GB2252663A

GB2252663A - Data transfer head

Info

Publication number: GB2252663A
Application number: GB9202997A
Authority: GB
Inventors: Alan Oliver
Original assignee: RUE CARD TECHNOLOGY Ltd DE
Current assignee: RUE CARD TECHNOLOGY Ltd DE
Priority date: 1989-08-29
Filing date: 1992-02-13
Publication date: 1992-08-12
Anticipated expiration: 2012-02-13
Also published as: GB2252663B; GB9202997D0

Abstract

A magnetic data transfer head for reading from or writing a substantially rigid magnetic medium comprises a support (15); a pair of pole pieces between which an elongate gap is defined, the pole pieces being mounted to the support; and a contact area (11', 12') including the pole piece faces along which the magnetic medium passes in use. The faces of the pole pieces define the contact area, the length of the gap constituting the largest dimension at the contact area.

Description

DATA TRANSFER HEAD The invention relates to a data transfer head of the kind comprising a support; a pair of pole pieces between which an elongate gap is defined, the pole pieces being mounted to the support; and a contact area including the pole piece faces along which the magnetic medium passes in use. Such a head is hereinafter referred to as of the kind described.

Heads of the kind described have been developed for use either in reading coded, usually magnetic, data held on rigid substrates such as plastic cards and the like or for recording such data. Such cards carry a strip of magnetic recording tape which typically contains gamma ferric oxide although other magnetic materials such as chrome dioxide may be used. The particles, generally acicular in shape, are coated with a binder onto a film base such as polyester to form magnetic recording tape.

This tape is thin and flexible.

"Soft" data is written in such tape by exposing the tape to a controlled magnetic field which causes remanent magnetism to be recorded in certain portions of the tape by magnetising the appropriate elements. This written code can then be read by moving the material past the data transfer head. The magnetic field may have a constant or varying strength.

For reliable writing and reading of high resolution magnetic information recorded on the tape it is necessary for there to be intimate contact between the magnetic medium and the magnetic reading/writing head. The magnetic coating must therefore be placed in intimate contact with the recording head. In general, with conventionally written data, if the gap is orthogonal to the direction of the tape a good contact can be achieved.

Recently, a special form of magnetic coding has been developed. This proprietory form of coding, which will be referred to as "permanent" coding assists in inhibiting the counterfeiting of security cards and the like to which the magnetic medium is bonded. Permanent coding is achieved during manufacture of the magnetic medium in which when the magnetic material is coated onto a substrate such as a tape, and the tape is subjected to controlled magnetic fields which cause the acicular particles to adopt controlled orientations. The coating is dried while the particles are in those orientations, causing the manufactured tape inherently to possess a permanent magnetic code. This embedded coding is extremely difficult for counterfeiters to reproduce or alter. The coding is read by monitoring changes in self-inductance in a circuit connected to the pole pieces.Typically, the particles are permanently set at a transverse angle to the length of the magnetic medium and consequently the angle of the gap defined between the pole pieces of the transfer head must also extend at the same transverse angle to the direction of movement of the magnetic medium past the head in use. We have found that with this arrangement there is a particular problem with achieving good contact between the magnetic medium and the pole pieces using conventional heads.

Furthermore, in conventional security cards and the like it is common to provide permanent data and soft data in which a magnetic code is superimposed on part of the permanently encoded particles, the permanent and soft data being read from separate tracks. These tracks need to be read individually and conventionally this has been achieved using a dual head or set of dual heads in series with each dual head having one head for each track. It has proved very difficult to ensure good contact between the magnetic medium and both heads so as to be able to read accurately the data in both tracks.

In some instances, three or four tracks may be used, the "permanent" data being read frond one track and the others having soft data recorded therein.

In current commercially available heads of this type, the contact area is 11.5 mm2.

This problem is particularly acute with magnetic media held on substantially rigid security substrates such as plastic cards. It has been found that a low percentage of magnetically encoded cards are not correctly read or recorded by the end user circuitry, resulting, in the case of reading, in their rejection by the system.

In particular there appear to be difficulties in securing good high resolution magnetic contact between the pole pieces and the rigid but partially deformable plastic card. Because both surfaces are essentially rigid at the point of contact it is difficult to achieve perfect coplanar contact under all circumstances. This is made more difficult because plastic cards may become bent or have debris on the magnetic surfaces. Any departure from good contact will lead to security data being lost and the card rejected by the system. If dual heads are employed the coplanarity requirement is even more difficulty to achieve.

In accordance with one aspect of the present invention, a magnetic data transfer head of the kind described is characterized in that the faces of the pole pieces define the contact area, the length of the gap constituting the largest dimension of the contact area.

In accordance with a second aspect of the present invention, a magnetic data transfer head of the kind described is characterized in that the contact area is less than 11.5mum2.

In accordance with a third aspect of the present invention, a magnetic data transfer head of the kind described is characterized in that the length of the gap defined between the pole pieces is less than 2mm.

In accordance with a fourth aspect of the present invention, a magnetic data transfer head of the kind described is characterized in that the ratio of contact area (xpjn2) to gap length (mm) is less than 3:1.

We have realized that a considerable improvement in reading data from or writing data to a magnetic medium can be achieved by reducing the size of the contact area which has been used in the past. The greatest degree of reduction is achieved by utilizing the first aspect of the invention in which not only do the pole pieces define the contact area but the largest dimension of the contact area is equal to the gap length. For example, contact areas which are rectangular, square or circular are possible. In the case of a square or rectangular contact area, the gap will define a diagonal of the square or rectangle and in the case of a circular contact area the gap will define the diameter of the circle. The direction of the gap will correspond to the direction of magnetisation. The contact area will typically be substantially planar but could also be slightly bowed.

Preferably the ratio of contact area to gap length is less than 3:1, more preferably less than 2:1, and most preferably less that 1:1 and in the currently preferred head, the ratio is 1:1.4.

A typical, commercially available magnetic data recording head for use in recording permanent magnetic 2 data has a contact area of dimension 11.5mm2 and we have realized surprisingly that this can be reduced significantly. Furthermore, such commercially available conventional heads have a gap length of 2.3mm and again we have realized surprising that this can be beneficially reduced to below 2mm.

The reduction in size of the gap length and the size of the pole piece faces will reduce signal strength from that achieved with conventional heads but this strength can be substantially restored by making the pole pieces from a material such as ferrite without the need for expensive alteration to the electronic circuitry attached to the head. This has considerable commercial and practical importance. Thus, by reducing the size of the contact area in accordance with any of the aspects of the invention mentioned above, a significant improvement in reading and writing accuracy has been achieved.

Typically, the gap defined between the pole pieces will extend at an angle of between 30 and 60 degrees, preferably 45 degrees to the direction of movement of the magnetic medium past the pole pieces in use. This enables permanent encoded data to be read where the acicular particles extend in similar directions.

Typically, the or each head will be sprung mounted onto the support so that it will be urged against the magnetic medium in use.

Preferably, the support includes a guide for guiding a substrate on which the magnetic medium is mounted past and in alignment with the head.

In the case of a dual head device, the contact areas of each head preferably have the same area in order to equalize wear. Typically, however, the direction of the gaps defined in each head will extend at different angles to the direction of movement of the magnetic medium past the heads. This is important where the tracks associated with each head encode data in elements which extend at different angles to the direction of movement.

In some cases, a dual head device may be simulated with one of the heads being replaced by a metal sheet or the like in order to equalize wear even though only one track is read in practice. In other arrangements, two dual head devices may be provided in series but only one head from each dual head device is used so that a respective track is read from or written to at different dual head devices.

A further problem which has arisen with commercially available dual head devices incorporating a permanent magnetic data head is the presence of cross-talk between the individual heads. The data tracks are relatively close together but with conventional contact areas, there is only a small gap between the two heads. This causes some interference between magnetic fields and disturbs the signals generated by or detected by the pole pieces.

Once again, by reducing the size of the contact areas, this cross-talk can be significantly reduced.

Preferably, however, the heads are separated by one or more suitable magnetic shields such as mu-metal. The contact (planar) area may be made less than the total cross-sectional area of the upper portions of the pole pieces by appropriate shielding.

Typically, the magnetic medium will be mounted or bonded on a security card such as a plastic security card or the like. Such cards are generally formed of a security printed PVC laminate to which a magnetically readable strip is bonded. The cards are generally rectangular in shape, for example being produced to an International Standards Organisation format.

Examples of plastic cards are financial cards, credit cards, charge cards, cheque guarantee cards, cash cards, service entitlement cards, decrementing value cards including telephone token prepayment cards, electricity prepayment cards, gas prepayment cards, parking meter prepayment cards, information storage cards, integrated circuit containing ("smart" or IC) cards, identification cards and proximity cards.

The head can be incorporated into a reader or writer but is particularly suitable for readers of permanent magnetic data. Such readers include swipe readers and motorised card readers, letterbox slot readers and token readers. Swipe readers present a slot along which the user's card is evenly swept. Within the slot is a magnetic head which enters into intimate contact with the magnetic medium so as to read the data already recorded on the strip. Motorised readers have a card receiving slot. When a card is presented the motorised drive is actuated and the card is taken into the equipment and then returned to the user by reverse action. During this process the data from the magnetic tape is read and if necessary new data applied.Motorised units are employed in automatic teller machines (ATMs) at point-of-sale terminals for financial applications, telephone prepayment systems, or toll control unit applications.

Letterbox slot readers are used generally for entrance access control. Token readers are used in electricity, parking, gas, telephone and other prepayment meters or for access control.

Some examples of magnetic heads according to the invention will now be described and contrasted with conventional heads with reference to the accompenyine drawings, in which: Figure 1 shows a conventional 90 degree permanent data head; Figure 2 is a plan of a conventional 45 degree permanent data head; Figure 3 is a plan of a permanent data head according to one example of the invention; Figure 4 is a plan of a conventional dual head; Figure 5 is a longitudinal section through the head of Figure 4; Figure 6 is a cross-section through one of the heads of Figure 4; Figures 7-9 are views similar to Figures 4-6 but of an example of a dual head according to the invention;; Figures 10 and 11 illustrate examples of output signals from heads for reading soft data in accordance with an example of an invention and according to the prior art respectively while simultaneously reading a permanent recording; Figures 12 to 14 are vertical sections through three different heads according to the invention; Figures 15 and 16 are orthogonal views of a reading device incorporating a dual head similar to that shown in Figures 7-9.

Figure 17 illustrates schematically the arrangement of magnetic particles on a typical magnetic strip; Figures 18A and 18B illustrate equipment for detecting permanent data; Figures 19A and 19B illustrate equipment for detecting soft data; and, Figure 20 illustrates an example of a head for reading four data tracks.

Figure 17 illustrates in a very schematic and enlarged form a portion of a magnetic strip 40 containing a large number of acicular elements or particles 41. The particles 41 are orientated at i450 to the length of the magnetic tape as shown in Figure 17. This orientation is achieved during manufacture of the tape and following manufacture these orientations are permanently fixed in the tape. Thus, as can be seen in Figure 17, a series of regions 42-46 are shown in each of which all the particles present are orientated in the same direction.

Thus, in the region 42 the particles 41 are orientated at +450 to the magnetic tape direction while in the region 43 the particles are orientated at -450. The orientations in the other regions 44-46 are shown in Figure 17.

Superimposed upon this permanent code are one or more "soft" data tracks, for example the data track 47 shown in Figure 17. This soft data is achieved by magnetising the particles in a predetermined manner again as shown in Figure 17. Thus, in a region 48 the soft data is magnetised so that the South-North direction runs generally from left to right while in a region 49 the South-North direction runs from right to left. In practice, there may be more than one such soft data track and typically there could be up to three such tracks. In order to read the permanent data it is necessary to provide a suitable head which typically will comprise a pair of pole pieces having an elongate gap between them and connected in a circuit whose self-inductance can be monitored. An example of such a circuit is shown in Figures 18A and 18B.The circuit comprises a C-shaped electrical member 50 terminating in a pair of pole pieces which define a gap 51 lying in the +45 direction as shown in Figure 17. An electrical coil 52 is wound around the member 50 and is connected to a monitor or detector 53. When the magnetic tape passes the detector shown in Figure 18A, any particles in regions 42, 44 or 46 will be aligned with the gap 51 as shown in Figure 18A.

Any particles aligned in the -450 direction, for example in the regions 43 or 45 will lie orthognal to the gap 51 as shown in Figure 18B. These different orientations will cause a different "shunting" effect across the gap 51 thus causing there to be differences in the self-inductance in the circuit connecting the pole pieces. This is sensed in a conventional manner by the detector 53 and provides an indication of the direction in which the particles 41 are orientated and thus enables the encoding defined by those orientations to be obtained.

The elongate form of the particles 41 accentuates differences between the coding directions since in the Figure 18A example the effective dimension of the particles is about one micron corresponding to the width of a particle whereas in the case of Figure 18B the effective dimension is the length of the particle, typically about ten microns.

Figures l9A and 19B illustrate a system for detecting the soft data. In this case, the International standards lay down that the gap between pole pieces of a circuit for detecting the soft data should lie in a directional orthogonal to the direction of movement of the magnetic strip. The gap 54 is shown in Figure 19 between pole pieces integrally formed with a C-shaped magnetic member 55. Once again, an electrical coil 56 is wound around the member 55 and is connected to a monitor or detector 57 which monitors e.m.f.s generated in the member 55. As the magnetic stripe passes the pole pieces defining the gap 54, the magnetic flux defined by the particles 41 will cause an e.m.f. to be generated in the circuit which is detected by the monitor or detector 57.

The direction of this e.m.f. will depend on the North-South direction of magnetisation of the particles 41 but will not vary in accordance with the direction in which the particles themselves are orientated. Thus, Figure 19A illustrates the particles in region 42 while Figure 19B illustrates the detection of particles in the region 44. However, it should be noted that particles in the region 43 will cause the same magnetic effect to be detected by the detector 57 as particles in the region 42 even though the particle orientations themselves are different.

In the following description, heads for reading both permanent and soft data will be described. It should be understood, however, that the heads described as being suitable for reading soft data could also be used for writing or recording such data in the magnetic medium.

The conventional head shown in Fig. 1 is for sensing previously encoded permanent data defined by acicular elements which extend at 90 degrees to the direction of movement A of the magnetic medium (not shown). The head comprises a pair of pole pieces 2 which define between them a gap 1 having a thickness of a few thousandths of an inch and a length of 1.52me.

Figure 2 illustrates another conventional head for reading permanent data which has been recorded by orienting the acicular elements at 45 degrees to the direction of movement A of the magnetic medium (similar to Figure 17). In this case, the gap 1 is also inclined at 45 degrees to the direction of travel of the medium while pole pieces 4 which define the gap are mounted to a support (not shown) with their faces flush with a rectangular contact area 5. Certain material 6 of the pole pieces has to be machined away so that it is below the contact plane. The dimensions (in millimetres) of the contact area and the pole pieces are shown in Fig. 2.

As has been explained above, we have managed to reduce significantly the area of contact 5. An example of a head according to the invention for reading data from a permanently encoded track is shown in Fig. 3. As can be seen, a gap 7 is provided inclined at 45 degrees to the direction of movement A of the magnetic medium, the gap 7 being defined by triangular cross-section pole pieces 8, typically of ferrite material, which are arranged to define a square contact area 5'. It will be noted that the contact area 5' is defined solely by the faces of the pole pieces 8 since pole piece material 9 is removed from the plane of contact.

It should also be noted that the dimensions of the pole pieces 8 have been reduced considerably from those of the pole pieces 4 of Fig. 2 so that a contact area of lmm2 is defined together with a gap of length 1.4mm. This significant reduction in size of gap length and contact area has been achieved without significantly reducing signal strength over conventional soft iron heads by making the pole pieces 8 from ferrite.

Figure 4 illustrates a conventional dual head device. The device comprises an outer housing 10 having planar, elevated contact areas 11, 12 for the permanent data and soft data heads respectively. The adjacent metal is recessed at 13, as shown in Fig. 5. The head 11 has the same form as the head shown in Fig. 2 while the head 12 is similar to the head 11 but with the gap 1 arranged perpendicular to the direction of movement A of the magnetic medium.

The contact areas 11, 12 have the same dimensions so as to equalize wear and are positioned in alignment with Track 0 and ISO Track 2 respectively. According to the ISO Standard for financial cards, there are three tracks, Tracks 1, 2 and 3 which are used for the "soft" data.

The permanent data extends across the full width of the magnetic medium, Tracks 1, 2, and 3 being spaced across that width and the remainder of the width being termed as Track 0 from which the permanent data is read. The magnetic particles will be orientated perpendicular i.e.

900 (or at some other angle, e.g. 3CO, 450 or 600) to the direction of travel A, and be selectively magnetically encoded in areas corresponding to one or more of Tracks 1, 2 and 3 to provide soft data as explained in more detail above. The gap 1 may also be perpendicular and of length about 1.52mm. In practice ISO Track 2 is used if there is a single track of soft data.

The conventional dual heads have a significant gap between the permanent data head and the soft head. The soft head generally is capable of reading and writing three tracks and there is a single gap.

Since the Track 0 gap may be inclined at, say, 45 degrees, a planar contact area must be produced as curved contacts are unsuitable. The balancing contact area for ISO Track 2 will normally be made identical to reduce wear. The gap length will be 2.3mm corresponding to a vertical height of 1.65mm. Using such an arrangement in a dual head it has been found with trial cards that failure rates up to 8% can be induced on ISO Track 2 and up to 5% on Track 0.

Typical dimensions for the contact area for the permanent magnetic data head are 4.6mm x 2.5mm. That is the area is 11.5 sq.rnm.

The contact area for the ISO Track 2 gap which will normally be perpendicular will also be 11.5 sq.mm. This area is very large relative to the gap dimension.

One of the problems with these large contact areas is cross-talk between the two heads. This is reduced by introducing one or more magnetic shields 14 of mu-metal between the heads.

Figure 6 is a cross-section through either of the heads showing how the contact area 11 or 12 is proud of the head support 15.

Figure 7 illustrates a dual head device incorporating examples of heads according to the invention. As can be seen, the dual head device is very similar to that shown in Fig. 4 but it will be noted that the contact areas, all', 12' are much smaller than in the Fig. 4 case. This can also be seen from Figs. 8 and 9.

The recessed parts of the pole faces are covered with non-radiating materials to reduce radiation.

Typically, the support 15 will be sprung to a housing (not shown) by a compression spring or the like so that upon the approach of a card carrying the magnetic medium, the heads will be urged against the magnetic medium. An alternative mounting is described below in connection with Figures 15 and 16.

An advantage of reducing the pole face area as is previously mentioned, is that the "radiating" area of the high energy oscillating permanent data section is reduced as is the "receiving" area of the soft data head, thus virtually eliminating "through-the-air" coupling. This reduction of cross-talk can be seen in the sharpening of the signal from the temporary (soft) data when comparing dual heads according to the invention (Fig. 10) and a previous, known design (Fig. 11).

It should also be noted that the contact areas 11', 12' are co-planar and it has been found that compared with an 8% failure rate of the conventional dual head device shown in Fig. 4, the Fig. 7 device has a 0% failure rate.

The electronic circuitry associated with the head shown in Fig. 7 is of conventional form and where the pole pieces are defined by a C-shaped member, this will be wrapped with a coil of wire to transfer electrical signal pulses to and from the head (see for example Figures 18 and 19). Generally, such pole pieces are machined from cast soft iron or multi layer laminates of soft iron although other inductive materials may also be used.

While heads with gaps oriented at 900 will not generally have flat surfaces, such 900 heads having a gap length of less then 2mm, preferably about 1.0 to 1.5mm and a contact area of the length of the gap squared, may be useful for security track reading. Generally such a head will be used in combination with another head having a gap at 900 of less, such as 450. The head however may be used separately as part of a two head recording assembly where the heads are spaced part along the tracks. Thus for example a head with a 900 gap of length lmm may present a contact and radiating area of 1mum2. The gap would normally be chosen to bisect the area.

Figure 12 is a vertical section through a magnetic head taken parallel to the direction of tape motion.

The section is taken through the middle of a head having a gap inclined at 450. The core is made of two ferrite pole pieces 16 with the windings held on a bobbin 17.

The upper surface 18 of the core is flat and represents the contact area and radiating area. The gap is indicated by 19. The whole assembly is contained within a mu-metal or other screening casing 20 which presents sloping approaches 21. Wires 22 enable connection to the recording circuitry. The casing in practice may be essentially solid, being made from machined metal, or it may be filled with screening composition.

In Figure 13 the surface area 18 is reduced by recessing the pole pieces. The approaches 21 screen the lower portion of the core so that the radiating and contact areas coincide at surface 18. In this case wires 22 are shown connected to a detector/generator 40. In the case of a reading head, unit 40 will be a detector and in the case of a soft data recording head unit 40 will be a voltage generator. It should be understood that each of the other heads described herein will be connected to a detector or generator.

Figure 14 shows a similar device except that the exposed pole piece area is greater than the contact area.

The contact area is surface 23. In combination with recessed areas 24 they give the total radiating area.

Figure 16 illustrates an example of a reading device, the device comprising a pair of support brackets 28 between which is mounted a dual head 25 similar to that shown in Figures 7-9. The head 25 can be seen in more detail in Figure 15 which is a part sectional view taken on the line 15-15 in Figure 16. As can be seen in Figure 15, the dual head 25 has two magnetic data transfer heads, one 26 having a gap inclined at 45 degrees to the direction of travel of a plastic card 34 and the other 27 having a perpendicular gap.

The head 25 is mounted on the support brackets 28 by a solid gimbal pin assembly 29 which locates with an indentation 29' on one side of the head 25 and by a sprung gimbal pin 30 which locates in an indentation 30' on the opposite side of the head 25 and is urged into the indentation by a compression spring 37. The heads 26,27 are isolated from one another by a central spacing unit 31 positioned between mu metal isolaters 32.

The support brackets 28 define an elongate slot 33 through which a plastic card 34 is guided in use.

Opposite the head 25 is an idling roller 35 which is held by a tensioning spring 36 so as to ensure good contact between the card 34 and the head 25 at all times.

The heads described up until now have comprised dual head devices in which one head is arranged to read permanent data while the other reads soft data. We have found, however, that it is possible to provide a twin device for reading all four Tracks of the magnetic strip, as shown in Figure 20. In this case, a first dual head device 60 similar to the device shown in Figure 7 is provided for reading Tracks O and 1 (permanent and soft data respectively) while a second device 61 is positioned adjacent the device 60. The second device comprises a pair of heads 62, 63 each of which has a pair of pole pieces defining gaps 64, 65 respectively which are orthogonal to the direction of the movement of the magnetic strip so that the soft data in Tracks 3 and 4 can be read.

Claims

l. A data transfer head for reading from or writing to a substantially rigid magnetic medium, the head comprising a support (15); a pair of pole pieces between which an elongate gap is defined, the pole pieces being mounted to the support; and a contact area (11',12') including the pole piece faces along which the magnetic medium passes in use; characterized in that the faces of the pole pieces define the contact area, the length of the gap constituting the largest dimension of the contact area.
2. A head according to claim 1, wherein the contact area is square or rectangular, the gap constituting a diagonal of the square or rectangle.
3. A data transfer head for reading from or writing to a substantially rigid magnetic medium, the head comprising a support (15); a pair of pole pieces between which an elongate gap is defined, the pole pieces being mounted to the support; and a contact area (11',12') including the pole piece faces along which the magnetic medium passes in use; characterized in that the contact area is less than 11.5mum2.
4. A head according to claim 3, wherein the contact area is substantially lmm2.
5. A data transfer head for reading from or writing to a substantially rigid magnetic medium, the head comprising a support (15); a pair of pole pieces between which an elongate gap is defined, the pole pieces being mounted to the support; and a contact area (11',12') including the pole piece faces along which the magnetic medium passes in use; characterized in that the length of the gap defined between the pole pieces is less than 2mm.
6. A head according to claim 5, wherein the gap length is substantially 1.4mm.
7. A data transfer head for reading from or writing to a substantially rigid magnetic medium, the head comprising a support (15); a pair of pole pieces between which an elongate gap is defined, the pole pieces being mounted to the support; and a contact area (11',12') including the pole piece faces along which the magnetic medium passes in use; characterized in that the ratio of contact area (win2) to gap length (mm) is less than 3:1.
8. A head according to claim 7, wherein the ratio is substantially 1:1.4.
9. A head according to any of the preceding claims, wherein the pole pieces are made of ferrite.
10. A dual head device having a pair of data transfer heads, at least one of which is constructed in accordance with any of the preceding claims.
11. A device according to claim 10, wherein the gaps defined by each head extend in different directions.
12. A data reading system comprising a head according to any of the preceding claims; an electrical circuit connected to the pole pieces; and monitoring means (40) for monitoring variations in the circuit due to the passage of a magnetic medium past the head.
13. A system according to claim 12, wherein the monitoring means (40) senses the generation of an e.m.f.

in the circuit.
14. A system according to claim 12, wherein the monitoring means (40) senses changes in self-inductance in the circuit.
15. A method of operating a system according to claim 12, when dependant on claim 10, for use with a magnetic medium having magnetic particles permanently oriented in a predetermined manner, the particles being selectively magnetized along at least one track, wherein a first one of the heads is aligned with the one track and a second one of the heads is aligned with the magnetic medium spaced from the first head, the method comprising causing relative movement between the heads and the magnetic medium; and monitoring e.m.f.s generated in the circuit connected to the pole pieces of the first head and monitoring changes in self-inductance in the circuit connected to the pole pieces of the second hand.
16. A magnetic data recording system comprising a head according to any of claims 1 to 9; an electrical circuit connected to the pole pieces; and generating means for generating e.m.f.s in the circuit so to cause corresponding changes in magnetic flux between the pole pieces.
17. A method of operating a data recording system according to claim 16, the method comprising causing relative movement between a substrate carrying a magnetic medium and the head; and causing predetermined changes in magnetic flux between the pole pieces so that said changes are recorded in the magnetic medium on the substrate.
18. A method according to claim 17, wherein the magnetic particles in the magnetic medium are permanently oriented in a predetermined manner.
19. A method according to claim 15 or any of claims 17 and 18, wherein the magnetic medium is mounted on a plastics substrate.
20. A method according to claim 19, wherein the plastics substrate comprises an identification or financial card.
21. A method according to claim 15 or any of claims 17 to 20, wherein the or each gap extends transversely to the direction of movement of the substrate in use.
22. A method according to claim 21, wherein the or one of the gaps extends at an angle other than 900 to the direction of movement of the substrate in use.
23. A method according to claim 22, wherein the gap extends at 450.