Abstract: A magnetic writer includes a first write element and a second write element. The first write element produces a first field when a first current is passed through a first coil. The second write element, which is disposed relative to the first write element, produces a second field when a second current is passed through a second coil such that the second field at least partially opposes the first field.

Abstract: A disk substrate for a perpendicular magnetic recording medium comprises a disk base member, a soft magnetic layer formed on the disk base member, and a protection layer formed on the soft magnetic layer. The soft magnetic layer and the protection layer are deposited by sputtering. A perpendicular magnetic recording disk comprises the above-mentioned disk substrate and at least a perpendicular magnetic recording layer formed thereon. The perpendicular magnetic recording layer is deposited on the disk substrate by sputtering after the disk substrate is heated.

Abstract: A thin film magnetic head includes a magnetic lower core layer, a magnetic upper core layer, a coil layer, for inducing a recording magnetic field in the lower core layer and the upper core layer, formed so as to go around the base of the upper core layer, and a planarizing layer formed in the periphery of the lower core layer, excluding the section exposed to the air bearing surface, so that the surface of the planarizing layer is level with the surface of the lower core layer, the coil layer being formed from on the lower core layer to on the planarizing layer. A method of producing a thin film magnetic head is also disclosed.

Abstract: A magnetic recording medium includes a cylindrical substrate and pores embedded with a magnetic substance that are provided on the outer circumferential surface of the cylindrical substrate and aligned in tracks that are in parallel with one another along the center axis of the cylindrical substrate. The tracks have as their center the center axis of the cylindrical substrate, and form circular orbits defined by planes perpendicular to the center axis. Alternatively, the tracks are in a spiral orbit or in spiral orbits whose center is the center axis of the cylindrical substrate. The pores at intersections of vertical and horizontal rows are aligned in quadrilaterals such as squares, rectangles or rhombi.

Abstract: A perpendicular magnetic recording medium has a magnetic layer comprised of two stacked sublayers each in the form of ferromagnetic metal thin film consisting of columnar crystal grains wherein the angle .theta. between the average growth direction of the columnar crystal grains and a normal to the substrate is .theta..gtoreq.45.degree. for the lower sublayer and .theta..ltoreq.30.degree. for the upper sublayer, and the mean maximum diameter of columnar crystal grains of the lower sublayer is not smaller than the mean maximum diameter of columnar crystal grains of the upper sublayer. An apparatus suitable for the evaporation of the magnetic layer is arranged as shown in the Figure. Magnetic flux associated with the upper sublayer finds an escape to the lower sublayer, preventing formation of a closed loop of magnetic flux within the columnar crystal grains of the upper sublayer. The lower sublayer is magnetized, thus contributing to an improvement in reproduction output.

Abstract: A magnetic recording medium has recorded thereon a series of discrete time-limited servo signal bursts. To suppress residual fields emanating from each servo signal burst, at least one half-cycle of a compensating signal is recorded immediately prior to and immediately after each signal burst of a polarity opposite the polarity of the immediately adjacent half-cycle of the servo signal burst. Furthermore, each half-cycle of each compensating signal has a magnetic moment that is weaker than the magnetic moment of one half-cycle of the immediately adjacent servo signal burst.

Abstract: A method for fabricating in a mold a rotatable recording drum is disclosed. Initially, the mold is rotated and a predetermined quantity of a flowable substance is poured into the mold cavity and is centrifugally flung against the mold wall to form a mold layer with a smooth inner surface. Next, a surface substance is poured into the mold cavity and is allowed to harden against the mold layer. A core outer layer substance is then poured into the mold cavity and allowed to harden against the surface substance, and rotation of the mold is stopped. The mold layer fluid is removed, and the cavity inside the outer layer is filled with a core inner layer substance which is allowed to harden before the drum core is removed from the mold cavity. Following removal of the core from the mold cavity, the hardened surface substance may be vacuum coated, and a protective overcoat applied to the core. The drum core and the surface substance can also be formed separately.

Abstract: A magnetic disk comprises an annular resilient magnetic recording mediumm, an outer ring having an outer affixing surface, and an annular frame disposed radially inwardly of the outer ring. The annular frame comprises an annular thrusting surface protruded axially for thrusting the recording medium to exert tension to it, an inner affixing surface inwardly affixed to the recording medium, and a base axially receded from the thrusting surface.

Abstract: A duplicating, printing or recording surface is produced by attaching a strip (10) of magnetic or other web material to a support such as a drum (20) or flat support element provided with a layer (22) of low durometer elastomer, the strip of material and the elastomeric material being mutually adherent. With the strip of material wrapped circumferentially around the drum, a sharp knife is used to cut the magnetic material and form a precise butt joint (35,36), diagonal or straight, with a minimal gap. The strip of material forms a secure bond with the low durometer elastomeric layer because of the inherent mutual adherence between the two materials, without the need for an intervening adhesive material. A compliant magnetic drum of this type has a low radial stiffness and a high circumferential stiffness which are well suited for a master recording drum in an anhysteretic duplicating apparatus.

Abstract: A data storage device having two distended and stretched magnetic media surfaces. The magnetic media has a backing which is distended when secured to a core body of the device. A shell surrounds the core body to further stretch the backing resulting in a flat and smooth surface. The flat and smooth surface of the backing is metal coated to form the magnetic media. The shell contains a metallic raceway which serves as a drive locator.

Abstract: A magnetic storage medium is composed of a non-wettable substrate upon which a transient liquid metal layer is deposited and maintained as a distribution of discontinuous liquid features. A magnetic film layer is deposited on the transient liquid metal layer resulting in a reaction of the liquid metal with the magnetic film. The topology of the magnetic film is controllable by adjusting the thickness of the transient liquid metal layer.

Abstract: A perpendicular magnetic recording and reproducing system records and reproduces a signal on and from a perpendicular magnetic recording medium comprising a magnetic layer which is made from one magnetic material and is constituted by a layer having a low coercivity and a layer having a high coercivity on top of the layer having the low coercivity, by use of a ring core head and utilizes the layer having the low coercivity as a high permeability layer and the layer having the high coercivity as a perpendicular magnetization layer.

Abstract: Disclosed is a perpendicular magnetic recording medium wherein an underlayer made of a material containing Si and/or Ge as its principal ingredient is deposited on a substrate, and a perpendicular magnetic film made of a Co-based alloy is deposited on the underlayer. With the perpendicular magnetic recording medium, the perpendicular magnetic film has an enhanced degree of C-axis orientation, and the bond strength between the perpendicular magnetic film and the substrate is also enhanced when the substrate is non-metallic.

Abstract: Magnetic recording media having a coating containing harsh pigment like CrO.sub.2 particles and adapted for frequent contact with a head, with good durability, abrasivity, etc., accommodated by using a lubricant system including a fluoro-silicone oil and a compatible fatty-acid-ester like butoxy ethyl stearate.

Abstract: A magnetic recording medium having good durability comprising a substrate and one or two magnetic layers at least a surface of which contains at least one compound selected from the group consisting of a compound of the formula:F--(CF.sub.2).sub.n --COO--(RO).sub.m --CO--R'--COOH (I)wherein R is a C.sub.2 -C.sub.10 alkyl group, R' is a C.sub.2 -C.sub.10 alkyl or alkenyl group, n is an integer of 4 to 20 and m is an integer of 5 to 50 and salts thereof.

Abstract: An improved cobalt-platinum (CoPt) thin film metal alloy media for horizontal magnetic recording has a coercivity substantially greater than prior CoPt thin film metal alloy media. A tungsten underlayer between the substrate and the CoPt magnetic layer improves the coercivity above that of media wiht conventional underlayers, such as chromium. The coercivity of the CoPt layer can be increased even further if the CoPt film is deposited in such a manner as to form an intermetallic compound of Co.sub.3 W in the interface region between the tungsten underlayer and the CoPt magnetic layer. The tungsten underlayer also improves the magnetic properties of the media when the magnetic layer is an alloy of cobalt-platinum-chromium (CoPtCr).

Abstract: An improved cobalt-platinum (CoPt) thin film metal alloy media for horizontal magnetic recording has a squareness greater than prior CoPt thin film metal alloy media. An underlayer of a body-centered-cubic (BCC) chromium-based alloy with a lattice cell constant greater than chromium (Cr), such as chromium-vanadium (CrV), is formed between the substate and the CoPt magnetic layer. The underlayer also improves the magnetic properties of the media when the magnetic layer is an alloy of cobalt-platinum-chromium (CoPtCr).

Abstract: Disclosed is a magnetic recording medium which comprises a support, a magnetic recording layer and optionally a back-coating layer, at least of said magnetic recording layer and said back-coating layer containing a urethane resin having a yield point.The tape performances of the magnetic recording medium according to the present invention can be markedly improved by incorporating the urethane resin having a yield point.

Abstract: Flexible, self-cross-linking binders, useful for, e.g., paints and magnetic media composites, are prepared from hydroxyl-containing thermoplastic polymers reacted with condensates of diisocyanates and unsaturated esters. They can be cured with an electron beam.

Abstract: A magnetic recording medium wherein a thin layer comprising an organic compound having at least one carboxyl or mercapto group, at least one fluoroalkyl group having 3 or more carbon atoms and at least one aliphatic alkyl group having 8 or more carbon atoms on its molecular terminals is formed on an oxygen-containing thin film of a ferromagnetic metal.The magnetic recording medium of the present invention is suitable for use as the magnetic recording media of magnetic tape, magnetic disk and the like and most suitable for use in rotating head type video tape recorders.

Abstract: Magnetic recording tape comprising a flexible substrate with magnetic particles secured to the substrate by means of a binder comprising a reaction product of a diisocyanate with a blend of a chain extender and a hydroxyl terminated polycarbonate, said binder having high tensile strength, is hydrolytically stable, and is soluble in selected commercial solvents.

Abstract: In a magnetic recording medium having a magnetic recording coat formed on one side of a base and a back coat on the other side, the back coat consists of a dispersion in a binder of an inorgainc pigment powder having an average particle diameter of 0.2 to 0.5 .mu.m and a Mohs hardness of not greater than 6. The binder consists of a specific copolymer, a rubber-like binder, and a polyisocyanate ingredient. The copolymer consists of vinyl chloride, a vinyl alkyl carboxylate, a monomer copolymerizable with vinyl chloride, and a saponified vinyl alkyl carboxylate and has an OH/CH absorption ratio as determined from its infrared absorption spectrum in the range of 0.2 to 0.7.

Abstract: A magnetic recording medium comprising a flexible support and a magnetic layer formed on the flexible support. The flexible support is made of a resin composition which comprises thermoplastic resins and particles of titanium monoxide or dititanium trioxide. The particles are dispersed in the thermoplastic resin in an amount of 0.03 to 10 wt % of the composition and have an average size below 2.5 .mu.m.

Abstract: A magnetic recording medium comprising a plastic base film, a magnetic layer formed on one side of the base film and a backing layer formed on the other side of the base film, characterized in that said magnetic layer has a surface roughness of at most 0.08 .mu.m, and said backing layer is composed essentially of first particles of an inorganic pigment having an average particle size of from 0.04 to 0.5 .mu.m and a Mohs hardness of from 2 to 4 and second particles of an inorganic pigment having an average particles size of from 0.04 to 0.5 .mu.m and a Mohs hardness of from 5 to 7, dispersed in a thermoset or radiation-cured binder.

Abstract: Disclosed herein is a magnetic recording medium, (1) in which a magnetic layer is formed by coating a magnetic paint containing a magnetic powder and a binder as its main components on a non-magnetic base material; (2) which is featured by containing as said binder a polymer having an alkenyl phenol as one of the components of the polymer; and (3) which has markedly improved effects in the dispersibility of the magnetic powder in said medium and in the abrasion resistance and flexibility of the magnetic layer.

Abstract: In a magnetic recording medium comprising a non-magnetic substrate having opposed major surfaces, a non-magnetic undercoat on one major surface of the substrate, a magnetic layer on the undercoat, and a non-magnetic backcoat on the other major surface of the substrate, at least one of said undercoat and said backcoat is formed of a coating composition comprising a radiation curable base resin preferably having a radiation-sensitive double bond such as acrylic, maleic or allylic double bond, an optional thermoplastic resin, and an effective amount of an anti-static agent selected from known surface-active agents, the composition being cured through crosslinking by exposure to radiation. The magnetic recording medium is prepared by mixing the resins with the anti-static agent, coating the substrate with the composition, and exposing the coating to radiation.

Abstract: A magnetic recording medium comprised of a non-magnetic substrate and a magnetic layer formed on one surface of the substrate and consisting essentially of a binder and magnetic powders. As the binder, a thermoplastic polyurethane-urea resin is employed, in which hydrophilic polar groups are introduced. The thermoplastic polyurethane-urea resin contains urethane and urea bonds in the molecule thereof and may be synthesized from a long-chain diol with a molecular weight of about 500 to 5000, a short-chain diol with a molecular weight of about 50 to 500, an organic diisocyanate and an organic diamine.

Abstract: The present invention concerns a process for producing a novel polymeric thin film having high density, high hardness and high toughness on the surface of a substrate by plasma polymerization. This process comprises providing a gas containing at least one compound selected from halogenated alkanes, alkanes, hydrogen and halogens in specific combinations for plasma polymerization, the atomic ratio of halogen/hydrogen in the aforesaid gas being 0.1 to 5 and the electron temperature of the plasma in the reaction zone being 6,000.degree. K. or higher and lower than 30,000.degree. K. The polymeric thin film obtained is useful on various articles for the purpose of protection, surface hardening, rust proofing, scratch proofing, providing gas barrier, etc. In particular, it is suitable as the protective film of magnetic recording media.

Abstract: A magnetic powder for magnetic recording media comprises a hexagonal crystalline system ferrite containing, as essential components for said hexagonal crystalline system ferrite, iron and oxygen, and at least one of the elements selected from the group of Ba, Sr, Ca and Pb, said iron being substituted by at least one of the elements selected from the group of Ti, Co, Ni, Mn, Cu, Zn, In, Ge, Nb, Zr and Sb; and at least one of Ba, Sr, Ca, Pb being substituted by at least one of the rare earth elements selected from the group of Ce, Pr, Nd, Pm, Sm and Eu. The magnetic powder has a mean particle size of 0.01 to 0.3 .mu.m and a coercive force of 200 and 2000 Oe. The magnetic powder is improved in its saturation magnetization while retaining a suitable coercive force. Accordingly, high reproduction output can be obtained in a magnetic recording medium employing the magnetic powder.

Abstract: A magnetic recording medium comprising a nonmagnetic support, and a magnetic recording layer formed on said support. The magnetic recording layer comprises a magnetic powder and an abrasive powder dispersed in a resin binder. The abrasive powder has a coating of an isocyanate compound having at least two isocyanate groups in one molecule thereof and is used in an amount of from 0.5 to 20 parts by weight per 100 parts by weight of the magnetic powder.

Abstract: A magnetic recording medium has a magnetic layer consisting essentially of a binder and magnetic particles on one surface of a non-magnetic substrate. The thermoplastic polyurethane-urea resin in which unsaturated bonds sensitive to radiation are introduced is used as the binder.The thermoplastic polyurethane-urea resin in which the radiation-sensitive unsaturated bonds are introduced has both urethane and urea bonds in the molecular structure thereof. It is synthesized from long chain diol with a molecular weight of approximately 500 to 5000, a short chain diol with a molecular weight of approximately 50 to 500, an organic diisocyanate, an organic diamine, and a compound containing radiation-sensitive unsaturated double bonds, and cured by radiation.The thermoplastic polyurethane-urea resin in which not only the radiation-sensitive double bonds but also hydrophilic polar groups are introduced may also be used as the binder. These hydrophilic polar groups may for example include --SO.sub.3 M, --OSO.sub.

Abstract: A magnetic recording medium having excellent electromagnetic properties and satisfactory running durability prepared by mixing a highly concentrated solution of copolymers for vinyl chloride, vinyl acetate and vinyl alcohol with ferromagnetic particles and kneeding them, and then adding a solution of rubber type resins and additives thereto and dispersing them, and if desired, by adding and mixing polyisocyanate compounds therewith.

Abstract: The present invention relates to a magnetic recording medium prepared by the process comprising (1) coating a magnetic coating composition mainly containing ferromagnetic particles and a binder on a support and (2) irradiating the resulting support with electron beams, wherein the binder contains a compound represented by the following formula, ##STR1## wherein A represents a radical of a hydroxy compound containing at least one isocyanurate ring and at least one (meth)acryloyl group; DI represents a radical of an organic diisocyanate compound; B represents a radical of a polyol having at least two hydroxy groups; C represents a radical of a polybasic acid having at least two carboxy groups; m is a number of 1 to 7; and N is a positive number of 7 or less.

Abstract: A magnetic recording medium comprising a plastic base of polyester film or the like having a magnetic coat formed on one side, the magnetic coat consisting of a dispersion of a ferromagnetic powder in a resin binder, characterized in that the magnetic coat contains a nonmagnetic composite oxide powder of a spinel structure in an amount of 0.5 to 15% by weight on the basis of the ferromagnetic powder weight. The composite oxide in powder form ranges in particle diameter from 0.1 to 0.7 .mu.m and has a Mohs hardness of not less than 5.

Abstract: A magnetic recording medium comprising a nonmagnetic supporting substrate and a magnetic layer provided thereon by a binder, said magnetic layer contains a ferromagnetic alloy powder having a specific surface area (S.sub.BET) of no less than 45 m.sup.2 /g, and copper oleate in an amount of no less than 0.001 part by weight per 100 parts by weight of said ferromagnetic alloy powder.

Abstract: A recording medium operative with a recording and/or reproducing apparatus comprising a rotational motor and recording and/or reproducing transducer. The recording medium comprises a main body including a flexible recording carrier in the form of a sheet or film, and a support member supporting one end of the main body to maintain a circular shape for the body and being engageable with the motor. The recording carrier has a hollow cylindrical-shape and is provided with an opening at the end remote from the support member. The carrier has a recording surface around its periphery and a hollow interior with a surface which is parallel to the outer peripheral surface. The recording medium is rotated by the motor while maintaining a predetermined minute gap between the main body and a rotation stabilizing member which has a cylindrical surface substantially parallel to the main body.

Abstract: In a coating type magnetic recording medium, a base film is coated with a magnetic paint composition comprising a binder and a magnetic powder mixture of magnetic metal particles with magnetic metal carbide particles or magnetic metal particles having a metal carbide coating on the surface thereof. Preferably, the metal is iron and the metal carbide is iron carbide.

Abstract: A magnetic recording medium comprising a non-magnetic substrate, a magnetic recording layer formed on one side of the substrate and a back coating layer formed on the other side of the substrate, characterized in that the back coating layer contains carbon black having an ash content of at most 0.05%, a specific surface area of from 20 to 350 m.sup.2 /g, an oil absorption of at most 200 ml/100 g and an average particle size of from 13 to 60 m.mu.m.

Abstract: A magnetic recording medium which comprises a plastic base film, a thin magnetic metal layer formed on one side of the base film and a thermoset backing layer formed on the other side of the base film, wherein the backing layer is made of a thermosetting composition comprising a vinyl chloride-vinyl acetate-vinyl alcohol copolymer, a polyurethane prepolymer and a polyisocyanate and has a surface roughness of from 0.05 to 0.6 .mu.m.

Abstract: In a magnetic recording medium, double magnetic layers are formed on one side of a substrate with a lower magnetic layer containing metallic magnetic particles formed on the substrate and an upper magnetic layer containing plate-shaped ferrite magnetic particles of the hexagonal structure with the magnetization axis oriented in a direction vertical to the surface of the upper magnetic layer formed on the lower magnetic layer for enabling high output level recording over a wide wave length range.

Abstract: A magnetic recording medium which comprises a first magnetic layer on a base and a second magnetic layer over the first magnetic layer, the second magnetic layer containing magnetic particles having an average particle length (long axis diameter) of not more than 0.35.mu. , with a coercive force of from 660 to 760 oersteds, and the ratio of the coercive force of the upper or second magnetic layer to that of the lower or first magnetic layer being 1.1/1 to 1.6/1. The resulting magnetic recording medium has excellent sensitivity and frequency characteristics over the entire frequency bands with less noise.

Abstract: A magnetic recording medium is described, comprising a support and a magnetic layer disposed thereon, said magnetic layer containing as its binder component a combination of at least one member of each of the following groups of compounds (1) to (3) that has been exposed to electron beams:(1) vinyl chloride type resin, vinyl acetate type resin, vinyl chloride-vinyl acetate type resin, each containing from 5.times.10.sup.-5 to 2.times.10.sup.-3 equivalents of --CO.sub.2 H groups per gram of copolymer, and nitrocellulose resin;(2) urethane resins and urethane-acrylate oligomers; and(3) compounds containing an isocyanurate ring and at least two groups selected from acryloyl and methacryloyl groups.

Abstract: A magnetic recording medium comprising a transparent support and a magnetic layer formed on the support, said magnetic layer comprising Co-adsorbed Fe.sub.3 O.sub.4 and Co-adsorbed .gamma.-Fe.sub.2 O.sub.3 in a weight ratio of from 40:60 to 80:20, each of said Co-adsorbed Fe.sub.3 O.sub.4 and Co-adsorbed .gamma.-Fe.sub.2 O.sub.3 having a particle size of at least 25 m.sup.2 /g as a BET value.

Abstract: By inclusion of a fluorinated graphite material in conjunction with carbon black in the magnetic layer of a magnetic recording medium, the medium is improved significantly in its surface properties and electromagnetic performance properties as well as magnetic characteristics.

Abstract: A magnetic recording medium for use in video and other applications is presented which comprises a substrate, a continuous thin magnetic layer of cobalt and at least one selected from nickel, chromium, and oxygen, preferably obliquely evaporated on the substrate, and a topcoat formed over the magnetic layer and made of a phosphate compound having the formula: ##STR1## where R is selected from the group consisting of substituted or unsubstituted alkyl radicals having 10 to 22 carbon atoms which may be saturated or have an unsaturated double bond in their backbone, and substituted or unsubstituted aryl, alkaryl, aralkyl and alkylarylalkyl radicals having 6 to 20 carbon atoms, andY.sup.1 and Y.sup.2 may be the same or different and selected from the group consisting of hydrogen atom, cations, and substituted or unsubstituted alkyl and aryl radicals, the alkyl radicals being saturated or having an unsaturated double bond in their backbone, and Y.sup.1 and Y.sup.2 may together represent a divalent cation.

Abstract: A magnetic recording medium is disclosed. The medium is comprised of a support base having an average center line roughness (Ra) of 0.035 .mu.m or less which has coated thereon a magnetic layer. The magnetic layer is comprised of ferromagnetic fine particles dispersed in a binder comprising (1) a polymer selected from the group consisting of nitrocellulose resins, vinyl chloride-vinyl acetate resins and vinyl chloride-vinyl propionate type resins; (2) a urethane resin; and (3) a compound having 2 or more acrylate groups or methacrylate groups in the molecule thereof. The magnetic layer is cured by radiation with electron beams and has excellent electromagnetic properties as well as high durability. The magnetic medium provides less abrasiveness and the magnetic coating composition is stable and homogeneous during storage.

Abstract: A magnetic recording medium is described, comprising a nonmagnetic support, a magnetic layer, and a nonmagnetic intermediate layer provided between the support and the magnetic layer, the nonmagnetic intermediate layer containing a radiation-polymerizable compound and finely divided particles, the nonmagnetic intermediate layer being polymerized by irradiation and having a surface roughness from about 0.001 to 0.02 .mu.m. This medium provides reduced gap loss between the medium and a magnetic head, and is suitable for use in high density recording.

Abstract: A magnetic recording medium comprised of a support having a magnetic layer coated thereon is disclosed. The magnetic layer is comprised of a binder composition having a ferromagnetic powder dispersed therein. The binder composition is cured by an electron beam. The composition is comprised of 50 to 400 parts by weight of a compound having 2 or more acryloyl groups or methacryloyl groups in the molecule and 100 parts by weight of a mixture of 20 to 80 parts by weight of a thermoplastic resin having a thermal deformation temperature of 70.degree. C. or more and 20 to 80 parts by weight of a rubber type resin. The material has excellent electromagnetic properties after repeated use. Further, the medium has high physical strength and excellent wear resistance.

Abstract: A recording medium comprising a substrate, a recording layer formed thereupon, and an anticorrosive layer which covers the recording layer and contains a chemical to prevent the corrosion of the recording layer.

Abstract: A magnetic recording medium comprising a plastic base film, a thin magnetic metal layer formed on one side of the base film and a backing layer formed on the other side of the base film, characterized in that the backing layer is composed essentially of a thermoset or radiation-cured resin and carbon black and inorganic compound powder uniformly dispersed in the resin, and has a surface roughness of from 0.05 to 0.4 .mu.m, said carbon black having an average particle size of from 10 to 150 m.mu.m as measured by an electron microscope, said inorganic compound powder having a Mohs hardness of at most 6 and a particle size of from 10 to 500 m.mu.m, and the ratio of the carbon black to the inorganic compound powder being from 1:9 to 9:1.