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
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:
- 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.
- 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.
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 *
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.
1917 edited by Harold Pender - dead beat
Coilx\Sensitive Research Inst Co Fluxmeter
and Magnetism for Advanced Students 1920
by Sydney George Starling - equations for ballistic galvo
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.
- 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
Back to Brooke's: PRC68
, Products for Sale
, F.W. Bell Gaussmeter