© Brooke Clarke 2015

    Field Strength
    Magnetic Recording
            Earth Inductor
        Measuring Thickness


This is a way to collect things magnetic in the same way that the Optics or Battery pages collect those things.


Field Strength

There are two areas of interest: 

Earth's magnetic filed

 Which is about 50,000 nano Tesla or 0.5 Gauss and important for things like compass determination of direction.

Permanent & Electromagnets

These develop much higher forces than the Earth's field and typically are measured by different instruments.


There are many magnetic sensors some of which measure only the total magnitude of the magnetic field and others which measure the field in a specific direction and so have a vector output, like for a compass.

1925872 Apparatus for making magnetic tests, Hermann Peter C, Gen Electric, Sep 5, 1933 324/227 - when a ring of material can not be used.
2415808 Detection of large magentic bodies, Oliver E Buckley, Bell Labs, App: 1941-07-31, TOP SECRET, Pub: 1947-02-18, - "The invention is particularly applicable to the detection and location of submerged submarine or other vessels having hulls of magnetic material but it is applicable as well to the detection and location of other magnetic bodies, Such as large magnetic Ore deposits."
2427014 Orienting device, Winthrop J Means, Bell Labs, App: 1943-07-30, W.W.II, Pub: 1947-09-09, - magnatometer mounted on non-magnetic gimbal
2485931 Magnetic field strength indicator, Slonczewski Thaddeus, Bell Labs, App: 1943-04-20, W.W. II, Pub: 1949-10-25, 324/247; 102/427; 102/417; 340/870.33 - three flux gate magnetometers mounted mutually orthogonality.
2687507 Method and apparatus for measuring the earth's total magnetic field vector (from airplane), Erick O Schonstedt (Wiki), 1954-08-24, - makes use of a magnatometers, Pendulum, Vertical Gyroscope and Directional Gyroscope.
2829338 Test transformer for ring-type magnetic cores, Lord Harold W, Gen Electric, Apr 1, 1958, 324/223, 336/220, 324/260, 324/211, 336/212, 336/73, 336/174, 336/222 -  intended for measuring strip wound cores of high permeability material.
5008621 Multiparameter magnetic inspection system with magnetic field control and plural magnetic transducers, David C. Jiles, Iowa State U, Apr 16, 1991, 324/227, 324/232, 324/243, 324/223 - many citations and references, just the test set for developing magnetostriction materials.
5012189 Method for deriving information regarding stress from a stressed ferromagnetic material, David C. Jiles Iowa State U, Apr 30, 1991, 324/209 -


Magnetic Viewing Film is green in color and allows visualizing magnetic fields.  The prior way to do this involved fine iron filings.  But these stick to magnets and are impossible to remove.
3320523 Method for visibly indicating and recording magnetic fields, Lyne S Trimble, 1967-05-16, 324/214; 346/74.2; 359/280; G9B/5.233 - maybe invented to view magnetic ink (Wiki) by NCR.


Magnetic Recording

Wire recording (Wiki) was the first type of magnetic recording.
661619 Method of recording and reproducing sounds or signals, Valdemar Poulsen, Nov 13, 1900, 360/87; 235/449; 360/136; 379/70 - for signals, messages or speech

The label on a "Webster Chicago (Wiki) Model 80-1 RMA 375" wire recorder says "This unit licensed under Armour Research Foundation patents"  So I looked for them:
2456767 Combination of magnetic transducing and erasing heads, Camras Marvin, Armour Res Found, Filed: Nov 29, 1945 Pub: Dec 21, 1948, 360/123.17, 360/121, 360/125.1, 361/151, 29/603.9 - This is for a single head where one coil is for recording/playback and the other is the erase head for a wire recorder.  Referenced by 38 other patents.

Tape recording works pretty much as wire recording except since the tape is much wider the signal to noise ratio is very much improved.  When learning how to adjust the Magnacord M1024 tape recorder I
discovered that the material being recorded acts like the bias signal and so the amount of bias depends on what's being recorded.  Very little bias is needed to record white noise whereas more bias is needed to record a single tone.

Magnetostriction (Wiki)

Technology used in the CRT-1 Sonobuoy for the hydrophone.
A lot of work was done on magnetostriction at Iowa State University, Ames Iowa and the nearby Ames Lab.

Note that magnetostriction is complementary to the piezoelectric effect.  In one case mechanical motion comes about by magnetic fields and in the other by electric fields.
YouTube: How anti-theft tags work - magnetostriction (MetGlass)


Iron (Wiki) and a number of iron alloys change dimensions when a magnetic filed is applied.  This is why transformers make noise.
Nickel (Wiki) was used in W.W.II About -50 ppm
Cobalt (Wiki) highest Magnetostriction of any pure element, about 60 ppm.
Metglas (Wili) - Fe81Si3.5B13.5C2 - see Magnetic Shielding & Fluxgate Core Material. don't know the coefficient (maybe 27 ppm for 2605SA1 & 2605HB1M Alloy, DataSheet.pdf)
Saturates about 1.6 Tesla.
Terfenol-D (Wiki)
Terfenol-D Rods  TbxDy1−xFe2 (x ~ 0.3), named after terbium, iron (Fe), Naval Ordnance Laboratory (NOL), and the D comes from dysprosium.  1000 ppm from room temperature Patent search on Tefenol-D.  Saturates at about 1 Tesla.

8.60 mm dia x 50.0 mm long  slightly too large in diameter to fit into Tattoo machine coil forms.

Galfenol (Wiki) is an alloy of iron and Gallium.  About 350 ppm in ~100 Oe magnetic field. Patent search on Galfenol
There is a list of patents for the CRT-1 sonobuoy hydrophone on that web page.


942897 Apparatus for receiving submarine sounds, Thomas Alexander Garrett, William Lucas, Dec 14, 1909, 367/168; 114/21.2; 318/118; 381/180 - " We make use of the well known fact that subjecting a longitudinally magnetized nickel rod or wire to variations of longitudinal stress causes variations in its magnetization and consequently causes variations of electric current in a coil of insulated wire around the rod or wire."

Patents issued on Oct 11, 1932 in a block to G.W. Pierce (Wiki) all on Magnetostrictive technology

1750124 Vibratory system and method,  George W. Pierce, Filed: Jan 3, 1927, Pub: Mar 11, 1930, 331/157, 310/26, 367/168, 318/118 - based on magnetostriction w/application to ships.
2014410  Electromagnetostrictive vibrator, Pierce George W, Priority: Jan 3, 1927, Pub: Sep 17, 1935, 318/118, 367/168, 73/662 - parallel rods or tubes

The below 9 patents 1882393 through 1882401 reference 2014410.

1882393 Magnetostrictive vibrator, Washington Pierce George, Priority: Mar 23, 1928,  Pub: Oct 11, 1932, 318/118, 367/168, 601/2, 29/607 -
1882394 Magnetostrictive vibrator, Washington Pierce George, Priority: Mar 23, 1928, Pub: Oct 11, 1932, 318/118, 310/26, 307/151, 367/176, 333/201, 322/3, 367/168, 331/157, 310/15 -
1882395 Frequency indicator, George W Pierce, Priority: Dec 31, 1927, Pub: Oct 11, 1932, 324/76.49, 318/118, 331/64, 29/607, 361/182, 331/157, 367/168, 367/176, 361/206, 601/2 -
1882396 Magnetostrictive transformer, Pierce George W, Priority: Dec 31, 1927, Pub: Oct 11, 1932, 333/201, 335/215, 318/118, 336/20 -  
1882397 Magnetostrictive vibrator, George W. Pierce, Filed: Aug 17, 1928, Pub: Oct 11, 1932, 333/201, 318/118, 361/206 - Nickel-Steel rod 0.5 cm (5mm or 0.196") dia X 10 cm (100mm or 3.937") long vibrates at 21 kHz. A rod 10X longer (1 meter) vibrates at 2.1 kHz.  Iron-Chromium vibrate at 27 kHz and 2.7 kHz respectively. By adding other metals with different coefficients of temperature a net zero temperature coefficient can be obtained.  Includes equations for the frequency of some standard configurations.
Magnetostrictive vibrator, Pierce George Washington, Priority: Aug 17, 1928, Pub: Oct 11, 1932,  318/118, 361/206, 428/595, 381/190, 261/DIG.480, 428/680, 261/1, 367/168, 428/684, 601/2 -
1882399 Magnetostrictive vibrator, Washington Pierce George, Priority: Aug 17, 1928, Pub: Oct 11, 1932, 318/118, 148/312, 367/168, 324/93, 148/315, 324/76.49, 451/165, 361/206, 148/310 -
1882400 Vibratory device, Alvord Buckingham Stephen, Washington Pierce George, Oct 11, 1932, 369/146, 381/190, 367/185, 318/118 - phonograph pickup using magnetostrictive element
1882401 Loud speaker, Washington Pierce George, Priority: Aug 17, 1928, Pub: Oct 11, 1932, 381/190, 318/118, 310/26, 340/384.73, 366/127 -

1962154 Magnetostrictive vibrator, George W. Pierce (Wiki), Priority Dec 31, 1927, Pub: Jun 12, 1934, 331/157, 116/137.00A, 318/118, 367/168 -
1962155 Vibratory system and apparatus, Pierce George W, Priority: Dec 31, 1927, Pub: Jun 12, 1934, 329/347, 318/118, 330/61.00R, 310/317, 310/328 - piezo-electric crystals

2044807 Transducer, Jr Atherton Noyes, Jun 23, 1936, 367/153, 367/176, 310/26, 318/118, 114/67.00R - underwater telephone - magnetostrictive type see" Sumarine - Echoscope (Gertrude)
2064911 Sound generating and directing apparatus, Harvey C Hayes, 1936-12-22, - magnetostrictive devices and mechanical impedance transformation.
Cited by 37 patents
2116522 Compressional wave sender and receiver, Willy Kunze, Submarine Signal Co, May 10, 1938, 367/168, 318/118, 335/215, 310/26, 29/609 -
2422425 Art of tuning magnetostrictive elements, Edward Lane Albert, Rca Corp, Jun 17, 1947, 310/26, 318/118, 367/168 -   
2487815 Magnetostrictive rod unit, Lee Norman E, Filed: May 13, 1944, Pub: Nov 15, 1949, 310/26, 331/157, 318/118 - oscillating rod frequency standard
2249835 Magnetostrictive vibrator, Emory Lakatos, Bell Telephone Labor Inc, Jul 22, 1941, 367/168, 381/190, 156/516, 335/215, 318/118, 310/26 - closing magnetic path
2166359 Magnetostrictive device, Emory Lakatos, Bell Telephone Labor Inc, Jul 18, 1939, 333/201, 420/581, 252/62.55, 148/312, 367/168, 420/459, 310/26, 148/315, 335/215, 318/118, 367/176 - hi Q circuit elements
2170206 Electrical and electromechanical system employing magnetostrictive devices, Warren P Mason, Bell Telephone Labor Inc, Aug 22, 1939, 333/201, 381/190, 367/156, 310/26 - circuit elements & loud speaker + submarine sound projector.
2398117 Magnetostrictive oscillator, Elias Claesson Per Harry, Fabian Rost Helge, Priority: May 3, 1941, Pub: Apr 9, 1946, 367/151, 381/190, 310/26, 367/156 - Submarine SONAR -
Cone microphone, Laymon N Miller, Filed: Jan 1, 1944, Pub (12 year delay): Oct 30, 1956, 367/141, 102/418, 114/20.1 - magnetostrictive hydrophone for torpedo
Magnetostrictive transducer, Miller Laymon N Cambridge, MA, Sec of Navy,  Filed: Jul 14 (17 year delay) 1944, Pub:   May 16, 1961 - 18 citations
4907209 Low frequency sound transducer, Martin Marietta Corporation, Mar 6, 1990, 367/168, 310/26, 367/156 -
8155344 Vehicle speaker, Toyota, Dec 23, 2008, 381/86, -  magnetostrictive vibrator used as part of speaker system.

Submarine Signal Co. patents using the word

8126169 Super magnetostriction speaker, Hirofumi Onohara, Foster Electric Company, 28 Feb 2012, 381/190 - Vibration Actuators (all for reference only =  discontinued? ) -


A Magnetostrictive device which will turn any movable flat surface like wallboard into a speaker.

                      Magnetostrictive speaker


Magnetic (See Navigation)

Earth Inductor Compass (Wiki)

An early type of aircraft compass was the Earth Inductor Compass that used a rotating coil acting as a generator where the magnetic field is due to the Earth.
Pioneer made a number of aircraft instruments including Earth Inductor Compasses. (5 sequential patents!)

1770243 Rotary coil compass, George A Titterington, PIONEER INSTR CO Inc, Filed: 1925-05-21, Pub: 1930-07-08 -
1770244 Aircraft-steering system, George A Titterington, PIONEER INSTR CO Inc, Filed: 1925-05-21, Pub: 1930-07-08 -
1770245 Earth inductor compass, George A Titterington, PIONEER INSTR CO Inc, Filed: 1925-05-21, Pub: 1930-07-08 -
1770246 Compass direction controller, George A Titterington, PIONEER INSTR CO Inc, Filed: 1925-05-21, Pub: 1930-07-08 -
1770247 Inductor-compass generator, George A Titterington, PIONEER INSTR CO Inc, Filed: 1925-05-21, Pub: 1930-07-08 -
1819488 Inductor compass generator, George A Titterington, PIONEER INSTR CO Inc, 1931-08-18 -vacuum-pressure driven using toothed wheel (see Train Gyroscope)

1963551 Inductor compass, Gunn Ross, BENDIX RES CORP, 1934-06-19 -
2025897 Earth inductor compass, Wladimir A Reichel, PIONEER INSTR CO Inc, Filed: 1929-05-22, Pub: 1935-12-31 -
2206018 Earth inductor compass, Bechberger Paul Franklin, Bendix Aviation Corp, 1940-07-02 -
2240680 Earth inductor compass, Jr Alfred A Stuart, Bendix Aviation Corp, Filed: 1938-12-29, Pub: 1941-05-06 - 3-phase for servo systems
  RE22699 Magnetic compass, A.A. Stuart Jr., Bendix Aviation, Filed May 21, 1940, Reiuued Nov. 27, 1945
2376883 Dynamic earth inductor compass, Alger S Riggs, Herbert H Thompson, Sperry Gyroscope, Filed: 1942-01-30, W.W.II, Pub: 1945-05-29, - uses servos (probably 400 Hz)
2434324 Earth inductor compass, Lehde Henry, Control Instrument Co, Filed: 1944-10-06, W.W.II, Pub: 1948-01-13, -
2710942 Electromagnetic induction device, John F Emerson, Bendix Aviation Corp, 1955-06-14 - 3-phase for servo systems
2925662 Compass system, Harry B Wattson, John C Karlson, Bendix Aviation Corp, Filed: 1955-02-10, Pub: 1960-02-23 -

Measuring Thickness

There's a wide assortment of "magnetic coating thickness gauge" on eBay.

2021 July 11 - I have one on order for under $20 including shipping.
"Painting Thickness Gauge Non-magnetic Coat Thickness Meter Automotive Tester"
They claim it works on both steel and Aluminum car bodies.

By using the magnetic attraction force to stretch a spring it's possible to measure the thickness of non magnetic coatings, like paint on a steel bodied car using a magnet.  Note that Chromium (Wiki) used in Chrome Plating (Wiki) exhibits Antiferromagnetism (Wiki).  That's to say it is not magnetic and so it's thickness can be measured using this magnetic method.

Tinsley Thickness Gauge

Tinsley & Co also holds a patent related to Tuning Forks.
According to NBS Circular 585  (paragraph 4.11 Magnetic attractive force thickness measurement, pg 36) the British Standards Association (BSA) developed this gauge.

Fig 1
                      Thickness Gauge
Fig 2
                      Thickness Gauge
Fig 3 Scale Calibrated in mills.
                      Thickness Gauge
Fig 4 Linear Scale for custom calibrations
                      Thickness Gauge
Fig 5 Method of working.
                      Thickness Gauge

The American Instrument Company made the Tinsley gauge in the US.  (American Instrument Co patents) but did not have a patent on it. The case for this Tinsley Thickness Gauge is marked Biddle.
                      Apparatus for measuring coating thickness, Lipson
                      Samuel, App: 1945-02-06
US2507529 Apparatus for measuring coating thickness, Lipson Samuel, App: 1945-02-06, Pub: 1950-05-16, - uses an AC line powered electromagnet.

Calibration data based on N.B.S. chrome plated steel standards.
                      Magnetic measuring gauge, Glen N Krouse,
US2625585 Magnetic measuring gauge, Glen N Krouse, 1953-01-13, - Uses a drum micrometer to lift the magnetic probe and you stop turning the drum when the magnet pulls free of the device under test.  A calibration chart is used to convert the micrometer reading into a thickness.
image not available
Let me know if it's on line.

GB1312763 Film thickness gauges, H Tinsley and Co Ltd,  App: 1971-03-08, Pub: 1973-04-04, -

A gauge for measuring the thickness of a non- magnetic film on a ferrous body 25 comprises, a sleeve 2 slidably positioned for datum purposes at locations 3a 3b of a dial gauge casing, and a tube 6 of Aluminium or its alloy slidable in the sleeve 2 and carrying a magnet 7 at one end and at its other end having a plug 8 provided with a brass pin 9 sliding in a slot 10 in the sleeve. The tube 6 has a tension spring 11 fixed to the plug 8 and at its other end the spring is tension adjusted by being fixed to a screw in the end of the sleeve 2. An indicating needle 12 counter-balanced at 14 and pivoted at 13 may be moved from the Fig.1 datum position by sliding movement of the abutting pin 9, which movement takes place when the magnet 7 is attracted to the end of the sleeve 2 by placing the gauge with its end 5 against the ferrous body 25. If the gauge is now drawn away from the body 25 the magnet 7 will be held by the attraction and withdrawn slightly from the end 5, and this movement will slide the pin 9 and move the pointer 12 over the gauge markings. Further movement of the apparatus finally breaks the magnet away from the ferrous body 25 and the magnet then retracts into the tube 6 and the pin 9 leaves the balanced pointer behind which remains in position indicating a reading on the gauge, the final pointer position depending on and indicating the thickness of the non- magnetic film on the body 25. A pointer reset mechanism 19, 20 operated by a plunger 23 is also provided.
5006799 Low
                      cost magnetic coating thickness gauge with holding
                      magnet, bias spring and holding force indicator,
                      John G. Pfanstiehl, 1991-04-09
5006799 Low cost magnetic coating thickness gauge with holding magnet, bias spring and holding force indicator, John G. Pfanstiehl, 1991-04-09, - talks about the Tinsley gauge.


3am 3 Axis Magnetometer
Aircraft Pilot's Standby Magnetic Compass
Collins PN-101 Pictorial Navigation System (gyroscope & flux valve)
CRT-1B Expendable Radio SonoBuoy (ESRB) - uses a magnetostriction (Wiki) sensor as the hydrophone  
DC Gaussmeter Model 1
    AMY6 Magnetic Polarity Tester
    GE Gauss Meter & Reference Magnet
EM_Toy Engine  Electromagnet Toy Engine
FGPat Fluxgate (& other) Patents
Helmholtz Coil
    Sensitive Research Instrument Co. Fluxmeter
    Annis M25 Pocket Magnetometer
    Cenco Scientific 79860 Dynamo Analysis Apparatus - works with Fluxmeter
HT20 2000 mT Magnetic Flux Meter
Home Built Magnetometers
HP 428 Clip-on Milliammeter Magnetometer
KVHC100FG KVH C100 Flux Gate Compass module
MC1 MC-1 Magnetic Compass Calibration Set similar to AN/ASM-344
Permanent Magnet DC Motors
Sonobuoys use magnatometers
Telegraph Telegraph Equipment, Stock Ticker, District Telegraph, Teletype, Keys, Relays, Sounders, Veedre Counter, Early Connectors, Electro-magnets
Torpedoes use magnetic exploders
Weeden-El-Mtr  Weeden DC Electro0magnetic Machine (Motor or Generator)
Wireless Driveway Monitor - STI-34100 solar power magnetometer
YouTube: Applied Science: Engineering magnetics -- practical introduction to BH curve -


PRC68, Alphanumeric Index of Web pages, Contact, Products for Sale