Fig. 211. DIAGRAM OF THE PILSEN DIFFERENTIAL ARC LAMP.

321 STANDARD ELECTRICAL DICTIONARY.

Lamp, Holophote. A lamp designed for use alone upon its own circuit. These have the regulating mechanism in series with the carbon and arc, so that the whole current goes through both. (See Lamp, Arc.)

Synonym—Monophote Lamp.

Lamp-hour. A unit of commercial supply of electric energy; the volt-coulombs required to maintain an electric lamp for one hour. A sixteen-candle power incandescent lamp is practically the lamp alluded to, and requires about half an ampere current at 110 volts, making a lamp-hour equal to about 198,000 volt-coulombs.

[Transcriber's note: 0.55 KW hours.]

Lamp, Incandescent. An electric lamp in which the light is produced by heating to whiteness a refractory conductor by the passage of a current of electricity. It is distinguished from an arc lamp (which etymologically is also an incandescent lamp) by the absence of any break in the continuity of its refractory conductor. Many different forms and methods of construction have been tried, but now all have settled into approximately the same type.

The incandescent lamp consists of a small glass bulb, called the lamp-chamber, which is exhausted of air and hermetically sealed. It contains a filament of carbon, bent into a loop of more or less simple shape. This shape prevents any tensile strain upon the loop and also approximates to the outline of a regular flame.

Fig. 212. INCANDESCENT ELECTRIC LAMP.

322 STANDARD ELECTRICAL DICTIONARY.

The loop is attached at its ends to two short pieces of platinum wire, which pass through the glass of the bulb and around which the glass is fused. As platinum has almost exactly the same coefficient of heat-expansion as glass, the wires do not cause the glass to crack.

The process of manufacture includes the preparation of the filament. This is made from paper, silk, bamboo fibre, tamidine, q. v., or other material. After shaping into the form of the filament the material is carbonized at a high heat, while embedded in charcoal, or otherwise protected from the air. The flashing process (see Flashing of incandescent Lamp Carbons) may also be applied. The attachment to the platinum wires is effected by a minute clamp or by electric soldering. The loop is inserted and secured within the open globe, which the glass blower nearly closes, leaving one opening for exhaustion.

The air is pumped out, perhaps first by a piston pump, but always at the end by a mercurial air pump. (See Pump, Geissler—and others.) As the exhaustion becomes high a current is passed through the carbons heating them eventually to white heat so as to expel occluded gas. The occluded gases are exhausted by the pump and the lamp is sealed by melting the glass with a blowpipe or blast-lamp flame. For the exhaustion several lamps are usually fastened together by branching glass tubes, and are sealed off one by one.

The incandescent lamps require about 3.5 watts to the candle power, or give about 12 sixteen-candle lamps to the horse power expended on them.

Generally incandescent lamps are run in parallel or on multiple arc circuits. All that is necessary in such distribution systems is to maintain a proper potential difference between the two leads across which the lamps are connected. In the manufacture of lamps they are brought to an even resistance and the proper voltage at which they should be run is often marked upon them. This may be fifty volts and upward. One hundred and ten volts is a very usual figure. As current one ampere for a fifty-volt, or about one-half an ampere for a one hundred and ten volt lamp is employed.

Lamp, Incandescent, Three Filament. A three filament lamp is used for three phase currents. It has three filaments whose inner ends are connected, and each of which has one leading-in wire. The three wires are connected to the three wires of the circuit. Each filament receives a current varying in intensity, so that there is always one filament passing a current equal to the sum of the currents in the other two filaments.

Lamp, Lighthouse. A special type of arc light. It is adapted for use in a lighthouse dioptric lantern, and hence its arc has to be maintained in the same position, in the focus of the lenses. The lamps are so constructed as to feed both carbons instead of only one, thereby securing the above object.

323 STANDARD ELECTRICAL DICTIONARY.

Lamp, Pilot. A lamp connected to a dynamo, and used by its degree of illumination to show when the dynamo on starting becomes excited, or builds itself up.

Lamp, Polyphote. An arc lamp adapted to be used, a number in series, upon the same circuit. The electric regulating mechanism is placed in shunt or in parallel with the carbons and arc. (See Lamp, Arc.)

Lamps, Bank of. A number of lamps mounted on a board or other base, and connected to serve as voltage indicator or to show the existence of grounds, or for other purposes.

Lamp, Semi-incandescent. A lamp partaking of the characteristics of both arc and incandescence; a lamp in which the imperfect contact of two carbon electrodes produces a part of or all of the resistance to the current which causes incandescence.

The usual type of these lamps includes a thin carbon rod which rests against a block of carbon. The species of arc formed at the junction of the two heats the carbons. Sometimes the upper carbon or at least its end is heated also by true incandescence, the current being conveyed near to its end before entering it.

Semi-incandescent lamps are not used to any extent now.

Lamp Socket. A receptacle for an incandescent lamp; the lamp being inserted the necessary connections with the two leads are automatically made in most sockets. The lamps may be screwed or simply thrust into the socket and different ones are constructed for different types of lamps. A key for turning the current on and off is often a part of the socket.

Latent Electricity. The bound charge of static electricity. (See Charge, Bound.)

Law of Intermediate Metals. A law of thermo-electricity. The electro-motive force between any two metals is equal to the sum of electro-motive forces between each of the two metals and any intermediate metal in the thermo-electric series, or the electro-motive force between any two metals is equal to the sum of the electromotive forces between all the intermediate ones and the original two metals; it is the analogue of Volta's Law, q. v.

Law of Inverse Squares. When force is exercised through space from a point, its intensity varies inversely with the square of the distance. Thus the intensity of light radiated by a luminous point at twice a given distance therefrom is of one-fourth the intensity it had at the distance in question. Gravitation, electric and magnetic attraction and repulsion and other radiant forces are subject to the same law.

324 STANDARD ELECTRICAL DICTIONARY.

Law of Successive Temperatures. A law of thermo-electricity. The electro-motive force due to a given difference of temperature between the opposite junctions of the metals is equal to the sum of the electro-motive forces produced by fractional differences of temperature, whose sum is equal to the given difference and whose sum exactly fills the given range of temperature.

Law, Right-handed Screw. This rather crude name is given by Emtage to a law expressing the relation of direction of current in a circuit to the positive direction of the axis of a magnet acted on by such current. It is thus expressed: A right-handed screw placed along the axis of the magnet and turned in the direction of the current will move in the positive direction, i. e., towards the north pole of the axis of the magnet.

Lead. A metal; one of the elements; symbol Pb. Atomic weight, 207; equivalent, 103-1/2; valency, 2. Lead may also be a tetrad, when its equivalent is 51.75. The following data are at 0 C. (32 F.) with compressed metal: Relative Resistance, (Silver = l) 13.05 Specific Resistance, 19.63 microhms. Resistance of a wire, (a) 1 ft. long, weighing 1 grain, 3.200 ohms. (b) 1 meter long, weighing 1 gram, 2.232 " (c) 1 meter long, 1 millimeter thick, .2498 " Resistance of 1 inch cube, 7.728 microhms. Electro-Chemical Equivalent (Hydrogen = .0105) 1.086 mgs.

Leading Horns. The tips of pole pieces in a dynamo, which extend in the direction of movement of the armature.

Leading-in Wires. The platinum wires passing through the glass of an incandescent lamp-chamber, to effect the connection of the carbon filament with the wires of the circuit.

Lead of Brushes, Negative. In a motor the brushes are set backwards from their normal position, or in a position towards the direction of armature rotation or given a negative lead instead of a positive one, such as is given to dynamo brushes.

Leak. A loss or escape of electricity by accidental connection either with the ground or with some conductor. There are various kinds of leak to which descriptive terms are applied.

Leakage. The loss of current from conductors; due to grounding at least at two places, or to very slight grounding at a great many places, or all along a line owing to poor insulation. In aerial or pole telegraph lines in wet weather there is often a very large leakage down the wet poles from the wire. (See Surface Leakage—Magnetic Leakage.)

325 STANDARD ELECTRICAL DICTIONARY.

Leakage Conductor. A conductor placed on telegraph poles to conduct directly to earth any leakage from a wire and thus prevent any but a very small portion finding its way into the other wires on the same pole. It presents a choice of evils, as it increases the electrostatic capacity of the line, and thus does harm as well as good. It consists simply of a wire grounded and secured to the pole.

Leg of Circuit. One lead or side of a complete metallic circuit.

Lenz's Law. A law expressing the relations of direction of an inducing current or field of force to the current induced by any disturbance in the relations between such field and any closed conductor within its influence. It may be variously expressed.

(a) If the relative position of two conductors, A and B, be changed, of which A is traversed by a current, a current is induced in B in such a direction that, by its electro-dynamic action on the current in A, it would have imparted to the conductors a motion of the contrary kind to that by which the inducing action was produced. (Ganot.)

(b) The new (induced) current will increase the already existing resistances, or develop new resistance to that disturbance of the field which is the cause of induction. (Daniell.)

(c) When a conductor is moving in a magnetic field a current is induced in the conductor in such a direction as by its mechanical action to oppose the motion. (Emtage.)

(d) The induced currents are such as to develop resistance to the change brought about.

Letter Boxes, Electric. Letter boxes with electrical connections to a bell or indicator of some sort, which is caused to act by putting a letter into the box.

Leyden Jar. A form of static condenser.

In its usual form it consists of a glass jar. Tinfoil is pasted around the lower portions of its exterior and interior surfaces, covering from one-quarter to three-quarters of the walls in ordinary examples. The rest of the glass is preferably shellacked or painted over with insulating varnish, q. v. The mouth is closed with a wooden or cork stopper and through its centre a brass rod passes which by a short chain or wire is in connection with the interior coating of the jar. The top of the rod carries a brass knob or ball.

If such a jar is held by the tinfoil-covered surface in one hand and its knob is held against the excited prime conductor of a static machine its interior becomes charged; an equivalent quantity of the same electricity is repelled through the person of the experimenter to the earth and when removed from the conductor it will be found to hold a bound charge. If the outer coating and knob are both touched or nearly touched by a conductor a disruptive discharge through it takes place.

326 STANDARD ELECTRICAL DICTIONARY.

Fig. 213. LEYDEN JAR WITH DISCHARGER.

If one or more persons act as discharging conductors they will receive a shock. This is done by their joining hands, a person at one end touching the outer coating and another person at the other end touching the knob.

From an influence machine a charge can be taken by connecting the coating to one electrode and the knob to the other.

Fig. 214. SULPHURIC ACID LEYDEN JAR.

327 STANDARD ELECTRICAL DICTIONARY.

Leyden Jar, Sir William Thomson's. An especially efficient form of Leyden jar. It consists of a jar with outer tinfoil coating only. For the interior coating is substituted a quantity of concentrated sulphuric acid. The central rod is of lead with a foot, which is immersed in the acid and from which the rod rises. A wooden cover partly closes the jar, as the central tube through which the rod passes is so large as not to allow the wood to touch it. Thus any leakage from inner to outer coating has to pass over the inside and outside glass surfaces. In the common form of jar the wooden cover may short circuit the uncoated portion of the inner glass surface. In the cut a simplified form of Thomson's Leyden jar is shown, adapted for scientific work.

Lichtenberg's Figures. If the knob of a Leyden jar or other exited electrode is rubbed over the surface of ebonite, shellac, resin or other non-conducting surface it leaves it electrified in the path of the knob. If fine powder such as flowers of sulphur or lycopodium is dusted over the surface and the excess is blown away, the powder will adhere where the surface was electrified, forming what are called Lichtenberg's Figures, Lycopodium and sulphur show both positive and negative figures, that is to say, figures produced by a positively or negatively charged conductor. Red lead adheres only to negative figures. If both positive and negative figures are made and the surface is sprinkled with both red lead and flowers of sulphur each picks out its own figure, the sulphur going principally to the positive one.

The red lead takes the form of small circular heaps, the sulphur arranges itself in tufts with numerous diverging branches. This indicates the difference in the two electricities. The figures have been described as "a very sensitive electrosope for investigating the distribution of electricity on an insulating surface." (Ganot.)

Life of Incandescent Lamps. The period of time a lamp remains in action before the carbon filament is destroyed. The cause of a lamp failing may be the volatilization of the carbon of the filament, causing it to become thin and to break; or the chamber may leak. The life of the lamp varies; 600 hours is a fair estimate. Sometimes they last several times this period.

The higher the intensity at which they are used the shorter is their life. From their prime cost and the cost of current the most economical way to run them can be approximately calculated.

[Transcriber's note: Contemporary incandecent buls are rated for 1000 hours; flourescent bulbs up to 24000 hours; LED lamps up to 100000 hours.]

Lightning. The electrostatic discharge to the earth or among themselves of clouds floating in the atmosphere. The discharge is accompanied by a spark or other luminous effect, which may be very bright and the effects, thermal and mechanical, are often of enormous intensity.

The lightning flash is white near the earth, but in the upper regions where the air is rarefied it is of a blue tint, like the spark of the electric machine. The flashes are often over a mile in length, and sometimes are four or five miles long. They have sometimes a curious sinuous and often a branching shape, which has been determined by photography only recently. To the eye the shape seems zigzag.

328 STANDARD ELECTRICAL DICTIONARY.

In the case of a mile-long flash it has been estimated that 3,516,480 De la Rue cells, q. v., would be required for the development of the potential, giving the flash over three and one-half millions of volts. But as it is uncertain how far the discharge is helped on its course by the rain drops this estimate may be too high.

There are two general types of flash. The so-called zigzag flash resembles the spark of an electric machine, and is undoubtedly due to the disruptive discharge from cloud to earth. Sheet lightning has no shape, simply is a sudden glow, and from examination of the spectrum appears to be brush discharges (see Discharge, Brush) between clouds. Heat lightning is attributed to flashes below the horizon whose light only is seen by us. Globe or ball lightning takes the form of globes of fire, sometimes visible for ten seconds, descending from the clouds. On reaching the earth they sometimes rebound, and sometimes explode with a loud detonation. No adequate explanation has been found for them.

The flash does not exceed one-millionth of a second in duration; its absolute light is believed to be comparable to that of the sun, but its brief duration makes its total light far less than that of the sun for any period of time.

If the disruptive discharge passes through a living animal it is often fatal. As it reaches the earth it often has power enough to fuse sand, producing fulgurites, q. v. (See also Back Shock or Stroke of Lightning.)

Volcanic lightning, which accompanies the eruptions of volcanoes, is attributed to friction of the volcanic dust and to vapor condensation.

[Transcriber's note: The origin of lightning is still (2008) not fully understood, but is thought to relate to charge separation in the vertical motion of water droplets and ice crystals in cloud updrafts. A lightning bolt carries a current of 40,000 to 120,000 amperes, and transfers a charge of about five coulombs. Nearby air is heated to about 10,000 C (18,000 F), almost twice the temperature of the Suns surface.]

Lightning Arrester. An apparatus for use with electric lines to carry off to earth any lightning discharge such lines may pick up. Such discharge would imperil life as well as property in telegraph offices and the like.

Arresters are generally constructed on the following lines. The line wires have connected to them a plate with teeth; a second similar plate is placed near this with its teeth opposite to those of the first plate and nearly touching it. The second plate is connected by a low resistance conductor to ground. Any lightning discharge is apt to jump across the interval, of a small fraction of an inch, between the oppositely placed points and go to earth.

Another type consists of two plates, placed face to face, and pressing between them a piece of paper or mica. The lightning is supposed to perforate this and go to earth. One plate is connected to the line, the other one is grounded.

The lightning arrester is placed near the end of the line before it reaches any instrument. (See Alternative Paths.)

329 STANDARD ELECTRICAL DICTIONARY.

Fig. 215. COMB OR TOOTHED LIGHTNING ARRESTER.

Fig. 216. FILM OR PLATE LIGHTNING ARRESTER.

Lightning Arrester, Counter-electro-motive Force. An invention of Prof. Elihu Thompson. A lightning arrester in which the lightning discharge sets up a counter-electro-motive force opposed to its own. This it does by an induction coil. If a discharge to earth takes place it selects the primary of the coil as it has low self-induction. In its discharge it induces in the secondary a reverse electro-motive force which protects the line.

Lightning Arrester Plates. The toothed plates nearly in contact, tooth for tooth, or the flat plates of a film lightning arrester, which constitute a lightning arrester. Some advocate restricting the term to the plate connected to the line.

Lightning Arrester, Vacuum. A glass tube, almost completely exhausted, into which the line wire is fused, while a wire leading to an earth connection has its end fused in also.

A high tension discharge, such as that of lightning, goes to earth across the partial vacuum in preference to going through the line, which by its capacity and self-induction opposes the passage through it of a lightning discharge.

It is especially adapted for underground and submarine lines.

330 STANDARD ELECTRICAL DICTIONARY.

Lightning, Ascending. Lightning is sometimes observed which seems to ascend. It is thought that this may be due to positive electrification of the earth and negative electrification of the clouds.

Lightning, Globe or Globular. A very unusual form of lightning discharge, in which the flashes appear as globes or balls of light. They are sometimes visible for ten seconds, moving so slowly that the eye can follow them. They often rebound on striking the ground, and sometimes explode with a noise like a cannon. They have never been satisfactorily explained. Sometimes the phenomenon is probably subjective and due to persistence of vision.

Lightning Jar. A Leyden jar whose coatings are of metallic filings dusted on to the surface while shellacked, and before the varnish has had time to dry. In its discharge a scintillation of sparks appears all over the surface.

Line of Contact. The line joining the points of contact of the commutator brushes in a dynamo or motor.

Synonym—Diameter of Commutation.

Lines of Force. Imaginary lines denoting the direction of repulsion or attraction in a field of force, q. v. They may also be so distributed as to indicate the relative intensity of all different parts of the field. They are normal to equipotential surfaces. (See Electro-magnetic Lines of Force—Electrostatic Lines of Force—Magnetic Lines of Force.)

Lines of Induction. Imaginary lines within a body marking the direction taken within it by magnetic induction. These are not necessarily parallel to lines of force, but may, in bodies of uniform agglomeration, or in crystalline bodies, take various directions.

Synonym—Lines of Magnetic Induction.

Lines of Slope. Lines in a field of force which mark the directions in which the intensity of force in the field most rapidly falls away.

Links, Fuse. Links made of more or less easily fusible metal, for use as safety fuses.

Listening Cam. In a telephone exchange a cam or species of switch used to connect the operator's telephone with a subscriber's line.

331 STANDARD ELECTRICAL DICTIONARY.

Lithanode. A block of compressed lead binoxide, with platinum connecting foils for use as an electrode in a storage battery. It has considerable capacity, over 5 ampere-hours per pound of plates, but has not met with any extended adoption.

Load. In a dynamo the amperes of current delivered by it under any given conditions.

Local Action. (a) In its most usual sense the electric currents within a battery, due to impurities in the zinc, which currents may circulate in exceedingly minute circuits, and which waste zinc and chemicals and contribute nothing to the regular current of the battery. Amalgamated or chemically pure zinc develops no local action.

(b) The term is sometimes applied to currents set up within the armature core or pole pieces of a dynamo. (See Currents, Foucault.)

Local Battery. A battery supplying a local circuit (q. v.); in telegraphy, where it is principally used, the battery is thrown in and out of action by a relay, and its current does the work of actuating the sounder and any other local or station instruments. (See Relay.)

Local Circuit. A short circuit on which are placed local apparatus or instruments. Such circuit is of low resistance and its current is supplied by a local battery, q. v. Its action is determined by the current from the main line throwing its battery in and out of circuit by a relay, q. v., or some equivalent.

Local Currents. Currents within the metal parts of a dynamo. (See Currents, Foucault.) In a galvanic battery. where there is local action, q. v., there are also local currents, though they are not often referred to.

Localization. Determining the position of anything, such as a break in a cable, or a grounding in a telegraph line. In ocean cables two typical cases are the localization of a break in the conductor and of a defect in the insulation admitting water. The first is done by determining the static capacity of the portion of the line which includes the unbroken portion of the conductor; the other by determining the resistance of the line on a grounded circuit.

Locus. A place. The word is used to designate the locality or position of, or series of positions of definite conditions and the like. Thus an isogonic line is the locus of equal declinations of the magnetic needle; it is a line passing through all places on the earth's surface where the condition of a given declination is found to exist.

332 STANDARD ELECTRICAL DICTIONARY.

Lodestone. Magnetic magnetite; magnetite is an ore of iron, Fe3 04 which is attracted by the magnet. Some samples possess polarity and attract iron. The latter are lodestones.

Synonym—Hercules Stone

Logarithm. The exponent of the power to which it is necessary to raise a fixed number to produce a given number. The fixed number is the base of the system. There are two systems; one, called the ordinary system, has 10 for its base, the other, called the Naperian system, has 2.71828 for its base. The latter are also termed hyperbolic logarithms, and are only used in special calculations.

Log, Electric. An apparatus for measuring the speed of a ship. A rotating helical vane of known pitch is dragged behind the vessel. As the helix rotates its movements may actuate electric machinery for registering its rotations. The number of these in a given time, multiplied by the pitch of the vane, gives the distance traversed in such time.

Loop. A portion of a circuit introduced in series into another circuit. The latter circuit is opened by a spring-jack, q. v. or other device, and the loop inserted. By loops any number of connections can be inserted into a circuit in series therewith, and in series or in parallel with one another.

Loop Break. A double bracket or similar arrangement for holding on insulators the ends of a conductor which is cut between them, and to which are connected the ends of a loop. The space between the insulators may be about a foot.

Luces. This may be used as the plural of lux, q. v. It is the Latin plural.

Luminous Jar. A Leyden jar whose coatings are of lozenge-shaped pieces of tinfoil between which are very short intervals. When discharged, sparks appear all over the surface where the lozenges nearly join.

Lux. A standard of illumination, q. v., as distinguished from illuminating power.

It is the light given by one candle at a distance of 12.7 inches—by a carcel, q. v., at a distance of one meter—-or by 10,000 candles at 105.8 feet.

It was proposed by W. H. Preece. All the above valuations are identical.

M. (a) Symbol of gaseous pressure equal to one-millionth of an atmosphere.

(b) The Greek m, , is used as the symbol of magnetic permeability.

333 STANDARD ELECTRICAL DICTIONARY.

Machine, Cylinder Electric. A frictional electric machine whose rotating glass is in the shape of a cylinder instead of a disc as in the more recent machines.

Fig. 217. PLATE FRICTIONAL ELECTRIC MACHINE.

Machine, Frictional Electric. An apparatus for development of high tension electricity by contact action, brought about by friction.

It consists of a plate or cylinder of glass mounted on insulating standards and provided with a handle for turning it. One or more cushions of leather are held on an insulated support, so as to rub against the plate or cylinder as it is turned. A metal comb or combs are held on another insulating support so as to be nearly in contact with the surface of the glass plate at a point as far removed as possible from the rubbers. The combs are attached to a brass ball or round-ended cylinder, which is termed the prime conductor.

In use either the prime conductor or cushions are connected by a chain or otherwise with the earth. Assume it to be the cushions. As the machine is worked by turning the plate, the glass and cushion being in contact develop opposite electricities. The glass is charged with positive electricity, and as it turns carries it off and as it reaches the prime conductor by induction and conduction robs it of its negative electricity. Meanwhile the cushions negatively excited deliver their charge to the earth. The action thus goes on, the prime conductor being charged with positive electricity.

334 STANDARD ELECTRICAL DICTIONARY.

If the prime conductor is connected to the earth and the cushions are left insulated, negative electricity can be collected from the cushions.

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 11 30' E. 1663 0 1700 8 10' W. 1780 19 55' W. 1785 22 00' W. 1805 22 5' W. 1814 22 34' W. 1825 22 22' W. 1830 22 12' W. 1835 22 4' W. 1850 20 30' W. 1855 19 57' W. 1860 19 32' W. 1865 18 44' W. 1875 17 21' W. 1878 17 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. (662F.) 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 Amprian 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, Ampre'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. AMPRIAN CURRENTS IN MAGNETS.

At the north pole of the magnet the direction of the Amprian 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 Amprian 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 AMPRIAN 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.

Magnetism or Magnetization, Temporary. When a mass of iron is magnetized by a current, when the current ceases the portion of its magnetism which disappears is the temporary magnetism; the portion retained is the residual or permanent magnetism.

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

Synonym—North Magnetic Fluid.

358 STANDARD ELECTRICAL DICTIONARY.

Magnetic Remanence. The residual magnetism left in a bar of steel or other paramagnetic material after the application of a powerful magnet. It is distinguished from coercive force, as the latter is the amount of negative magnetizing or of demagnetizing force required to reduce the remanent magnetism to zero.

Synonym—Remanence—Residual Magnetism.

Magnetism, Solenoidal Distribution of. The distribution of magnetism in such a way that the poles are very far apart in proportion to their area. The magnetization of a long thin bar of steel illustrates solenoidal distribution.

Magnetism Sub-permanent. The magnetism of a paramagnetic substance which presents a considerable degree of permanency, but which gradually disappears, leaving the permanent magnetism present. It is noticeable in iron or steel ships whose magnetism gradually reduced in quantity, eventually becomes fully permanent.

Magnetism, Weber's Theory of. The molecules of a magnetizable material by this theory are supposed to be magnets with their poles lying in every direction, and hence neutralizing each other. By magnetization these are supposed to be turned with their similar poles in the same direction, and their axis parallel, hence acting like a group of magnets. It is practically identical with Hughes' theory.

Magnetism, Terrestrial. The magnetism of the earth. (See Magnetic Elements.)

Fig. 228. MAGNETIZATION BY DOUBLE TOUCH.

Magnetization by Double Touch. The process of magnetizing a steel bar by simultaneously stroking it with two poles of a horseshoe magnet or with two opposite poles of two bar magnets. The poles must be close but not touching. A block of wood may be placed between the ends if single magnets are used. The poles are placed on the middle of the bar and carried back and forth to one end, then to the other, and so on, ending at the middle of the bar in such direction as to give each end the same number of strokes. The poles must be close together or consequent poles will be produced. If bar magnets are used they may be held inclined at an angle of 15 to 20 with the horizontal bar to be magnetized. The ends of the latter may rest on poles of two other magnets, each end on a pole of the same name as that of the magnetizing magnet on its side. (See Magnetization, Hoffer's Method.)

359 STANDARD ELECTRICAL DICTIONARY.

Magnetization by Separate Touch. A method of magnetization. Two magnets are used. Held in an inclined position two opposite poles are touched to the bar near its centre, and are drawn off to the two ends. They are returned through the air and the process is repeated.

Magnetization by Single Touch. A method of polarizing or magnetizing steel bars, by stroking them always in one direction with one pole of a magnet, returning it through the air. The stroking is best done on both sides. The stroking may begin at one end and end at the other, or it may be commenced in the center of the bar and be carried to one end with one pole, and the same done for the other half with the other pole.

Magnetization by the Earth. The earth imparts magnetism to iron masses. If a rod of steel is held parallel to the inclination and in the magnetic meridian it exhibits polarity, which by jarring or hammering, can be made to some extent permanent. A piece of soft iron held vertically, or still better in the line of the dip as above, and which is twisted when in that position, becomes magnetized with some degree of permanence. Many other instances are cited, such as fire-irons, lamp-posts, iron gates, lathe turnings, all of which often exhibit polarity, having been magnetized by the earth's field.

[Transcriber's note: The earth's magnetic field is believed to originate it electric currents in the moving molten core.]

Magnetization, Coefficient of Induced. The coefficient (q. v.) expressing the relation between the specific intensity of magnetization of a particle and the magnetizing force. The magnetizing force is measured by the lines of force it can produce in a field of air. The coefficient of induced magnetization is the factor by which the intensity of a magnetizing field must be multiplied to produce the magnetization imparted by it to a particle of any substance. This coefficient varies for different substances, and is also called magnetic susceptibility. It is distinguished from permeability as referring only to a particle isolated from influence of a mass of surrounding particles of its own kind. It is definable as the intensity of the magnetization assumed by an exceedingly long and exceedingly thin bar placed in a unit field. If a mass of metal were placed in such a field all its particles would become affected and within the mass no unit field could exist. Hence magnetic susceptibility (another name for this coefficient) does not apply to the case of large cores of electro-magnets and dynamo-armatures, but is really a theoretical rather than a practical figure.

The sign of the coefficient of diamagnetic bodies is negative; of paramagnetic bodies is positive.

Synonym—Magnetic Susceptibility.

360 STANDARD ELECTRICAL DICTIONARY.

Magnetization, Cycle of. A cycle of positive or of positive and negative magnetization represents the application of a magnetizing force beginning at a fixed value, generally zero, rising to a maximum, or to a value of maximum distance from the initial and then returning to the original basis. It is virtually a full wave of magnetization and may extend on both sides of a zero line giving positive and negative values.

Cycles of magnetization apply especially to transformers and other apparatus of that character used with the alternating current system.

Magnetization, Hoffer's Method. For horseshoe bars an armature is placed against the poles of the magnet bar to be treated. The poles of a strong horseshoe magnet are stroked over it from poles to bend and returned through the air, or vice versa. In the first case the poles will be the same as those of the inducing magnet; in the second case they will be opposite. A maximum effect is produced in ten strokes. The stroking should be applied to both sides. An electro-magnet may be used as inducer as shown, but an armature should be used; in the cut it is omitted.

Fig. 229. MAGNETIZING A HORSESHOE MAGNET.

Magnetization, Intensity of. The amount of magnetism induced in or present in a body. It is expressed in Magnetic Lines of Force, q. v., per cross-sectional area.

Magnetization, Isthmus Method of. A method used by Ewing in a research on the magnetization of iron in very strong fields. He used samples of iron turned down in the centre to a narrow neck, and thus concentrated the lines of force greatly.

Magnetization, Elias' Method. The bar to be magnetized is surrounded by a magnetizing coil, q. v. A strong current is passed through it, and the coil is moved back and forth a few times.

Magnetization, Jacobi's Method. For horseshoe bars. The bar is placed with its poles against those of a horseshoe magnet. A bar of soft iron, long enough to reach from outside to outside of the legs, is laid across near the junction and is drawn along towards the bend of the new bar and away from it. This is repeated a few times on both sides.

361 STANDARD ELECTRICAL DICTIONARY.

Magnetization, Limit of. As the induction of magnetizing force increases, magnetization of paramagnetic metals tends towards a limit, the increase in magnetization being continually less and less as the metal becomes more highly magnetized. In diamagnetic substances no limit is discernible.

Synonym—Maximum Magnetization.

Magnetization, Specific. The magnetic moment per gram of a substance.

Magnet-keeper. A bar of iron connecting the two poles of a permanent magnet. Often the same bar serves as armature and keeper.

Magnet, Lamination of. It is advantageous to make magnets of laminated construction, or of thin plates of steel. The thin metal can be better tempered or hardened than thick metal. A slight separation of the plates is advantageous from some points of view. If in actual contact there is some danger that the weaker members will have their polarity reversed by the stronger ones. This is counteracted to some extent by separation.

Magnet, Long Coil. A high resistance electro-magnet; one whose coil is of thin wire of considerable length.

Magnet, Natural. The lodestone, q. v.; a variety of magnetite or magnetic oxide of iron, exhibiting permanent magnetism, attracting iron, and possessing north and south poles.

Magnet, Neutral Line of. A line at right angles to the magnetic axis of a magnet, q.v., and nearly or quite at the centre, so situated with reference to the poles on either end that it marks the locus of no polarity. It has been called the equator of the magnet. It is defined by the intersection of the plane of no magnetism with the surface of the bar.

Synonym—Magnetic Equator.

Magnet, Normal. A bar or compound bar magnet, magnetized to such an extent that the curves of the lines of force run into each other in the middle, is thus termed by Jamin.

Magneto. Abbreviation for Magneto-electric Generator. (See Magneto-electric Generator.)

Magneto Call Bell. A call operated by current from a magneto-electric generator. It is very generally used in telephone systems.

362 STANDARD ELECTRICAL DICTIONARY.

Magneto-electric. adj. Relating to induced electric effects due to the cutting of true magnetic lines of force by, or equivalent action of or upon a conductor. These effects are identical with electro-magnetic effects and are only distinguished from them by the field being due to a permanent magnet instead of an electromagnet.

Magneto-electric Brake. A device for bringing to rest an oscillating galvanometer needle. It consists essentially of a coil in circuit with a key and with the galvanometer. On opening the circuit an inverse current is established by induction, tending to bring the needle to rest.

Magneto-electric Generator. A current generator operating by maintaining a potential difference at its terminals, by reactions in a field of force, which field is established by a permanent magnet.

The cut, Fig. 230, shows the general principle of construction of a direct current generator. The armature is rotated between the poles of a permanent magnet. Any of the regular types of dynamo armature can be used. From its commutator the current is taken by brushes.

Fig. 230. MAGNETO-ELECTRIC GENERATOR.

Fig. 231. MAGNETO-ELECTRIC GENERATOR.

363 STANDARD ELECTRICAL DICTIONARY.