Collector type article surveillance marker with continuous keeper

A dual-status marker for use in electronic article surveillance systems having an alternating magnetic field within an interrogation zone. The marker comprises a piece of a high permeability, low coercive force magnetic material substantially coextensive with a piece of remanently magnetizable material. The first piece is rectangular and exhibits lengthwise sections at which the material is removed, thus leaving narrow width regions forming switching sections, portions adjacent each end forming flux collectors. The marker is desensitized by uniformly magnetizing the piece of remanently magnetizable material.

FIELD OF THE INVENTION 
This invention relates to electronic article surveillance (EAS) systems of 
the general type in which an alternating magnetic field is produced in an 
interrogation zone and in which a magnetically responsive marker present 
in the zone results in the production of a characteristic signal which is 
detected and processed to create a suitable response such as an audible or 
visible alarm. 
BACKGROUND OF THE INVENTION 
Modern magnetically based electronic article surveillance systems generally 
derive their parentage from 1934 French Patent No. 763,681. That patent 
depicts the use of markers formed of a piece of low coercive force, high 
permeability alloy, such as permalloy, and teaches that when the 
magnetization of such a piece is reversed by a magnetic field alternating 
at a fundamental frequency, detectable harmonics of that frequency will be 
produced. 
More recently, various investigators have developed magnetic markers which 
have dual-status capabilities. Typically, as first disclosed in U.S. Pat. 
Nos. 3,665,449 (Elder et al.) and 3,747,086 (Peterson), such dual-status 
markers include at least one piece of low coercive force, high 
permeability material together with at least one piece of remanently 
magnetizable material. When the latter piece is magnetized it has 
associated therewith a magnetic field which biases the low coercive force, 
high permeability material so as to alter the signal produced when the 
biased material is in the interrogation field. It is also disclosed in the 
'449 patent that such dual-status markers may comprise coextensive strips 
of magnetizable material and high permeability, low coercive force 
material, and while not preferred, that the magnetizable material could be 
uniformly magnetized. 
Similarly, one marker embodiment depicted in the '086 patent comprises two 
coextensive strips. While that patent indicates that magnetization of one 
strip alters the harmonic content of the signal produced by the other, the 
exact nature of the magnetization is not specified. The disclosure 
pertaining to FIG. 6D of the '086 patent suggests only that magnetization 
be such as to leave the responder strip in a fully magnetized condition, 
thereby causing the marker to be completely silent. 
The '449 and '086 patents thus suggest that single directionally responsive 
markers may be deactivated by a magnetic bias field extending the full 
length of the responder strip, but fail to enable that suggestion. Rather, 
by following the teaching in those and subsequent patents it has become 
well recognized that reliable deactivation is obtained by providing 
discontinuous fields so that the responder strip essentially responds as a 
number of strips of shorter length. This is effected in typical, 
commercially viable systems by providing a number of magnetizable pieces 
spaced along the responder strip or by providing a continuous strip of 
magnetizable material which is magnetized in bands of alternating 
polarity. 
More recently, multi-directionally responsive magnetic markers have also 
been developed. Thus, for example, as set forth in a prior patent of the 
present inventor, U.S. Pat. No. 4,710,754, such markers may comprise a 
square piece of low coercive force, high permeability material fabricated 
to have regions with narrow widths centered along each edge of the 
squares, thereby providing switching sections, and extensive regions in 
each corner which collect and channel flux into the switching sections. 
The markers of the '754 patent are made dual-status by adding discrete 
pieces of magnetizable material adjacent each switching section. 
A further embodiment of a dual-status, multi-dimensionally responsive 
marker is disclosed in U.S. Pat. No. 4,825,194 (Church et al.) in which 
discrete magnetizable pieces are positioned adjacent flux collector 
sections of a sheet of responder material. Optionally, that patent also 
suggests that additional pieces of magnetizable material may be positioned 
adjacent the switching sections, but that the separation between the 
respective magnetizable pieces be sufficient to prevent appreciable 
magnetic coupling therebetween. 
Multi-dimensionally responsive markers in which a coextensive sheet of 
magnetizable material is provided together with a sheet of low coercive 
force, high permeability responder material are disclosed in a second 
patent of the present inventor, U.S. Pat. No. 4,746,908. However, the 
markers of the '908 patent function in a significantly different manner 
and utilize a piece of responder material configured so as not to create a 
desired response. The coextensive sheet of magnetizable material is 
magnetized with a predetermined pattern which biases only adjacent 
portions of the responder material, thereby inhibiting response from those 
portions. The magnetized pattern is such that the dimensions of the 
unbiased, remaining portion can then produce the desired response. Such 
markers thus function oppositely to those in typical use, i.e., that the 
marker is magnetized when in its sensitive state. 
A third patent of the present inventor, U.S. Pat. No. 4,967,185, discloses 
that multi-dimensionally responsive markers somewhat similar to those 
preferred in the '754 patent may be reliably changed from a first, active 
state, to a second, deactive state, by applying a magnetic field to 
uniformly magnetize a coextensive magnetizable sheet in any direction in 
the plane of the sheet. The marker may be subsequently changed, or 
switched back to the active state by demagnetizing the magnetizable sheet. 
Such a marker thus may comprise two coextensive magnetic sheets in which 
the width of the sheets is not less than one-half the length. The first 
sheet may be selected of a material having a high permeability and low 
coercive force, which is configured to have at least two, mutually 
perpendicular elongated areas proximate to the periphery of the sheet. 
Each of the elongated areas is capable of responding to an alternating 
magnetic field in an interrogation zone generally applied along the length 
of the area to result in the production of an alarm. Each area thus 
includes a narrow width region forming a switching section and extends on 
each end along the length into extensive regions forming flux collector 
sections for the adjacent switching section. 
The second sheet is selected of a remanently magnetizable material, which 
overlies and is magnetically coupled to the sheet of responder material. 
This magnetizable sheet, when substantially uniformly magnetized in the 
plane of the sheet, causes alternate polarity switching pulses resulting 
from a reversal of magnetization of the switching sections when exposed to 
alternating fields, to be shifted in time and/or altered in amplitude. 
Markers having the magnetizable sheet alternatively magnetized or 
demagnetized can then be distinguished from each other. 
As noted above, the two states of the marker of the '185 patent are 
manifested by differences in the time at which alternate polarity pulses 
are produced and by differences in the amplitude of the respective pulses, 
depending upon whether or not the magnetizable sheet is magnetized. That 
patent invention thus also includes an EAS system for use with such 
markers. In addition to the markers themselves, the system thus comprises 
means, such as a drive oscillator, amplifier, and field coils, for 
generating within an interrogation zone an alternating magnetic field, 
means for receiving marker produced signals and ultimately producing an 
alarm signal when appropriate and means for magnetizing the magnetizable 
material in the markers. The magnetizing means preferably provides a 
single, substantially uniform magnetic dipole in the magnetizable sheet, 
one edge of the sheet having one magnetic polarity and an opposite edge 
having the opposite polarity. 
The receiving means receives signals resulting from flux changes in the 
marker produced when the marker is exposed to the alternating field in the 
zone. Means are also included for distinguishing between signals from the 
markers when the piece of magnetizable material is either magnetized to 
have a said single magnetic dipole or is demagnetized, and from other 
signals as may be caused by ambient effects, random ferromagnetic objects 
and the like. The distinguishing means further comprises means responsive 
to differences in the amplitude of marker produced signals and to relative 
displacements of alternate signal components for producing an alarm signal 
when appropriate. 
SUMMARY OF THE INVENTION 
While similar to the multi-dimensionally responsive marker of the '185 
patent, the marker of the present invention is primarily responsive in but 
one direction, but is significantly less expensive. The marker comprises a 
substantially rectangular sheet of high permeability, low coercive force 
ferromagnetic responder material having a predetermined width and length. 
This sheet has sections along its length at which the material is removed, 
the remaining material thus forming at least one area of narrow width 
which may function as a switching section. Reversal of the magnetic state 
in that section by an alternating field of an EAS system interrogation 
zone may thus create a characteristic response while the remaining 
lengthwise portions function to collect and channel flux into the 
switching section. 
The marker further includes a sheet of remanently magnetizable material 
having substantially the same overall dimensions as the sheet of responder 
material, overlying and magnetically coupled to the sheet of responder 
material. When the magnetizable sheet is substantially uniformly 
magnetized in its plane, the associated external field causes alternate 
switching pulses resulting from a reversal of magnetization of the 
switching section to be shifted in time and/or altered in amplitude, 
thereby changing the characteristic response and enabling markers having 
the magnetizable sheet magnetized or demagnetized to be distinguished from 
each other. 
In a preferred embodiment, opposing edges along which the material is 
removed are defined by a continuous narrow band in which the material is 
absent, the remaining portions of the sheet outside that band thus being 
substantially magnetically isolated from the rest of the sheet, but 
physically present. This enables the sheet to provide a substantially 
uniformly thick, homogeneous appearance to a complete marker. 
As noted above, the two states of the marker of the present invention are 
manifested by differences in the time at which alternate polarity pulses 
are produced and by differences in the amplitude of the respective pulses, 
depending upon whether or not the magnetizable sheet is magnetized. The 
present invention thus also includes an EAS system for use with the 
markers described above. In addition to the markers themselves, the system 
thus comprises apparatus such as a drive oscillator, amplifier, and field 
coils, for generating within an interrogation zone an alternating magnetic 
field, circuits for receiving marker produced signals and ultimately 
producing an alarm signal when appropriate and apparatus for changing the 
magnetization state of the magnetizable material in the markers. This 
latter apparatus preferably magnetizes the magnetizable material to 
provide a single, substantially uniform magnetic dipole in the 
magnetizable sheet, one edge of the sheet having one magnetic polarity and 
an opposite edge having the opposite polarity. 
The receiving circuits receive signals resulting from flux changes in the 
marker produced when the marker is exposed to the alternating field in the 
zone. Circuits are also included for distinguishing between signals from 
the markers when the piece of magnetizable material is either magnetized 
to have a said single magnetic dipole or is demagnetized, and from other 
signals as may be caused by ambient effects, random ferromagnetic objects 
and the like. The distinguishing circuits further respond to differences 
in the amplitude of marker produced signals and to relative displacements 
of alternate signal components for producing an alarm signal when 
appropriate.

DETAILED DESCRIPTION 
One embodiment of a marker of the present invention is set forth in FIGS. 
1A and 1B. As may there be seen, such a marker 10 comprises two sheets 12 
and 14 of magnetic material. The first sheet 12 is formed of a 
ferromagnetic material having high permeability and low coercive force 
properties, such as permalloy, supermalloy or the like. This sheet may 
also be any of a number of amorphous ferromagnetic compositions, such as 
an iron-nickel composition, Type 2628MB2 or a high cobalt containing 
composition, Type 2705M, both of which are manufactured by the 
Allied-Signal Corporation. The sheet 12 is configured in a rectangle 
having semicircular portions 16 and 18 removed from each lengthwise edge, 
thus leaving a centermost area 20 of restricted cross-section. This area 
thus forms a switching section in which magnetic flux will be concentrated 
by the extensive areas 21 and 22 at the respective ends of the rectangle. 
As shown in FIG. 1B, the second sheet 14 of the marker 10 is coextensive 
with the first sheet 12 and comprises a solid sheet of a magnetizable 
material, such as vicalloy, magnetic stainless steel, Chromendur II or the 
like. A preferred construction utilizes Arnokrome.TM., an iron, cobalt, 
chromium and vanadium alloy marketed by Arnold Engineering Co., Marengo, 
Ill., such as the Alloy "A" described in U.S. Pat. No. 4,120,704, which is 
assigned to that company. In a particularly desired configuration, a sheet 
of such material may be heat treated to provide a coercive force of 
approximately 80 Oersteds. Other alloys having coercive forces in the 
range of 40 to 200 Oersteds are likewise acceptable. To ensure the same 
response to both desensitizing (magnetizing) fields and to interrogating 
fields, regardless of the orientation of the marker with respect to those 
fields, it is also desirable that the sheets exhibit the same magnetic 
properties in all directions in the plane of the sheet. 
The two sheets 12 and 14 are then preferably joined together via a 
pressure-sensitive adhesive or the like and the combined layers in turn 
are sandwiched between an underlying layer of pressure-sensitive adhesive 
and release liner in order to allow the markers to be dispensed and fixed 
to articles to be protected. A suitable top layer may also be included, 
enabling customer indicia, price information etc. to be provided on the 
marker. 
In a preferred embodiment as shown in FIGS. 1A and 1B, the first sheet 12 
was made of a one inch (2.54 cm) long and one-third inch (0.85 cm) wide 
section of permalloy, 0.0006 inches (15.2 micrometers) thick. The sheet 
was further formed with the removed sections 16 and 18 having a radius of 
about 0.154 inches (0.39 cm), thus leaving the switching section 20 to be 
about as 0.025 inches (635 micrometers) wide. The second sheet 14 was a 
one inch (25.4 cm) by one-third inch (0.85 cm) section of Arnochrome.TM. 
alloy 0.0008 inches (20.3 micrometers) thick, treated to have a coercive 
force of about 80 Oe (6400 A/m), as described above. 
It has now been found that such a marker may be reliably switched from a 
first, active state into a second, deactivated state, by substantially 
uniformly magnetizing the magnetizable sheet in its plane so as to exhibit 
a first magnetic polarity along one edge of the sheet and an opposite 
polarity at the opposite edge of the sheet. By so magnetizing the 
magnetizable sheet, the switching element becomes biased so that alternate 
polarity switching pulses from the respective elements occur at different 
times than that occurring from an unbiased marker, and/or the respective 
switching pulses are significantly altered in amplitude. 
An unbiased switching element saturates or switches in an alternating 
magnetic field when the field reaches a given intensity, depending upon 
the coercive force of the switching element. Accordingly, if the time 
between a negative and positive pulse is substantially the same as the 
time between a positive and negative pulse when the marker is interrogated 
by a sinusoidal alternating field, the marker will be deemed to be 
sensitized. In contrast, if the magnetizable sheet is magnetized, the time 
between adjacent positive and negative pulses will be different than that 
between adjacent negative and positive pulses. The detection logic in a 
system may then be used to detect such time differences, and thus 
differentiate between an unbiased (sensitized) marker and a biased 
(desensitized) marker. As the amplitudes of harmonics generated by a 
marker when interrogated by an alternating magnetic field are also 
substantially altered, and for the most part, decreased by the presence of 
the bias due to the magnetized sheet, detection logic may also be utilized 
to respond to such differences in amplitude. 
It has also been found that when the sheet of magnetizable material is 
magnetized by an unidirectional field so as to exhibit a single magnetic 
dipole extending from one edge to the opposite edge of the sheet, that 
magnetization may be affected by the configuration of the adjacent high 
permeability, low coercive force sheet. By selecting the sheet of 
magnetizable material to have a relatively low coercive force, i.e., in 
the range of 60-90 Oersteds, the magnetizable material may be magnetically 
imprinted with the configuration of the sheet of responder material. Such 
a magnetization pattern can, for example, be seen by separating the sheet 
of responder material from the magnetizable sheet and thereupon viewing 
the magnetization pattern with a magnetic viewer. The magnetization 
pattern arises during the magnetization process because some of the flux 
coming out of the flux collector and switching sections enters the 
relatively low coercive force sheet of magnetizable material and thereby 
alters the magnetization therein. The collector and switching elements 
thus ultimately become more highly saturated and the state of 
desensitization of the marker is thereby enhanced. 
As shown in FIG. 2, in an alternative embodiment, the marker 24 may be 
formed of a sheet 25 of high permeability, low coercive force material in 
which the most of the removed portions 26 and 28 along length-wise edges 
are still present, but are separated from the remainder of the sheet by 
narrow bands 30 and 32 in which the magnetic material has been removed. 
The narrow band of removed material 30 and 32 thus magnetically isolates 
the portions 26 and 28 from the magnetically active switching section 34 
and flux collector sections 36 and 38 respectively. 
Another embodiment of the marker of the present invention is shown in FIGS. 
3A and 3B. As shown in FIG. 3A, the marker 50 is formed of a sheet 52 of 
high permeability, low coercive force responder material like that 
described in conjunction with FIGS. 1A and 2. In the embodiment of FIG. 
3A, magnetically inactive region 60 are provided by removing regions 54 
and 56, the remaining material thus being magnetically isolated from both 
the adjacent regions and from the remainder of the sheet, and unable to 
significantly affect the concentration of flux within the centermost 
region 62. By thus subdividing the remaining material in the inactive 
region 60, the propensity for flux directed toward the marker to pass 
through those regions is further lessened. The flux collecting 
capabilities of the end regions 66 and 68, and redirection of that flux 
into the switching section 64 is thereby maximized. 
A preferred manner in which the markers of the present invention may be 
manufactured is set forth in FIG. 4. It will there be recognized that a 
plurality of markers 70 extending in orthogonal directions from each other 
may be formed from large sheets of the respective materials, the sheet of 
responder material having been first processed to have a plurality of 
equally spaced-apart holes formed therein, the spacing between which 
defines the width of the switching sections of the resultant markers. 
After the respective sheets are laminated together, the respective markers 
may then be cut into strips as shown in FIG. 5, in a manner suitable for 
dispensing with conventional label guns and the like. In FIG. 4, the 
respective markers are shown spaced apart to clarify that the sheets are 
cut so as to generally bisect the holes. 
As shown in FIG. 5, a strip 100 contains a plurality of markers 102. The 
strips 100 of markers 102 include a sheet 104 of high permeability, low 
coercive force material in which the appropriate configuration has been 
formed, adhered via a layer of pressure sensitive adhesive (not shown) to 
a sheet of magnetizable material 106. An outermost layer 108 of paper or 
the like on which customer indicia may be printed may, in turn be adhered 
to the top of the magnetizable material 106. An underlying layer of 
pressure-sensitive adhesive between the bottom most layer 104 and release 
liner 110 may be provided in order to affix the markers to objects to be 
protected. Such an adhesive layer is nominally invisible. 
The benefit provided by the semi-circular holes 112 along the periphery of 
each of the markers may further be appreciated from FIG. 5, as it will 
there be noted that as the individual markers are cut from the larger 
sheets from which they are formed, any variance in the location of the 
separation lines will only affect the relative location of the switching 
sections. As the widths of the respective switching sections are precisely 
determined by the distance between adjacent holes, the exact location of 
the separation line becomes much less important. 
The configuration in the sheets of high permeability, low coercive force 
material may be provided in a number of ways, such as die cutting, etching 
or the like. When sheets of crystalline materials, such as permalloy or 
the like, are utilized, such materials being notoriously sensitive to 
mechanical working, it may be desired that the respective regions of 
removed material be formed via chemical etching techniques in a manner 
well known to those skilled in the art. Similarly, if sheets of material 
relatively immune to mechanical workings, such as amorphous alloys, are 
utilized, conventional die cutting techniques and the like may similarly 
be employed. 
A system in which the markers of the present invention are preferably 
utilized is set forth in the combined pictorial and block diagram of FIG. 
6. As is typical in magnetic electronic article surveillance systems, the 
system 120 comprises two spaced apart panels 122 and 124 between which 
persons carrying objects protected by the markers may be directed. Within 
the panels are positioned appropriate field coils 126 and detector coils 
128. In the present system, the field coil is powered by a suitable 
oscillator 130 coupled through a drive amplifier 132, producing a magnetic 
field oscillating at a predetermined frequency, such as approximately 10 
kilohertz, within the interrogation zone extending between the panels. The 
detector coil 128 is in turn coupled through a sense amplifier and filter 
134 and thence to a pair of level detectors 136 and 138, respectively, and 
to a phase sensitive detector 140. The common outputs of the respective 
detectors are in turn coupled to an alarm logic network 142, which is 
basically an exclusive AND gate, such that an appropriate signal from all 
three detectors must be present for the production of a signal to activate 
an alarm 144. Thus if a patron 146 carrying objects 148 having markers 
affixed thereto which are in a sensitized condition passes between the 
panels 122 and 124, the presence of the sensitized markers will be 
detected and an alarm produced by the alarm unit 144. 
Conversely, if prior to entering the interrogation zone, the markers are 
desensitized at a checkout counter 150, at which time the respective 
markers are placed within a desensitization apparatus 152 within which a 
substantially continuous magnetization state is impressed upon the 
magnetizable sheets within each of the markers, thereby rendering the 
marker desensitized, egress through the interrogation zone may be possible 
without generating an alarm. Such an apparatus may preferably comprise a 
permanent magnet having at a top, or working surface, a substantially 
uniform field of a single polarity. The magnetizable sheets of the markers 
are then magnetized by passing the marker across the working surface of 
the apparatus. 
Alternatively, if objects are desired to be returned to the protected area 
and removal thereafter again detected, the markers may be resensitized by 
passing them through a demagnetization apparatus 153. Such an apparatus 
may comprise a permanent magnet assembly configured to have a series of 
alternating polarity fields of decreasing intensities so that as a marker 
is moved thereover, any remanent magnetization state is gradually removed. 
The desirability of the detector circuits operating both in response to 
phases, so as to respond to the respective time between alternate polarity 
pulses and also to the respective amplitude of the signal pulses, will be 
further appreciated as it is recognized that as an object is presented for 
deactivation, the orientation of the marker with respect to the 
magnetizing fields in the desensitization apparatus 148 will generally be 
unknown and uncontrolled. Similarly, as an object is carried through the 
interrogation zone, the orientation of the marker with respect to the 
interrogating fields will generally be unknown and uncontrolled. Thus it 
is important that markers be unambiguously recognized as being deactivated 
regardless of whether the direction of the magnetic dipole impressed on 
the sheet of magnetizable material is aligned with the interrogating 
fields, is oriented at 90.degree. with respect to the interrogating 
fields, or is at any other random angle therebetween. 
Taking the two extremes, it will be recognized that if the magnetic dipole 
is in alignment with an interrogating field, the field associated with the 
dipole will alternately aid and oppose the interrogating field. In such a 
case, the time at which the requisite field at which the magnetization in 
the respective aligned switching elements will reverse will be shifted in 
time relative to the switching times when no biasing field is present. 
Such a shift in the spatial position of the signal pulses may then be 
detected by the phase sensitive detector 140. Conversely, if the field 
associated with the magnetic dipole is at right angles to the 
interrogating field, the overall amplitude of the switching pulses will 
generally be decreased. Such a condition may be recognized by the level 
detectors 136 and 138, which require signal pulses to exceed a minimum 
threshold and not to exceed a maximum threshold level in order to create 
the requisite alarm signal. 
In one set of experiments, the performance of a marker as shown in FIG. 2, 
was compared with the performance of a marker of comparable dimensions 
(1"by 1"), prepared according to the disclosure in U.S. Pat. No. 4,967,185 
and marketed by 3M Company as a Quadratag.TM. marker. The tested marker of 
the present invention thus had overall dimensions of one inch (2.54 cm) by 
one-third inch (0.85 cm) and was formed of a 0.0006 inch (15.2 
micrometers) thick sheet of permalloy laminated to a 0.0008 inch (20.3 
micrometers) thick sheet of Arnochrome.TM.. The permalloy sheet was formed 
to have semicircular sections along the length-wise edges separated from 
the remainder of the permalloy sheet by a narrow semicircular band. When 
tested in a representative EAS system, e.g., a Model 3300 EAS system 
marketed by 3M Company, the marker of the present invention was detected 
53% of the time when randomly oriented and variously located throughout 
the interrogation zone. When similarly tested, the Quadratag marker was 
detected 76% of the time. It was thus confirmed that the marker of the 
present invention exhibited reasonable detectability regardless of 
orientation. The marker was also found to be reliably deactivated when a 
desensitizer, formed of strips of Neodymium permanent magnets arranged in 
an X configuration along a band, with the magnets positioned at right 
angles with respect to each other, and at 45.degree. with respect to the 
band, was positioned so that the marker passed along the surface at right 
angles with respect to the band. 
In another set of tests, markers having the configurations shown in FIGS. 7 
and 8 were compared. The marker 160 of FIG. 7 was substantially like that 
of FIG. 1, and was formed of the same materials. It differed only in that 
material was removed from the length-wise edges of the permalloy sheet so 
as to leave an approximately one-third of an inch long (0.85 cm), 0.22 
inch (0.56 cm) wide center section 162. The single resultant dipole was 
thereby positioned as far as it could be from the length-wise edges of the 
marker. 
In contrast, the marker 164 of FIG. 8, while having the same overall 
dimensions as that of FIGS. 1 and 7, was shaped so that the permalloy 
sheet had two dipoles 166 and 168, each being in close proximity to a 
length-wise edge. 
These two markers were then desensitized by passing them through magnetic 
fields in the plane of the markers, but with the long axis of the markers 
being variously positioned at 0.degree. (parallel to the field), 30, 45, 
60, and 90 with respect to the field. When tested in apparatus simulating 
the field and detection parameters used in the Model 3300 EAS System 
acknowledged above, the data presented in the PG,19 following table were 
obtained. In such a system, marker produced signals will only result in 
the production of an alarm when the signal amplitude is in excess of a 
given level and at the same time, the time difference is less than a given 
amount. That is, a sensitized marker in which the magnetizable sheet is 
unmagnetized will produce high amplitude, harmonic related signals, and 
the relative time between adjacent positive-negative transitions will be 
substantially the same as that between adjacent negative-positive 
transitions. The combination of both signal characteristics will result in 
an alarm. Contrariwise, alarms will not result even though the signal 
amplitude is high (i.e., in excess of 2.0 volts, on an arbitrary scale) if 
at the same time a time difference is also high (i.e., in excess of 5.0 
microseconds). An alarm will also be prevented from occurring even though 
the time difference is less than a minimum amount (i.e., 0-4 
microseconds), if at the same time the signal amplitude is low (i.e., less 
than approx. 0.5 volts). 
TABLE I 
______________________________________ 
MEASUREMENTS OF DESENSITIZED MARKERS OF 
FIGS. 7 AND 8 
FIG. 7 Marker FIG. 8 Marker 
Signal Time Signal Time 
Marker Amplitude Difference 
Amplitude 
Difference 
Orientation 
(Volts) (u-sec) (Volts) (u-sec) 
______________________________________ 
Parallel to 
Field 
(0 Degrees) 
2.4 12.5 2.0 13.5 
30 Degrees 
0.8 0 0.3 0 
45 Degrees 
1.0 9.5 0.44 9.0 
60 Degrees 
0.88 8.0 0.3 6.0 
90 Degrees 
1.8 11.5 0.62 6.5 
______________________________________ 
FIG. 8, having two dipoles near opposite edges were most reliably 
desensitizable, as they exhibited the lowest amplitude signal and/or the 
largest time difference between positive-negative versus negative-positive 
pulses. As noted above, the magnetizable sheets utilized in the markers of 
the present invention are desirably formed of materials having a coercive 
force in the range between 40 and 200 Oersteds. Thus, for example, 
materials such as Arnokrome.TM. have been evaluated and found to be 
acceptable. Other materials having similar coercive forces may also be 
used. Materials having coercive forces in the range of 60-90 Oersteds are 
particularly desired. The non-uniform magnetization patterns resulting 
from flux shunting effects of the adjacent, configured piece of responder 
material are more pronounced. Also, lower intensity magnetizing fields may 
be employed, thereby lessening the danger of affecting magnetically 
sensitive objects such as prerecorded magnetic tapes and credit cards.