US2076230A - Insulated magnetic core and method of making insulated magnetic cores - Google Patents
Insulated magnetic core and method of making insulated magnetic cores Download PDFInfo
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- US2076230A US2076230A US702289A US70228933A US2076230A US 2076230 A US2076230 A US 2076230A US 702289 A US702289 A US 702289A US 70228933 A US70228933 A US 70228933A US 2076230 A US2076230 A US 2076230A
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- cores
- core
- insulated magnetic
- magnetic
- insulated
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Links
- 238000004519 manufacturing process Methods 0.000 title description 5
- 239000011162 core material Substances 0.000 description 41
- 239000000428 dust Substances 0.000 description 11
- 239000006249 magnetic particle Substances 0.000 description 10
- 239000011248 coating agent Substances 0.000 description 9
- 238000000576 coating method Methods 0.000 description 9
- 239000000919 ceramic Substances 0.000 description 7
- 239000011810 insulating material Substances 0.000 description 7
- 238000000034 method Methods 0.000 description 6
- 239000002245 particle Substances 0.000 description 6
- 238000000137 annealing Methods 0.000 description 5
- 210000003298 dental enamel Anatomy 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- 229910045601 alloy Inorganic materials 0.000 description 3
- 239000000956 alloy Substances 0.000 description 3
- 238000010438 heat treatment Methods 0.000 description 3
- ZPPSOOVFTBGHBI-UHFFFAOYSA-N lead(2+);oxido(oxo)borane Chemical compound [Pb+2].[O-]B=O.[O-]B=O ZPPSOOVFTBGHBI-UHFFFAOYSA-N 0.000 description 3
- 239000000696 magnetic material Substances 0.000 description 3
- 238000003825 pressing Methods 0.000 description 3
- 238000005507 spraying Methods 0.000 description 3
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 2
- UQSXHKLRYXJYBZ-UHFFFAOYSA-N Iron oxide Chemical compound [Fe]=O UQSXHKLRYXJYBZ-UHFFFAOYSA-N 0.000 description 2
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 2
- CDBYLPFSWZWCQE-UHFFFAOYSA-L Sodium Carbonate Chemical compound [Na+].[Na+].[O-]C([O-])=O CDBYLPFSWZWCQE-UHFFFAOYSA-L 0.000 description 2
- XLOMVQKBTHCTTD-UHFFFAOYSA-N Zinc monoxide Chemical compound [Zn]=O XLOMVQKBTHCTTD-UHFFFAOYSA-N 0.000 description 2
- KGBXLFKZBHKPEV-UHFFFAOYSA-N boric acid Chemical compound OB(O)O KGBXLFKZBHKPEV-UHFFFAOYSA-N 0.000 description 2
- 239000004327 boric acid Substances 0.000 description 2
- 238000005524 ceramic coating Methods 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 238000007598 dipping method Methods 0.000 description 2
- 238000009413 insulation Methods 0.000 description 2
- NUJOXMJBOLGQSY-UHFFFAOYSA-N manganese dioxide Chemical compound O=[Mn]=O NUJOXMJBOLGQSY-UHFFFAOYSA-N 0.000 description 2
- 230000035699 permeability Effects 0.000 description 2
- 239000000243 solution Substances 0.000 description 2
- 239000000454 talc Substances 0.000 description 2
- 229910052623 talc Inorganic materials 0.000 description 2
- 235000012222 talc Nutrition 0.000 description 2
- 238000004804 winding Methods 0.000 description 2
- BPQQTUXANYXVAA-UHFFFAOYSA-N Orthosilicate Chemical compound [O-][Si]([O-])([O-])[O-] BPQQTUXANYXVAA-UHFFFAOYSA-N 0.000 description 1
- 239000004115 Sodium Silicate Substances 0.000 description 1
- 235000010407 ammonium alginate Nutrition 0.000 description 1
- 239000000728 ammonium alginate Substances 0.000 description 1
- KPGABFJTMYCRHJ-YZOKENDUSA-N ammonium alginate Chemical compound [NH4+].[NH4+].O1[C@@H](C([O-])=O)[C@@H](OC)[C@H](O)[C@H](O)[C@@H]1O[C@@H]1[C@@H](C([O-])=O)O[C@@H](O)[C@@H](O)[C@H]1O KPGABFJTMYCRHJ-YZOKENDUSA-N 0.000 description 1
- 239000007864 aqueous solution Substances 0.000 description 1
- 229910010293 ceramic material Inorganic materials 0.000 description 1
- KRVSOGSZCMJSLX-UHFFFAOYSA-L chromic acid Substances O[Cr](O)(=O)=O KRVSOGSZCMJSLX-UHFFFAOYSA-L 0.000 description 1
- 229910000428 cobalt oxide Inorganic materials 0.000 description 1
- IVMYJDGYRUAWML-UHFFFAOYSA-N cobalt(ii) oxide Chemical compound [Co]=O IVMYJDGYRUAWML-UHFFFAOYSA-N 0.000 description 1
- 230000006835 compression Effects 0.000 description 1
- 238000007906 compression Methods 0.000 description 1
- 229910001610 cryolite Inorganic materials 0.000 description 1
- 230000001066 destructive effect Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- AWJWCTOOIBYHON-UHFFFAOYSA-N furo[3,4-b]pyrazine-5,7-dione Chemical compound C1=CN=C2C(=O)OC(=O)C2=N1 AWJWCTOOIBYHON-UHFFFAOYSA-N 0.000 description 1
- 238000000227 grinding Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 229910001004 magnetic alloy Inorganic materials 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 239000002923 metal particle Substances 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 239000000843 powder Substances 0.000 description 1
- 229910000029 sodium carbonate Inorganic materials 0.000 description 1
- 235000017550 sodium carbonate Nutrition 0.000 description 1
- NTHWMYGWWRZVTN-UHFFFAOYSA-N sodium silicate Chemical compound [Na+].[Na+].[O-][Si]([O-])=O NTHWMYGWWRZVTN-UHFFFAOYSA-N 0.000 description 1
- 229910052911 sodium silicate Inorganic materials 0.000 description 1
- 239000000375 suspending agent Substances 0.000 description 1
- XOLBLPGZBRYERU-UHFFFAOYSA-N tin dioxide Chemical compound O=[Sn]=O XOLBLPGZBRYERU-UHFFFAOYSA-N 0.000 description 1
- 229910001887 tin oxide Inorganic materials 0.000 description 1
- 239000000037 vitreous enamel Substances 0.000 description 1
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
- 239000011787 zinc oxide Substances 0.000 description 1
Images
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
- H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
- H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
- H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
- H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
- H01F1/20—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys in the form of particles, e.g. powder
- H01F1/22—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys in the form of particles, e.g. powder pressed, sintered, or bound together
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S428/00—Stock material or miscellaneous articles
- Y10S428/90—Magnetic feature
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T29/00—Metal working
- Y10T29/49—Method of mechanical manufacture
- Y10T29/49002—Electrical device making
- Y10T29/4902—Electromagnet, transformer or inductor
- Y10T29/49075—Electromagnet, transformer or inductor including permanent magnet or core
- Y10T29/49076—From comminuted material
Definitions
- This invention relates to insulated magnetic cores and methods of making insulated magnetic cores and more particularly to magnetic cores of the compressed dust type having an integral insulating coating and methods of making such COIES.
- Objects of the invention are to provide effective and efficient insulated magnetic cores and effective and economical'methods of making such 10 cores.
- a magnetic core made of compressed finely divided magnetic particles is sprayed with an aqueous solution of lead borate and dried.
- coated core is then heated to a temperature sufflciently high to fuse the coating to form a glazed vitreous insulated coating-on the core.
- FIG. 1 is a perspective view insulated coating made in accordance with the invention.
- Fig. 2 is a fragmentary sectional view of the core shown in Fig. 1.
- the magnetic material is preferably prepared from brittle alloys containing various percentages of nickel and iron. These alloys may be prepared as fully described in the patent to C. P. Beath and H. M. E. Heinicke, No. 1,669,649, issued May 15, 1928.
- An alloy thus prepared is rolled while hot into 'relatively thin slabs and quenched to produce a fine crystalline structure, which is very desirable since the distintegration of the material takes place at the crystal boundaries and consequently the smaller the size of the crystals, the finer the dust which can be produced from the finished product.
- the magnetic alloy thus received in slab form is reduced to a finely divided form or dust, in any well known manner, as for example, in a hammer mill or other suitable reducing apr paratus and subsequently pulverized in an attri tion mill.
- the dust from the attrition mill is sifted and the portion passing through a 120 mesh sieve is placed in a closed container and annealed at a temperature of approximately 885 C.
- the annealed dust is removed from the container in the form of a cake which is again reduced to a powder by crushing in a rotary crusher and subsequently grinding in an attrition mill.
- the ground dust is again sifted through a standard 120 mesh of a core having an screen and the dust passing through such a screen is insulated with a solution of chromic acid, sodium silicate, powdered talcum and water. This mixture is heated and stirred until dry, as fully described in Patent No. 1,669,643, issued May 15, 1928, to J. W. Andrews and R. Gillis.
- the desired permeability can be obtained by varying the amount of the insulating material.
- other methods of insulating the dust well known to the art, could also be employed.
- the insulated dust is sifted through a 16 mesh screen and placed in a mold and compressed into core parts under a pressure of approximately 200,000 pounds per square inch.
- the core parts are then covered with a ceramic slip either by dipping or spraying the slip on the parts.
- the ceramic material may consist of 83 parts of red lead and 17 parts'of boric acid.
- a suspending agent, such as ammonium alginate, may be used to form an aqueous colloidal solution for dipping or spraying the parts.
- the cores thus coated are then heated at a temperature from 575 C; to 690 C., which is sufflcient to fuse these low'melting point ceramic compositions.
- the cores are also annealed to relieve the stresses arising from the compression of the dust particles into cores and the annealing temperature must be low enough not to damage the insulating material on the dust particles.
- the ceramic coating protects the core material from the atmosphere and prevents oxidation of the materials which effects a considerable decrease in the hysteresis losses of the cores and improves their permeability.
- the enamel serves as an insulating material for the cores.
- it is customary to apply a serving of paper to the cores before applying the windings thereto.
- the present invention eliminates the necessity for wrapping the core with paper or other insulating material and the windings may be wound directly on the insulating cores.
- vitreous enamels While several types of vitreous enamels have been referred to, it is obvious that any of the low fusing point enamels may be used and the enamels described are merely illustrative of the invention.
- a method 0! making magnetic cores by pressing individually insulated magnetic particles into a mass, applying an additional ceramic envelop around the formed mass oi magnetic material, and applying heat to fuse the ceramic envelop and anneal the magnetic particles.
- a magnetic core comprising a compressed mass of finely divided insulated magnetic particles and a fused ceramic insulating coating over the core, the particles being insulated with ma terial having a different composition than the core coating material, and the core coating material having a fusing temperature within the annealing range of the magnetic particles.
- a core comprising particles of metal individually coated with insulating material and pressed into form and a fused envelop of a different insulating material having a fusing temperature of the same order as the annealing temperature of the metal particles.
- a method of making cores comprising coating finely divided magnetic particles with insu lating material, pressing the coated particles into a mass, applying to the mass a ceramic slip having a lower fusing temperature than the destructive temperature of the insulation on said particles, and heating the cores to fuse the slip and anneal the cores.
- a magnetic core comprising a compressed mass of finely divided magnetic particles individually coated with insulation compounded 01 a metallic silicate and talcum, and a fused ceramic insulating coating over the core, the ceramic coating having a fusing temperature within the annealing range of the magnetic particles.
- a method oi making magnetic cores by pressing individually insulated magnetic particles into a mass, applying an additional envelope of lead borate around the formed mass of magnetic material, and applying heat to fuse the lead borate and anneal the magnetic particles.
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- Chemical & Material Sciences (AREA)
- Dispersion Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Soft Magnetic Materials (AREA)
Description
April 6, 19 7. R, ms 2,076,230
I INSULATED MAGNETIC GORE AND METHOD OF MAKING INSULATED MAGNETIC CORES Filed Dec. 14, 1935 (2754M!!! GMT/N6 Patented Apr. 6, 1937 PATENT OFFICE INSULATED MAGNETIC CORE AND METHOD OF MAKING CORES INSULATED MAGNETIC Randall Gillis, La Grange, Ill., assignor to Western Electric Company,
Incorporated, New York,
N. Y., a corporation of New York Application December 14, 1933, Serial No. 702,289
6 Claims.
This invention relates to insulated magnetic cores and methods of making insulated magnetic cores and more particularly to magnetic cores of the compressed dust type having an integral insulating coating and methods of making such COIES.
Objects of the invention are to provide effective and efficient insulated magnetic cores and effective and economical'methods of making such 10 cores.
In accordance with one embodiment of the invention, a magnetic core made of compressed finely divided magnetic particles is sprayed with an aqueous solution of lead borate and dried. The
coated core is then heated to a temperature sufflciently high to fuse the coating to form a glazed vitreous insulated coating-on the core.
Other features and advantages of the invention will become apparent from the following detailed descriptions, reference being bad to the accompanying drawing, wherein Fig. 1 is a perspective view insulated coating made in accordance with the invention, and
Fig. 2 is a fragmentary sectional view of the core shown in Fig. 1.
In carrying out the present inventionthe magnetic material is preferably prepared from brittle alloys containing various percentages of nickel and iron. These alloys may be prepared as fully described in the patent to C. P. Beath and H. M. E. Heinicke, No. 1,669,649, issued May 15, 1928.
An alloy thus prepared is rolled while hot into 'relatively thin slabs and quenched to produce a fine crystalline structure, which is very desirable since the distintegration of the material takes place at the crystal boundaries and consequently the smaller the size of the crystals, the finer the dust which can be produced from the finished product. The magnetic alloy thus received in slab form is reduced to a finely divided form or dust, in any well known manner, as for example, in a hammer mill or other suitable reducing apr paratus and subsequently pulverized in an attri tion mill.
The dust from the attrition mill is sifted and the portion passing through a 120 mesh sieve is placed in a closed container and annealed at a temperature of approximately 885 C. The annealed dust is removed from the container in the form of a cake which is again reduced to a powder by crushing in a rotary crusher and subsequently grinding in an attrition mill. The ground dust is again sifted through a standard 120 mesh of a core having an screen and the dust passing through such a screen is insulated with a solution of chromic acid, sodium silicate, powdered talcum and water. This mixture is heated and stirred until dry, as fully described in Patent No. 1,669,643, issued May 15, 1928, to J. W. Andrews and R. Gillis. As clearly described therein, the desired permeability can be obtained by varying the amount of the insulating material. Obviously, other methods of insulating the dust, well known to the art, could also be employed.
The insulated dust is sifted through a 16 mesh screen and placed in a mold and compressed into core parts under a pressure of approximately 200,000 pounds per square inch. The core parts are then covered with a ceramic slip either by dipping or spraying the slip on the parts. The ceramic material may consist of 83 parts of red lead and 17 parts'of boric acid. A suspending agent, such as ammonium alginate, may be used to form an aqueous colloidal solution for dipping or spraying the parts.
Another composition which has been found to be effective for this purpose comprises red lead 125 parts, boric acid 63 parts, flint parts, cryolite 15 parts, iron oxide 10 parts, soda ash '1 parts, tin oxide 5 parts, zinc oxide 4 parts, manganese dioxide 1.04 parts, cobalt oxide 1.16 parts.
The cores thus coated are then heated at a temperature from 575 C; to 690 C., which is sufflcient to fuse these low'melting point ceramic compositions. During this heat-treatment the cores are also annealed to relieve the stresses arising from the compression of the dust particles into cores and the annealing temperature must be low enough not to damage the insulating material on the dust particles. During this annealing heat-treatment the ceramic coating protects the core material from the atmosphere and prevents oxidation of the materials which effects a considerable decrease in the hysteresis losses of the cores and improves their permeability. Improvements of the order of 20% in hysteresis losses have been noted in cores treated in this manner over cores annealed in the atmosphere. In additionto the improvement in the magnetic characteristics of the cores in spraying or coating the cores with vitreous enamel during the an ealing of the cores, the enamel serves as an insulating material for the cores. In accordance with one practice used in the manufacture of loading coils, it is customary to apply a serving of paper to the cores before applying the windings thereto. The present invention eliminates the necessity for wrapping the core with paper or other insulating material and the windings may be wound directly on the insulating cores.
While several types of vitreous enamels have been referred to, it is obvious that any of the low fusing point enamels may be used and the enamels described are merely illustrative of the invention.
What is claimed is:
1. A method 0! making magnetic cores by pressing individually insulated magnetic particles into a mass, applying an additional ceramic envelop around the formed mass oi magnetic material, and applying heat to fuse the ceramic envelop and anneal the magnetic particles.
2. A magnetic core comprising a compressed mass of finely divided insulated magnetic particles and a fused ceramic insulating coating over the core, the particles being insulated with ma terial having a different composition than the core coating material, and the core coating material having a fusing temperature within the annealing range of the magnetic particles.
3. A core comprising particles of metal individually coated with insulating material and pressed into form and a fused envelop of a different insulating material having a fusing temperature of the same order as the annealing temperature of the metal particles.
4. A method of making cores comprising coating finely divided magnetic particles with insu lating material, pressing the coated particles into a mass, applying to the mass a ceramic slip having a lower fusing temperature than the destructive temperature of the insulation on said particles, and heating the cores to fuse the slip and anneal the cores.
5. A magnetic core comprising a compressed mass of finely divided magnetic particles individually coated with insulation compounded 01 a metallic silicate and talcum, and a fused ceramic insulating coating over the core, the ceramic coating having a fusing temperature within the annealing range of the magnetic particles.
6. A method oi making magnetic cores by pressing individually insulated magnetic particles into a mass, applying an additional envelope of lead borate around the formed mass of magnetic material, and applying heat to fuse the lead borate and anneal the magnetic particles.
RANDALL GILLIS.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US702289A US2076230A (en) | 1933-12-14 | 1933-12-14 | Insulated magnetic core and method of making insulated magnetic cores |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US702289A US2076230A (en) | 1933-12-14 | 1933-12-14 | Insulated magnetic core and method of making insulated magnetic cores |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| US2076230A true US2076230A (en) | 1937-04-06 |
Family
ID=24820605
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| US702289A Expired - Lifetime US2076230A (en) | 1933-12-14 | 1933-12-14 | Insulated magnetic core and method of making insulated magnetic cores |
Country Status (1)
| Country | Link |
|---|---|
| US (1) | US2076230A (en) |
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2467101A (en) * | 1941-03-08 | 1949-04-12 | Western Electric Co | Magnetic core |
| US2503947A (en) * | 1947-03-10 | 1950-04-11 | Comm Engineering Pty Ltd | Method of molidng magnetic powder |
| US2576679A (en) * | 1939-08-02 | 1951-11-27 | Electro Chimie Metal | Permanent magnet and method of manufacture thereof |
| US2601212A (en) * | 1948-11-09 | 1952-06-17 | Gen Aniline & Film Corp | Heat resistant magnetic cores and method of making |
| US2818551A (en) * | 1953-01-07 | 1957-12-31 | Philips Corp | Ferromagnetic body |
| US2847333A (en) * | 1956-07-10 | 1958-08-12 | Western Electric Co | Methods of making magnetic cores |
| US2958819A (en) * | 1952-03-25 | 1960-11-01 | Western Electric Co | Method and apparatus for adjusting magnetometers |
-
1933
- 1933-12-14 US US702289A patent/US2076230A/en not_active Expired - Lifetime
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2576679A (en) * | 1939-08-02 | 1951-11-27 | Electro Chimie Metal | Permanent magnet and method of manufacture thereof |
| US2467101A (en) * | 1941-03-08 | 1949-04-12 | Western Electric Co | Magnetic core |
| US2503947A (en) * | 1947-03-10 | 1950-04-11 | Comm Engineering Pty Ltd | Method of molidng magnetic powder |
| US2601212A (en) * | 1948-11-09 | 1952-06-17 | Gen Aniline & Film Corp | Heat resistant magnetic cores and method of making |
| US2958819A (en) * | 1952-03-25 | 1960-11-01 | Western Electric Co | Method and apparatus for adjusting magnetometers |
| US2818551A (en) * | 1953-01-07 | 1957-12-31 | Philips Corp | Ferromagnetic body |
| US2847333A (en) * | 1956-07-10 | 1958-08-12 | Western Electric Co | Methods of making magnetic cores |
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