south.
The other phenomenon is mentioned by Greek and Roman writers of the 1st century: a piece of iron, when brought into contact with a magnet, or even held near one, itself becomes " inductively " magnetized, and acquires the power of lifting iron.
If the iron is soft and fairly pure, it loses its attractive property when removed from the neighbourhood of the magnet; if it is hard, some of the induced magnetism is permanently retained, and the piece becomes an artificial magnet.
Magnetism may be imparted to a bar of hardened steel by stroking it several times from end to end, always in the same direction, with one of the poles of a magnet.
Such a combination constitutes an electromagnet, a valuable device by means of which a magnet can be instantly made and unmade at will.
Steel articles, such as knitting or sewing needles and pieces of flat spring, may be readily magnetized by stroking them with the bar-magnet; after having produced magnetism in any number of other bodies, the magnet will have lost nothing of its own virtue.
The compass needle is a little steel magnet balanced upon a pivot; one end of the needle, which always bears a distinguishing mark, points approximately, but not in general exactly, to the north,' the vertical plane through the direction of the needle being termed the magnetic meridian.
The north-seeking end of a magnet is in English-speaking countries called the north pole and the other end the south pole; in France the names are interchanged.
The wire will in fact become temporarily magnetized by induction, that end of it which is nearest to the pole of the magnet acquiring opposite polarity, and behaving as if it were the pole of a permanent magnet.
Even a permanent magnet is susceptible of induction, its polarity becoming thereby strengthened, weakened, or possibly reversed.
If one pole of a strong magnet is presented to the like pole of a weaker one, there will be repulsion so long as the two are separated by a certain minimum distance.
At shorter distances the magnetism induced in the weaker magnet will be stronger than its permanent magnetism, and there will be attraction; two magnets with their like poles in actual contact will always cling together unless the like poles are of exactly equal strength.
Induction is an effect of the field of force associated with a magnet.
Magnetic force has not merely the property of acting upon magnetic poles, it has the additional property of producing a phenomenon known as magnetic induction, or magnetic flux, a physical condition which is of the nature of a flow continuously circulating through the magnet and the space outside it.
Inside the magnet the course of the flow is from the south pole to the north pole; thence it diverges through the surrounding space, and again converging, re-enters the magnet at the south pole.
When the magnetic induction flows through a piece of iron or other magnetizable substance placed near the magnet, a south pole is developed where the flux enters and a north pole where it leaves the substance.
Outside the magnet the direction of the magnetic induction is generally the same as that of the magnetic force.
A map indicating the direction of the force in different parts of the field due to a magnet may be constructed in a very simple manner.
A sheet of cardboard is placed above the magnet, and some iron filings are sifted thinly and evenly over the surface: if the cardboard is gently tapped, the filings will arrange themselves in a series of curves, as shown in fig.
ir`"' lines of force," of which the curves formed by the filings afford a rough indication; Faraday's lines are howeve confined to the plane of the cardboard, but occur in the whole of the space around the magnet.
A line of force is regarded as proceeding from the north pole towards the south pole of the magnet, its direction being that in which an isolated north pole would be urged along FIG.
When the compass is far from the magnet, the vibrations will be comparatively slow; when it is near a pole, they will be exceedingly rapid, the frequency of the vibrations varying as the square root of the magnetic force at the spot.
It is to the non-uniformity of the field surrounding a magnet that the apparent attraction between a magnet and a magnetizable body such as iron is ultimately due.
Gilbert in 1600, that the earth itself is a great magnet, having its poles at the two places where the dipping needle is vertical.
To be consistent with the terminology adopted in Britain, it is necessary to regard the pole which is geographically north as being the south pole of the terrestrial magnet, and that which is geographically south as the north pole; in practice however the names assigned to the terrestrial magnetic poles correspond with their geographical situations.
If, for example, a knitting needle is stroked with the south pole of a magnet, the strokes being directed from the middle of the needle towards the two extremities alternately, the needle will acquire a north pole at each end and a south pole in the middle.
It is also possible that a magnet may have no poles at all.
But no magnet can have a single pole; if there is one, there must also be at least a second, of the opposite sign and of exactly equal strength.
This experiment proves that the condition of magnetization is not confined to those parts where polar phenomena are exhibited, but exists throughout the whole body of the magnet; it also suggests the idea of molecular magnetism, upon which the accepted theory of magnetization is based.
The process of magnetization consists in turning round the molecules by the application of magnetic force, so that their north poles may all point more or less approximately in the direction of the force; thus the body as a whole becomes a magnet which is merely the resultant of an immense number of molecular magnets.
In every magnet the strength of the south pole is exactly equal to that of the north pole, the action of the same magnetic force upon the two poles being equal and oppositely directed.
A magnet attached to a cork and [[[Terminology And Principles]] floated upon water will set itself with its axis in the magnetic meridian, but it will be drawn neither northward nor southward; the forces acting upon the two poles have therefore no horizontal resultant.
The poles at the ends of an infinitely thin uniform magnet, or magnetic filament, would act as definite centres of force.
An actual magnet may generally be regarded as a bundle of magnetic filaments, and those portions of the surface of the magnet where the filaments terminate, and socalled " free magnetism " appears, may be conveniently called poles or polar regions.
A more precise definition is the following: When the magnet is placed in a uniform field, the parallel forces acting on the positive poles of the constituent filaments, whether the filaments ' For the relations between magnetism and light see Magnetooptics.
terminate outside the magnet or inside, have a resultant, equal to the sum of the forces and parallel to their direction, acting at a certain point N.
The point N, which is the centre of the parallel forces, is called the north or positive pole of the magnet.
The opposite and parallel forces acting on the poles are always equal, a fact which is sometimes expressed by the statement that the total magnetism of a magnet is zero.
The line joining the two poles is called the axis of the magnet.
A magnetic field is generally due either to a conductor carrying an electric current or to the poles of a magnet.
The moment, M, M or V, of a uniformly and longitudinally magnetized bar-magnet is the product of its length into the strength of one of its poles; it is the moment of the couple acting on the magnet when placed in a field of unit intensity with its axis perpendicular to the direction of the field.
If 1 is the length of the magnet, M = ml.
The action of a magnet at a distance which is great compared with the length of the magnet depends solely upon its moment; so also does the action which the magnet experiences when placed in a uniform field.
The moment of a small magnet may be resolved like a force.
If the magnet is not uni - form, the magnetization at any point is the ratio of the moment of an element of volume at that point to the volume itself, or I = m.ds/dv.
If the direction of the magnetization at the surface of a magnet makes 3 The C.G.S.
The potential due to a thin magnet at a point whose distance from the two poles respectively is r and r' is V =m(l/r=l/r') (8) When V is constant, this equation represents an equipotential surface.
2 shows the lines of force and the plane sections of the equipotential surfaces for a thin magnet with poles concentrated at its ends.
The potential due to a small magnet of moment M, at a point whose distance from the centre of the magnet is r, is V=M cos 0/r 2, (io) where 0 is the angle between r and the axis of the magnet.
For a point in the line OY bisecting the magnet perpendicularly, 0 =42 therefore cos 0 =0, and the point D is at an infinite distance.
Although the above useful formulae, (io) to (15), are true only for an infinitely small magnet, they may be practically applied whenever the distance r is considerable compared with the length of the magnet.
If a small magnet of moment M is placed in the sensibly uniform field H due to a distant magnet, the couple tending to turn the small magnet upon an axis at right angles to the magnet and to the force is MH sin 0, (17) where 0 is the angle between the axis of the magnet and the direction of the force.
4 S'N' is a small magnet of moment M', and SN a distant fixed magnet of moment M; the axes of SN and S'N' make angles of 0 and 4 respectively with the line through their middle points.
A magnet may be regarded as consisting of an infinite number of elementary magnets, each having a pair of poles and a definite magnetic moment.
A magnet which consists entirely of such solenoids, having their ends either upon the surface or closed upon themselves, is called a solenoidal magnet, and the magnetism is said to be distributed solenoidally; there is no free magnetism in its interior.
If the constituent solenoids are parallel and of equal strength, the magnet is also uniformly magnetized.
A magnet which can be divided into simple magnetic shells, either closed or having their edges on the surface of the magnet, is called a lamellar magnet, and the magnetism is said to be distributed lamellarly.
A magnet consisting of a series of plane shells of equal strength arranged at right angles to the direction of magnetization will be uniformly magnetized.
Since 7ra'I is the moment of the sphere (=volume X magnetization), it appears from (10) that the magnetized sphere produces the same external effect as a very small magnet of equal moment placed at its centre and magnetized in the same direction; the resultant force therefore is the same as in (14).
When it is desired to have a uniform magnet with definitely situated poles, it it usual to employ one having the form of an ovoid, or elongated ellipsoid of revolution, instead of a rectangular or cylindrical bar.
If the magnetization is parallel to the major axis, and the lengths of the major and minor axes are 2a and 2C, the poles are situated at a distance equal to 3a from the centre, and the magnet will behave externally like a simple solenoid of length 3a.
Kohlrausch 2 the distance between the poles of a cylindrical magnet the length of which is from io to 30 times the diameter, is sensibly equal to five-sixths of the length of the bar.
In the case of a straight uniformly magnetized bar the direction of the magnetic force due to the poles of the magnet is from the north to the south pole outside the magnet, and from the south to the north inside.
In certain cases, as, for instance, in an iron ring wrapped uniformly round with a coil of wire through which a current is passing, the induction is entirely within the metal; there are, consequently, no free poles, and the ring, though magnetized, constitutes a poleless magnet.
Forces acting on a Small Body in the Magnetic Field.-If a small magnet of length ds and pole-strength m is brought into a magnetic field such that the values of the magnetic potential at the negative and positive poles respectively are V 1 and the work done upon the magnet, and therefore its potential energy, will be W =m(V2-Vi) =mdV, which may be written W =m d s- = M d v= - MHo = - vIHo, ds ds where M is the moment of the magnet, v the volume, I the magnetization, and Ho the magnetic force along ds.
The small magnet may be a sphere rigidly magnetized in the direction of Ho; if this is replaced by an isotropic sphere inductively magnetized by the field, then, for a displacement so small that the magnetization of the sphere may be regarded as unchanged, we shall have dW = - vIdHo = v I+-, whence W = - 2 I + H2 ° (37) The mechanical force acting on the sphere in the direction of displacement x is 1 Hopkinson specified the retentiveness by the numerical value of the " residual induction " (=47rI).
A substance of which the real susceptibility is will, when surrounded by a medium having the susceptibility k', behave towards a magnet as if its susceptibility were - -}-4,rK').
- The moment M of a magnet may be determined in many ways,' the most accurate being that of C. F.
The product MH is first determined by suspending the magnet horizontally, and causing it to vibrate in small arcs.
M/H = (d 2 -1 2) tan 0/2d, where 1 is half the length of the magnet, which is placed in the " broadside-on " position as regards a small suspended magnetic needle, d the distance between the centre of the magnet and the needle, and 0 the angle through which the needle is deflected by the magnet.
The moment of a magnet may also be deduced from a measurement of the couple exerted on the magnet by a uniform field H.
Thus if the magnet is suspended horizontally by a fine wire, which, when the magnetic axis points north and south, is free from torsion, and if 0 is the angle through which the upper end of the wire must be twisted to make the magnet point east and west, then MH = CB, or M = C6/H, where C is the torsional couple for r 0.
If P is the weight of the magnet, l the length of each of the two threads, 2a the distance between their upper points of attachment, and 2b that between the lower points, then, approximately, MH = P(ab/l) sin 0.
It is often sufficient to find the ratio of the moment of one magnet to that of another.
Let 0 be the angle which the standard magnet M makes with the meridian, then M'/R = sin 0, and M/R = cos 0, whence M' = M tan 0.
10 The magnet is laid on a table with its north pole pointing northwards.
A compass having a very short needle is placed on the line which bisects the axis of the magnet at right angles, and is moved until a neutral point is found where the force due to the earth's field H is balanced by that due to the magnet.
The distance between the poles may with sufficient accuracy for a rough determination be assumed to be equal to five-sixths of the length of the magnet.
Of the three methods which have been described, the first two are generally the most suitable for determining the moment or the magnetization of a permanent magnet, and the last for studying the changes which occur in the magnetization of a long rod or wire wl?E:n subjected to various external magnetic forces, or, in other words, for determining the relation of I to H.
The sample, arranged as a bundle of rectangular strips, is caused to rotate about a central horizontal axis between the poles of an upright C-shaped magnet, which is supported near 'its middle upon knife-edges in such a manner that it can oscillate about an axis in a line with that about which the specimen rotates; the lower side of the magnet is weighted, to give it some stability.
When the specimen rotates, the magnet is deflected from its upright position by an amount which depends upon the work done in a single complete rotation, and therefore upon the hysteresis.
Ewing has described an arrangement in which the test bar has a soft-iron pole piece clamped to each of its ends; the pole pieces are joined by a long well-fitting block of iron, which is placed upon them (like the " keeper " of a magnet), and the induction is measured by the force required to detach the block.
The sample to be inserted between the magnet poles was prepared in the form of a bobbin resembling an ordinary cotton reel, with a short narrow neck (constituting the " isthmus ") and conical ends.
It is shown in the paper that the greatest possible force which the isthmus method can apply at a point in the axis of the bobbin is F = 11, 137 I, log i n b/a, I, being the saturation value of the magnet pores, a the radius of the neck on which the cones converge, and b the radius of the bases of the cones.
Tractive Force of a Magnet.-Closely connected with the results just discussed is the question what is the greatest tractive force that can be exerted by a magnet.
The stress in question seems, however, to be quite unconnected with the " stress in the medium " contemplated by Maxwell, and its value is not exactly B 2 /87r except in the particular case of a permanent ring magnet, when H = O.
It is suggested that a permanent magnet might conveniently be " aged " (or brought into a constant condition) by dipping it several times into liquid air.
For additional information regarding the composition and qualities of permanent magnet steels reference may be made 6 The marked effect of silicon in increasing the permeability of cast iron has also been noticed by F.
Weber's theory, the molecules of a ferromagnetic metal are small permanent magnets, the axes of which under ordinary conditions are turned indifferently in every direction, so that no magnetic polarity is exhibited by the metal as a whole; a magnetic force acting upon the metal tends to turn the axes of the little magnets in one direction, and thus the entire piece acquires the properties of a magnet.
It can be shown that if a current i circulates in a small plane circuit of area S, the magnetic action of the circuit for distant points is equivalent to that of a short magnet whose axis is perpendicular to the plane of the circuit and whose moment is iS, the direction of the magnetization being related to that of the circulating current as the thrust of a right-handed screw to its rotation.
The creation of an external magnetic field H will, in accordance with Lenz's law, induce in the molecule an electric current so directed that the magnetization of the equivalent magnet is opposed to the direction of the field.
If S is the area of the orbit described in time T by an electron of charge e, the moment of the equivalent magnet is M = eST; and the change in the value of M due to an external field H is shown to be OM = - He'S/47rm, m being the mass of the electron.
If the structure of the molecule is so perfectly symmetrical that, in the absence of any external field, the resultant magnetic moment of the circulating electrons is zero, then the application of a field, by accelerating the right-handed (negative) revolutions, and retarding those which are left-handed, will induce in the substance a resultant magnetization opposite in direction to the field itself; a body composed of such symmetrical molecules is therefore diamagnetic. If however the structure of the molecule is such that the electrons revolving around its atoms do not exactly cancel one another's effects, the molecule constitutes a little magnet, which under the influence of an external field will tend to set itself with its axis parallel to the field.
The property of orientation, in virtue of which a freely suspended magnet points approximately to the geographical north and south, is not referred to by any European writer before the 12th century, though it is said to have been known to the Chinese at a much earlier period.
The downward tendency of the north pole of a magnet pivoted in the usual way had been observed by G.
The greatest of Gilbert's discoveries was that the globe of the earth was magnetic and a magnet; the evidence by which he supported this view was derived chiefly from ingenious experiments made with a spherical lodestone or lerrella, as he termed it, and from his original observation that an iron bar could be magnetized by the earth's force.
C. Oersted 6 that a magnet placed near a wire carrying an electric current tended to set itself at right angles to the wire, a phenomenon which indicated that the current was surrounded by a magnetic field.
Ampere's experimental and theoretical investigation of the mutual action of electric currents, and of the equivalence of a closed circuit to a polar magnet, the latter suggesting his celebrated hypothesis that molecular currents were the cause of magnetism.
Throughout his researches Faraday paid special regard to the medium as the true seat of magnetic action, being to a large extent guided by his pregnant conception of " lines of force," or of induction, which he considered to be " closed curves passing in one part of the course through, the magnet to which they belong, and in the other part through space," always tending to shorten themselves, and repelling one another when they were side by side (Exp. Res.
C. Oersted (1777-1851) had shown that a magnetic needle is deflected by an electric current, he attempted, in the laboratory of the Royal Institution in the presence of Humphry Davy, to convert that deflection into a continuous rotation, and also to obtain the reciprocal effect of a current rotating round a magnet.
It was probably in Paris, the chief intellectual centre of his time, that Neckam heard how a ship, among its other stores, must have a needle placed above a magnet (the De utensilibus assumes a needle mounted on a pivot), which needle would revolve until its point looked north, and thus guide sailors in murky weather or on starless nights.
rer., a sort of manual of the scientific knowledge of the 12th century, is much the most important: the magnet passage herein is in book ii.
- Iron, nickel and cobalt are the only metals which are attracted by the magnet and can become magnets themselves.
His prize subjects were, the capstan, the propagation of light, and the magnet.
The field magnet of the dynamo has two gaps in it.
By rot ting a small electro-magnet in water, between the poles of ano her magnet, and then measuring the heat developed in the wat r and other parts of the machine, the current induced in the coils, and the energy required to maintain rotation, he cal b lated that the quantity of heat capable of warming one you d of water one degree F.
The thin disk of mercury is therefore traversed perpendicularly by lines of magnetic force when the magnet is excited.
In its improved form this meter consists of a single horseshoe permanent magnet formed of tungsten-steel having a strong and constant field.
The driving force is balanced against a retarding force produced by the rotation of a copper disk fixed on the armature shaft, which rotates between the poles of a permanent magnet.
By the use of a permanent magnet instead of a shunt coil as the bob of one pendulum, the meter can be made up as an ampere-hour meter.
In this instrument there is a fixed permanent magnet, producing a.
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.
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.
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 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.
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.
A second setting with the magnet inverted is generally made, and then another setting with the magnet in its original position.
For this reason some observers use a thin strip of phosphor bronze to suspend the magnet, considering that the absence of a variable torsion more than compensates for the increased difficulty in handling the more fragile metallic suspension.
The method of measuring the horizontal component which is almost exclusively used, both in fixed observatories and in the field, consists in observing the period of a freely suspended magnet, and then obtaining the angle through which an auxiliary suspended magnet is deflected by the magnet used in the first part of the experiment.
By the vibration experiment we obtain the value of the product of the magnetic moment (M) of the magnet into the horizontal component (H), while by the deflexion experiment we can deduce the value of the ratio of M to H, and hence the two combined give both M and H.
In the case of the Kew pattern unifilar the same magnet that is used for the declination is usually employed for determining H, and for the purposes of the vibration experiment it is mounted as for the observation of the magnetic meridian.
The temperature of the magnet must also be observed, for which purpose a thermometer C (fig.
The auxiliary magnet has a plane mirror attached, the plane of which is at right angles to the axis of the magnet.
An image of the ivory scale B is observed after reflection in the magnet mirror by the telescope A.
The magnet K used in the vibration experiment is supported on a carriage L which can slide along the graduated bar D.
The axis of the magnet is horizontal and at the same level as the mirror magnet, while when the central division of the scale B appears to coincide with the vertical cross-wire of the telescope the axes of the two magnets are at right angles.
During the experiment the mirror magnet is protected from draughts by two wooden doors which slide in grooves.
What is known as the method of sines is used, for since the axes of the two magnets are always at right angles when the mirror magnet is in its zero position, the ratio M/H is proportional to the sine of the angle between the magnetic axis of the mirror magnet and the magnetic - = meridian.
When conducting a deflexion experiment the de flecting magnet K is placed with its centre at 30 cm.
from the mirror magnet and to the east of the latter, and the whole instrument is turned till the centre division of the scale B coincides with the cross-wire of the telescope, when the readings of the verniers on the azimuth circle are noted.
The magnet K is then reversed in the support, and a new setting taken.
The difference between the two sets of readings gives twice the angle which the magnetic axis of the mirror magnet makes with the magnetic meridian.
In order to eliminate any error due to the zero of the scale D not being exactly below the mirror magnet, the support L is then removed to the west side of the instrument, and the settings are repeated.
Further, to allow of a correction being applied for the finite length of the magnets the whole series of settings is repeated with the centre of the deflecting magnet at 40 cm.
from the mirror magnet.
Omitting correction terms depending on the temperature and on the inductive effect of the earth's magnetism on the moment of the deflecting magnet, if 0 is the angle which the axis of the deflected magnet makes with the meridian when the centre of the deflecting magnet is at a distance r, then zM sin B=I+P+y2 &c., in which P and Q are constants depending on the dimensions and magnetic states of the two magnets.
Thus it is usual, if the magnets are of similar shape, to make the deflected magnet 0.467 of the length of the deflecting magnet, in which case Q is negligible, and thus by means of deflexion experiments at two distances the value of P can be obtained.
Mag., 1904 , 7, p. 113.) In the case of the vibration experiment correction terms have to be introduced to allow for the temperature of the magnet, for the inductive effect of the earth's field, which slightly increases the magnetic moment of the magnet, and for the torsion of the suspension fibre, as well as the rate of the chronometer.
If the temperature of the magnet were always exactly the same in both the vibration and FIG 2.
The fact that the moment of inertia of the magnet varies witli the temperature must, however, be taken into account.
In the deflexion experiment, in addition to the induction correction, and that for the effect of temperature on the magnetic moment, a correction has to be applied for the effect of temperature on the length of the bar which supports the deflexion magnet.
In order to obtain the declination a pivoted magnet is used to obtain the magnetic meridian, the geographical meridian being obtained by observations on the sun or stars.
The principle of the method consists in deflecting the compass needle by means of a horizontal magnet supported vertically over the compass card, the axis of the deflecting magnet being always perpendicular to the axis of the magnet attached to the card.
The method is not strictly an absolute one, since it presupposes a knowledge of the magnetic moment of the deflecting magnet.
In practice it is found that a magnet can be prepared which, when suitably protected from shock, &c., retains its magnetic moment sufficiently constant to enable observations of H to be made comparable in accuracy with that of the other elements obtained by the instruments ordinarily employed at sea.
By 1891 he had designed and erected at the Royal Institution an apparatus which yielded liquid oxygen by the pint, and towards the end of that year he showed that both liquid oxygen and liquid ozone are strongly attracted by a magnet.
The mariner's compass, with which this article is concerned, is an instrument by means of which the directive force of that great magnet, the Earth, upon a freely-suspended needle, is utilized for a purpose essential to navigation.
high, and fitted to receive and alter at pleasure the several magnet and soft iron correctors.
The hull of an iron or steel ship is a magnet, and the distribution of its magnetism depends upon the direction of the ship's head when building, this result being produced by induction from the earth's magnetism, developed and impressed by the hammering of the plates and frames during the process of building.
With the deflector any inequality in the directive force can be detected, and hence the power of equalizing the forces by the usual soft iron and magnet correctors.
Johnson, R.N., showed from experiments in the iron steamship "Garry Owen" that the vessel acted on an external compass as a magnet.
806-820); and there is no evidence that they contained any magnet.
The Chinese name for the compass is ting-nan-ching, or needle pointing to the south; and a distinguishing mark is fixed on the magnet's southern pole, as in European compasses upon the northern one."
The Italian name of calamita, which still persists, for the magnet, and which literally signifies a frog, is doubtless derived from this practice.
The Arabic geographer, Edrisi, who lived about r roo, is said by Boucher to give an account, though in a confused manner, of the polarity of the magnet (Hallam, Mid.
c. 89, he writes, - "Mariners at sea, when; through cloudy weather in the day which hides the sun, or through the darkness of the night, they lose the knowledge of the quarter of the world to which they are sailing, touch a needle with the magnet, which will turn round till, on its motion ceasing, its point will be directed towards the north" (W.
The magnetical needle, and its suspension on a stick or straw in water, are clearly described in La Bible Guiot, a poem probably of the r3th century, by Guiot de Provins, wherein we are told that through the magnet (la manette or l'amaniere), an ugly brown stone to which iron turns of its own accord, mariners possess an art that cannot fail them.
In Scandinavian records there is a reference to the nautical use of the magnet in the Hauksbok, the last edition of the LandndmabOk (Book of the Colonization of Iceland): - "Floki, son of Vilgerd, instituted a great sacrifice, and consecrated three ravens which should show him the way (to Iceland); for at that time no men sailing the high seas had lodestones up in northern lands."
All that is certain is a knowledge of the nautical use of the magnet at the end of the r3th century.
c. 4, p. 345, Hafniae, 1711); and it is probable that the use of the magnet at sea was known in Scotland at or shortly subsequent to that time, though King Robert, in crossing from Arran to Carrick in 1306, as Barbour writing in 1375 informs us, "na nedill had na stane," but steered by a fire on the shore.
Prior to this clear description of a pivoted compass by Peregrinus in 1269, the Italian sailors had used the floating magnet, probably introduced into this region of the Mediterranean by traders belonging to the port of Amalfi, as commemorated in the line of the poet Panormita: "Prima dedit nautis usum magnetis Amalphis."
In 1511 Baptista Pio in his Commentary repeats the opinion as to the invention of the use of the magnet at Amalfi as related by Flavius.
Gyraldus, writing in 1540 (Libellus de re nautica), misunderstanding this reference, declared that this observation of the direction of the magnet to the poles had been handed down as discovered "by a certain Flavius."
A specimen of one of these heavy glasses afterwards became historically important as the substance in which Faraday detected the rotation of the plane of polarization of light when the glass was placed in the magnetic field, and also as the substance which was first repelled by the poles of the magnet.
This led him in the beginning of September to discover the method of producing the continuous rotation of the wire round the magnet, and of the magnet round the wire.
He did not succeed in making the wire or the magnet revolve on its own axis.
Ampere, Wollaston and others, the realization of the continuous rotation of the wire and the magnet round each other was a scientific puzzle requiring no mean ingenuity for its original solution.
For on the one hand the electric current always forms a closed circuit, and on the other the two poles of the magnet have equal but opposite properties, and are inseparably connected, so that whatever tendency there is for one pole to circulate round the current in one direction is opposed by the equal tendency of the other pole to go round the other way, and thus the one pole can neither drag the other round and round the wire nor yet leave it behind.
The thing cannot be done unless we adopt in some form Faraday's ingenious solution, by causing the current, in some part of its course, to divide into two channels, one on each side of the magnet, in such a way that during the revolution of the magnet the current is transferred from the channel in front of the magnet to the channel behind it, so that the middle of the magnet can pass across the current without stopping it, just as Cyrus caused his army to pass dryshod over the Gyndes by diverting the river into a channel cut for it in his rear.
In December 1824 he had attempted to obtain an electric current by means of a magnet, and on three occasions he had made elaborate but unsuccessful attempts to produce a current in one wire by means of a current in another wire or by a magnet.
He endeavoured, but in vain, to detect any change in the lines of the spectrum of a flame when the flame was acted on by a powerful magnet.
On the 3rd of November a new horseshoe magnet came home, and Faraday immediately began to experiment on the action in the polarized ray through gases, but with no effect.
A bar of heavy glass was suspended by silk between the poles of the new magnet.
From Aristotle we learn (I) that Thales found in water the origin of things; (2) that he conceived the earth to float upon a sea of the elemental fluid; (3) that he supposed all things to be full of gods; (4) that in virtue of the attraction exercised by the magnet he attributed to it a soul.
In Se p tember of that year he discovered that the force required for the rotation of a copper disk becomes greater when it is made to rotate with its rim between the poles of a magnet, the disk at the same time becoming heated by the eddy or "Foucault currents" induced in its metal.
M-P; because, though both end in a universal conclusion, the limits of experience prevent induction from such inference as: Every experienced magnet attracts iron.
Every magnet whatever is every experienced magnet..
Every magnet whatever attracts iron.
This view makes inference easy: induction is all over before it begins; for, according to Bradley, " every one of the instances is already a universal proposition; and it is not a particular fact or phenomenon at all," so that the moment you observe that this magnet attracts iron, you ipso facto know that every magnet does so, and all that remains for deduction is to identify a second magnet as the same with the first, and conclude that it attracts iron.
" this magnet attracts iron," is not another, e.g.
" that magnet attracts iron," and neither is universal; on the other hand, a universal judgment, e.g.
" every magnet attracts iron," means, distributively, that each individual magnet exerts its individual attraction, though it is similar to other magnets exerting similar attractions.
magnets, each of which is separately though similarly a magnet, not magnet in general.
Hence also induction is a real process, because, when we know that this individual magnet attracts iron, we are very far from knowing that all alike do so similarly; and the question of inductive logic, how we get from some similars to all similars, remains, as before, a difficulty, but not to be solved by the fallacy that inference is identification.
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.
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.
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 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.
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, 0, corresponding to unit twist of the suspension system remain constant.
Since such a decrease in 0 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 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.
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 1 Report British Association, Bristol, 18 9 8, P. 741.
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.
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.
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.
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.
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.
cap. 2.2 He invented the versorium or 1 Gilbert's work, On the Magnet, Magnetic Bodies and the Great Magnet, the Earth, has been translated from the rare folio Latin edition of 1600, but otherwise reproduced in its original form by the chief members of the Gilbert Club of England, with a series of valuable notes by Prof. S.
- Noticing an analogy between the polarity of the voltaic pile and that of the magnet, philosophers had long been anxious to discover a relation between the two, but twenty years elapsed after the invention of the pile before Hans Christian Oersted (1777-1851), professor of natural philosophy in the university of Copenhagen, made in 1819 the discovery which has immortalized his name.
Oersted's important discovery was the fact that when a wire joining the end plates of a voltaic pile is held near a pivoted magnet or compass needle, the latter is deflected and places itself more or less transversely to the wire, the direction depending upon whether the wire is above or below the needle, and on the manner in which the copper or zinc ends of the pile are connected to it.
In 1821 Michael Faraday (1791-1867), who was destined later on to do so much for the science of electricity, discovered electromagnetic rotation, having succeeded in causing a wire conveying a voltaic current to rotate continuously round the pole of a permanent magnet.
Trans., 1823) showed that when two wires connected with the pole of a battery were dipped into a cup of mercury placed on the pole of a powerful magnet, the fluid rotated in opposite directions about the two electrodes.
The study of the relation between the magnet and the circuit conveying an electric current then led Arago to the discovery of the " magnetism of rotation."
He found that a vibrating magnetic compass needle came to rest sooner when placed over a plate of copper than otherwise, and also that a plate of copper rotating under a suspended magnet tended to drag the magnet in the same direction.
Faraday and others then discovered, as already mentioned, means to make the conductor conveying the current rotate round a magnetic pole, and Ampere showed that a magnet could be made to rotate on its own axis when a current was passed through it.
The word usage examples above have been gathered from various sources to reflect current and historial usage. They do not represent the opinions of YourDictionary.com.