WO1990010291A1

WO1990010291A1 - Re-usable information recording sheet material

Info

Publication number: WO1990010291A1
Application number: PCT/GB1990/000280
Authority: WO
Inventors: Richard Graham Outram; D. J. Mapps
Original assignee: Remote Image Systems Limited
Priority date: 1989-02-23
Filing date: 1990-02-22
Publication date: 1990-09-07
Also published as: AU5160190A; GB8904198D0

Abstract

A reusable information recording material comprises a layer (10) of transparent plastics formed with dimples (16) on one side, a further plastics layer (34), having a magnetic coating (14), being bonded to the dimpled side of layer (10), with the magnetic coating (14) directly adjoining the layer (10), so that respective closed cavities are defined by the dimples (16) and the layer (34, 14). Each said cavity contains one or more indicator particles (18) in the form of permanent magnets, each indicator particle (18) being dark on the side thereof corresponding with one pole and light on the side thereof corresponding with the other pole. The indicator particles are visible from one side of the material and the arrangement is such that the orientation of each particle (18) mirrors closely the orientation of the magnetic field, due to the magnetic layer (14), in the immediate vicinity of the particle. Consequently the particles provide an accurate visual indication of the corresponding state of magnetisation of the magnetic layer (14) and therefore the particles (18) can be made to represent, in visible form, information recorded magnetically on the magnetic layer, and thus, for example can be made to display a text or graphics.

Description

Title: "Re-usable information recording sheet material"

THIS INVENTION relates to a re-usable information recording sheet material which may be used as a substitute for paper.

Paper is used to carry a great deal of information in various forms, for example in the forms of writing, printing, drawn or photographic pictures and so on. Some of this paper is stored as a record of the information thereon, but much, for example in the form of newspapers and magazines, has an ephemeral information content and is discarded or disposed of after only a short period as a bearer of information.

An object of the invention is to provide a sheet material which can be "written" on, erased, and "written" on over and over again.

It is another object to provide such a sheet material having some of the desirable characteristics of paper in that it may be made of similar thickness, may be made white or pale in colour, as most paper, and handy to use.

It will be appreciated that even though paper is really very cheap and is relatively easy to dispose of, there are ecological objections to the continued conversion of forests on an ever increasing scale into a material destined to be disposed of after a very short useful life. Furthermore, modern technology is effecting rapid and significant changes in the way in which information is recorded and stored, and it is anticipated that, in the future, information of all sorts will be in digital form which can be transmitted around the world at will and stored electronically. Provided that quality is adequate, printing can then be done "on-demand" where it is needed. Complete "reusability" of the medium upon which such printing is carried out would eliminate much of the need to distribute tons of wood or paper over long distances.

Conventionally, most images (text and pictures) are transferred to paper by selectively coating the paper with a material of contrasting colour, such as ink. In its most primitive form a person may write on the paper with pencils, crayons, inks or paints. It is only with certain of these coatings, e.g. pencil, that the markings so made may be erased. This is a cumbersome process. Mechanical forms of printing include typing and lithography where selected patterns are inked and transferred to the paper. None of these images can be readily erased. More recent means for writing on paper include the dot-matrix printer, photostatic and laser printing and inkjet printing. All of these processes involve the selective transfer of "inks" (includ¬ ing toners) to the paper. One other type of printing is the "thermal" process, where selective heating will convert special paper from white to black. Thermal colour printing involves the transfer of a dye to the paper. Images produced by the last-noted process again cannot readily be erased.

According to the invention there is provided a reusable information recording material comprising a re¬ usable information recording material comprising a matrix layer supporting a plurality of indicator particles, the arrangement being such that said indicator particles are visible from one side of said material, each of said indicator particles being free to undergo rotational motion relative to one another between a first and a second opposite orientation and wherein each said particle, in said first orientation, presents a visual aspect, to an observer or sensing instrument surveying the material through the said one side of the material, which contrasts with the corresponding aspect in said second orientation thereof, the arrangement being such that information may be recorded on the material by changing the positions of said indicator particles locally by applying appropriate physical forces to the particles.

According to a further aspect of the invention there is provided a method of making a reusable information recording material, comprising forming an array of recesses or dimples in a plastics sheet, providing a plurality of magnetic indicator particles of permanently magnetised material and marked to present different aspects from opposite ends thereof, introducing said indicator particles into said dimples, and enclosing the particles in said dimples by bonding to the dimpled surface of said plastics sheet a further plastics sheet, one of said sheets bearing a magnetic coating.

According to a yet further aspect of the invention, there is provided a writing head for recording on a material in accordance with the first-noted aspect of the invention, the writing head being in the form of a sheet-like structure one edge of which forms the operative face of the head adapted to cooperate with the information recording material, the sheet-like laminated structure comprising a layer of ferro-magnetic material which, in the region of said operative face, is contoured to form a plurality of individual fingers or teeth terminating in said operative face, each said finger or tooth forming a respective pole of the head, the sheet-like structure further comprising a conductor layer including, for each said pole, a respective conductor section extending transversely across the respective tooth or finger, adjacent said operative surface, means for passing electrical currents selectively through said conductor sections whereby selected said poles can be magnetised longitudinally, and thus perpendicular to said operative face, by passing current through the conductor section traversing that pole.

Embodiments of the invention are described below by way of example with reference to the accompanying drawings wherein:-

FIGURE 1 is a perspective sectional view showing a fragment of an information recording material embodying the invention,

FIGURE 2 is a diagrammatic view illustrating steps in the manufacture of the material of Figure 1 ,

FIGURE 3 is a schematic view illustrating the manu¬ facture of the material of Figure 1 ,

FIGURE 4 is a diagrammatic perspective view of a writing head for the material,

FIGURE 5 is an exploded perspective view showing part of the head of Figure 4,

FIGURE 6 is a fragmentary plan view of the part shown in Figure 5,

FIGURE 7 is a corresponding end view and,

FIGURE 8 is a corresponding view in section on the line VIII-VIII of Figure 7.

Referring to Figure 1 , the information-recording sheet material shown comprises a layer of transparent plastics film 10, for example, of PVC. The lower surface of the layer 10 (as viewed in the drawing) is formed with minute dimples or depressions 16 and each said dimple or depression 16 contains one or more magnetic indicator particles 18 (described in more detail below). Bonded to the lower side (as viewed in Figure 1) of the film 10 is a thin plastics film 34, for example of PET, which has, on its upper side (as illustrated) a magnetic coating 14, similar to that conventionally used on magnetic recording tapes, but preferably having the magnetic particles therein so oriented that they tend to adopt magnetisation substantially per¬ pendicular to the general plane of the layer, as used in so- called "perpendicular recording". The layer 34 thus closes off the dimples 16 and defines, with each dimple 16, a respective cavity containing the respective magnetic particle or particles 18, the magnetic coating 14 thus providing the lower wall of each said cavity. The layer 10 is covered by a thin transparent .protective layer 20 of PET.

The regions of the layer 10 between the dimples serve to maintain a relatively uniform spacing between the magnetic coating 14 and the upper end (as viewed in Figure 1) of each depression 16. As will appear from what follows, it is preferable that the said regions between the dimples 16, where the lower surface of the layer 10 contacts the coated side of the layer 34 (i.e. the regions in which there are no indicator particles between the layer 10 and the layer 34), should be as small as possible and should represent as small a proportion of the total area of the material as possible. Because the particles are effectively held in position by the magnetic field due to the layer 14, there is little tendency for the particles to migrate from one area of the material to another, so that containment of the particles is not really a significant function of the dimples 16.

Each indicator particle ideally comprises a minute sphere which has opposite hemispheres thereof of contrasting colours. Thus, for example, each sphere or spheroid may be black over one hemisphere and white over the other. Each sphere is a minute permanent magnet, with the magnetic axis of the sphere being so oriented that, when aligned with a magnetic field extending perpendicular to the plane of the layer 10, either the black hemisphere or the white hemi¬ sphere is exclusively exposed to view through the layer 20, depending upon the direction of that magnetic field, that is to say, the magnetic axis of the particle is substantially perpendicular to the equatorial line separating the black and white hemispheres thereof. In practice, it is difficult to ensure that the particles 18 are precisely spherical, and the particles may be spheroidal, in the form of cubes with rounded corners, or in the form of discs or of irregular shape, although ideally having a longest dimension of the same order of size as the shortest dimension.

Various materials may be used for making the indicator particles and various methods of manufacture are possible. The material for the indicator particles may comprise a plastics material, such as PVC with a filler constituted by very fine magnetic particles, for example of barium ferrite, making up a large proportion of the material, e.g. 70$ by weight of the indicator particle material.

Such material may be, for example, extruded in fine rods chopped to provide cylinders of around the same length as their diameters, or may be cast in minute cavities in a mould or die surface, the fluid material being subjected to an appropriately oriented magnetic field during extrusion or casting to obtain the desired alignment of the magnetic filler particles in the medium. The indicator particles thus formed may then be spread on a support surface and oriented perpendicularly to such surface by an applied magnetic field before being coated, on the exposed halves of their surfaces, with an "ink" of colour contrasting with the colour of the filled plastics material. Where barium ferrite is used as a filler, the filled PVC material is black, so that the contrasting ink used should be white, although a metallised deposit may be utilised instead of white ink to enhance reflectivity. It is currently envisaged, however, that the indicator particles may be "punched" or "pressed" from a ttiin sheet comprising a black layer (e.g. around 10 μ thick) of PVC with a barium ferrite filler making up 70% by weight of the filled plastics, and a superimposed uniform thin (e.g. 5 μ thick) coating of white ink on one surface of the PVC layer, the magnetic particles in the PVC/barium ferrite layer being oriented perpendicularly to the plane of the film. To form the indicator particles, this sheet material is passed, as indicated schematically at 54 in Figure 2, through the nip between cooperating rollers, referenced 50 and 52, the roller 50 being formed on its surface at regular spacings, with a plurality of minute projections forming, in effect punches, and the cooperating rollers 52 being formed with corresponding minute recesses, forming dies, each said recess registering with a respective projection on the roll 50 as the rollers rotate in synchronism, whereby, as each projection or punch on roller 50 moves into the nip area, it punches a minute disc or cylinder from the sheet 54, into the respective recess or die in roller 52. The indicator particles, constituted by said minute discs or cylinders may be extracted from the recesses in roller 52 by being attracted magnetically therefrom.

The dimples 16 may be formed in the layer 10 by passing a PVC film, destined to form said layer, in a heated and softened condition, through the nip between cooperating chilled pressure rolls, one of which is formed on its surface with an array of regularly spaced projections similar to those on the roller 50, whilst the other is a smooth counter-roll. Preferably, and as illustrated in Figure 3, in which the chilled roller with projections is referenced 60 and the chilled counter-roller is referenced 62, the PVC film 10, already bonded to the protective PET layer 20, is drawn from a supply roll 64, through an oven 66 to heat and soften the PVC layer 10 and thence through the nip between the rolls 60 and 62. The projections (not shown) on the roll 60 of course form the dimples 16. In the process illustrated diagrammatically in Figure 3, after the layer 10 has been dimpled, the material passes through a "printing" station 68, where the indicator particles are inserted in the respective dimples 16 by a process analogous to printing, in that individual indicator particles, or small clumps of such particles, are deposited in pre¬ determined locations on the layer 10, which locations are arranged to correspond with the locations of the dimples 16.

The "printing" station 68 preferably incorporates the rollers 50 and 52 of Figure 2 and means for feeding the sheet 54 of inked magnetic PVC material to the rollers 50 and 5^'2. In this preferred arrangement, the sheet comprising the layer 10 and PET backing 20 engages the circumference of roller 52 at a position opposite the nip between rollers 50 and 52, so that, as the sheet 10, 20 meets the roller 52, the dimples in the layer 10 register accurately with the recesses in the surface of roller 52. A magnet 70 on the opposite side of the sheet 10, 20 from the roller 52 attracts the indicator particles from the recesses in roller 52 into the dimples 16 as the latter come into register with the recesses.

After passing the "printing" station 68, the web 10, 20, with the indicator particles in the recesses 16, is bonded to the layer 34, having the magnetic coating 14, by passing a corresponding coated film, with the web 10, 20, through the nip between further pressure rollers 72, after which the product is wound on a take-up roll 76.

It will be appreciated that the size of the pro¬ jections on roll 50, the recesses in roll 52, the spacing between adjoining projections and between adjoining recesses and the thicknesses of the films, in relation to the overall dimensions of the rolls 50 and 52 is much exaggerated in Figure 2 and that in practice, the pro- jections on roller 50 and the recesses in roller 52 are, like the dimples 16, minute, e.g. of the order of 25 μ in diameter and are spaced apart by distances of the same order. It will be appreciated that the manufacturing method proposed requires accurate registration between the rolls 50 and 52 and between the rolls and the web 10, but the difficulties involved are of the same order as are met, for example, in conventional high quality printing techniques.

It will be appreciated that sundry variants of the re¬ usable sheet material described above are possible. Thus, for example, the layer 10 may have the magnetic coating 14 applied directly to its surface remote from the dimpled surface with the open ends of the dimples being covered by a transparent layer of plastics foil.

In another variant, the magnetic coating 14 may be applied to the side of film 34 which is further from the layer 10, in order to ensure that the magnetic coating 14 will be directly adjacent the operative surface of the writing head (see below). This, however, means that the particles 18 will be on the opposite side of the film 34 from the magnetic coating 14. In either case in order to achieve good resolution the magnetic coating on the film 34 must be relatively thin.

In order to improve the magnetic resolution or definition in the magnetic coating 14, the coating may be arranged as an array of dots, each underlying a respective depression 16.

In order to write on the recording material, the material, indicated at 8 in Figure 4, is passed longitudin¬ ally, in the direction indicated by arrow Y, under a recording head 30 which provides an operative face, very narrow, (about 25μ) in the direction of advance of the material 8, and extending transversely for substantially the whole width of the material 8. The head 30 is a perpendicular recording head and serves to "write" onto the magnetic coating 14 a magnetic pattern corresponding to the desired visual pattern. The material 8 is oriented upside down, as compared with Figure 1, in Figure 5, so that the coated film 34 is uppermost.

The head 30 comprises, in principle, a respective magnetic pole for every dimple 16 across the width of the material 8 and the apparatus is capable of reversing the magnetic polarisation of each pole in the time taken for successive dimples 16 to pass such pole in the direction of arrow Y. As indicated above, the magnetisation applied by the poles of head 30 to layer 14 of the material is aligned vertically, i.e. perpendicular to the plane of the material 8. This vertical magnetisation in turn affects the orientation of the magnetic particles 18. The force on each particle 18 due to the magnetic medium 14 is greater than that due to the adjoining particles 18 so that the orientation of the particles is determined exclusively by the state of magnetisation of the layer 14. Furthermore, because the magnetic head 30 is close to the magnetic medium 14, resolution of the system is relatively high.

For reasons explained below, the head is formed as a series of head segments, each occupying a respective section of the width (measured transverse to the direction Y of transport of the recording material), of the head 30.

Referring to Figures 5 to 8, the construction of each segment of the head is as follows.

Each segment of the head is of laminar construction, comprising a continuous substrate layer 90 of ferrite. Along one edge of the ferrite layer 90, corresponding with the operative face of the head, the layer 90 has a shallow rebate on its upper surface which receives a copper strip 92, the upper surface of which is flush with the upper surface of the layer 90. The upper surface of the layer 90 and the copper strip 92 are covered by a layer 94 of insulating plastic film, on top of which is laid a "comb" of copper foil, indicated at 96 and which comprises a narrow strip extending parallel with the strip 92 and directly above the latter, so that the last-noted part of the "comb" has a longitudinal edge which also lies in the operative face of the head, and a plurality of "teeth" extending over the upper surface of the layer 94 away from the operative face of the head to respective terminals of an electronic writing device (not shown).

. Superimposed on the layer of foil 96 is a . further "comb" of soft magnetic metal foil, for example of nickel iron alloy, indicated at 98. The "comb" 98 is, however, orientated oppositely from the "comb" 96 in that the "back" of the comb is located remote from the operative face of the head and the "teeth" 100 of the comb extend, from the "back" of the comb 98 towards the operative face of the head, the "teeth" 100 of the comb terminating in end faces forming part of the operative face of the head. The teeth 100 overlie the gaps between the teeth of the comb 96, as best shown as Figure 6. Thus, adjoining the operative face of the head, the free end portion of each tooth of the comb 98 overlies a short length of copper conductor, afforded by a respective part of the back of the comb 96, connecting the adjacent "teeth" of the comb 96 electrically. If, for example, an electrical current is passed between the teeth 97a and 97b shown as Figure 6, by the electronic drive circuitry, there will be a corresponding flow of current, in the plane of the sheet of foil 96, parallel with the longitudinal direction of the edge of the reading head, that is to say perpendicular to the paper in Figure 8, and the magnetic field due to the current in this section will temporarily magnetise in the tip of the overlying Ni-Fe comb in the longitudinal direction of the tooth (i.e. per- pendicular to the operative face of the head).

The copper strip 92, extending across an entire width of the head, is intended to carry a bias current. Prefer¬ ably the bias current and, (when applied) the current between adjoining "teeth" of the copper comb 96 are each of such a strength as to produce slightly more than half of the magnetic field required to reverse the magnetisation, locally, of the magnetic backing of the "new paper material" in contact with the head, or to rotate the magnetic particles in the adjoining dimples. Consequently, the magnitude of the currents which have to be switched by the electronic drive circuitry for the individual "poles" of the head is substantially reduced, allowing a cost saving in the manufacture of such drive circuitry.

It will be understood that the planes of the various laminae making up the head 30 are perpendicular to the plane of the material 8 as the latter passes under the head in Figure 4.

The construction of the printing head 30 as a series of segments is adopted partially to facilitate manufacture and partially to allow adjustment of the head 30 to variations in pitch between dimple 16 in the material 8, either with temperature or as a result of manufacturing variations.

Each segment 30 preferably carries, bonded to the ferrite substrate, for example, and directly connected with the conductors formed by the "teeth" of the respective copper foil "comb" 92, a respective semi-conductor chip comprising the drive circuitry for that particular segment of the head 30. It is envisaged that each segment of the head may provide around 100 poles (i.e. may have 100 teeth in the Ni-Fe comb for that segment) and, correspondingly 100 teeth in the copper foil comb 92, forming respective conductor leads of the respective drive "chip". The associated electronic circuitry is preferably arranged to energise the individual poles of the individual head segments in parallel, with the individual "chip" on each head segment being arranged to receive data in serial form and to determine the appropriate signals to be supplied in parallel form to the "teeth" of the respective comb 92 to produce the required pattern. The function of the necessary circuitry will be evident to those skilled in the art of computer-controlled printing devices and will not be dis¬ cussed in detail here.

In order to secure optimum resolution, the head 30 should, at the time each set of magnetic pulses is applied to switch the indicator particles directly below the head 30, be in exact register with the respective transverse row of dimples 16, with each pole of the head 30 being in correct register with the respective - longitudinal row of dimples 16 and Figure 4 illustrates various mechanisms which may be utilised to ensure appropriate alignment. Thus, the head 30 may be supported by links 104, 108, permitting lateral adjustment of the head 30 by an appropriate servo mechanism, the mounting of the link 108, for example, being adjustable in the "Y" direction by a further servo mechanism, to adjust the skew of the head 30. Likewise, in order to minimise head wear and facilitate high-speed writing, it is preferably that a very slight spacing should be maintained between the operative surface of the head 30 and the adjacent surface of the material 8, and to this end further servo mechanisms 110, 112 are preferably provided for adjusting the height of the head 30, above the material 8, at each end of the head 30. The servo mechanisms utilised may be similar to those used in, for example, existing compact disc players or the like and may be controlled by sensors sensing reference markings or tracks provided along the edges of the material 8, for example. Furthermore, in order to compensate for variations in the transverse pitch between dimples 16, means may be provided for adjusting the lateral spacing between adjoining print head segments, such adjustment representing a compromise between an ideal but probably impracticable arrangement in which the spacing of each individual pole of the head with respect to the adjoining pole is variable and an arrangement where no provision at all is made for adjustment to variations in transverse pitch between the dimple 16.

It will be appreciated that if it possible to accept a minimum resolution of the printing system which is sub¬ stantially greater than the pitch between individual dimples 16, so that, for example, each "pole" of the head 30 is wide enough to overlap two or more rows of dimples 16, then the alignment, skew, etc. of the head 30 with respect to the material becomes substantially less critical.

Whilst it is preferred to utilise magnetic indicator particles, held in their desired orientations by an associated "permanent" magnetic layer 14, it may be advantageous to utilise a magnetically "soft" layer as the layer 14, the induced magnetism in this layer being maintained by the magnetism of the adjoining indicator particles until the material is "written" on afresh by the writing head.

Furthermore it is envisaged that electrostatically polarised indicator particles might be used instead of the magnetic indicator particles, for example with some form of stabilising polarised dielectric layer, for example being utilised in place of the magnetic layer 14, or with a photoconductive layer or photo electret layer in place of the magnetic layer, which could be "written to" by laser light, so changing the charge and rotating the electro¬ statically charged indicator particles. Whilst in such an arrangement, the indicator particles would ideally be permanently charged, some form of charge "refresh" arrange- ment might be adopted if it proves impracticable to attain this ideal. Alternatively, the indicator particles may take the form of electret particles.

In a further variant, a re-usable information recording material comprises a plurality of indicator particles accommodated in a cavity or cavities in a said matrix layer, the matrix layer including, or being contiguous with, a transparent layer bounding said cavities on one side of the material, each said cavity being fitted with an opaque or light-filtering fluid presenting, as viewed from said one side of the material, a contrasting aspect as compared with said indicator particles, said particles being movable, by externally applied magnetic or electrostatic forces acting generally perpendicular to said material, between positions at the ends of said cavities closest to said one side, in which the particles are visible from said one side, and positions at the ends of said cavities furthest from said one side, in which the particles are obscured from view by said fluid, whereby by selective application of such magnetic or electrostatic forces, a desired pattern or text may be recorded on said material.

As in the preceding embodiments, a recording or stabilising layer may be incorporating material to maintain the magnetic or electrostatic field pattern which corres¬ ponds with the positions of the indicator particles which render the recorded information visually readable or view¬ able.

Claims

1. A reusable information recording material comprising a matrix layer supporting a plurality of indicator particles, the arrangement being such that said indicator particles are visible from one side of said material, each of said indicator particles being free to undergo rotational motion relative to one another between a first and a second opposite orientation and wherein each said particle, in said first orientation, presents a visual aspect, to an observer or sensing instrument surveying the material through the said one side of the material, which contrasts with the corresponding aspect in said second orientation thereof, the arrangement being such that information may be recorded on the material by changing the positions of said indicator particles locally by applying appropriate physical forces to the particles.

2. A reusable information recording material according to claim 1 wherein said indicator particles are magnetic indicator particles accommodated in a cavity or cavities in said matrix layer, the arrangement being such that said indicator particles are visible from one side of said material, the material including a magnetic recording layer disposed on the opposite side of said cavity or cavities from said one side of the material, the arrangement being such that the orientation of each said particle mirrors closely the orientation of the magnetic field, due to the magnetic layer, in the immediate vicinity of the particle, whereby the particles provide an accurate visual indication of the corresponding state of magnetisation of the magnetic layer and therefore the particles can be made to represent, in visible form, information recorded magnetically on the magnetic layer.

3- A reusable information recording material according to claim 2 comprising a first layer of transparent plastics film formed with a plurality of dimples in one face of said first layer, said dimples defining said cavities, a further layer of plastics film supporting a magnetic recording layer and bonded to said one face of said first layer so that the magnetic recording layer directly adjoins said dimples and thus closes off said cavities, to retain the indicator particles therein, the indicator particles being visible from the face of said first layer remote from said one face.

4. A material according to claim 3₅ providing an array of such cavities, each containing a respective magnetic particle or particles, and wherein said magnetic recording layer is in the form of a discontinuous layer comprising a corresponding array of spots, each spot being disposed at the mouth of a respective said cavity.

5. A material according to claim 3 wherein said magnetic recording layer is in the form of a continuous layer.

6. A material according to any preceding claim, wherein the material of said magnetic layer is adapted to be magnetised perpendicularly to the planes of the layers or films and each said particle in each of said first and second opposite orientations, has its magnetic axis sub¬ stantially perpendicular to the magnetic layer.

7. A method of making a reusable information recording material, comprising forming an array of recesses or dimples in a plastics sheet, providing a plurality of magnetic indicator particles of permanently magnetised material and marked to present different aspects from opposite ends thereof, introducing said indicator particles into said dimples, and enclosing the particles in said dimples by bonding to the dimpled surface of said plastics sheet a further plastics sheet, one of said sheets bearing a —Io-

magnetic coating.

8. A method according to claim 7 wherein said magnetic coating is applied to said further sheet before the further sheet is bonded to said dimpled sheet, and is applied on the surface of said further sheet adjacent said dimpled sheet.

9. A writing head for recording on a material according to any of claims 2 to 6, the writing head being in the form of a sheet-like structure one edge of which forms the operative face of the head adapted to cooperate with the information recording material, the sheet-like laminated structure comprising a layer of ferro-magnetic material which^', in the region of said operative face, is contoured to form a plurality of individual fingers or teeth terminating in said operative face, each said finger or tooth forming a respective pole of the head, the sheet-like structure further comprising a conductor layer including, for each said pole, a respective conductor section extending trans¬ versely across the respective tooth or finger, adjacent said operative surface, means for passing electrical currents selectively through said conductor sections whereby selected said poles can be magnetised longitudinally, and thus perpendicular to said operative face, by passing current through the conductor section traversing that pole.

10. A writing head according to claim 9 wherein said sheet-like laminated structure further comprises a base layer of ferro-magnetic material also terminating in said operative face.

11. A writing head according to claim 10 wherein said ferro-magnetic layer affording said poles is disposed between said conductor layer and said base layer of ferro- magnetic material and wherein, adjacent said operative face, a further conductive strip extends along said face and thus across said poles, between the layers of material affording said poles and the base layer.

12. A reusable information recording material according to claim 1 wherein said indicator particles are electro¬ statically charged in such a way as to form electrostatic dipoles, said indicator particles being accommodated in a cavity or cavities in said matrix layer, the arrangement being such that said indicator particles are visible from one side of said material, the material including an electrostatic recording layer disposed on the opposite side of said cavity or cavities from said one side of the material, the arrangement being such that the orientation of each said particle mirrors closely the orientation of the electrostatic field, due to the electrostatic recording layer, in the immediate vicinity of the particle, whereby the particles provide an accurate visual indication of the corresponding state of charge of the recording layer and therefore the particles can be made to represent, in visible form, information recorded electrostatically on the electrostatic recording layer.

13- A reusable information recording material comprising a plurality of indicator particles accommodated in a cavity or cavities in a said matrix layer, the matrix layer including, or being contiguous with, a transparent layer bounding said cavities on one side of the material, each said cavity being fitted with an opaque or light-filtering fluid presenting, as viewed from said one side of the material, a contrasting aspect as compared with said indicator particles, said particles being movable, by externally applied magnetic or electrostatic forces acting generally perpendicular to said material, between positions at the ends of said cavities closest to said one side, in which the particles are visible from said one side, and positions at the ends of said cavities furthest from said one side, in which the particles are obscured from view by said fluid, whereby by selective application of such magnetic or electrostatic forces, a desired pattern or text may be recorded on said material.

14. A material substantially as hereinbefore described with reference to, and as shown in Figures 1 to 3 of the accompanying drawings.

15. A method of making a reusable information recording material, substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

16. A writing apparatus for a material according to claim 9 and substantially as hereinbefore described with reference to, and as shown in, Figures 4 to 8 of the accompanying drawings.