Abstract:
A reading head is presented for use in a security system for reading an intermittent code pattern, when the code pattern is displaced in a reading direction with respect to the reading head. The code pattern is formed of a plurality of spaced-apart magnetic elements made. The reading head comprises a magnetic material producing a high-gradient static magnetic field, and a sensing element of a kind responsive to signals produced by the magnetic elements. The magnetic material is designed such that it defines an extended narrow region where the static magnetic field vector is substantially equal to zero. The sensing element is located substantially within the zero-field region, and is thereby responsive to signals generated by each of the magnetic elements, when the magnetic element is located in the zero-field region.

Description:
FIELD OF THE INVENTION 
     This invention is generally in the field of security techniques, and relates to a security system for encoding documents and other valuable items with code patterns, and a method for reading these code patterns. 
     BACKGROUND OF THE INVENTION 
     It is often the case that documents or other valuable items need to be protected from tampering, falsification and unauthorized use. The accepted way of protection consists of introducing one or more security means into, or attaching these means to, a document or an item. The documents and items to be protected include ID cards, passports, licenses, security passes, currency, checks, travel tickets, keys and key cards, and the like. The most widely used encoded security means are so-called “optical bar codes” and magnetic strips. Such encoded security means may be either visible or hidden from view. 
     Conventional optical bar codes suffer from the drawback associated with the fact that dust or dirt incidentally appearing on either a data recording medium or a data reader device may cause read errors. Additionally, in magnetic strips, the recorded data may be damaged by the influence of an ambient magnetic field or an elevated temperature. 
     U.S. Pat. No. 4,883,949 discloses a system with increased code reliability. In this system, data recording means are defined by intermittent patterns of different materials that can affect the high frequency impedance of the magnetic coil of a reading head. The pattern materials may differ in electric conductivity and/or magnetic permeability. 
     Another common drawback of the optical bar codes and magnetic strips, is that they can be easily read out and duplicated by conventional means. To increase the security of important documents and valuable items from unauthorized use or falsification, it was proposed to incorporate a magnetic bar code strip or other pattern hidden within the document or item substrate or concealed under other “dummy” security elements such as optical bar codes or holograms. 
     In the known devices of the kind specified, magnetic materials were used mostly in the form of magnetic inks containing powders of high magnetic coercivity, e.g., Fe 2O   3 , BaFe 12 O 19  and the like. However, it is recognized that the strong persisting magnetic field of such materials allows for reading out and duplicating the encoded pattern. 
     In U.S. Pat. No. 5,616,911, it is proposed that the hidden magnetic bar code pattern be formed using the low concentration magnetic ink exhibiting very low, approximately zero, remanance. Such magnetic inks may be formed from soft magnetic materials including Fe-powder, ferrites, Sendust alloy powder, amorphous alloys in a powder form and the like. The concentration of these materials in an ink or binider medium of less than 10% by volume is preferred. A reading head utilizing an in-field magneto-resistive sensing element is employed to magnetize the bar code pattern bands and detect their width or the space between them, whilst they move with respect to the head. After being read, the code pattern exhibits practically no residual magnetic field and is not readable by a magnetic viewer. One of the disadvantages of the proposed security system is in that the applied soft magnetic powders are easily available and their magnetic signature is not specific, and therefore the document counterfeiting is still possible. Another disadvantage is in that the sensitive area of the reading head with a magneto-resistor sensing element is large. The size of the code pattern elements and the distance between them cannot be smaller than the reading head sensitive area, otherwise these elements would not be unambiguous. Therefore, the information density of such a coding system is intrinsically low. Moreover, the reading head resolution deteriorates strongly with the increase in the distance between the magneto-resistive sensing element reading head and the code pattern. The same is relevant for the bar code system disclosed in the above-indicated U.S. Pat. No. 4,883,949. 
     U.S. Pat. No. 5,801,630 discloses a method for preparing a magnetic material with a highly specific magnetic signature, namely with a magnetic hysteresis loop having large Barkhausen discontinuity at low coercivity values. This material is prepared from a negative-magnetostrictive metal alloy by casting an amorphous metal wire, processing the wire to form longitudinal compressive stress in the wire, and annealing the processed wire to relieve some of the longitudinal compressive stress. One disadvantage of such material in security pattern applications is the relatively large wire diameter, which is approximately 50 μm. Another disadvantage is the complicated multi-stage process of the wire preparation. Yet another disadvantage of the material is the amorphous wire brittleness which appears due to wire annealing Such brittleness will prevent using the material in security patterns formed in paper documents like currency, checks, passports, etc. 
     According to another known technique, a glass-coated magnetic microwire is used as a magnetic material having unique magnetic properties. This microwire is cast directly from the melt by a modified Taylor method, which is well known in the art, and is disclosed, for example, in the article of I. W. Donald and B. L. Metcalf “ The Preparation Properties and Applications of Some Glass-Coated Metal Filaments Prepared by the Taylor - Wire Process ”, Journal of Materials Science, Vol. 31, pp 1139-1149, 1996. It is an important feature of the Taylor process that it enables pure metals and alloys to be produced in the form of a microwire in a single operation, thus offering an intrinsically inexpensive method for the manufacture of microwire. The glass-coated microwires can be produced with very small diameters (ranging from 1 μm to several tens micrometers), from a variety of magnetic and non-magnetic alloys and pure metals. Magnetic glass-coated microwires may be prepared also with amorphous metal structures, as disclosed in the article of H. Wiesner and J. Schneider “ Magnetic properties of Amorphous Fe - P Alloys Containing Ga, Ge and As ”, Physica Status Solidi, Vol. 26, pp 71-75, 1974, and other publications cited as references in the above-indicated review of Donald and Metcalf. These amorphous magnetic glass-coated microwires have good mechanical strength, flexibility and corrosion resistance, so that they can be easily incorporated in paper, plastic and other document substrate materials, Amorphous magnetic glass-coated microwires are characterized by their unique response which may resemble, inter alia, that of die-drawn amorphous wires of the above cited US &#39;630. 
     SUMMARY OF THE INVENTION 
     There is accordingly a need in the art to improve security systems by providing a novel security system utilizing a code pattern formed by magnetic elements with extremely low coercivity and high permeability, and a novel reading head and method for reading the code pattern. 
     It is a major feature of the present invention to provide such a system that has a high density magnetic read-only code pattern, which is not visible to the Individual&#39;s eyes, and a reading head capable of reading this code pattern 
     It is another feature of the present invention to provide such a reading head that is designed to match the unique response characteristic of the code pattern material. 
     The main idea of the present invention is based on the following. A read-only code pattern, to be attached to a document that is to be protected, is formed by magnetic elements having extremely low coercivity (substantially less than 10 μm) and high permeability (substantially higher than 20000). A reading head, in addition to the conventionally used magnetic sensing element (e.g., coils), is provided with a magnetic means (e.g., at least two permanent magnets) creating a static magnetic field of a specific configuration. This static magnetic field, on the one hand, affects the magnetic elements so as to provide their magnetic response to this static filed, and, on the other hand, has such a configuration as to define an extended narrow region (plane) where the static magnetic field vector is substantially equal to zero. The sensing element is located substantially within his zero-field plane. In other words, the magnetic field produced by the magnetic means has a high gradient in the sensitivity zone of the sensing element. Due to the above magnetic characteristics of the magnetic elements, they will produce an effective (noticeable by the sensing element) response only within this narrow (i.e., plane-like) zone, while being saturated by the static magnetic field at any location out of this plane. The magnetic response of these magnetic elements would not be readable with sufficient resolution by the sensing element only, without the use of the high-gradient static magnetic field. Moreover, to read the code pattern, the magnetic elements should be displaced with respect to the sensing element and located outside the magnetic means, provided during this displacement each of the magnetic elements passes the zero-field plane in a reading direction perpendicular to this plane. 
     The pattern may be representative of a binary code, wherein binary “1” and “0” are defined either by different distances between the adjacent magnetic elements, or by different parameters of the magnetic elements (e.g., materials or dimensions). 
     There is thus provided according to one aspect of the present invention a security system for protecting an item by forming it with a predetermined code pattern, the system comprising: 
     (a) a plurality of spaced-apart magnetic elements forming said code pattern, wherein said magnetic elements are made of at least one magnetic material having a coercivity substantially less than 10 μm and a permeability substantially higher than 20000; 
     (b) a reading head comprising a sensing element of a kind responsive to signals generated by the magnetic elements, and a magnetic means of a kind producing a high gradient static magnetic field and defining an extended narrow region where the static magnetic field vector is substantially equal to zero, said sensing element being mounted with respect to the magnetic means so as to be located substantially within said region to detect the signals produced by the magnetic elements when they pass said extended narrow region in a reading direction perpendicular to said region; and 
     (c) a detector detecting the response of said sensing element and generating data representative thereof 
     The term “ Hexended narrow region ” used herein signifies a plane-like zone. Generally speaking, the zero-field extended narrow region is located within a plane perpendicular to a reading direction in which the magnetic elements are to be displaced with respect to the reading head. Preferably, the zero-field region, a sensing area, is less than 50 μm in the axis along the reading direction. 
     The sensing element may be a conventional magnetic head, e.g., a coil wound onto a core, or a magnetoresistor element. The magnetic means producing the high gradient static magnetic field may include at least two spaced-apart permanent magnets. The magnets are oriented with respect to each other such that their axes of magnetization are aligned in a spaced-apart parallel relationship, and each pole of one permanent magnet faces an opposite pole of the other permanent magnet. 
     The magnetic elements are, preferably, glass-coated microwires, which may be produced in a one-stage casting process from an alloy having substantially zero magnetostriction. Preferably, the alloy is cobalt based, including more than 70% cobalt by atomic percent. 
     The microwires have very small diameters, so that they can be embedded into the document substrate without causing noticeable changes in the document thickness or form. The position of the microwires in the document cannot be detected visually or by touch. The code pattern formed by microwires is not visible with the magnetic viewer, owing to the fact that the microwire is a soft magnetic material and demonstrates a very low remanence. The microwire pattern density is considerably higher than that obtained with the known magnetic materials like magnetic inks. The microwire code pattern cannot be read with known readers, including those which detect soft magnetic materials, since the sensing area of known readers is much larger than the distance between the microwire pieces in the code pattern of the present invention. The microwire code pattern can be read only and solely with a specific reader device (reading head) according to the present invention. Hence, the microwire code pattern increases substantially the document security. 
     The code pattern may be attached directly to the item, or to a separate label attached to the item. 
     According to another aspect of the invention, there is provided a reading head for reading an intermittent code pattern formed of a plurality of spaced-apart magnetic elements made of at least one magnetic material, when the magnetic elements are displaced in a reading direction with respect to the reading head, the reading head comprising: 
     a magnetic means producing a high gradient static magnetic field defining an extended narrow region where the static magnetic field vector is substantially equal to zero; and 
     a sensing element mounted with respect to the magnetic means so as to be located substantially within the zero-field region, the sensing element being thereby responsive to signals generated by the magnetic elements, when each of the magnetic elements, while being displaced in said reading direction, is located in said zero-field region. 
     According to yet another aspect of the present invention, there is provided a method for reading a code pattern formed by a plurality of spaced-apart magnetic elements made of at least one magnetic material having a coercivity substantially less tan 10 A/m and a permeability substantially higher than 20000, the method comprising the steps of: 
     (i) creating a static magnetic field with substantially high gradient, such that the static magnetic field vector is substantially equal to zero within an extended narrow region; 
     (ii) providing a sensing element of a kind responsive to signals generated by the magnetic elements and locating said sensing element substantially wit the zero-field region; 
     (iii) providing relative displacing of the code pattern relative to said sensing element in a reading direction perpendicular to said extended narrow region, such that the code pattern passes through said static magnetic field, thereby exciting each of the magnetic elements to generate the signals whilst passing the zero-field region; and 
     (iv) detecting the response of the sensing element to said signals generated by the magnetic elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be more fully understood from the descriptions of preferred embodiments of the invention set forth below, together with accompanying drawings, in which: 
     FIG. 1 is a schematic illustration of a security system according to the 
     FIG. 2 graphically illustrates the magnetic characteristics of magnetic elements suitable to be used in a marker of the system of FIG. 1; 
     FIG. 3 more specifically illustrates the layout of the magnetic elements and reading head in the system of FIG. 1; 
     FIG. 4 graphically illustrates the main operational principles of the system of FIG. 1; 
     FIG. 5 illustrates one possible example of the implementation of the reading head suitable to be used in the security system of FIG. 1; 
     FIG. 6 graphically illustrates advantageous features of the present invention, as compared to a conventional one; and 
     FIG. 7 illustrates another possible example of the implementation of the reading head suitable to be used in the security system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is illustrated a security system  10  having such main constructional parts as a magnetic marker or code pattern  12  and a magnetic reading head  14 . The code pattern  12  is formed by a set of glass-coated magnetic microwires  16  aligned in a spaced-apart parallel relationship along the surface region of a document-substrate D covered by an obscuring layer  18 . The microwires  16  (pieces) are either attached to the document (or item) itself or are contained in a label attached to the item. The reading head  14  comprises a sensing element  20 , whose construction and operation will be described more specifically further below. As the reading head  14  passes over the microwire code pattern  12 , each microwire piece  16  produces a specific response, which is detected by the sensing element  20 . 
     In this specific example, the code pattern  12  is formed by several 8 mm-length microwire pieces  16  with different spaces between them, namely d 1  of 0.2 mm and d 2  of 0.5 mm. Generally, security information (code) may be written in a binary form, for example, by establishing two different distances d i  and d 2  between the microwire pieces  16 . The shorter distance di may be interpreted as binary “0”, and the larger distance d 2  as binary “1”, or vice versa. While the reading head  14  passes over the microwire pattern  12  at a constant speed, the time intervals between the detected microwire responses precisely correspond to the distances between the microwire pieces  16 . 
     It should be noted, although not specifically shown, that the code information may be written in a binary form by using the microwire pieces  16  of two different types. These different microwires may have different diameters or permeabilities, so that they produce considerably different responses to a magnetic field produced by the reading head  14 . Accordingly, one type of the microwires may present binary “0” and the other binary “1”. 
     The microwires  16  are manufactured from the Co-Fe-Si-B alloy containing 77.5% Co, 4.5% Fe, 12% Si and 6% B by atomic percentage. The outer diameter of the microwire  16  is about 20 μm, and its metal core diameter is about 15 μm. A measured hysteresis curve H of the microwire material is shown in FIG.  2 . The hysteresis loop L has a small coercivity value H c , which is less than 20 A/m (generally less than 10 A/m), a large Barkhausen discontinuity (i.e., high permeability value, i.e., higher than 20000), and the saturation induction value B equal to 0.8T. 
     Generally, for the purposes of the present invention, the coercive force values are in the range of 5-10 A/m (0.06-0.12 Oe). In other words, a glass-coated magnetic microwire material to be used for the preparation of the code pattern should be characterized by small magnetic coercivity and high permeability values. Such a microwire can be fabricated from amorphous alloys having approximately zero magnetostriction. This technique has been developed by the inventors of the present application, and is disclosed in the article “ High Frequency Properties of Glass-Coated Microwire ”, A. N. Antonenko, E. Sorkine, A. Rubshtein, V. S. Larin and V. Manov, Journal of Applied Physics, Vol. 83, No. 11, 1998, pp.  6587--6589 . To is end, an alloy containing 77.5% Co, 4.5% Fe, 12% Si and 6% B by atomic percentage may be used. 
     Turning back to FIG. 1, the reading head  14  comprises two or more permanent magnets—two such magnets  22  and  24  in the present example (constituting a magnetic means). The magnets  22  and  24  are arranged in a spaced-apart parallel relationship, being oriented with respect to each other such that their magnetization directions are perpendicular to a plane P (i.e., the plane of the drawing). The poles orientation of the magnet  22  is inverse to that of the magnet  24 . In other words, the axes of magnetization of the magnets  22  and  24  are aligned in a spaced-apart parallel relationship, and each pole of one magnet faces the opposite pole of the other magnet. Therefore, a plane  26  exists between the magnets  22  and  24 , in which the induction of the sum (interpolated) static magnetic field vector created by both magnets is equal to zero. The sensing element  20 , which is responsive to the microwire magnetic response, is secured in such a position that the zero-field plane  26  passes therethrough. It should be noted that the sensing element may and may not be accommodated between the permanent magnets, provided that it is located in the plane  26 . 
     FIG. 3 schematically illustrates the layout of the system  10 . The permanent magnets  22  and  24  are preferably identical in size and magnetization value, and are aligned in a line in a manner to create the zero-field plane  26  between the magnets  22  and  24 . The sensing element  20  is accommodated such that the zero-field plane preferably coincides with a symmetry plane SP of the sensing element  20 . The magnetic field lines configuration for both magnets is partly shown in the figure. 
     FIG. 4 illustrates the main principles of locating the microwire  16  (i.e., detecting its presence and determining its position) by the gradient of the magnetic field created by the permanent magnets  22  and  24 . A graph G presents the distribution of the intensity H of the static magnetic field of the permanent magnets along the x-axis, which is the axis along which the reading is carried out (i.e., the axis of movement of the reading head with respect to the pattern), In the vicinity of the zero-field plane  26  (FIG.  3 ), the magnetic field intensity H actually presents a linear function of the distance relative to the plane  26  along the x-axis. 
     It is generally known that the microwire of the type selected for the purposes of the present invention is highly anisotropic and provides a noticeable response only to a magnetic field directed along the microwire axis (perpendicular to the x-axis). As indicated above, the microwire hysteresis loop is nearly rectangular, and therefore it will be saturated whenever the external magnetic field intensity exceeds the microwire coercivity value H c . To this end, the intermittent code pattern  12  and the reading head  14  should be oriented with respect to each other daring the reading process such that the microwire pieces  16  are substantially parallel to the zero-field plane  26  and are displaced in a reading direction perpendicular to the plane  26 . In this orientation, the given microwire piece will be noticed by the sensing element  20 , only when the microwire piece  16  is positioned within the internal {x 1 , x 2 }, where the intensity of the magnetic field created by the permanent magnets is less than H c . 
     Reference is made to FIG. 5, illustrating a security system  30  utilizing a sensing element of an inductive type. To facilitate understanding, same reference numbers are used for identifying those components, which are identical in the systems  10  and  30 . Thus, the system  30  comprises the reading head  14  and the microwire-based code pattern  12  attached to the item D. An indicative sensing element  31  of the reading head  14  is in the form of a coil  32  wound on a ring-like core  34  made from a soft magnetic material, e.g., fentite. As shown, spaced-apart ends of the core  34  form an air gap  36  therebetween. An inductance change detector  38  supplies high frequency alternating current to the coil  32 . When the microwire piece  16  of the intermittent code pattern  12  approaches the air gap  36 , it lowers the gap reluctance causing the corresponding change in the inductance of the coil  32 , which is detected by the inductance change detector  38 . By moving the reading head  14  with the inductive sensing element  31  relative to the pattern  12 , and by analyzing the corresponding inductance changes of the coil  32 , the position of the microwire pieces  16  in the pattern  12  can be detected. 
     The coil  32  is placed in the plane  26  where the intensity of the static magnetic field vector created by the permanent magnets  22  and  24  is close to zero, While the reading head  14  passes over the microwire code pattern  12 , the microwire pieces  16  successively enter the strong magnetic field created by the permanent magnets  22  and  24 . This strong magnetic field saturates the microwire pieces, so that they do not produce any change in the impedance of the coil  32  until the microwire piece  16  enters a very narrow region where the magnetic field intensity is between (−H c ) and (H c ). Only in this narrow region (about 50 μm), the microwire piece  16  will produce a response, and the impedance of the coil  32  will be affected (i.e., lowered) so as to be detected by the detector  38 . 
     The output of the induction change detector  38  may be in the form of high frequency alternating voltage with variable amplitude proportional to the inductance of the coil  32 . As further shown in FIG. 5, connected to the detector  38  is a waveform shaping circuit  40  that transforms the high frequency signal into a low frequency signal proportional to the amplitude of the high frequency signal. This low frequency signal is transmitted to an analog-to-digital converter  42 , and the digital output is further processed by a processing unit  44 . The results of the code analysis may be presented on a display  46 , or supplied to an external computer (not shown). 
     In this specific example, the reading head  14  comprises two identical FeNdB permanent magnets  22  and  24 , each with a 3 mm length, a 1 mm height, and a 2 mm width. The magnets have the remanence induction of 1.2T. The distance a between the magnets is 4 mm. The inductance coil  32  is fabricated by winding 500 turns of a 0.05 mm copper wire on a 3F3 Phillips ferrite ring core with a 6 mm outer diameter, a 4 mm inner diameter, and a 2 mm width. The 1 mm air gap  36  is made between the core&#39;s ends, The coil  32  is supplied with a 100 kHz, 3V alternating voltage. 
     It is an important feature of the present invention that the reading head  14  allows for reading the code pattern  12  formed of glass-coated microwires at a considerable distance between the reading head  14  and the code pattern  12 . This is due to the fact that the substantially zero-field region extends along the plane between the magnets, rather than being a small region within a physical gap between the magnets. The code pattern  12  to be read need not be moved through the gap between the magnets, but is displaced with respect to the reading head outside this gap. The resolution of responses coming from adjacent microwire pieces  16  does not depend substantially on the distance between the reading head  14  and the code pattern  12 . The code pattern  12  is read reliably when this distance varies from 0.05 to 1 mm, as will be described more specifically fiber below. 
     To more clearly demonstrate the principally different performance of the impedance changes based reading head  14  utilizing the permanent magnets  22  and  24 , let us compare the operation of this reading head to a reading head (not shown) having the coil  32  and no permanent magnets, both being applied to the same glass-coated microwire based code pattern. As indicated above, the provision of the permanent magnets creates the zero-field region to be passed by the microwire pieces. Hence, what we are actually comparing, are the operation of the reading head in the presence and in the absence of the zero-passing field created by the permanent magnets  22  and  24 . FIG. 6 shows the item D with the code pattern  12 , and graphically illustrates the main principles of the operation of the system  30 . Two graphs G 1 , and G 2  present the output signal U OUT  of the waveform shaping circuit  40  (FIG.  5 ), both measured at the coil&#39;s location close to the pattern (e.g., about 30 μm). Graph G 1  corresponds to the signal recorded whilst passing the reading head with the coil  32  only (i.e., having no permanent magnets) over the microwire code pattern  12 . Graph G 2  corresponds to the signal recorded whilst passing the reading head  14  (with the coil  32  and permanent magnets  22  and  24 ) over the pattern  12 . 
     It is clear from graph G 1 , that when using the reading head having no permanent magnets (i.e., having no zero-field region), the sensor resolution is insufficient for the pattern reading. Experiments have shown that when the distance between the air gap and the microwire pieces increases to 0.3 mm and more, it becomes impossible to detect the response of the microwires from the sensor output. On the contrary, when using the reading head  14  constructed as described above (i.e., with the permanent magnets  22  and  24 ), sharp peaks in the sensor output are detected (graph G 2 ), each sharp peak corresponding to each microwire position in the scanned code pattern  12 . When moving the reading head away from the pattern to a distance of 0.5 mm, the amplitude of the output signal decreases by 30%, while the form of the peaks remain unchanged. 
     The above performance of the reading head  14  can be easily understood in terms of the above considerations regarding the microwire response in the vicinity of the zero-field plane  26 . Indeed, the measured field gradient of the reading head sample  14  at the zero-field plane is approximately 40 Oe/mm, whereas the microwire piece coercivity value H, is less than 0.2 Oe. This means that the microwire pieces  16  are not saturated only if they are located at a distance of 5 μm or less from the zero-field plane  26 , that is the resolution of the head sample being about 10 μm. 
     Reference is now made to FIG. 7, illustrating, by way of a block diagram, a security system  50 , which is constructed generally similar to the above-described system  30 , but has a somewhat different design of its sensing element  52 , which is of a magnetoresistor type. Similarly, those components, which are common in the systems  30  and  50 , are identified by the same reference numbers. The output circuit of the magnetoresistor element  52  is coupled to a resistance change detector  54 , which, in turn, may be connected to the waveform shaping circuit  40 , if necessary. The output signal of the circuit  40  is then processed, for example, by a custom LS 1  unit  56 , and the code analysis results may be represented on the display  46 . 
     When the reading head  14  moves with respect to the intermittent code pattern  12 , the gradient of the magnetic field created by the permanent magnets  22  and  24  causes re-magnetization of each microwire piece  16  after its passage through the zero-field plane  26 . It is generally known in the art, that the re-magnetization of microwire having large Barkhausen discontinuity, i.e., like to those needed for the purposes of the present invention, occurs within a microseconds time period producing a sharp magnetic field pulse. The magnetoresistor element  52  is secured in the zero-filed plane  26 , and, consequently, is not affected by the permanent magnetic field produced by the magnets  22  and  24 . On the contrary, the magnetic field perturbations caused by the magnetization reversals of the microwire pieces  16  will be sensed by the magnetoresistor element  52 , and then detected by the resistance change detector  54 . 
     Those skilled in the art will readily appreciate that various modifications applied to the preferred embodiment of the invention as hereinbefore exemplified, without departing from its scope defined in and by the appended claims.