Chapter I: W. G (2)

In some machines both prime conductor and cushions are kept insulated and without ground contact. Electrodes connecting with each are brought with their ends close enough to maintain a sparking discharge.

Machine Influence. A static electric machine working by induction to build up charges of opposite nature on two separate prime conductors. In general they are based on the principle of the electrophorous. Work is done by the operator turning the handle. This rotates a disc and draws excited parts of it away from their bound charges. This represents a resistance to mechanical motion. The work absorbed in overcoming this mechanical resistance appears as electric energy. There are various types of influence machines, the Holtz, Toeppler-Holtz and Wimshurst being the most used. The electrophorous, q. v., is a type of influence machine.

Machine, Holtz Influence. A static electric machine. It includes two plates, one of which is rapidly rotated in front of the other. Two armatures of paper are secured to the back of the stationary plate at opposite ends of a diameter. To start it one of these is charged with electricity. This charge by induction acts through the two thicknesses of glass upon a metal bar carrying combs, which lies in front of the further side of the movable plate. The points opposite the armature repel electrified air, which strikes the movable disc and charges it. A second rod with comb at the opposite end of the same diameter acts in the reverse way. Thus opposite sections of the disc are oppositely charged and the combs with them. By induction these portions of the disc react upon the two armatures. The opposite electricities escape from the armatures by paper tongues which are attached thereto and press against the back of the movable plate. As the plate rotates the opposite electricities on its face neutralize the electricity repelled from the combs. The charges on the back strengthen the charges of the armatures and brass combs. Thus the machine builds up, and eventually a discharge of sparks takes place from the poles of the brass combs.

335 STANDARD ELECTRICAL DICTIONARY.

Machine, Toeppler-Holtz. A modification of the Holtz machine. The priming charge of the armatures is produced by friction of metallic brushes against metallic buttons on the face of the rotating plate. (See Machine, Holtz.)

Machine, Wimshurst. A form of static influence machine. It consists of two plates of glass, on which radial sectors of tinfoil are pasted. Both plates are rotated in opposite directions. The sectors of the two plates react one upon the other, and electric charges of opposite sign accumulate on the opposite sides of the plates and are collected therefrom by collecting combs.

Mack. A name, derived from Maxwell, and suggested for the unit of inductance. It is due to Oliver Heaviside, but has never been adopted. (See Henry.)

Magne-Crystallic Action. The action of a supposed force of the same name, proposed by Faraday. It relates to the different action of a magnetic field upon crystalline bodies, according to the position of their axes of crystallization. A needle of tourmaline, normally paramagnetic, if poised with its axis horizontal, is diamagnetic. Bismuth illustrates the same phenomenon. The subject is obscure. Faraday thought that he saw in it the action of a specific force.

Magnet. A body which tends when suspended by its centre of gravity to lay itself in a definite direction, and to place a definite line within it, its magnetic axis, q. v., in a definite direction, which, roughly speaking, lies north and south. The same bodies have the power of attracting iron (Daniell), also nickel and cobalt.

Magnets are substances which possess the power of attracting iron. (Ganot.)

[Transcriber's note: Edward Purcell and others have explained magnetic and electromagnetic phenomenon as relativistic effects related to electrostatic attraction. Magnetism is caused by Lorentz contraction of space along the direction of a current. Electromagnetic waves are caused by charge acceleration and the resulting disturbance of the electrostatic field. (Electricity and Magnetism: Berkeley Physics Course Volume 2, 1960)]

Magnet, Anomalous. A magnet possessing more than the normal number (two) of poles. If two straight magnets are placed end to end with their south poles in juxtaposition the compound bar will seem to possess three poles, one at each end and one in the middle. The apparent pole in the middle is really made up of two consequent poles, q. v. It sometimes happens that when a single long thin bar is magnetized consequent poles are produced, although such magnet is in one piece. This may be accidental, as in such case it is quite hard to avoid anomalous poles, or, as in the field magnets of some forms of dynamos, anomalous poles may be purposely produced.

Magnet, Artificial. A magnet formed artificially by any method of magnetization (see Magnetism) applicable to permanent magnets, electro-magnets and solenoids. It expresses the distinction from the natural magnets or lodestone, q. v. It is made of steel in practice magnetized by some of the methods described under Magnetization.

336 STANDARD ELECTRICAL DICTIONARY.

Magnet, Axial. A straight-solenoid with axial core.

Magnet, Bar. A bar magnet is one in the shape of a bar, i. c., straight with parallel sides and considerably longer than wide or deep.

Magnet, Bell-shaped. A form of permanent magnet used in some galvanometers. In shape it is a thick-sided cylindrical box with two slots cut out of opposite sides, so as to make it represent a horseshoe magnet. Its shape enables it to be surrounded closely by a mass of copper, for damping its motion, to render the instrument dead-beat. Such a magnet is used in Siemens & Halske's galvanometer.

Magnet Coil. A coil to be thrust over an iron core, to make an electro-magnet. They are often wound upon paper or wooden bobbins or spools, so as to be removable from the core if desired.

Magnet, Compensating. (a) A magnet fastened near a compass on an iron or steel ship to compensate the action of the metal of the ship upon the magnetic needle. The ship itself always has some polarity and this is neutralized by one or more compensating magnets.

(b) See below.

Magnet, Controlling. A magnet attached to a galvanometer by which the directive tendency of its magnetic needle is adjusted. In the reflecting galvanometer it often is a slightly curved magnet carried by a vertical brass spindle rising from the center of the instrument, and which magnet may be slid up and down on the spindle to regulate or adjust its action.

Synonym--Compensating Magnet.

Magnet, Compound. A permanent magnet, built up of a number of magnets. Small bars can be more strongly magnetized than large. Hence a compound magnet may be made more powerful than a simple one.

Magnet Core. The iron bar or other mass of iron around which insulated wire is wound for the production of an electro-magnet. The shapes vary greatly, especially for field magnets of dynamos and motors. For these they are usually made of cast iron, although wrought iron is preferable from the point of view of permeability.

Magnet, Damping. A damping magnet is one used for bringing an oscillating body to rest. The body may be a metallic disc or needle, and the action of the magnet depends on its lines of force which it establishes, so that the body has to cut them, and hence has its motion resisted.

337 STANDARD ELECTRICAL DICTIONARY.

Magnet, Deflection of. The change of position of a magnet from the plane of the earth's meridian in which it normally is at rest into another position at some angle thereto, by the effect of an artificial magnetic field, as the deflection of a galvanometer needle.

Magnet, Electro-. A magnet consisting of a bar of iron, bundle of iron wires, iron tube or some equivalent, around which a coil of insulated wire is wound. Such combination becomes polarized when a current is passed through it and is an active magnet. On the cessation of the current its magnetism in part or almost completely disappears. (See Electro-magnet.)

Magnet, Equator of. In a magnet the locus of points of no attractive power and of no polarity. In a symmetrical, evenly polarized magnet it is the imaginary line girdling the centre. The terms Neutral Point or Neutral Line have displaced it.

Synonyms--Neutral Line--Neutral Point.

Magnet, Field. A magnet, generally an electro-magnet, used to produce the field in a dynamo or motor.

Magnet, Haarlem. Celebrated magnets made in Haarlem, Holland. Logeman, Van Wetteren, Funckler and Van der Willigen were the makers who gave the celebrity to the magnets. They were generally horseshoe magnets, and would carry about twenty times their own weight.

Magnet, Horseshoe. A magnet of U shape--properly one with the poles brought a little closer together than the rest of the limbs. For direct lifting and attractive effects it is the most generally adopted type. Its advantage as regards lifting effect is due to small reluctance, q. v., offered by a complete iron circuit, such as the armature and magnet together produce. As the term is now used it is applied to any U shaped magnet.

Fig. 218. JOULE'S ELECTRO-MAGNET.

Magnet, Joule's Electro. An electro-magnet of the shape of a cylinder with a longitudinal segment cut-off. It is wound with wire as shown. The segment cut-off is a piece of the same shape as the armature. It is of high power.

338 STANDARD ELECTRICAL DICTIONARY.

Magnetic Adherence. The tendency of a mass of iron to adhere to the poles of a magnet. It is best figured as due to the virtual shortening of lines of force, as the more permeable iron gives a better path for them than the air can afford, and consequently a virtually shorter one.

Magnetic Attraction and Repulsion. The attraction of a magnet for iron, steel, nickel and cobalt and of unlike poles of magnets for each other. It is identical with electro-magnetic attraction, q.v. (Also see Electro-magnetism.)

Magnetic Attraction and Repulsion, Coulomb's Law of. Magnetic attraction and repulsion are inversely as the square of the distance. (Ganot.)

While theoretically true in the case of isolated poles, in practise it does not generally apply on account of the large diameter and relative shortness of magnets.

Magnetic Axis. The line connecting the poles of a magnet. It does not generally coincide exactly with any symmetrical axis of figure. In such cases an error is introduced into the indications of the needle which must be determined and allowed for in compasses. To determine it with a magnetic needle the suspension cup is made removable, so that the needle can be reversed. Readings are taken with one side of the needle and then with the other side of the needle up, and the average corresponds with the position of the magnetic axis in both positions of the needle.

Magnetic Azimuth. The angle, measured on a horizontal circle, between the magnetic meridian and a great circle of the earth passing through the observer and any observed body. It is the astronomical azimuth of a body referred to the magnetic meridian and therefore subject to the variation of the compass. The angle is the magnetic azimuth of the observed body.

Magnetic Battery. A name for a compound permanent magnet; one made up by bolting or clamping together, or to single soft iron pole pieces, a number of single permanent magnets. There are a number of forms of compound magnets. In making them care has to be taken to have them of even strength. It is also well to have them slightly separated. The object of both these precautions is to prevent a stronger element or magnet from depolarizing its neighbor.

Synonym--Compound Magnet.

Magnetic Bridge. An apparatus for testing the relative permeability of iron. It consists of a rectangular system of iron cores. Three of the sides are wound with wire as shown. The other side is built up of double bars, and from the centre two curved arms rise, as shown in the cut. The arms do not touch. Between them a short magnet is suspended by a filament, which also carries a mirror and an index.

339 STANDARD ELECTRICAL DICTIONARY.

Fig. 219. MAGNETIC BRIDGE.

A lamp and scale are provided as in the reflecting galvanometer. When adjusted the magnetic needle hangs as shown in the cut, Fig. 219, without any tendency to turn towards either curved pole piece. If all iron parts are symmetrical and of similar metal, a current through the coils will make no difference. It will work in magnetic opposition upon the two arms, or, in other words, will maintain both arms at identical potential.

Fig. 220. POLE PIECES, MAGNETIC NEEDLE AND MIRROR OF MAGNETIC BRIDGE.

If there is the least difference in permeability, length or thickness between any of the iron bars the magnetic potential of the two curved arms will differ, and the magnetic needle will turn one way or the other. In practical use different samples of iron are substituted for the unwound members of the fourth side of the parallelogram, and the needle by its motions indicates the permeability.

In the cut, Fig. 220, D D are the ends of the curved pole pieces; A the wire carrying the mirror B and magnetic needle N, and E is the index which shows the larger deflections.

340 STANDARD ELECTRICAL DICTIONARY.

Magnetic Circuit. A magnetic field of force is characterized by the presence of lines of force, which, while approximately parallel, curve around and tend to form closed curves. The polarity of a field of force is referred to an imaginary direction of the lines of force from the north pole through space to the south pole, and in the part of the field corresponding to the body of the magnet, from the south to the north pole. The cut indicates these features. Hence the magnetic field of force is termed the magnetic circuit, and to it are attributed a species of resistance termed reluctance, q. v., and the producing cause of the field or lines of force is termed sometimes magneto-motive force, q. v.) corresponding to the electro-motive force. The modern treatment of the magnetic circuit is similar to the application of Ohm's law and the laws of resistance and conductivity to the electric circuit.

Magnetic Circuit, Double. A magnetic circuit which virtually represents two horseshoe magnets placed with their like poles in contact. It is used for field magnets, the armatures occupying a place between the consequent poles.

Fig. 221. ONE-HALF PORTION OF A DOUBLE MAGNETIC CIRCUIT.

Magnetic Concentration of Ores. The concentration of ores or the freeing them from their gangue by magnetic attraction. It is only applicable to those cases in which either the ore itself or the gangue is attracted by the magnet. Its principal application is to the concentration of magnetic iron sands. (See Magnetic Concentration.)

Magnetic Concentrator. An apparatus similar to a magnetic separator, q. v., but used to concentrate magnetic iron sands. By the action of electro-magnets the magnetic iron sand (magnetite) is separated from the sand with which it is mixed.

Magnetic Conductivity and Conductance. The first notion of permeance and of the magnetic circuit included the idea of magnetic conductivity, which conducted lines of force urged by magneto-motive force through a magnetic circuit. The terms are displaced by permeability and permeance.

341 STANDARD ELECTRICAL DICTIONARY.

Magnetic Continuity. The completeness of a magnetic circuit, as when the armature of a horseshoe magnet is in contact with both poles. It is an attribute of a paramagnetic substance only and is identical for permanent magnets or for electro-magnets. An air space intervening between armature and magnet poles, or a space filled with any diamagnetic substance prevents continuity, although the lines of force to some extent still find their way around. The leakage is increased by discontinuity.

Magnetic Control. Control of a magnetic needle, magnet, iron index or armature, in a galvanometer, ammeter or voltmeter by a magnetic field; the restitutive force being derived from a permanent magnet.

Magnetic Couple. The couple of magnetic force which tends to bring the magnetic needle into the plane of the magnetic meridian. One force is represented by the imaginary pull upon the north pole, and the other by the opposite pull upon the south pole of the needle. The moment of the couple varies from a maximum when the needle is at right angles to the plane of the magnetic meridian to zero when it is in such plane.

Magnetic Creeping. Viscous hysteresis; the slow increase of magnetism in a paramagnetic body when exposed to induction.

Fig. 222. MAGNETIC CURVES OR FIGURES.

Magnetic Curves. The pictorial representation of magnetic lines of force. It is generally produced by scattering filings on a sheet of paper or pane of glass held over a magnet. The filings arrange themselves in characteristic curves. Tapping the paper or pane of glass facilitates the arrangement, or jarring the filings off a smaller magnet, so that they fall polarized upon the paper, is thought by some to improve the effect. The group of curves forms what are termed magnetic figures, q. v.

342 STANDARD ELECTRICAL DICTIONARY.

Magnetic Declination. The angular deviation of the magnetic needle, causing it to rest at an angle with the true meridian; the variation of the compass. (See Magnetic Elements.)

Magnetic Density. The intensity of magnetization expressed in lines of force per stated area of cross-section in a plane at right angles to the lines of force.

Magnetic Dip. The inclination from the horizontal assumed by a magnetic needle free to move in the vertical plane. (See Magnetic Elements.) The angle of dip or inclination is entirely a function of the earth, not of the needle.

Magnetic Discontinuity. A break or gap in a magnetic circuit. To make a complete circuit the iron or other core must be continuous. If the armature of a horseshoe magnet is in contact with both poles the continuity is complete. If the armature is not in contact magnetic continuity gives place to discontinuity. It is an attribute of a paramagnetic substance only, and is identical for permanent magnets, or for electro-magnets.

Magnetic Elements. The qualities of the terrestrial magnetism at any place as expressed in its action upon the magnetic needle. Three data are involved.

I. The Declination or Variation. II. The Inclination or Dip. III. The Force or Intensity.

I. The Declination is the variation expressed in angular degrees of the magnetic needle from the true north and south, or is the angle which the plane of the magnetic meridian makes with that of the geographical meridian. It is expressed as east or west variation according to the position of the north pole; east when the north pole of the needle is to the east of the true meridian, and vice versa. Declination is different for different places; it is at present west in Europe and Africa, and east in Asia and the greater part of North and South America. The declination is subject to (a) secular, (b) annual and (c) diurnal variations. These are classed as regular; others due to magnetic storms are transitory and are classed as irregular, (a) Secular variations. The following table shows the secular variations during some three hundred years at Paris. These changes are termed secular, because they require centuries for their completion.

343 STANDARD ELECTRICAL DICTIONARY.

Table of Declination or Variation at Paris. Year. Declination. 1580 11º 30' E. 1663 0° 1700 8° 10' W. 1780 19º 55' W. 1785 22º 00' W. 1805 22º 5' W. 1814 22º 34' W. 1825 22° 22' W. 1830 22º 12' W. 1835 22º 4' W. 1850 20º 30' W. 1855 19º 57' W. 1860 19º 32' W. 1865 18º 44' W. 1875 17º 21' W. 1878 17º 00' W. [Transcriber's note The value for 2008 is about 0° 48' W, changing by 0° 7' E/year.]

On scrutinizing these figures it will be seen that there is part of a cycle represented and that the declination is slowly returning to the zero point after having reached its maximum western variation in 1814. Upwards of 300 years would be required for its completion on the basis of what is known. In other places, notably the coast of Newfoundland, the Gulf of the St. Lawrence and the rest of the North American seaboard and in the British Channel, the secular variations are much more rapid in progress. (b) Annual variations--These were first discovered in 1780 by Cassini. They represent a cycle of annual change of small extent, from 15' to 18' only. In Paris and London the annual variation is greatest about the vernal equinox, or March 21st, and diminishes for the next three months, and slowly increases again during the nine following months. It varies during different epochs. (c) Diurnal variations were discovered in 1722 by Graham. A long needle has to be employed, or the reflection of a ray of light, as in the reflecting galvanometer, has to be used to observe them. In England the north pole of the magnetic needle moves every day from east to west from sunrise until 1 or 2 P. M.; it then tends towards the east and recovers its original position by 10 P. M. During the night the needle is almost stationary. As regards range the mean amplitude of diurnal variations at Paris is from April to September 13' to 15'; for the other months from 8' to 10'. On some days it amounts to 25' and sometimes is no more than 5'. The amplitude of diurnal variations decreases from the poles to the equator. Irregular variations accompany earthquakes, the aurora borealis and volcanic eruptions. In Polar regions the auroral variations may be very great; even at 40° latitude they may be 1° or 2°. Simultaneous irregularities sometimes extend over large areas. Such are attributed to magnetic storms. II. The Inclination is the angle which the magnetic needle makes with the horizon, when the vertical plane in which the needle is assumed to be free to move coincides with the magnetic meridian. It is sometimes called the dip of the needle. It varies as does the declination, as shown in the following table of inclinations of London.

344 STANDARD ELECTRICAL DICTIONARY.

Table of Inclination or Dip at London Year. Inclination. 1576 71° 50' 1600 72° 1676 73° 30' 1723 74° 42' 1773 72° 19' 1780 72° 8' 1790 71° 33' 1800 70° 35' 1821 70° 31' 1828 69° 47' 1838 69° 17' 1854 68° 31' 1859 68° 21' 1874 67° 43' 1876 67° 39' 1878 67° 36' 1880 67° 35' 1881 67° 35'

III. Force or Intensity is the directive force of the earth. It varies with the squares of the number of oscillations the magnetic needle will make if caused to oscillate from a determined initial range. The intensity is supposed to be subject to secular change. According to Gauss the total magnetic intensity of the earth is equal to that which would be exerted if in each cubic yard there were eight bar magnets, each weighing one pound. This is, of course, a rough way of expressing the degree of intensity. Intensity is least near the magnetic equator and greatest near the magnetic poles; the places of maximum intensity are termed the magnetic foci. It varies with the time of day and possibly with changes in altitude.

Magnetic Elongation. The elongation a bar of iron or steel undergoes when magnetized. By magnetization it becomes a little longer and thinner, there being no perceptible change in volume. The change is accompanied by a slight sound--the magnetic tick. An exceedingly delicate adjustment of apparatus is required for its observation.

Magnetic Equator. A locus of the earth's surface where the magnet has no tendency to dip. It is, approximately speaking, a line equally distant from the magnetic poles, and is called also the aclinic line. It is not a great circle of the earth.

345 STANDARD ELECTRICAL DICTIONARY.

Magnetic Field of Force. The field of force established by a magnet pole. The attractions and repulsions exercised by such a field follow the course of the electro- magnetic lines of force. (See also Field of Force.) Thus the tendency of a polarized needle attracted or repelled is to follow, always keeping tangential to curved lines, the direction of the lines of force, however sweeping they may be. The direction of magnetic lines of force is assumed to be the direction in which a positive pole is repelled or a negative one attracted; in other words, from the north pole of a magnet to its south pole in the outer circuit. The direction of lines of force at any point, and the intensity or strength of the field at that point, express the conditions there. The intensity may bc expressed in terms of that which a unit pole at unit distance would produce. This intensity as unitary it has been proposed to term a Gauss. (See Weber.)

The direction of the lines of force in a magnetic field are shown by the time-honored experiment of sprinkling filings of iron upon a sheet of paper held over a magnet pole or poles. They arrange themselves, if the paper is tapped, in more or less curved lines tending to reach from one pole of the magnet to the other. Many figures may be produced by different conditions. Two near poles of like name produce lines of force which repel each other. (See Magnetic Curves.)

A magnetic and an electro-magnetic field are identical in all essential respects; the magnetic field may be regarded as a special form of the electro-magnetic field, but only special as regards its production and its defined north and south polar regions.

Synonyms--Magnetic Spin (not much used).

Magnetic Field, Uniform. A field of identical strength in all parts, such as the earth's magnetic field. If artificially produced, which can only be approximately done, it implies large cross-section of magnet pole in proportion to the length of the magnetic needle affected by it, which is used in determining its uniformity.

Magnetic Figures. The figures produced by iron filings upon paper or glass held near magnetic poles. By these figures the direction of lines of force is approximately given, and a species of map of the field is shown. (See Magnetic Field of Force--Magnetic Curves.)

Magnetic Filament. The successive rows of polarized molecules assumed to exist in magnetized iron. Each molecule represents an infinitely small magnet, and its north pole points to the south pole of the next molecule. Such a string or row is a theoretical conception based on the idea that the molecules in a magnet are all swung in to parallelism in the magnetizing process. A magnetic filament may be termed the longitudinal element of a magnet. (See Magnetism, Hughes' Theory of.)

[Transcriber's note: This description parallels the modern notion of electron spin as the basis of magnetism in materials.]

Magnetic Fluids. A two-fluid theory of magnetism has been evolved, analogous to the two-fluid theory of electricity. It assumes north fluid or "red magnetism" and a south fluid or "blue magnetism." Each magnetism is supposed to predominate at its own pole and to attract its opposite. Before magnetization the fluids are supposed to neutralize each other about each molecule; magnetization is assumed to separate them, accumulating quantities of them at the poles.

Magnetic Flux. Magnetic induction; the number of lines of force that pass through a magnetic circuit.

Synonym--Magnetic Flow.

346 STANDARD ELECTRICAL DICTIONARY.

Magnetic Force. The forces of attraction and repulsion exercised by a magnet. By Ampere's theory it is identical with the forces of attraction and repulsion of electric currents.

Magnetic Friction. The damping effect produced on the movements of a mass of metal by proximity to a magnet; the phenomenon illustrated in Arago's wheel, q. v. When a mass of metal moves in the vicinity of a magnet it cuts the lines of force emanating from its poles, thereby producing currents in its mass; as the production of these currents absorbs energy a damping effect is produced upon the movements of the mass.

Magnetic Gear. Friction gear in which electro-magnetic adherence is employed to draw the wheels together. (See Adherence, Electro-magnetic--Electro-magnetic Friction Gear.)

Magnetic Inclination. The inclination from the horizontal of a magnetic needle placed in the magnetic meridian. (See Magnetic Element--Inclination Map.)

Synonym--Magnetic Dip.

Magnetic Induction. The force of magnetization within an induced magnet. It is in part due to the action of the surrounding particles of polarized material; in part to the magnetic field. (See Magnetic Induction, Coefficient of.)

In a more general way it is the action of a magnet upon bodies in its field of force. In some cases the magnetism induced causes the north pole of the induced magnet to place itself as far as possible from the north pole of the inducing magnet and the same for the south poles. Such substances are called paramagnetic or ferromagnetic. They lie parallel or tangential to the lines of force. In other cases the bodies lie at right angles or normal to the lines of force. Such bodies are called diamagnetic.

Some bodies are crystalline or not homogeneous in structure, and in them the lines of magnetic induction may take irregular or eccentric paths. (See AEolotropic.)

Synonym--Magnetic Influence.

Magnetic Induction, Apparent Coefficient of. The apparent permeability of a paramagnetic body as affected by the presence of Foucault currents in the material itself. These currents act exactly as do the currents in the coils surrounding the cores of electro-magnets. They produce lines of force which may exhaust the permeability of the iron, or may, if in an opposite direction, add to its apparent permeability.

Magnetic Induction, Coefficient of. The number, obtained by dividing the magnetization of a body, expressed in lines of force produced in it, by the magnetizing force which has produced such magnetization, expressed in lines of force producible by the force in question in air. It always exceeds unity for iron, nickel and cobalt. It is also obtained by multiplying the coefficient of induced magnetization by 4 PI (4 * 3.14159) and adding 1. (See Magnetic Susceptibility--Magnetization, Coefficient of Induced.)

347 STANDARD ELECTRICAL DICTIONARY.

The coefficient of magnetic induction varies with the material of the induced mass, and varies with the intensity of the magnetizing force. This variation is due to the fact that as the induced magnetism in a body increases, the magnetizing force required to maintain such induction, increases in a more rapid ratio. The coefficient of magnetic induction is the same as magnetic permeability, and in a certain sense is the analogue of conductivity. It is also termed the multiplying power of the body or core magnetized. It is the coefficient of induced magnetization (see Magnetization, Coefficient of Induced) referred to a mass of matter. For diamagnetic bodies the coefficient has a negative sign; for paramagnetic bodies it has a positive sign.

Synonyms--Permeability--Multiplying Power--Magnetic Inductive Capacity.

Magnetic Induction, Dynamic. The induction produced by a magnetic field which moves with respect to a body, or where the body if moving moves at a different rate, or where the body moves and the field is stationary. In the case where both move, part of the induction may be dynamic and part static. (See Magnetic Induction, Static.)

Magnetic Induction, Static. Magnetic induction produced by a stationary field acting upon a stationary body.

Magnetic Induction, Tube of. An approximate cylinder or frustrum of a cone whose sides are formed of lines of magnetic induction. (See Magnetic Induction, Lines of.) The term tube is very curiously applied in this case, because the element or portion of a magnetic field thus designated is in no sense hollow or tubular.

Magnetic Inertia. A sensible time is required to magnetize iron, or for it to part with its magnetism, however soft it may be. This is due to its magnetic inertia and is termed the lag. Permanent or residual magnetism is a phase of it. It is analogous to self-induction of an electric circuit, or to the residual capacity of a dielectric.

Magnetic Insulation. Only approximate insulation of magnetism is possible. There is no perfect insulator. The best ones are only 10,000 times less permeable than iron. Hence lines of force find their way through air and all other substance, being simply crowded together more in paths of iron or other paramagnetic substance.

348 STANDARD ELECTRICAL DICTIONARY.

Magnetic Intensity. The intensity of the magnetization of a body. It is measured by the magnetic lines of force passing through a unit area of the body, such area being at right angles to the direction of the lines of force.

Magnetic Lag. In magnetism the tendency of hard iron or steel especially to take up magnetism slowly, and to part with it slowly. (See Magnetic Inertia.) The lag affects the action of a dynamo, and is a minor cause of those necessitating the lead of the brushes.

Synonym--Magnetic Retardation.

Magnetic Latitude. Latitude referred to the magnetic equator and isoclinic lines.

Magnetic Leakage. The lines of force in a field magnet which pass through the air and not through the armature are useless and represent a waste of field. Such lines constitute magnetic leakage.

Magnetic Limit. The temperature beyond which a paramagnetic metal cannot be magnetized. The magnetic limit of iron is from a red to a white heat; of cobalt, far beyond a white heat; of chromium, below a red heat; of nickel at about 350° C. (662°F.) of manganese, from 15° C. to 20° C. (59° to 68° F.)

Magnetic Lines of Force. Lines of force indicating the distribution of magnetic force, which is due presumably to whirls of the ether. A wire or conductor through which a current is passing is surrounded by an electro-magnetic field of force, q. v., whose lines of force form circles surrounding the conductor in question. A magnet marks the existence of a similar electro-magnetic field of force whose lines form circuits comprising part of and in some places all of the body of the magnet, and which are completed through the air or any surrounding paramagnetic or diamagnetic body. They may be thought of as formed by the Ampérian sheet of current, and analogous to those just mentioned as surrounding a conductor.

Fig. 223. MAGNETIC LINES OF FORCE, DIRECTION OF.

A magnetic line of force may be thought of as a set of vortices or whirls, parallel to each other, and strung along the line of force which is the locus of their centres.

If as many lines are drawn per square centimeter as there are dynes (per unit pole) of force at the point in question, each such line will be a unitary c. g. s. line of force.

349 STANDARD ELECTRICAL DICTIONARY.

Magnetic Mass. A term for a quantity of magnetism. Unit mass is the quantity which at unit distance exercises unit force.

Magnetic Matter. Imaginary matter assumed as a cause of magnetism. Two kinds, one positive and one negative, may be assumed as in the two fluid theory of electricity, or only one kind, as in the single fluid theory of electricity. Various theories of magnetic matter have been presented whose value is only in their convenience.

[Transcriber's note: See "magnet" and Edward Purcell's explanation of magnetism using general relativity.]

Magnetic Memory. The property of retaining magnetism; coercive force; magnetic inertia; residual magnetism.

[Transcriber's note: Small ferrite magnetic donuts were used as computer main memory from 1950 to 1970.]

Magnetic Meridian. A line formed on the earth's surface by the intersection therewith of a plane passing through the magnetic axis. It is a line determined by the direction of the compass needle. The meridians constantly change in direction and correspond in a general way to the geographical meridians.

Magnetic Moment. The statical couple with which a magnet would be acted on by a uniform magnetic field of unit intensity if placed with its magnetic axis at right angles to the lines of force of the field. (Emtage.) A uniformly and longitudinally magnetized bar has a magnetic moment equal to the product of its length by the strength of its positive pole.

Magnetic Needle. A magnet with a cup or small depression at its centre and poised upon a sharp pin so as to be free to rotate or oscillate in a horizontal plane. The cup is often made of agate. Left free to take any position, it places its magnetic axis in the magnetic meridian.

Magnetic Parallels. Lines roughly parallel to the magnetic equator on all parts of each of which the dip of the magnetic needle is the same; also called Isoclinic Lines. These lines mark the places of the intersection of equipotential surfaces with the earth's surface. They are not true circles, and near the poles are irregular ellipses; the magnet there points toward their centres of curvature. They correspond in a general way with the Geographical Parallels of Latitude.

Magnetic Permeability. The specific susceptibility of any substance, existing in a mass, for magnetic induction. (See Magnetic Induction, Coefficient of, synonym for Magnetic Permeability and Magnetization, Coefficient of Induced.)

Synonyms--Magnetic Inductive Capacity--Multiplying Power--Coefficient of Magnetic Induction.

350 STANDARD ELECTRICAL DICTIONARY.

Magnetic Perturbations. Irregular disturbances of the terrestrial magnetism, as by the aurora and in electric storms.

Magnetic Poles. The points where the equipotential surfaces of the terrestrial field of force graze the earth's surface; the points toward which the north or south poles of the magnetic needle is attracted. Over a magnetic pole the magnetic needle tends to stand in a vertical position. There are two poles, Arctic or negative, and Antarctic or positive. Magnetic needles surrounding them do not necessarily point toward them, as they point to the centres of curvature of their respective magnetic parallels. The poles constantly change in position. The line joining them does not coincide with anything which may be termed the magnetic axis of the earth.

Magnetic Poles, False. Poles on the earth's surface other than the two regular magnetic poles. There seem by observation to be several such poles, while analogy would limit true magnetic poles to two in number.

Magnetic Potential. The potential at any point of a magnetic field is the work which would be done by the magnetic forces of the field upon a positive unit of magnetism as it moves from that point to an infinite distance. (Emtage.)

Magnetic Proof Piece. A piece of iron used for testing magnets and the distribution of magnetism in bars, by suspending or supporting above or near the magnet, by detaching after adherence, and in other ways.

Magnetic Proof Plane. An exploring coil used for testing the distribution of magnetism. It is connected in circuit with a galvanometer, and exposed to alternation of current, or to other disturbing action produced by the magnet or field under examination. This affects the galvanometer, and from its movements the current produced in the coil, and thence the magnetic induction to which it was exposed, are calculated.

Synonym--Exploring Coil.

Magnetic Quantity. The magnetism possessed by a body; it is proportional to the action of similar poles upon each other, or to the field produced by the pole in question. It is also called the strength of a pole.

The force exercised by two similar poles upon each other varies with their product and inversely with the square of the distance separating them; or it may be expressed thus (m * m) / (L^2). This is a force, and the dimensions of a force are ML/(T^2). Therefore, (m^2)/(L^2) = ML/(T^2) or m = (M^.5)*(L^1.5)/T.

351 STANDARD ELECTRICAL DICTIONARY.

Magnetic Reluctance. The reciprocal of permeance; magnetic resistance; the relative resistance to the passage of lines of force offered by different substances. The idea is derived from treating the magnetic circuit like an electric one, and basing its action on magneto-motive force acting through a circuit possessing magnetic reluctance.

Magnetic Reluctivity. The reciprocal of magnetic permeability, q. v.

Synonym--Magnetic Resistance.

Magnetic Retentivity. The property of steel or hard iron by which it slowly takes up and slowly parts with a magnetic condition--traditionally (Daniell) called coercitive force.

Magnetic Rotary Polarization. If a plane polarized beam of light is sent through a transparent medium in a magnetic field its plane of polarization is rotated, and this phenomenon is denoted as above. (Compare Refraction, Electric, and see Electro-magnetic Stress.) This has been made the basis of a method for measuring current. A field of force varies with the current; the polarization produced by such field is therefore proportional to the current. (Becquerel & Rayleigh.)

A plane polarized beam of light passing through the transparent medium in the magnetic field by the retardation or acceleration of one of its circular components has its plane of polarization rotated as described. The direction of the lines of force and the nature of the medium determine the sense of the rotation; the amount depends upon the intensity of the field resolved in the direction of the ray, and on the thickness and nature of the medium.

Magnetic Saturation. The maximum magnetic force which can be permanently imparted to a steel bar. A bar may be magnetized beyond this point, but soon sinks to it. The magnetism produced in a bar is prevented from depolarization by the retentivity or coercive force of the bar. The higher the degree of magnetization the greater the tendency to depolarization.

It is also defined as the maximum intensity of magnetism produced in a paramagnetic substance by a magnetic field as far as affected by the permeability of the substance in question. The more lines of force passed through such a substance the lower is its residual permeability. It is assumed that this becomes zero after a certain point, and then the point of saturation is reached. After this point is reached the addition of any lines of force is referred entirely to the field and not at all to the permeability of the substance. But such a zero is only definable approximately.

Magnetic Screen. A box or case of soft iron, as thick as practicable, for protecting bodies within it from the action of a magnetic field. The lines of force to a great extent keep within the metal of the box on account of its permeability, and but a comparatively few of them cross the space within it.

Such screens are used to prevent watches from being magnetized, and are a part of Sir William Thomson's Marine galvanometer.

A magnetic screen may be a sphere, an infinite or very large plane, or of the shape of any equipotential surface.

Synonym--Magnetic Shield.

352 STANDARD ELECTRICAL DICTIONARY.

Magnetic Self-induction. The cause of a magnet weakening is on account of this quality, which is due to the direction of the lines of force within a magnet from the positive towards the negative pole. "A magnet thus tends to repel its own magnetism and to weaken itself by self-induction." (Daniell.)

Magnetic Separator. An apparatus for separating magnetic substances from mixtures. Such separators depend on the action of electro-magnets. In one form the material falls upon an iron drum, magnetized by coils. Any magnetic substance adheres to the drum and is thereby separated. They are used by porcelain makers for withdrawing iron particles from clay, by machinists to separate iron filings and chips from brass, and for similar purposes.

Fig. 224. MAGNETIC SEPARATOR.

Magnetic Shell. A theoretical conception of a cause of a magnetic field or of a distribution of magnetism. If we imagine a quantity of very short magnets arranged in contact with their like poles all pointing in the same direction so as to make a metal sheet, we have a magnetic shell. Its magnetic moment is equal to the sum of the magnetic moment of all its parts. If the shell is of uniform strength the magnetic moment of a unit area gives the strength of the shell; it is equal to the magnetic quantity per unit of area, multiplied by the thickness of the shell.

If its strength is uniform throughout a magnetic shell is called simple; if its strength varies it is termed complex.

Emtage thus defines it: A magnetic shell is an indefinitely thin sheet magnetized everywhere in the direction normal to itself.

Magnetic Shell, Strength of. The magnetic quantity per unit of area of the shell multiplied by the thickness of the shell.

353 STANDARD ELECTRICAL DICTIONARY.

Magnetic Shield. In general a magnetic screen, q. v. Sometimes a strong local field is made to act as a shield, by its predominance overcoming any local or terrestrial field to which the needle to be protected may be exposed.

Magnetic Shunt. The conception of a magnetic circuit being formed, the shunt is a corollary of the theory. It is any piece of iron which connects points of a magnet differing in polarity, so as to divert part of the lines of force from the armature or yoke. The shunt is especially applicable in the case of horseshoe magnets. Thus a bar of iron placed across from limb to limb a short distance back from the poles would act as a shunt to the armature and would divert to itself part of the lines of force which would otherwise go through the armature and would weaken the attraction of the magnet for the latter. In dynamos a bar of iron used as a magnetic shunt has been used to diminish the lines of force going through the armature and hence to weaken the field and diminish the electro-motive force. By moving the shunt nearer or further from the poles the dynamo is regulated.

In the cut the projections between the yoke and poles of the magnet shown act as a shunt to the yoke, taking some lines of force therefrom.

Fig. 225. MAGNETIC SHUNT.

Magnetic Storms. Terrestrial magnetic disturbances sometimes covering very wide areas, and affecting the magnetic declination and inclination. One such disturbance was felt simultaneously at Toronto, Canada, the Cape of Good Hope, Prague and Van Diemen's Land. (Sabine.)

354 STANDARD ELECTRICAL DICTIONARY.

Magnetic Strain. The strain produced by magnetic lines of force in substances exposed to their action. It is observed in substances placed between the poles of a strong electro-magnet, and evinces itself in the alteration of the optical properties of transparent substances.

Magnetic Stress. The stress produced by magnetic lines of force on substances through which they pass, evidenced in alteration of the optical properties of transparent bodies thus treated.

Magnetic Susceptibility. The specific intrinsic susceptibility of any material for magnetic induction. It refers to the particle of matter, and not to the mass, as in the latter its own particles react on each other and bring about what is termed permeability, q. v. (See also Magnetization, Coefficient of Induced, and Magnetic Induction, Coefficient of.)

Synonym--Coefficient of Induced Magnetization.

Magnetic Tick. When a bar of iron is suddenly magnetized or demagnetized it emits a slight sound, called the Page sound, or the magnetic tick. This has been utilized in a telephone by Reiss. The telephone will receive sound, but is very weak. It consists of a bar surrounded with a coil of insulated wire. Variations in current produce sounds, which may be articulate if the currents are produced by a telephonic transmitter.

Magnetic Twist. A bar of iron held in the magnetic meridian and pointing to the pole and twisted becomes to some extent permanently magnetized. Conversely a bar when magnetized seems to have a twist set up in it. The latter is magnetic twist.

Magnetic Variations. Changes in the value of magnetic declination or inclination. (See Magnetic Elements.)

Magnetism, Ampére's Theory of. A theory accounting for magnetic phenomena by assuming the existence of currents circulating around the molecules of permanent magnets. If such currents so circulate and all in the same direction, the result is the same as if the body of the magnet was enveloped in currents representing those of an electro-magnet or solenoid. This is because in the interior the current around one molecule would counteract the current around its neighboring ones in part, so that the only virtual currents left would be represented by those on the outer surfaces of the outer shell of molecules, and these virtually resolve themselves into one general current sheet, surrounding the magnet and coinciding with its surface.

The theory assumes that such currents permanently circulate around the molecules of paramagnetic substances. Under ordinary conditions there is no coincidence in their direction and no resultant current is produced. When magnetized or polarized the molecules are brought into order, so that the direction of their current coincides and the body becomes a magnet.

355 STANDARD ELECTRICAL DICTIONARY.

Fig. 226. AMPÉRIAN CURRENTS IN MAGNETS.

At the north pole of the magnet the direction of the Ampérian currents is the reverse of that of a watch when the observer faces the pole; the reverse obtains for the south pole.

The attraction of opposite and repulsion of similar poles is explained by the actions of the Ampérian currents upon each other. If north and south pole are placed together these currents will coincide in direction and hence will attract each other. If two like poles are put together the currents will have opposite directions and will repel each other.

No energy is supposed to be required to maintain currents around or in a single molecule.

Fig. 227. NORTH AND SOUTH POLES OF A MAGNET SHOWING DIRECTION OF AMPÉRIAN CURRENTS.

Magnetism, Blue. A term arising from the two fluid theory of magnetism; the magnetism of the south pole of a magnet. (See Magnetic Fluids.) The magnetism of the north pole is termed red magnetism. Both terms originated presumably in the painting of magnets, and are little used.

Synonym--South Magnetic Fluid.

356 STANDARD ELECTRICAL DICTIONARY.

Magnetism, Components of Earth's. The magnetic force of the earth acts in the plane of the magnetic meridian and in direction generally lies oblique to the plane of the horizon. It can be resolved into two components, one vertical, which has no directive effect upon the magnetic needle, the other horizontal, which represents the directive element for the usual compass needle. For the dipping needle, q. v., the vertical component is the only active one. A magnetic needle mounted on a universal joint at its centre of gravity would be acted on by both components.

Magnetism, Creeping of. The gradual increase of magnetism when a magnetic force is applied with absolute steadiness to a piece of iron. It is a form of magnetic lag. It may last for half an hour and involve an increase of several per cent. of the total magnetism.

Synonym--Viscous Hysteresis.

Magnet, Iron Clad. A magnet with a casing of iron connected at one end to the core. The term is generally applied to electromagnets of this form.

Synonyms--Tubular Magnet--Jacketed Magnet.

Magnetism, Decay of. The gradual loss of magnetism by permanent magnets, due to accidental shocks, changes of temperature, slow spontaneous annealing of the iron and other similar causes.

Magnetism, Discharge of. The loosing of magnetization. Thus in a shunt-wound dynamo there is a critical resistance for the outer circuit, below which the field ceases to be magnetized, as enough current ceases to be shunted into it to magnetize it. The machine is said to unbuild itself, and a discharge of magnetism occurs from the field magnet.

Magnetism, Ewing's Theory of. Ewing found by a model consisting of a number of pivoted magnetic needles that the observed phenomena of magnetization could be represented thereby. Thus there would be no need of assuming internal frictional forces of Maxwell, nor the closed rings or chains of Hughes. The theory retains the notion, however, of paramagnetic matter, consisting of an assemblage of molecular magnets. The loss of energy by hysteresis is represented in the model by the energy lost by the needles in beating against the air.

357 STANDARD ELECTRICAL DICTIONARY.

Magnetism, Free. The magnetism or magnetic field outside of a magnetic circuit. It is due to escape of lines of force and to the magnetic leakage through the air. The lines of force are never, under the most favorable circumstances, confined to the metallic circuit of the magnet and armature. In a simple magnet without armature all the lines of force have to follow an air path, and the field is at its strongest. As the magnetism is strongest at the surface near the poles, the term is sometimes understood as applying to the surface attraction. In such case it is defined as the distribution, on a magnetized bar or mass, of magnetic lines of force as they emerge from its surface.

Synonym--Surface Magnetization.

Magnetism, Hughes' Theory of. A theory accounting for magnetic phenomena by assuming that each molecule is a magnet, and that in a polarized or magnetized body they are all arranged with their poles in the same direction, while in an unmagnetized body their poles, alternating in direction, neutralize each other.

Magnetization consists in a partial rotation of the molecules so as to make them agree in position, thus, as a resultant developing north and south poles at the ends of the bar.

The theory is in a certain sense simpler than Ampere's theory, but is not so generally adopted.

Magnetism, Lamellar Distribution of. The distribution of magnetism in thin and uniform or "simple magnetic shells," q. v. A given distribution is termed lamellar if the substance in which it exists can be divided into simple magnetic shells, which either form closed surfaces, or have their edges in the surface of the substance. In lamellar distribution the polar area is very large compared with the distance between opposite poles.

Magnetism of Gases. Faraday experimented on this point by coloring gases with a little vapor of iodine or other colored gas, and letting them flow between the two poles of a powerful electromagnet. In this way he found some are repelled, some attracted, and in the case of oxygen, it is attracted at one temperature and repelled at another. At ordinary temperatures a cubic yard of oxygen possesses the magnetism of 5.5 grains of iron and when liquefied it is strongly attracted.