Brooke Clarke 2005 - 2022
Shoplifting Prevention Tags


Magnetometers measure magnetic fields.  There are scalar types that measure the total field and there are vector types that measure the field strength only along one axis.  Some applications use 2 or 3 of the vector type sensors.  There are AC only units and more capable units that measure both DC and AC fields.  There quite a number of different sensor technologies but this page is aimed at home built magnetometers.

Shoplifting prevention tags

The shoplifting prevention tags used on many products contain strips of thin magnetic materials, one of which can be used as the basis of a home built magnetic flux gate type field sensor.  The common U.S. drinking straw has an O.D. of about 9 mm (0.238 inches) and will contain the sensor strip.


The Fluxgate (see my Fluxgate patents web page) is more sensitive than the common Hall devices.  It's used for satellite applications, sonobuoys, laboratory instruments and many other applications. 
A fluxgate has two operational states:
  1. The core is saturated by a drive coil.  In this state the magnetic field lines are undisturbed by the core as if there was no core. 
  2. The core is not saturated.  In this state the magnetic field lines are concentrated into the core.  The higher the permeability the more the flux lines are concentrated into the core, so Supermalloy or similar material is good.
As the fluxgate is switched between the two states the magnetic field lines cut the coil (or coils) that are a part of the magnetometer and cause variation in the the properties of the coil (or coils).  This shows up as an output signal at not only the drive frequency but also at harmonics of the drive frequency.

eBay: 79% Nickel - Permalloy - core with plastic case

The eBay seller (ttshungary) is offering these for $20 + $10 for shipping.  The appear to be would tape type toroidal cores with a plastic case.  The core is 28mm OD, 20mm ID, 5mm high with a rectangular cross section leading me to think they are wound tape.  The plastic case is: 30.5mm OD, 17.5mm ID and 7mm high.

YouTube: nikos 44317:  Evaluation of Permalloy cores for a fluxgate magnetometer, 2:19,
How to build and use a vector magnetometer, 4:16 - this is based on the HSCDTD008A (data sheet) a 3-axis compass sensor, Hall effect device, not flux gate.

Toroid Equations

These are in cgs units, but there are other units used making magnetic calculations complicated since the MKS units all have different names.

B = (E * 1E8) / (4 * A * N * f)
B is Flux Density in Gauss
E is RMS Voltage across coil
A is Core cross section area in cm^2
N is the number of turns
f is the frequency in Hz

H = (0.4 * PI * N * I) / L
H is the magnetic field strength in Oersted
PI is 3.14159...
N is the number of turns
I is the current in Amps
L is the magnetic path length in cm


The fluxmeter is a device that integrates the magnetic lines to come of with the total strength of a magnet.
See: Helmholtz Coilx\Sensitive Research Inst Co Fluxmeter

Handbook_for_Electrical_Engineers 1917 edited by Harold Pender - dead beat galvanometer
Electricity and Magnetism for Advanced Students 1920 by Sydney George Starling - equations for ballistic galvo fluxmeter.


2020 June - noticed a number of quack AKS Long Range Metal Detectors where the pitch is pure snake oil.  But I did find an interesting web page with some real information.
Geotech Technology for Treasure Hunting - where a number of topics include a list of related patents.

Magnetometers - Patents - Proton Precession (Wiki) magnetometers are very sensitive, but also expensive.  It's possible to home brew them, but a lot of work. Typically used in professional Archaeology and treasure hunting.
Geophysics - Patents -


DC Gaussmeters
DC PM Motors
Flux Gate Patents
Helmholtz Coil
Magnetic Locators (only ferrous, includes gradiometer)
Metal Locators (All metals)


Producing wound components -
International Coil Winding Association - mostly transformer related, but a couple of Strip Wound Core suppliers.

Permag Ltd -
Wiltan Ltd.  -
Telcon UK - Strip Wound Toroidal Cores -

Magnetics -

Publications at the Space Magnetometer Group, IAU-DTU

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