United States Patent 3665449

A non-contact method of and system for distinguishing the presence, identity or status of an object. The system comprises a marker including a ferro-magnetic material to accompany each object to be detected, means for producing an alternating magnetic field within a zone through which the objects are to pass, means for monitoring magnetic flux changes within the zone and a circuit for detecting a flux change within the zone which corresponds to a signal characteristically produced by magnetization reversal of the marker ferro-magnetic material. The marker includes a relatively long, relatively thin, open-strip of a ferro-magnetic material and may include a remanently magnetizable control element to provide a sensitized and desensitized marker for demagnetized and magnetized states, respectively, of the control element. A plurality of magnetic field producing means are employed to produce in the zone magnetic fields of different orientations, virtually assuring production of a characteristic signal when a marker is passed into the zone.

Elder, James T. (Shoreview, MN)
Wright, Donald A. (Woodbury, MN)
Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
324/243, 335/284, 340/572.3
International Classes:
G01N27/72; G08B13/24; (IPC1-7): G01R33/02; G08B13/24
Field of Search:
340/280,258,258C,149A 324
View Patent Images:

Foreign References:
Other References:

Luck, David G. C., Young, Charles J., "The Prison Gun Detector (Electromagnetic Devices Discloses Metal Generally)", Radio World, Dec. 1936, pp. 50-56,.
Primary Examiner:
Caldwell, John W.
Assistant Examiner:
Partridge, Scott F.
What is claimed is

1. A method for detecting the presence of an object within an interrogation zone, comprising

2. A method according to claim 1 wherein step (c) further comprises sequentially applying in said interrogation zone a plurality of alternating applied magnetic fields such that while said object is in said interrogation zone there is present in every direction at least one magnetic field component vector greater than a field sufficient to reverse the magnetization of said marker to thereby assure at least one magnetization reversal whenever said object is in said interrogation zone.

3. A method according to claim 1 wherein step (c) comprises applying a pulsed alternating applied magnetic field.

4. The method of claim 1 wherein said

5. The method of claim 4 wherein step (e) further comprises

6. The method of claim 1 wherein said applied magnetic field is a modulated sinusoidally varying magnetic field.

7. The method of claim 4 wherein step (c) further comprises applying in said interrogation zone at least a second alternating magnetic field of a frequency at least 5 Hz. different from said first applied alternating magnetic field, wherein

8. The method of claim 1 including the step of sensing for a potential carrier of said object within said interrogating zone and initiating step (c) only when a said carrier is sensed.

9. A method for detecting the presence of an object within an interrogation zone, comprising

10. A system for detecting the presence of an object within an interrogation zone, comprising

11. A system for detecting the presence of an object within an interrogation zone, comprising

12. A system according to claim 11 wherein the means of paragraph (c) further comprises means for sequentially applying in said interrogation zone a plurality of alternating applied magnetic fields such that while said object is in said interrogation zone there is present in every direction at least one magnetic field component vector greater than a field sufficient to reverse the magnetization of said marker to thereby assure at least one magnetization reversal whenever said object is in said interrogation zone.

13. A system according to claim 11, wherein the means of paragraph (c) comprises means for applying a pulsed alternating applied magnetic field.

14. A system according to claim 11 wherein the system further comprises

15. A system according to claim 11 wherein said alternating magnetic field applying means comprises a set of magnetic field producing means including

16. The system of claim 11 further comprising means for desensitizing said marker to render said marker incapable of producing said characteristic pulse when subjected to said applied field.

17. The system of claim 16 wherein said desensitizing means when in operation is out of contact with said marker.

18. The system of claim 16 further comprising means for altering said marker from a desensitized to a sensitized state to render said marker capable of producing said characteristic pulse when subjected to said applied field.

19. The system of claim 18 wherein each marker further comprises a control element of a ferro-magnetic material having a coercivity of at least 5 oersteds and capable of producing, when remanently magnetized, a static external magnetic field of at least 3/4 oersted over at least a portion of said open-strip to at least partially magnetize said portion wherein the disposition of said control element with respect to said open-strip is such that an electrical signal generated while the control element is demagnetized differs distinguishably from an electrical signal generated while the control element is magnetized and wherein

20. An automated checkout system employing the apparatus of claim 16 and further comprising

21. The system of claim 11 for detecting the presence, identity, or status of an object within an interrogation zone, wherein

22. The system according to claim 21 wherein the system comprises n groups of markers, each open-strip of all markers in a particular group having the same AC coercivity but the AC coercivity of each group being different; wherein

23. The system according to claim 21 wherein the system comprises n groups of markers, where n is an integral number greater than 1, each said marker in said group comprising m open-strips, wherein m is an integral number, each of the m open-strips of a said marker having different AC coercivities, all of said markers of a said group having the same combination of said open-strips but different groups having different combinations of open-strips, each marker of a said group producing a set of magnetic field pulses characteristic of said group when said each marker is passed into said applied field,


This invention relates in general to systems and materials for detection of an object or of the status of an object by use of alternating magnetic fields. More particularly, the invention relates to a system in which the object to be detected need not come in direct contact with the sensing apparatus. The object is provided with a specifically chosen piece of magnetic material, and an alternating magnetic field is provided in an area through which the object is to pass. Upon passage of the object through the area, the characteristic magnetic response of the specifically chosen piece of magnetic material is sensed to distinguish the presence of the object and either or both the identity or status of the object.


Theft of books from libraries has become a serious problem both in the form of expense to the taxpayer for replacing stolen books and in terms of impairment of service rendered by the libraries. The annual loss of books from all libraries in the U. S. exceeds 20 million dollars and is increasing.

Systems for preventing such theft, in which instruments sensing evidence of theft actuate alarms, have been known since at least 1934. These systems generally comprise a "marker" element secured to each object to be detected and instruments for sensing signals produced by the markers. Obviously, in light of the foregoing statistics, such systems have been ineffectual.

A particularly serious problem of such theft detection systems is false alarms. After one or more false alarms, those using the system tend to ignore all alarms rather than risk being personally embarrassed or subjecting their establishment to a lawsuit. In addition to this kind of unreliability, such systems are readily compromised either by deliberately producing false alarm or "masking" signals or by shielding the marker to prevent it from producing a signal.


French Pat. No. 763,681, issued to Pierre Arthur Picard, discloses a remote detection system which employs dynamic magnetic phenomena to detect the presence of an object, e.g. a library book being carried through a doorway. The system of Picard is based upon his discovery that when a piece of metal is subjected to a sinusoidally varied magnetic field, an induced voltage which is characteristic of the metal composition is produced in a pair of balanced coils in the vicinity of the applied field. Analysis of this characteristic voltage thus permits classification of a metal present in the applied field. Hence, detection of a book to which a piece of metal of a special class has been attached is possible.

Picard gives as one class of metals those which require a high applied field in order to magnetically saturate. He shows that the induced voltage from each such metal contains in addition to a fundamental component a certain number of harmonic components. One such metal, iron, is described as producing a voltage containing a third harmonic and a little bit of the fifth harmonic.

Picard also teaches that high permeability metals, i.e. those which will saturate in a weak applied field, produce an induced voltage including higher order harmonics than the harmonics of metals such as iron. He gives permalloy as one high permeability material and points out that the permalloy characteristic voltage contains ninth and eleventh harmonic components, unlike such common metals as copper, iron, or aluminum, which produce practically no harmonics of such a high order.

According to Picard, only the composition of a metal determines the order of the harmonics present in its characteristic voltage. He teaches that marker size and geometry affect the component amplitudes proportionately. Accordingly, the ratio of two individual harmonic components for a particular material would be the same regardless of the material's size or geometry. Further, Picard teaches that the ratio between at least certain selected components is characteristically different for different materials. Picard does, however, emphasize that size of the metal piece to be used as a marker is important; not to control the order of the harmonics present, but rather to provide a signal large enough to be detected.

Picard thus teaches that high permeability metals, and specifically permalloy, can be distinguished from other classes of magnetizable metal by both the presence of harmonics on the order of the ninth and eleventh and by the ratios of some of its characteristic harmonic components.

Picard also felt that permalloy was a good material for use as the special marker to be placed in library books because normally one would rarely, if ever, carry such a material.


The present invention provides a non-contact method of detecting the presence, identity, or status of an object within an interrogating zone such as an exit from a library. The method comprises securing to each object to be detected a marker comprising at least one open-strip of a ferro-magnetic material with an aggregate saturation magnetic moment of at least 0.1 electromagnetic units or "pole-centimeters". The open-strip is selected such that its magnetization, when the strip is within and has a major dimension oriented parallel to a 60 Hz. sinusoidally varying magnetic test field of a predetermined peak magnitude of less than 20 oersteds, reverses for each alternation of the test field.

In the strictest sense, a complete magnetization reversal from one saturated condition to a saturated condition of opposite polarity is not required. By reversal we mean any cyclic magnetization change of at least 0.2 electromagnetic units per cubic centimeter in response to an applied field alternation.

An alternating magnetic field is applied within the interrogating zone. This applied field is at least equal to said predetermined peak magnitude and is substantially free of frequency components exceeding a predetermined frequency of at least 1,000 Hz. When an object having an open-strip is passed into the applied magnetic field, and a major dimension of the open-strip and a vector component of the magnetic field become oriented with each other, the magnetization of the open-strip reverses at each alternation of the applied field. Each magnetization reversal produces a pulse of external polar magnetic field. Means are provided for monitoring in the vicinity of the interrogating zone an entire band of frequency components of magnetic flux. The lower cutoff frequency of this band is greater than the predetermined frequency and the band has a width of at least 600 Hz. Means are provided for detecting a signal corresponding to a said polar magnetic field pulse to verify the presence, identity, or status of the object by sensing the amplitude and time characteristics of at least one of the signals.

By an open-strip we mean one which when magnetized has separate poles, i.e. a strip which is not closed or wound upon itself. For a particular marker, the predetermined peak magnitude of the test field is the minimum field capable of reversing the marker's magnetization, i.e. the marker's "switching field." By oriented, we mean, for an applied field of a particular magnitude, the angular relationships between the applied field and marker major dimension are such that the applied field's vector component parallel to the marker major dimension is at least equal to the marker's switching field.

As used herein, "ferro-magnetic" includes both conductive and non-conductive materials. Materials of the former include iron and its alloys with nickel; the latter class of materials includes ferrites. Conductive materials are generally preferred; they are capable of producing external magnetic fields on the order of 10 times greater than those produced by the same amount of non-conductive materials. Accordingly, a marker of conductive material may be smaller than an equivalent non-conductive marker. Such small size has dual advantages; material cost and, perhaps more importantly for anti-pilferage markers, concealability.

We have found that an open-strip of a ferro-magnetic material, if made sufficiently thin relative to its major dimension, will produce a characteristic pulse of an external polar magnetic field when its magnetization is reversed by an alternating applied field having a maximum time rate of change in excess of 300 oersteds per second. Unlike the marker described by Picard, the open-strip marker we have described is not reliably distinguished by a single frequency or a ratio of two frequencies. We have found that the amplitude of a particular harmonic depends upon the amplitude and frequency of the applied field and upon the orientation of the marker with respect to the applied field. The composite of an appropriately selected band of frequencies from our marker, however, provides a signal which has distinguishing features which are essentially independent of the orientation of the marker relative to the field and of both the amplitude and frequency of the applied field. Indeed, our marker is frequency independent to the extent that it produces its characteristic signal in response to other than a sinusoidal applied field, e.g. a pulsed field.

When an open-strip of the present invention is subjected to a sinusoidal applied field, it produces an external polar pulse distinguishable by its gross flux components in a band whose lower bound (the predetermined frequency) exceeds 1,000 Hz. and whose width is at least 10 times the applied field frequency to insure a statistically representative group of frequencies. A signal corresponding to the gross time rate of change of the flux components within the band (a "magnetization reversal signal"), such as the voltage induced in a coil linked by the flux, is very narrow, less than 0.1 milliseconds at half amplitude.

A further distinguishing characteristic of the open-strip signal is that, for a particular composition, size and shape of material, the peak amplitude of the signal will occur for an applied alternating field of a particular waveform a predictable time after each applied field alternation. This signal characteristic can also be defined in terms of the absolute instantaneous value of the applied field at the instant when the marker's net magnetization is zero. Hypothetically, it is at this instant that magnetization "reversal" occurs and we believe it corresponds to the peak point of the magnetization reversal signal. We shall hereafter call this instantaneous applied field value the "AC coercivity" of the material, although it should be kept in mind that AC coercivity depends not only on material magnetic properties, but also on the waveform of the applied field. It is a convenient term for comparing responses of different markers subjected to the same applied field.

The open-strip of the present invention may take the form of a thin, flat ferro-magnetic ribbon or wire having a magnetic moment of at least 0.1 electromagnetic unit. The ratio of the major dimension, i.e. the length, to the square root of the cross-sectional area of the ribbon or wire should be at least 150. At ratios below this, internal self-demagnetizing field effects in highly magnetic materials may increase the switching field beyond 20 oersteds. Also, for ratios below 150, the magnetization reversal signal amplitude decreases radically and becomes noticeably dependent upon orientation of the open-strip within the applied field. The open-strip may have one or more major dimensions satisfying this criterion.

The corresponding criterion for a thin, flat disc of a ferro-magnetic material would be a ratio of its major dimension to thickness of at least 6,000. Conductive ribbon or disc markers should have a thickness of about 0.1 to 130 microns and conductive wire markers should have a diameter of 10 to 300 microns. For dimensions greater than these, the amplitude of the magnetization reversal signal decreases and the width at half-amplitude increases to become eventually indistinguishable from reversal signals of many common ferro-magnetic metals likely to be carried by a person.

A thin, flat, narrow marker is particularly amenable for use with library books as it may easily be concealed either by insertion into the book binder or between two of the book pages. Commonly, a book includes two pairs of flyleafs having a seam joining them along their entire length. Such seams are normally wider than an open-strip and thus a strip could easily be concealed in the seam. Or, by providing the marker with an adhesive coating on each face and a carrier web, a marker may be conveniently inserted near the binder between any two pages.

The preferred number and relative orientations of marker major dimensions depend upon the applied field characteristics in a manner which will be explained later. For reference purposes, we shall define an idealized marker having a single major dimension as a "one-dimensional" marker, a marker having two major dimensions perpendicular to each other, e.g. an "L", "T", or "plus" shaped marker, as a "two-dimensional" marker and a marker having three mutually perpendicular major dimensions as a "three-dimensional" marker.

The open-strip may be wholly inorganic or may comprise ferro-magnetic laminae held together with an organic adhesive; or, it may be a dispersion of ferro-magnetic particles in an organic binder such as vinyl chloride. It may even be closely spaced but physically separate ferro-magnetic strips held in fixed geometric relation to each other on or within a nonmagnetic substrate (such as very fine wire filaments or ribbons within a piece of paper).

Still another version of a marker is two or more open-strips of different AC coercivities employed as an integral unit. Such integrally joined open-strips, even if in physical contact with each other, do not magnetically influence each other enough to prevent each from providing its own characteristic pulse. Open strips may be selected to sequentially reverse their magnetization following an applied field and "n" is the number of open-strips alternation at equal intervals of time, each interval being less than or equal to T/4n, where "T" is equal to the period of the applied field. A sinusoidally varying field is only increasing during the first and third quadrants, thus such a series of pulses will occur in those quadrants, i.e., one-fourth of the total period T, at a point where the applied field has risen to a value sufficient to exceed the AC coercivity of the open-strip material and cause the magnetization to reverse. This series, or burst of pulses, actually becomes a short time signal having a characteristic frequency of occurrence whose period is the interval T/4n, and which can be unambiguously detected by unsophisticated apparatus.

Yet another suitable marker of the present invention would be the combination of one or more of the foregoing open-strips with at least one "control" element. The objective of a control element is to permit selective setting of a marker to either a sensitized or a desensitized state. By sensitized we mean a state in which the marker will produce a characteristic signal in response to an applied field. Conversely, by desensitized we mean a state in which the marker does not produce this characteristic signal in response to an applied field; instead, the marker will either produce a different distinguishable signal or fail entirely to produce a sensible signal.

We have found that a convenient way to prevent or alter magnetization reversal of a marker, i.e. to desensitize a marker, is to effectively bias the applied field at the open-strip by providing as the marker control element a remanently magnetizable material. When remanently magnetized, the remanent field of the control element alternately aids and opposes the applied field on successive half cycles. By providing a sufficiently large remanent field adjacent an open-strip, the net field to which the open-strip is subjected during each half cycle when the remanent and applied fields oppose each other is insufficient to reverse the open-strip magnetization in the characteristic manner. It is not necessary to completely prevent reversal of the open-strip magnetization for each applied field alternation. It is sufficient that reversal be so altered that the resulting signal is uncharacteristic of a marker. Accordingly, such a magnetic control element need not completely cover an open-strip surface. For example, a magnetized control element adjacent only a central portion of an open-strip causes the segments on either side to behave approximately as two independent open-strips. Thus, the central portion should be large enough and positioned such that neither of the segments satisfies the aforementioned ratio criteria.

The remanent magnetization of the control element need not be uniform; in fact, non-uniformly magnetized control elements are generally desired because they are less costly to use.

An example of a non-uniformly magnetized control element would be one magnetized to have a series of bands of remanent magnetization, adjacent bands being oppositely polarized. Preferably, to minimize internal demagnetization effects and to provide the greatest external magnetic field, the respective directions of magnetization of the bands of such an alternately magnetized control element should be parallel to the control element length.

Conventional ways of magnetizing a control element are acceptable for desensitizing the marker. For example, to uniformly magnetize a control element, it could be exposed to the field of a large permanent magnet. Or, to provide a "band" type non-uniform magnetized control, the element could be exposed to a series of permanent magnets wherein adjacent magnets in the series were oppositely polarized. Some care is required in removing the magnetizing magnets. Movement of the magnetizing magnets along the direction of an axis parallel to the magnet polarizations would alter or skew said magnet polarizations from that intended. Such skew might reduce the magnetic influence of the control element to an amount less than that required to control or desensitize an open-strip. The use of a single pulsed magnetic field, whose geometrical field distribution resembles that produced by permanent magnets, for magnetizing markers would avoid such skew difficulties since it does not require controlled relative movement between the marker and source of magnetization while the field is applied. Means for providing such a field are well known. An example is "overdamped" discharge of a capacitor through a coil.

To sensitize markers, conventional demagnetizing apparatus based on the well-known principle of applying a relatively high frequency and diminishing amplitude magnetic field may be employed. For example, an apparatus for providing an "underdamped" discharge of a capacitor through a coil (i.e. a coil-capacitor combination similar to that which may be employed for magnetizing a control element but having a high "Q"). Alternatively, a demagnetizing apparatus comprising a series of permanent magnets in which adjacent magnets are oppositely polarized may be employed. When a control element and such a demagnetizing apparatus are moved relative to each other along a coordinate common to the series of magnets, the control element is effectively subjected to an alternating magnetic field. Such a demagnetizing apparatus can also be made to provide a field of diminishing amplitude through proper selection and arrangement of the magnets. By selecting the magnets to be of different strengths and by arranging them in an order ranging from highest to lowest (relative to the direction of travel) the magnetic field will appear to diminish in amplitude when passed over a control element. Magnets of the same field strength arranged like inverted ascending steps or like an inclined plane so that the amplitude of the field is progressively diminished would also produce the same result.

Because the external magnetic field of a control element may vary depending on the pattern of magnetization, it is not ordinarily necessary to demagnetize the control element in the strictest sense; rather, the magnetic influence of the control element need only be reduced to an extent permitting magnetization reversal of the open-strip by the applied field.

The control element should have a coercivity of at least 5 oersteds and be capable of producing when remanently magnetized a static external magnetic field of at least three-fourths oersted over at least a portion of an adjacent open-strip. For convenience of manufacture, the control element may be a thin, magnetic, uniform coating of gamma-ferric-oxide powder in a vinyl chloride binder on the surface of the open-strip.

Alternative marker "desensitization" techniques include deformation or rupture of the open-strip such that its resulting longest linear section is less than that required to satisfy the aforementioned length-to-square-root of cross-section ratio. Or, a marker may be desensitized by stressing the open-strip to change its magnetic response. For example, an open-strip may be employed in conjunction with, or as one element of, a thermosensitive bi-metallic strip.

Hereafter, a marker including one or more open-strips alone shall be referred to as a "single-status" marker and a marker including the combination of one or more control elements with one or more open-strips shall be referred to as a "multi-status" marker.

The general requirement of the alternating applied field is that when a marker passes through the interrogation zone, the marker becomes oriented with the applied field at at least one, and preferably several, points in the zone to reverse the marker magnetization. Oriented was previously defined as the condition when the applied field vector component parallel to an open-strip major dimension was equal to or greater than the open-strip switching field. One combination of applied field and marker which would absolutely insure orientation would be the combination of a "one-dimensional" applied field and a "three-dimensional" marker wherein the strength of the applied field at every point in the zone was at least √2 times the marker switching field. By a one-dimensional field, it is meant one in which all magnetic lines of force in the zone are parallel.

In a one-dimensional field, there exist two mutually perpendicular directions, which are also perpendicular to the field direction, along which there are virtually no components of the applied magnetic field. Similarly, a "two-dimensional" field is one in which there exists only one direction along which there are virtually no components of the applied magnetic field; and, a three-dimensional field is one in which there is no direction devoid of applied magnetic field components. It can thus be seen that the combination of a one-dimensional marker and a three-dimensional field, a two-dimensional marker and a two-dimensional field, and a three-dimensional marker and one-dimensional field, would absolutely guarantee "orientation" at each point in the zone.

With such combinations, the length of the path traversed by a marker passing through the zone could be very short, only slightly longer than the length of an open-strip. By increasing the path length, such an ideal combination which would assure orientation of the marker at every point in the zone is not required. To virtually assure at least one magnetization reversal whenever an open-strip passes through a zone having a relatively long path length, it is only necessary that magnetic field component vectors greater than the marker switching field along every direction of the unit sphere be present at many points in the zone.

This condition may be satisfied by producing sequentially, in time, at each point in the zone three one-dimensional fields, each of the fields being oriented along a different coordinate axis of the unit sphere. Alternatively, the condition may be satisfied by providing along the path through the zone a plurality of regions in which the applied field orientation does not vary, the fields of successive regions, however, being oriented differently.

In addition to the length of the zone through which the open-strip is to pass, other significant interdependent variables for designing a particular system which will insure at least one magnetization reversal of an open-strip passing through the zone include: the open-strip velocity, the number and orientation of the open-strip major dimensions, the applied field alternation rate, the peak magnitude of the applied field, and the applied field vector components at each point in the zone at each instant in time. Such an alternating field may also be in the form of a damped oscillating pulse or a modulated sinusoidally varying field. One such embodiment is to provide a second alternating magnetic field having a frequency at least 5 Hz. different from the first applied alternating field.

The alternating magnetic field may be produced by conventional methods, such as by application of an alternating current to an air core loop or to a coil of an electromagnet or by moving a permanent magnet such that the permanent magnet's field is made to effectively alternate throughout the interrogation zone.

General requirements of the flux monitoring system and detecting circuit are that it monitor magnetic flux changes within the interrogating zone and discriminate between magnetic flux changes produced by a marker and all extraneous magnetic flux changes. Extraneous flux changes include the applied field, noise produced by electric motors, circuit breaker noise, etc., whose effect upon the sensor is dependent to some extent upon their strength and distance from the interrogating zone. Examples of magnetic flux monitoring means include types which indicate the rate of change of the field directly, as a coil, and types which indicate the instantaneous magnitude of the field from which the field rate of change can be derived. The latter includes magnetoresistive devices, Hall devices, and magneto-diode sensors. Both types may be used with flux gathering devices to improve their sensitivity. The coil type include at least one coil for inductively sensing magnetic flux. One large coil may be used with only a few turns, even as few as a single turn, or several small coils, each having relatively more turns, may be used. Small coils have the advantage of being less sensitive to magnetic noise such as that produced by electric motors and circuit breakers. On the other hand, one large coil responds more uniformly to a marker at different positions in the zone than do the individual small coils. Any number of coils may be arranged in opposition so that magnetic noise from a distant source can be minimized, while detecting a marker closer to one coil than the other. Further, when a plurality of coils are employed, they may be provided with individual detecting systems or may sequentially share a common detecting system.

The signal detecting circuit associated with the flux monitoring means, in its simplest form, will indicate the presence of a marker in the interrogating zone by sensing the time and amplitude characteristics of a signal corresponding to a single magnetization reversal of a marker open-strip. As previously stated, an open-strip produces an uncommonly narrow signal and a signal which occurs, for a particular applied field and a marker of a particular AC coercivity, at a predictable time following an alternation of the applied field. By sensing signals which occur within a prescribed interval and are of some minimum amplitude and which both begin and end during a minimal time interval, the detecting circuit distinguishes between noise, marker signals and extraneous signals from other objects which respond to an alternating magnetic field.

The predictability of the time occurrence of marker characteristic signals also permits building redundancy into a sensing circuit to provide a safety factor against false alarms. The reference for measuring successive signals may be a preceding signal or some phase or time of the applied field, conveniently the alternation points of the applied field. Also, a marker may be permanently secured within the zone to provide a reference signal. Accordingly, both the time intervals between signals as well as individual signal characteristics provide a reliable indication of the detection of a marker. In combination, the two provide a virtually false-alarm-free system.

Implicit in an anti-pilferage system is initiation of some prescribed action upon detecting or sensing of an apparently stolen object. The particular action initiated is incidental to the operation of the present invention but may include production of electromagnetic or sonic waves such as light, ultrasonic transmissions or radio transmissions, either or both immediate to or remote from the interrogation zone. Action may consist of making a video or photographic record of the persons present in the interrogation zone at the time of sensing an apparently stolen object, or disablement of the automatic door opening mechanism of one or more exits. To complicate intentional compromise of the system, visible or audible indications may be slightly delayed from the instant of detection.

The sensing means may take the form of a circuit which responds to at least one signal which is both of at least a minimum amplitude and less than a maximum width. Additionally, the circuit may check the time occurrence of the detected signal against the time characteristic of either the applied magnetic field signal or a reference signal correlated with the applied magnetic field signal. The same general sensing circuit may be employed for both a single-status and a multi-status marker. The amplitude and width of the signals produced by each open-strip when an associated control element is demagnetized are substantially the same as those produced when the strips are employed as a single-status marker. However, when the control element is magnetized, the shape, amplitude and time occurrence of the signals are changed. The sensing circuit, in detecting such changes, can thus in essence sense the magnetization of the control element.

As applied to protection of the books in a library, it is readily apparent that by employing a multi-status marker, books may be desensitized during checkout to permit removal of the book from the library. Conversely, upon return of the book, it may be conveniently sensitized to prevent undetected removal of the book from the library, until it is again properly checked out.

With such sensitization and desensitization, the exact location of a marker need not be known. Accordingly, clever concealment of one or more markers on an object renders a system of the present invention virtually invulnerable to compromise as by shielding or removal of the marker.


A preferred embodiment of a multi-status marker particularly amenable for use in protecting the stock of a library comprises an open-strip consisting of an annealed permalloy ribbon of composition 4 percent molybdenum, 79 percent nickel and 17 percent iron about 25 microns thick, 18 centimeters long and 0.6 centimeter wide. A suitable control element is formed of a gamma-ferric-oxide strip of the same width and length as the open-strip. Such a control element may be produced by first dispersing 100 parts by weight of a recording-tape grade gamma-ferric-oxide pigment and 2 parts by weight of a wetting agent such as Ross and Rowe Yelkin TTS with a solvent such as toluene to produce a mixture of 25 percent solids. To this is added 50 parts by weight of a resin composition, e.g. 75 percent of a copolymer of 89 parts vinyl chloride and 11 parts vinyl acetate (VYHH) and 25 percent dioctyl phthalate. A small amount of a mixture of equal parts of methyl ethyl ketone and toluene may be also added as required to make a coatable solution. The solution is knife-coated on a silicone coated release sheet. After the solution has dried, the release sheet is peeled away and strips of the dried solution of a uniform 230 micron thickness are selected for use as control elements. For the previously described open-strip, thicknesses of more than 230 microns have been found to be sufficient to desensitize the open-strip when the control element is magnetized by a 1-inch gap magnetron magnet. These elements are then laminated to an open-strip. For more efficient production of large quantities, the mixture may be coated on a wide sheet of the open-strip material which is then subsequently slit into 0.6 centimeter wide strips.

These and other characteristics and advantages of the invention will become more apparent when considered in light of the following description of preferred embodiments taken in light of the accompanying drawings wherein:

FIG. 1 is a combinational block diagram and schematic wiring diagram of an anti-pilferage system such as may conveniently be used at the exits of a building in which the objects to be protected are kept;

FIG. 2, View A, is a front elevational view of a preferred embodiment of the applied field producing means of the system of FIG. 1; and FIG. 2, View B, is a schematic illustration of the windings of a portion of the applied field producing means of View A;

FIGS. 3 and 4 show representations of magnetic fields produced by the applied field producing means of FIG. 2;

FIG. 5 in three Views, is a superimposition of two sets of magnetic field lines of the applied field producing means of FIG. 2;

FIG. 6 is a schematic diagram of an embodiment of the field sequencing circuit of FIG. 1;

FIG. 7 on two sheets labeled 7A and 7B is a combinational block diagram and schematic wiring diagram of preferred embodiment of the magnetic flux monitoring means and signal detecting circuit of the system of FIG. 1.

In FIG. 1, three zone units, 10A, 10B and 10C, are shown positioned to form a pair of exit ways, the spaces between opposing units 10A and 10B and between 10B and 10C each thus forming an "interrogation zone". For the exemplary embodiment shown, zone units 10A and 10C comprise field producing and flux monitoring means whereas unit 10B includes only field producing means.

Unit 10A is partially cut away, revealing a pair of electromagnets, 12A and 12B, an air core loop 14, and four smaller coils, 16A, 16B, 16C and 16D. The electromagnets and air core loop form an applied field producing means and each have a terminal (for reference purposes, an "input" terminal) coupled to a "hot" lead 18 of a filtered alternating current source and another "output" terminal respectively coupled to a field sequencing circuit 20 by leads 22, 24 and 26. Units 10A and 10C are identical, but unit 10B differs in that it does not contain the four smaller coils. These smaller coils, each formed of 900 turns of enameled wire 0.01 centimeter in diameter wound in a bundle around a form 10 centimeters in diameter, form a magnetic flux monitoring means, designated as 29. They are connected in series with RG 58 A/U coaxial cable, placed in separated locations on one side of the interrogating zone, and very carefully oriented to balance out as much as possible of the magnetic noise produced by the field producing means and other sources. In the embodiment shown, the upper coils 16A and 16B are shown oriented vertically and the lower coils 16C and 16D are shown horizontally oriented. Each coil is wrapped with one layer of 12.7-micron-thick aluminum foil (not shown) to shield it from electrostatic noise while permitting magnetic signals to pass. These foil shields are connected to the shield of the inter-connecting coaxial cable.

Coupled to the magnetic flux monitoring means of unit 10A by coaxial cable 28 is a signal detector circuit 32A; an identical signal detector 32B is coupled by coaxial cable 30 to the magnetic flux monitoring means of unit 10C. When a book carrying a sensitized marker passes into an applied field interrogating zone, the marker magnetization reverses at each applied field alternation to produce a pulse of external polar magnetic field. The flux monitoring means of that zone responds to this change in magnetic flux within the zone and provides a signal corresponding to the pulse to its associated detector circuit 32. The detector circuit responds to the unique amplitude and time characteristics of the marker signal and provides a signal for activating its alarm and indicator circuit 34.

FIG. 2, View A, is a front elevational view of a preferred embodiment of a zone unit 10A. The electro-magnets 12A and 12B of the applied field producing means are each formed by solenoids surrounding a 5.1-centimeter square bar. The bars are laminates formed of 0.0457-centimeter thick by 142-centimeter long by 5.1-centimeter wide sheets of transformer steel, type M-19. Each solenoid comprises a pair of 125-turn windings of 0.205-centimeter diameter enameled wire distributed uniformly along the length of the bar and an additional pair of 60-turn windings of enameled wire 0.259 centimeter in diameter at each end. Both additional 60-turn windings are uniformly wound in a bifilar-aiding fashion with the end-most 60 turns of the corresponding 125-turn windings.

The 60-turn windings 36A and 36B are wound in series with each other and in series with the parallel combination of 125-turn windings 38A and 38B as shown in FIG. 2, View B. This combination of windings is referred to herein as the coil "winding", and its "input" terminal is shown as 35 and its "output" terminal is shown as 37. We have found that the poles of the polar magnets are located approximately 7.5 centimeters in from the bar ends.

A pair of applied field producing means separated by about one meter and of the foregoing dimensions is suitable for providing an interrogation zone of about one meter by two meters by two meters.

An air core loop 14 of the preferred embodiment is simply a multiturn closed loop. For the preferred embodiment shown, loop 14 is circular and has a diameter equal to the pole separation of electromagnets 12, and is made of 80 turns of enameled wire 0.205 centimeter in diameter. If the electromagnets were not of the same length, the loop geometry would remain curvilinear but would be adjusted so that the loop periphery still passed adjacent the poles of both polar magnets for reasons which will become apparent following a discussion of the relationships between the electromagnet and air core loop magnetic fields.

FIGS. 3 and 4 illustrate free space characteristic magnetic fields of an electromagnet and an air core loop respectively, such as those of FIG. 2. (The figures illustrate the magnetic field lines lying in the plane of the paper.) As shown, the field of the vertically oriented electromagnet includes components which are vertical (parallel to the y axis) and also includes significant horizontal x components in the regions near its poles. Similarly, the magnetic field components of the air core loop include components parallel to the x axis and components parallel to the y axis.

By passing the air core loop adjacent the electro-magnet poles, the two forms of field producing means complement each other. This will become more apparent following a discussion of the views of FIG. 5 wherein a superimposition of two free space magnetic field lines from each of means 12A, 12B and 14 of one zone unit are shown. For the sake of clarity, the lines representing the field produced by means 12A are shown uniformly dashed; the lines representing the field produced by means 12B are shown in alternately long and short dashes; and the lines representing the field produced by means 14 are shown in solid lines. As shown, one set of lines intersects at point M; the other at point N. A vector representation of the angular relationship of the magnetic field strengths is shown at Views B and C of FIG. 5 for points M and N respectively. Components r, s, t of View B represent the magnetic field strength of the fields at point M respectively produced by means 12B, 14, and 12A; components r', s', t' of View C are corresponding magnetic field strength vectors of the fields at point N. As shown, the components of each set are almost mutually perpendicular even though the r, s, t component directions are different from the r', s', t' directions. Inspection of FIG. 5 makes it readily apparent that the vector components of intersections of other sets of lines will be nearly perpendicular, too. Accordingly, by proper selection of individual magnetic field strengths, means 12A, 12B and 14 would produce a nearly ideal field.

By producing fields of not less than √2 times the marker open-strip switching field, such fields, even in the interrogating zone extremities adjacent the ends of the polar magnets where the respective directions of the fields change rapidly, are nearly "ideal" or "three-dimensional". Of course, an absolutely uniform three-dimensional field is not required because it is virtually impossible that a marker could pass through an entire zone without the marker becoming oriented at least once with a component of the applied field which is greater than the marker switching field.

The fields of FIGS. 3, 4 and 5 are those which would be produced absent any external magnetic influence. Thus, for the embodiment of FIG. 1, wherein three zone units are shown, the middle zone unit cooperating with each of the other two to form a pair of interrogating zones, it would be necessary to consecutively energize each of the nine individual field producing means in order to produce fields as shown. Because of the relatively long path length, for reasons previously set forth, sequential production of the nine fields is unnecessary. Indeed, it may be undesirable, for the peak amplitude of currents driven through the windings of the electromagnets and air core loops, and hence the associated circuitry costs, can be reduced by simultaneously producing one field of each zone unit.

FIG. 6 is a schematic diagram of an embodiment of the field sequencing circuit 20 of FIG. 1. The particular circuit shown in FIG. 6 simultaneously turns on like field producing means of each of the three zone units, and sequentially turns on each of the three separate means. The basic sequencing cycle thus consists of three phases, each phase lasting 8 complete cycles of the AC line voltage.

With reference to FIG. 6, the field sequencing circuit is shown to comprise, each shown generally, a power line filter 39, a phase selector 40, phase timer 42 and a stepdown transformer 44. Power line filter 39 is included in the field sequencing circuit to minimize noise on the power line and is shown to comprise an inductor 41 in series with the hot line of a 117-volt power source 43 and a capacitor bank 45 coupled between the hot lead 18 and common line 47. Inductor 41 is comprised of 130 turns of enameled wire 0.259 centimeter in diameter wound in a bundle about 4 centimeters wide around to form 18 centimeters in diameter. Preferably such a coil would be cast in a potting compound such as an epoxy resin after winding. Capacitor bank 45 is comprised of a number of AC capacitors in parallel, each having a voltage rating greater than 117 VAC, and totaling about 450 microfarads capacitance. Although the primary function of these capacitors is to reduce the effects of harmonic noise on the power line, their exact value should be chosen so that they also operate to compensate for the poor power factor presented by the field producing coils. This is done by adding capacitors while observing the AC current drawn from the line, and stopping when this current reaches a minimum.

Phase selector 40 is shown to comprise a modulus 3 ring counter 46 and three identical line switches (one for each phase) shown generally as 48, 50 and 52, with switch 48 connected by lead 54 to the counter phase "1" output, switch 50 connected by lead 56 to the counter phase "2" output and switch 52 connected by lead 58 to the counter phase "3" output. The modulus 3-ring counter input is coupled to the output of phase timer 42. Phase timer 42 comprises a modulus 8 binary counter 62 (three cascaded integrated circuit toggle flip-flops). Counter 62 has its input driven by a Schmidt trigger 64 which in turn has its input coupled by lead 66 to the AC line voltage hot lead 18.

In operation, the Schmidt trigger circuit switches states at about zero volts for each negative-to-positive line voltage transition, switching just before the voltage crosses zero. At this instant, transistor 68 conducts and also switches on transistor 70. This causes the voltage at input pin 72 of integrated circuit flip-flop 74 to drop sharply, toggling the flip-flop and incrementing counter 62 by one. When the counter reaches a count of seven cycles, the next line voltage negative-to-positive transition will cause it to revert to zero. At that time, the output terminal of flip-flop 76 will drop sharply in voltage, sending a negative pulse through capacitor 78 to the base of transistor 80. Normally, transistor 80 is biased into saturation by resistor 82. However, this short negative-going pulse turns the transistor off momentarily, to increment by one the modulus 3 ring counter 46 formed by transistors 80, 84, 86 and 88, in a manner well known in the art. Thus, such negative pulses are the counter 46 "input" pulses. Only one of transistors 84, 86 and 88 is normally conducting, and upon receiving an input pulse that transistor stops conducting and the next transistor in the sequence conducts. Transistors 84, 86 and 88 respectively correspond to phases "1", "2" and "3" or phase selector circuit and thus it can be seen that said transistors provide a sequence of three phases, each phase lasting for the modulus of counter 62, which for the particular embodiment is 8 alternations of the line voltage.

The outputs of counter 46 are coupled to identical line switches 48, 50 and 52, only one of which, 48, is shown schematically; the counter phase "1" output is coupled to switch 48 by lead 54, the counter phase "2" output is coupled to switch 50 by lead 56, and the counter phase "3" output is coupled to switch 52 by lead 58. The line switch operation will be described with reference to switch 48, although it is to be understood that operation of switches 50 and 52 is similar. Switch 48 is shown to include normally nonconducting transistors 90, 92, 94, 96 and 98. The collectors of transistors 92, 94, 96 and 98 are respectively coupled to gate leads of triacs 100, 102, 104 and 106. "Load-switching" triacs 100, 102 and 104 are bi-directional conducting devices having their anodes respectively coupled to an output winding of like field producing means of zone units 10A, 10B and 10C and also having their anodes commonly resistively coupled to the anode of triac 106. Triacs 100, 102 and 104, when conducting, thus permit alternating current flow through the windings of each of horizontal electro-magnetic field producing means 12B of zone units 10A, 10B and 10C.

The function of triac 106 is very important to false-alarm-free operation of a field sequencing circuit such as that of FIG. 6, because of the noise spikes which the load switching triacs produce each time they pass from one quadrant of operation to another, i.e. to switch from a negative to a positive conduction state or vice versa. If such a switching instant occurs at nearly the same time as the corresponding load current alternation instant, the marker characteristic signal will be masked by the load switch noise spikes because it too occurs at about the load current alternation point. Triac 106 prevents such masking by causing the load switch to switch states sufficiently in advance of the load current alternation that the concommitant noise spikes have decreased adequately to prevent masking of a marker signal. Because of the inductance of the field producing means windings, the current from said coils appearing at the anodes of triacs 100, 102 and 104 will lag the current appearing in the secondary winding of transformer 44. By connecting the cathode of triac 106 to said secondary winding such that the polarity of the current passed through resistors 108, 110 and 112 to the anodes of the load switches opposes the load current for the part of each half cycle where the load current is approaching zero, the switching instants of said load switches are caused to lead the corresponding load current alternation in the field producing means windings. Because the marker magnetization reversal of the present embodiment lags the corresponding field producing means current alternation, it is preferable that the transformer secondary winding be connected across triac 106 as shown; were the connection reversed, the load switch switching instant would lag instead of lead line current and hence the load switch noise spikes might mask a marker signal.

FIG. 7A and B, shown on two sheets of drawings, is a combinational block diagram and schematic wiring diagram of one of the two identical combinations of magnetic flux monitoring means 29 and signal detecting circuits 32 of the system of FIG. 1. As shown, the coils 16A, 16B, 16C and 16D of magnetic flux monitoring means 29 are coupled to the input of detector 32, by coaxial cable 28.

Detector 32 comprises a signal shaping and amplifier section (SSAS) 128, a signal discriminator section (SDS) 130 and an output section 132, the output of the latter of which is provided by lead 134 to alarm and indicator 34 of the system of FIG. 1.

SSAS 128 comprises, in series, an input filter 136, a first stage preamp 138, second stage preamp 140, third stage preamp 142 coaxially coupled to a further filter 144, a further amplifier 146, and a limiter stage 148, each of which is of a well known construction. The pair of twin-T filters of filter 136 have a very high transfer impedance at 60 Hz., the frequency of the field producing means in this embodiment; they are made from low-cost, 10-percent tolerance components, require no tuning, almost eliminate the very strong 60 Hz. interference and reduce other interfering signals up to about 1,000 Hz. as well. In input filter 136, capacitors 201,202, 204 and 206 were selected to be 0.01 microfarads, capacitors 208 and 210 were selected to be 0.022 microfarads, resistors 212, 214, 216 and 218 were selected to be 270 × 103 ohms and resistors 220 and 222 were selected to be 120 × 103 ohms. Preamplifier stage 138 employs a transfer designed for very low noise, and is intended to contribute as little self-generated noise as possible to the amplified signal at its output.

Second and third preamplifier stages 140 and 142 each provide signal gain with good linearity. Their interstage coupling capacitors attenuate frequencies below about 1,000 Hz. A variable resistor 150 is coupled between preamplifier 140 and 142 to permit optimum adjustment of the system against false alarms in the form of random signals present in, and perhaps peculiar to, the particular system's environment. Optimum adjustment is made by increasing the AC collector voltage of transistor 152 until detector 32 detects a signal even though it is known that a marker characteristic signal has not been produced in the interrogation zone. The value of this AC collector voltage is observed and variable resistor 150 then set to provide a transistor 152 AC collector voltage of approximately one-half the observed value.

Filter 144 is essentially a high-pass filter with an additional band-reject filter centered upon the applied field fundamental frequency, the latter filter comprising a twin-T filter network between high-pass filter capacitors. The twin-T filter network provides a high transfer impedance at 60 Hz., the fundamental frequency of the applied field, and the high-pass capacitors substantially attenuate signals within the range of 60 Hz. to 1,000 Hz., the predetermined frequency of the instant preferred embodiment. In filter 144, capacitors 224 and 226 were selected to be 0.01 microfarads, resistors 228 and 230 were selected to be 270 × 103 ohms, resistor 232 was selected to be 120 × 103 ohms, and capacitor 234 was selected to be 0.022 ohms. Further, amplifier 146 provides additional amplification and limiter stage 148 provides still further gain and also provides both a high input impedance for amplifier 146 and unsymmetrically clipped signals to signal discriminator 130.

Signal discriminator 130 is designed to provide an output signal for enabling alarm and indicator circuit 34 each time three signals of an amplitude at least twice as great as the noise and less than about 0.3 milliseconds in duration occur at successive intervals of about 16 2/3 milliseconds. Each such signal corresponds to the characteristic signal of a marker of the foregoing described preferred embodiment. Further, the 16 2/3 millisecond interval corresponds to the period of a 60 cycle signal and thus represents alternate magnetization reversals of a sensitized marker. In general, signal discriminator 130 is synchronized with the applied field to sense for a characteristic signal only for about a 1.5 millisecond interval or time window beginning immediately following each applied field positive-to-negative alternation. If such a signal is detected, a signal is provided on lead 154 to the output section 132. Upon receipt of three signals on lead 154 at successive 16 2/3 millisecond second intervals, output section 132 provides a signal to alarm and indicator 34. After receiving a first signal, output section 132 blocks incrementation of the counter during the following 13 2/3 milliseconds and if another signal is not received with 6 milliseconds after that, the counter is cleared.

Signal discriminator section 130 is shown to comprise a disabling transistor 156 which, together with trigger circuit 158, delay timer 160 and coercivity gate 162 prevent passage of an output signal from limiter stage 148 to the combination of a signal shaper 164 and signal width discriminator 166 for most of each cycle. In operation, trigger 158 changes state at approximately the time the AC line voltage goes through zero. When this transistion is positive-to-negative, the trigger output makes a similar transistion and couples a pulse through capacitor 168 to trigger the delay timer, a monostable multivibrator, to its unstable state. The delay timer stays in this state for a period of time selected to be about 4 milliseconds, then switches off at approximately the time that the applied field crosses through zero (the applied field lags the line voltage by slightly less than 90°, or roughly 4 milliseconds). When the delay timer goes off, it sends a negative-going pulse through capacitor 170 to trigger the coercivity gate, also a monostable multivibrator, into its unstable state, which lasts about 1.5 milliseconds before reverting to the stable state. While in its unstable state, it supplies a low voltage for resistor 172 so that no current flows into the base of disabling transistor 156. Therefore, transistor 156 does not conduct during the unstable state of gate 162, thereby enabling the shaper circuit during this time. A high voltage level is simultaneously placed on resistor 174 which conducts current into the base of transistor 176 to in turn permit it to conduct current to similarly enable the signal width discriminator circuit. When the coercivity gate multivibrator returns to its stable state, the situation is reversed. A high voltage level appears on resistor 172 so that transistor 156 conducts current through resistor 177, causing transistor 179 to conduct current continuously, disabling the shaper circuit. At the same time, a low voltage level is impressed upon resistor 174, which then supplies no current to transistor 176, and the signal width discriminator circuits are disabled. Thus, it is only during the short unstable state of coercivity gate 162 of each 60 Hz. period that a signal will be accepted for processing. The characteristic signal pulses emanating from a marker of the type used in this invention will fall within this short interval.

When such a characteristic signal is timely received from limiter stage 148, it switches the shaper circuit state if its peak amplitude is greater than about 8 volts, transistors 179 and 181 switching from normal conducting and non-conducting states respectively to their new opposite states. This sends a pulse through capacitor 178 and diode 180 to trigger the signal width discriminator multivibrator 182 into its unstable state.

Transistors 179 and 181 remain in their new states for so long as the received signal exceeds about 6 volts, which for characteristic marker signals is less than about 0.3 milliseconds. In their new states, absence of current flow through the collector of transistor 179 cuts off transistor 184, which transistor will also be cut off whenever either or both transistors 176 and 186 are non-conducting. The non-conductive state of the former has been previously described in connection with the description of coercivity gate 162. Transistor 186 is non-conductive during the stable state of signal width multivibrator 182. Transistor 184 will thus conduct only during the unstable states of the coercivity gate and signal width multivibrator and during a normal state of shaper 164 when the collector current of transistor 179 is providing forward bias current to its base. Accordingly, the unstable state of multivibrator 182 must be greater than the width of a characteristic signal.

When a characteristic signal does switch transistor 184 into the conducting state, a negative-going signal is provided on lead 154, the output section 132 input lead.

The output section 132 is shown to comprise a counter 188, a period time 190, a consecutive-pulse gate 192 and a counter-clear gate 194.

Period timer 190 is simply a monostable multivibrator coupled to input lead 154 and adapted to switch into its unstable state in response to a negative going signal thereon. Each transition of the period timer from its stable to unstable state increments counter 188 by switching transistor 196. The period timer unstable state is about 13 2/3 milliseconds; accordingly, counter incrementing is limited to once per each 13 2/3 milliseconds or about once per a 60 Hz. cycle. Clearing of the counter is also inhibited during this 13 2/3 millisecond interval by the forward bias provided at node 198 because transistor 200 is cut off. Consecutive-pulse gate 192, another monostable multivibrator, is switched into its unstable state when the period timer reverts to its stable state. In its unstable state, lasting about 6 milliseconds, gate 192 prevents clearing of the counter by providing forward bias to the base of counter clear gate 194. If the period timer has not been switched into its unstable state when consecutive-pulse gate 192 reverts to its stable state, the counter is cleared.

Relay driver circuit 133 responds to an output produced when the counter is counted up to three to provide an output signal on lead 134 for activating alarm and indicator circuit 34.

The present invention has utilities other than protection against theft of library books or other articles of merchandise. For example, the system may be used for sortation. Objects belonging to one class may be each provided with a marker having an open-strip of one AC coercivity, objects belonging to another class would be provided with a marker having a second AC coercivity, etc. Accordingly, objects may be sorted into groups by the time occurrence of their magnetization reversal signals. Similarly, strips of different coercivities may be combined to form a binary code. Each bit of the code would correspond to a strip of a particular coercivity. The presence or absence of a strip of a particular AC coercivity in a marker would correspond to a "0" or "1" for the digit or "bit" position corresponding to that strip. A selectively alterable marker may be provided by including all strips in every marker and by providing each strip with a control element of the type previously described.

A further use of the marker of the present invention would be as a tamper-proof seal, whose integrity could be determined by instruments without direct contact as on a railway car moving past an interrogation point. The seal would be secured such that it could not be removed intact and subsequently be replaced. Moreover, the seal would be secured such that any attempt to remove it would result in a division of the seal such that the dimensional ratios of each of the divided pieces would be less than the aforementioned minimum value of 150.

The marker of a tamper-proof seal may comprise a high-strength adhesive on a flimsy length of an open-strip of ferro-magnetic material. The adhesive may be one such as that described in U.S. Pat. RE No. 24,906 and the open-strip may be a strip of permalloy 0.6 centimeter wide, 25 microns thick and 18 centimeters long. Applications of such a seal would include sealing railway car doors or containers. The seal would be applied such that opening the door would rupture the seal. Because of the high adhesive strength and the low open-strip tensile strength, it would be impossible to peel off and subsequently reapply a marker. The original length of the seal and the positioning of the seal on the door and door frame must be such that a piece of a ruptured marker would be sufficiently short that it would no longer produce its original characteristic magnetization reversal signal. Conveniently, the means for producing an applied field and sensing means may be used in conjunction with automated optical scanning equipment presently used by the railroads to record car identities.

It is to be recognized that each library book could be provided with machine readable indicia of the book's identity, e.g. the book's Library of Congress number. By automatically desensitizing a book's marker in response to decoding and recording both these indicia and the pertinent data on a user's identity card, an automated checkout system is provided.