US3562759A

US3562759A - Pattern forming apparatus

Info

Publication number: US3562759A
Application number: US705590*A
Authority: US
Inventors: Arthur Edward Brewster
Original assignee: International Standard Electric Corp
Current assignee: STC PLC
Priority date: 1966-11-11
Filing date: 1968-01-11
Publication date: 1971-02-09
Anticipated expiration: 1988-02-09
Also published as: SE339883B; GB1183587A; DE1524883A1; FR1553848A; CH480673A; NL6715257A

Abstract

AN APPARATUS IS PROVIDED WHEREIN A CHARGE PATTERN, WHICH IS REPRESENTATIVE OF INFORMATION CONTAINED IN A SIGNAL APPLIED TO THE APPARATUS, IS FORMED ON ONE SIDE OF A RECORDING MEDIUM BY RECORDING MEANS SITUATED ON THE OTHER SIDE OF SAID RECORDING MEDIUM. POWDER PARTICLES ARE APPLIED TO THE CHARGE PATTERN TO PROVIDE A TWODIMENSIONAL VISIBLE IMAGE OF THE CHARGE PATTERN ON THE ONE SIDE OF THE RECORDING MEDIUM. THE APPARATUS INCLUDES A CAVITY HOUSING THE POWDER PARTICLES, THE REAR WALL OF WHICH IS THE ONE SIDE OF THE RECORDING MEDIUM AND THE FRONT WALL IS TRANSPARENT TO FACILITATE VIEWING. POWDER APPLICATION MEANS INCLUDE A TRANSPARENT DIAPHRAGM CONSTITUTING PART OF THE FRON WALL.

Description

I Feb. 9, 1911 A. E; BREWSTER PATTERN FORMING APPARATUS Filed Jan. 11. 1968 FIG 7 11 Sheets-Sheet 1 38- 39 V l 'L" Flew). W, F/G.7@}f

Inventor Anna/k z. 8R5 wsrm A Home y ,1 A. E. mews-reg 5 PATTERN FORMING APPARATUS Filed Jan; 11. 1968 11 Sheets-Sheet 2 7 a M 15 I 8 16 21 10 72 I nnnnn or ARTHUR Z. BREWSTER PATTERN FORMING APPARATUS Filed Jan. 11, 1968 11 Sheets-Sheet 8 I ARTHUR E. Rlk/STER A Home y Feb. 9, 1971 A. E. BREWSTER PATTERN FORMING APPARATUS 11 Sheets-Sheet 4.

Filed Jan. 11. 1968 F/GS.

I nvenlor ARTHUR E. BREWSTER Feb. 9, 1971 A. E. BREWS TER I PATTERN FORMING APPARATUS 11 Sheets-Sheet 5 Filed Jan. 11. 1968 2 A f-7G8A.

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med Jan. 11, 1968 Feb. 9, 1971 AB. BREWSTER v PATTERN FORMING APPARATUS 11 Sheets-Sheet 6 Invenlor ARTHUR LEREWSTE'R A Home y Feb. 9, 1971 A. E. BREWSTER PATTERN FORMING APPARATUS l1 Sheets-Sheet 7 Filed Jan; 11, 1968 I!!! III II IIII I r I I I IIIIIIII m3 GI IIIIIIIIIIIIIIINIIIIIIIIIIIII/IIIfIIIIII/III I I I I B mm fi 3 wt- I nvenlor 7 AR THUR E. BREWSTE R Home y A. E. BREWSTER PATTERN FORMING APPARATUS Feb. 9 197 1 11 Sheets-Sheet 8 Filed Jan. 11, 1968 E OU/ VALE N T RECORDING POSITION BIA S AMPL/ -TUDE lnvenlor ARTHUR E. BREWSTER J Home y Feb. 9; 1971 QA. E. BREWSTER 3,562,759

PATTERN FORMING APPARATUS Filed Jan. 11. 196E 11 Sheets-Sheet 9 lnvenlor ARTHUR E, BREWSTER Attorney Feb. 9, .1971 I A. E. BREWSTER 3,552,759

7 PATTERN FORMING APPARATUS Filed Jan. 11. 1968 11 Sheets-Sheet 10 67 GAP FIGJQ LENGTH(L) I nvcnlor .4 Home y United States Patent 3,562,759 PATTERN FORMING APPARATUS Arthur Edward Brewster, Cheshunt, England, assignor to International Standard Electric Corporation, New York, N.Y., a corporation of Delaware Continuation-impart of application Ser. No. 675,542, Oct. 16, 1967. This application Jan. 11, 1968, Ser. No. 705,590 Claims priority, application Great Britain, Nov. 11, 1966, 50,574/66 Int. Cl. G01d 15/06, 15/12; Bb 5/02 US. Cl. 346-74 18 Claims ABSTRACT OF THE DISCLOSURE An apparatus is provided wherein a charge pattern, which is representative of information contained in a signal applied to the apparatus, is formed on one side of a recording medium by recording means situated on the other side of said recording medium. Powder particles are applied to the charge pattern to provide a twodimensional visible image of the charge pattern on the one side of the recording medium. The apparatus includes a cavity housing the powder particles, the rear wall of which is the one side of the recording medium and the front wall is transparent to facilitate viewing. Powder application means include a transparent diaphragm constituting part of the front wall.

This invention is a continuation-in-part of an application by Arthur E. Brewster, Ser. No. 675,542, filed Oct. 16, 1967, now abandoned.

The invention relates to pattern forming apparatus.

The invention provides apparatus wherein a charge pattern which is representative of information contained in a signal applied to said apparatus is formed on one side of a recording medium by recording means situated on the other side of said recording medium.

According to a feature of the invention apparatus as outlined in the preceding paragraph is provided wherein powder particles are applied to said charge pattern to provide a two-dimensional visible image on said one side of said recording medium.

The foregoing and other features according to the invention will be understood from the following description with reference to FIGS. 1 to of the drawings which accompanied the original application and to FIGS. 11 to 24 of the accompanying drawings, in which:

FIG. 1 shows diagrammatically a side elevation of a magnetic display device;

FIG. 2 shows a perspective view of a ferrite block slotted and drilled, to make a recording head assembly for the magnetic display device shown in the drawing. according to FIG. 1;

FIG. 3 shows a side elevation of the ferrite block shown in the drawing according to FIG. 2 on the line XX;

FIG. 4 shows a wound core which forms part of a recording head assembly for the magnetic display device shown in the drawing according to FIG. 1;

FIG. 5 shows the method of making a recording head assembly using the wound core shown in the drawing according to FIG. 4;

FIG. 6 shows in enlarged detail the magnetic recording gap provided by the construction shown in the drawing according to FIG. 5;

FIG. 7 shows a perspective view of a finished recording head assembly with five cores;

3,562,759 Patented Feb. 9, 1971 FIG. 8A shows a recording head assembly using the wound core shown in the drawing according to FIG. 4;

FIG. 8B shows diagrammatically a sectioned side view of FIG. 8A on the line ZZ;

FIG. 9 shows in enlarged detail the ends of the windings on the cores as depicted in the drawing according to FIG. 8B;

FIGS. 10A and 10B show respectively front and side elevations of a magnetic core which is used as part of a magnetic recording head for the magnetic display device shown in the drawing according to FIG. 1;

FIG. 11 diagrammatically illustrates in side elevation a modified arrangement of the magnetic display device shown in the drawing according to FIG. 1;

FIG. 12 diagrammatically illustrates part of the apparatus shown in the drawing according to FIG. 11 which has been modified to include a different arrangement for the recording medium.

FIG. 13 diagrammatically illustrates a plan view of part of the magnetic display device shown in the drawings according to either FIG. 1, FIG. 11 or FIG. 12 which has been modified to include a different viewing arrangement.

FIG. 14A shows a cross-sectioned end elevation of a diagrammatical representation of a magnetic recording head for the magnetic display device shown in the drawings according to either FIG. 1, FIG. 11 or FIG. 12.

FIG. 14B shows a side elevation of a diagrammatical representation of the magnetic recording head shown in the drawing according to FIG. 14A;

FIG. 15 shows an AC. bias signal that may be applied to the magnetic recording head shown in the drawings according to FIG. 14A and FIG. 14B;

FIG. 16 shows a rectangular hysteresis loop for the pole pieces of the magnetic recording head and/or the material on which the magnetic recording head shown in the drawings according to FIG. 14A and FIG. 1413 records;

FIG. 17A shows a cross-sectioned end elevation of a diagrammatical representation of another arrangement for the magnetic recording head shown in the drawings according to FIGS. 14A and 14B;

FIG. 17B shows a side elevation of a diagrammatical representation of the magnetic recording head shown in the drawing according to FIG. 17A;

FIG. 18A shows a pictorial view of a diagrammatical representation of a magnetic recording head for the magnetic display device shown in the drawings according to either FIG. 1, FIG. 11 or FIG. 12;

FIG. 1813 shows a cross-sectioned end elevation of the magnetic recording head shown in the drawing according to FIG. 18A;

FIG. 19 illustrates the scanning action of the magnetic recording head shown in the drawings according to FIGS. 18A and 18B;

FIG. 20 illustrates the operating principles of the magnetic recording head shown in the drawings according to FIGS. 18A and 1813;

FIG, 21 shows the M.M.F. waveforms which are applied to the magnetic recording head shown in the drawings according to FIGS. 18A and 18B;

FIG. 22 shows a plan view of a diagrammatical representation of a magnetic recording head for the magnetic display device shown in the drawings according to either FIG. 1, FIG. 11 or FIG. 12;

FIG. 23 shows a cross-sectional front elevation of the magnetic recording head shown in the drawing according to FIG. 22; and

FIG. 24 shows a cross-sectioned side elevation of the magnetic recording head shown in the drawing according to FIG. 23.

The formation of charge patterns which are representative of information, for example alpha-numeric characters or facsimile, contained in an electrical signal on one side of a recording medium is conventionally effected by recording means which are situated in close proximity to or in contact with said one side of the recording medium. It is however, according to the invention, preferable to position the recording means on the other side of the recording medium especially when the charge patterns are to be transformed to two-dimensional visible images by applying powder particles thereto since they are less likely to be contaminated with the powder particles.

The charge patterns may be formed either electrostatically or electromagnetically, and in order to increase the contrast between the two-dimensional visible images and the one side of the recording medium, the one side has a white, matt silver or reflecting coating applied thereto, This coating need only be a few microns thick and can be applied over the one side of the recording medium by for example spraying or vacuum aluminising.

In a practical arrangement, the one side of the recording medium, when the charge patterns are formed electromagnetically, would be in the form of a coating of magnetic material, for example iron oxide or nickel/ cobalt, on one side of a substrate which is of an insulating non-magnetic material, for example a plastics material, and which forms the said other side of the recording medium.

In order to ensure that the recording means, when situated on the other side of the recording medium, i.e. in close proximity to or in contact with the other side of the insulating non-magnetic substrate, will perform their function, it is necessary to arrange that the insulating nonmagnetic material used for the substrate is not more than approximately 0.001 inch thick.

The facility of positioning the recording means on said other side of the recording medium when the charge patterns are formed on said one side may be utilized in many applications, for example this facility may be utilized in the apparatus shown in the drawing according to FIG. 1 which diagrammatically illustrates in side elevation, a magnetic display device.

Referring to FIG. 1, the insulating non-magnetic substrate having thereon the magnetic coating with a reflecting, white or matt silver surface coating is formed by the backplate 4 of a shallow sealed chamber having a main body 1, a transparent frontplate 2, for example of glass, a recording head assembly 3, i.e. the recording means mentioned previously and agitating means 5, for example an air blower unit.

The sealed chamber contains magnetically attractive powder, for example carbonyl nickel dust which is dispersed in the space enclosed by the main body 1 between the frontplate 2 and the magnetic coating on the inside of the backplate 4 by the agitating means when required.

In operation, the output signal of say a computer is applied to the recording head assembly 3 which causes regions of the magnetic coating on the inside of the backplate 4 which are representative of the output signal to be energized i.e. forming the charge patterns. The magnetically attractive powder adheres to the energized regions of the magnetic coating on the inside of the backplate 4 to form a two dimensional visible image of the output signal which may be viewed through the transparent frontplate 2.

If very fine magnetically attractive powder is used and it gives rise to a transparent cloud when agitated then the powder may be continuously agitated by the agitating means 5 or suspended in a transparent fluid in which case the agitating means 5 would not be required. If other than fine powder is used then the agitating means would be intermittently operated to cause the powder to be dispersed in the space enclosed by the main body 1 between the frontplate 2 and the magnetic coating on the inside of the backplate 4 when it is required to obtain a two-dimensional visible image of the output signal.

Alternatively, the main body 1 and the transparent frontplate 2 of the magnetic transient display device shown in FIG. 1 could be replaced by a transparent diaphragm unit which with the backplate 4 would form a sealed chamber. In this arrangement the magnetically attractive powder which would be contained within the sealed chamber could be caused to be dispersed in the space enclosed by the transparent diaphragm unit by causing the transparent diaphragm unit to be continually compressed towards and expanded away from the backplate 4.

Alternatively, the isulating non-magnetic substrate having thereon the magnetic coating with a reflecting, white or matt silver surface coating may be formed by a band or closed loop 31 of insulating non-magnetic material having the magnetic coating on the surface 34, the band 1 being guided as shown in the drawing according to FIG. 11 by rollers 32 and driven by means not shown in the drawings in the direction of the arrows D.

Referring to FIG. 11 which diagrammatically illustrates in side elevation a modified arrangement of the magnetic display device shown in the drawing according to FIG. 1, the band 31 during rotation passes through sealing units 33 which form part of the main body 1. The sealing units 33 are arranged to allow the band 31 to be moved relative to the main body .1 and the recording head assembly 3 and at the same time prevent the loss of the magnetically attractive powder from within the main body 1.

Although the band 31 is shown in the form of a continuous loop, it may be in the form of a long, open-ened rolls, carried on take-up reels. As an example of this feature, FIG. 12 illustrates diagrammatically part of the apparatus of FIG. 11 which has been modified to show the band 31 in the form of a long, open'ended roll which is carried on take-up reels 35. This feature therefore enables the band 31 with the electromagnetic images thereon to be retained and be available for any future display.

While the magnetic display devices shown in the drawings according to FIGS. 1, l1 and 12 form the twodimensional visible images the right way round, it may be that the magnetic images from a practical standpoint need to be formed in reverse, in which case the arrangement shown in the drawing according to FIG. 13 would be required. FIG. 13 shows a plan view of part of the apparatus of either FIG. 1, FIG. 11 or FIG. 12 which has been modified in order that the reversed two-dimensional image on the recording medium when viewed through the transparent plate 2 is presented the right way round. This is achieved by replacing the transparent front plate shown in FIGS. 1, 11 or 12 with a front plate 36 which is inclined at an angle of the order of 45 relative to the horizontal axis of the magnetic display device and which has a reflecting inner surface, and modifyin the main body 1 to accommodate the inclined front plate 36 and the repositioning of the transparent plate 2. The two-dimensional visible image on the surface of the recording medium is reflected by the front plate 36 which presents it the right way round when viewed through the transparent plate 2.

An important feature of the display devices outlined in preceding paragraphs is that it could form part of a system which also provided a permanent record of the two-dimensional display in which case the same output signals could be fed simultaneously to the printing drum of a conventional non-percussive printing machine running without paper feed. The stored pattern on the printing drum would thus be amended constantly to correspond with the two dimensional visible display. A permanent record of any selected display could be obtained at will merely by initiating the paper feed for the printing machine.

Alternatively, the permanent record may be obtained directly from the band 31 shown in the drawings according to FIGS. 11 and 12 by utilizing standard non-percussive printing machine techniques or the permanent record may be obtained from any one of the magnetic display devices outlined in the preceding paragraphs utilizing the techniques outlined in our co-pending patent application No. 754,579, filed Aug. 27, 1968.

The recording head assembly 3 shown in the drawings according to FIGS. 1, 11 and 12 could be in the form of a block of ferrite material having a pattern of intersecting slots cut into the operative face, holes drilled through the block from the opposite face to intercept the bases of the slots at various points and energising windings passed through the holes and along the bases of each section of the slots to create a plurality of magnetic recording gaps on the operative face.

FIG. 2 shows a perspective view of a typical arrangement for a ferrite block Which is slotted and drilled to make the recording head assembly 3 and the head shown in this drawing is basically a rectangular block 11 of ferrite material having a front or operative face 7 and a rear face 8. The front face 7 has cut into it a pattern of slots 9. These slots are in fact very narrow and form the recording gaps which will produce sharply defined lines in a magnetic image when the latter is recorded in either the magnetic coating on the inside face of the backpla-te 4 or the surface 34 of the band 31 in contact with or closely spaced from the front face. The pattern of the slots 9 is such that by choosing various combinations of the slots, either full length or part length, the outline, for example of alpha-numeric characters, facsimile or any numerical digit can be formed in either the magnetic coating on the inside face of the backplate 4 or the surface 34 of the band 31. Some of the slots in fact are divided into a larger number of parts than are at first apparent; this will be described in greater detail below.

Each undivided portion of a slot has a separate energising Winding. The windings have been omitted from the drawings in the interests of clarity, with the exception of one single turn winding shown as an example in FIG. 3. To enable the windings to be wound the rear face 8 of the block is drilled with holes 12 which pass through to the bases of the slots 9. Also, since the slots are narrow, horizontal holes 6 are drilled along the bases of the slots 9. The

holes

12 and 6 intersect each other as shown. These

holes

12 and 6 therefore provided passageways in which the windings can be wound. The winding 10 is shown as a single turn of heavy wire, though in practice it would probably be a multi-turn coil of thin wire. When the winding 10 is energised it creates a magnetic recording field in the slot 9 extending over the portion NN. If the Winding for the next portion of the same slot is energised at the same time a longer magnetic field will be created. These magnetic fields will be recorded as narrow lines in the recording medium.

The recording head assembly illustrated must of course be coupled to an energising amplifier by means of a selector circuit. The function of the selector circuit is to translate the incoming signals representing for example, an alpha-numeric character into a selection of the appropriate slots or slot portions which, when energised simultaneously, will record the character in the magnetic recording medium. It will be appreciated that the use of such a recording head as described enables the complete signal to be displayed on either the magnetic coating on the inside face of the backplate 4 or the surface 34 of the band 31 by one pulse from the amplifier.

The selector circuit, although it is an essential adjunct of the recording head, is not illustrated or described in detail herein as it does not include any novel elements, being, for example, a gating circuit built according to well known techniques.

In practice the construction of a recording head as illustrated is facilitated by extending the holes 12 right through the block 1. Firstly this simplifies the actual wiring of the head if the wire used is thicker than the slots 9. Secondly the cutting of the slots may be by spark erosion techniques, which have been developed to fabricate certain solid materials which require extremely fine and intricate apertures to be formed therein. If the holes 12 extend through to the front face 7 thin wire electrodes can be stretched between rigid support members which can pass down the holes while the wire elec-; trodes cut the slots between the holes.

The completed and wired head is then potted, for example in an epoxy resin compound, which fills the slots and holes and protects and secures the wiring in the slots.

In an alternative embodiment the magnetic recording fields can be created by heavy single turn windings lying in shallow grooves flush with the front face of the head. The grooves are cut to the required pattern and holes are drilled through the block to intercept the grooves in a manner similar to that of the previous embodiment.

The recording head assembly 3 described in preceding paragraphs with reference to the drawings according to FIGS. 2 and 3 could be arranged such that the operative face of the ferrite block completely covers the actual storage plane of the magnetic display device of either FIG. 1, FIG. 11 or FIG 12 i.e. a selected area of the surface 34 of the band 31 or the whole of the surface area of the magnetic coating on the backplate 4. In this arrangement the magnetic storage coating serves as a data store and it is possible, as outlined in a preceding paragraph, to make the contents of the store visible either continously or on demand. The magnetically attractive powder held in liquid or air suspension would beredistributed continously to accommodate changes in the stored information, giving direct visibility whenever the stored condition remained long enough for the eye to follow it.

It may be advantageous to follow any energising writing signal with a part-erase signal in order to break the flux loop from the recording head assembly and thus en courage the stored field to emerge symmetrically. It can be seen from the above that with the recording head assembly 3 described in preceding paragraphs with reference to the drawings according to FIGS. 2 and 3 and the band 31, the actual storage plane of the display device is capable of being moved away from the recording head assembly and substituted by another magnetizable area. The storage capacity of any given arrangement of this recording head assembly is thus indefinitely extended, since by virtue of the band 31 any one of a large number of storage planes can be brought into contact with the recording head assembly at will. Thus with this recording head assembly, the magnetic display device is capable of being operated in a manner which enables an operator to not only view the incoming signal but also to select at the viewing position a particular magnetized area for print out. Exact re-registration of any given magnetized area could be achieved by providing special monitoring heads and positioning recordings on the edges of the band 31.

Alternatively, the recording head assembly 3 could be in the form of a matrix of magnetic dots. A single row of closely spaced very small magnetic gaps could be moved in steps or at a uniform speed across the band 31 or the backplate 4 and selectively energised at each step to build up a character or pattern.

The construction of a multi-gap recording head is basically the same as that of a single gap head. A piece of soft iron wire 13 (FIG. 4) has a single insulated winding 14 wound thereon, the winding 14 must not have the turns thereof so tight together that flexing of the wire will damage the insulation on the wire. Next, a metal plate 21 (FIG. 5) preferably of brass, has two holes 20 drilled in it, each hole being large enough to receive one end of the soft iron wire 13. The ends 15 of the wire 13 are pushed through the holes 20 and then pulled taut so that the two end turns 14 of the winding 14 are pressed against the brass plate 21.

The ends 15 of the soft iron wire 13 are then bent over to anchor the core against the brass plate 21. Two side plates 16 carrying contact pins 17 are glued or otherwise fixed to the edges of the brass plate 21, and the ends 18 of the winding 14 are soldered to appropriate contact pins 17. After all the cores have thus been placed in position the space 19 is filled under vacuum with an epoxy resin potted compound. Finally the brass plate 21 is ground away to expose the surface of the solidified epoxy resin which now forms a solid body. The end turns 14' of the winding 14 and the ends 13 of the core 13 are now flush with the surface 22 of the body as shown in FIG. 6. In fact the griding operation may even partially remove some of the end turns 14 of the winding 14, as has been depicted in FIG. 6. Provided that the continuity of the winding is not broken the closer the end turns 14 are brought to the surface, the better.

As an indication of the size of recording gap which can be obtained by this method, if the core wire 13 is 0.004 inch in diameter and the winding wire is 0.001 inch thick, the two ends of the core can be spaced on approximately 0.006 inch pitch, centre to centre. Similarly adjacent cores in a multi-gap head can be spaced at intervals of 0.006 inch approximately.

FIG. 7 shows a recording head assembly 3 having five cores 26 defining five gaps which can have the dimensions specified above. Each core winding 27 is terminated at contact pins 23 in the side of the body 24. It has been found that, even when the separate cores are virtually as close as the winding thickness will permit, crosstalk between adjacent cores when one of them is energised is so small as to be negligible. The efiiciency of each core is governed largely by the method of construction which allows each winding to be carried virtually to the pole tips.

The cores require low coercivity, high saturation magnetization and are not limited to soft iron. Other material such as Radiometal, mild steel and Permendur (registered trademark) are satisfactory, provided they are not too brittle to withstand the bending required to insert the core ends in the locating member.

A recording head assembly as described in the preceding paragraphs with reference to FIGS. 4 to 7 having only one core defining one gap could be used if the recording head assembly is caused to scan the complete surface of the backplate 4 or a selected area of the band 31 by being moved by traversing means not shown in the drawings for example in alternate directions along one axis of the backplate 4 or the band 31 and in steps along the other axis of the backplate 4 or the band 31. Each of the steps along the other axis would be carried out after the completion of each movement along said one axis and be selectively energised to build up a character or pattern.

Alternatively, a plurality of the wound cores shown in FIG. 4 could be arranged to form a matrix of magnetic dots to provide a recording head assembly 3 which either completely covers the backplate 4 Or a selected area of the band 31, thereby requiring no relative movement between the recording head assembly 3 and the backplate 4 or the band 31 at each record position, or covers only a section of the backplate 4 or the selected area of the i band 31, and therefore would require movement in steps across the backplate 4 or the band 31, whereby it is stationary at the record position and be selectively energised at each step to build up a character or pattern.

FIGS. 8A and 8B show a recording head assembly 3 with a single row of magnetic dots as outlined in the preceding paragraph.

FIG. 8B is a diagrammatic sectional view of FIG. SA on the line ZZ. This method of making a recording head assembly is the same as the method outlined in the preceding paragraphs with reference to FIGS. and 6 except that only one end of the cores 13 are passed through the plate 28 until the end turns of the winding 14 abut the plate 28. The other ends of the cores 13 are alternatively pulled to one side or the other as shown in FIG. 8B, and the free ends 18 of the windings 14 are soldered to the pins 29, which are fixed in thin sheets of non-magnetic material 30 which have previously been glued or otherwise fixed to the edges of the plate 28 to provide not only a support for the pins 29 but also to form a mould for the potting compound which is used to fill up the space within and hold the cores and windings as required.

FIG. 9 shows in greater detail the end turn 14 and the ends 18 of the windings 14 as depicted in FIG, 8B. Two cores 13 are shown, one behind the other as they would be seen in FIG. 8B.

When producing a recording head assembly, which completely covers the backplate 4 or a selected area of the band 31, by this method the cores would remain upright and would not be pulled to one side or the other as shown in the drawing according to FIG. 8B.

As an indication of the dimensions of these recording head assemblies, if each core wire 13 is 0.004 inch in diameter and the winding wire is 0.001 inch thick, then the two ends 13' of adjacent cores 13 can be spaced on approximately 0.006 inch pitch, centre to centre. Thus a row of individual recording magnetic cores can be spaced at intervals of 0.006 inch approximately.

Although potting of the cores in a plain epoxy resin compound is quite satisfactory it is possible to impregnate the compound with fillers such as amorphous silica to reduce wear of the recording surface. Alternatively air ducts may be drilled or otherwise provided to enable the head to be supported and located on an air hearing if the circumstances permit.

It will be appreciated that the problem of wear is important since the ends of the windings are located virtually at the surface of the head. Yet another method of combating wear is to leave the end turns of the windings exposed, as in FIG. 6, and to cover the recording surface with a replaceable film of wear resistant material which should not exceed 0.0005" in thickness. Materials which may be used include wear resistant plastic films. Alternatively the recording surface may have a film of material such as silicon nitride deposited thereon by a glow discharge process. Other potting compounds may be used where the properties of the potting compound may effect the performance of the head. For example, temperature considerations may call for the use of compounds other than epoxy resins.

In order to provide a high-definition two-dimensional visible image on the magnetic transient display device it would be advantageous to use a recording head assembly which utilizes only one core 37 as shown in the drawings according to FIGS. 10A and 10B, defining one gap 38 of width W and length L. The recording head assembly would be caused to scan the complete surface of the backplate 4 or a selected area of the band 31 at each record position by being moved by traversing means not shown in the drawings, for example, in alternate directions along one axis of the backplate 4 or band 31 and in steps along the other axis of the backplate 4 or band 31. Each of the steps along the other axis which are a fraction of the width W of the gap 38 are carried out after the completion of each movement along the one axis. The recording head assembly is initially energised to a saturated state at say one corner of the backplate 4 or the selected area of the band 31 to provide part of a character or pattern, and every time the recording head assembly is moved a distance equivalent to a fraction of the length L of the gap 38 the saturating energising signal is reversed thereby eliminating all but a fraction of that part of the character or pattern which was previously impressed on the magnetic coating on the inside of the backplate 4 or the surface 34 of the band 31 by the recording head assembly and impressing thereon the next part of the character or pattern. Similarly, since the recording head assembly is moved in steps along the other axis by an amount equivalent to a fraction of the width W of the gap 38 then as the recording head assembly is moved along the one axis after each step along the other axis all but a fraction of the last length W of the preceding pattern will be eliminated and the next part of the character or pattern will be impressed on the magnetic coating on the inside of the backplate 4 or the surface 34 of the band 31. Thus is can be seen from the above that providing the trailing edge 39 of the gap 38 and the trailing end 40 of the core 37 have steep flux gradients, a highdefinition two-dimensional magnetic image, which is transformed to a visible image by the magnetically attractive powder, may be obtained using this traversing method and recording head assembly as part of the magnetic transient display device.

While the magnetic display devices outlined in preceding paragraphs have been described using either a ferrite block recording arrangement, a multiplicity of heads which necessitates the use of an associated complex signal distribution network, or a single moving head which is limited in high speed magnetic recording application due to its mechanical limitations, it may be advantageous to utilize the magnetic recording heads shown in the draw ings according to FIGS. 14 to 24 which have non-mechanical means for scanning the surface of a recording medium at an angle to its normal direction of motion.

Referring to FIG. 14A, a cross-sectioned end elevation of a diagrammatical representation of a magnetic recording head is shown which comprises an AC. bias winding 41, which is wound on a former 42 and which is terminated at each end thereof at the

terminals

50 and 51,

pole pieces

46 and 47, which are made of a high-coercivity magnetic material having a rectangular hysteresis loop 54 as shown in the drawing according to FIG. 16 and which are spaced apart by the side members 45 to form a single recording gap 58 which extends the full width of the band 31 or the backplate 4, and a signal winding 48 which is shown as a single wire and which as shown in FIGS. 14A and 14B passes through an aperture 56 which is provided between the

pole pieces

46 and 47.

The side members 45, which also support the former 42, the flanges 43 of which are spaced from the side members 45 by means of the spacing members 44, are made of a material which provides a saturable magnetic path for the AC. bias signal applied by way of the A.-C. bias winding 41, thereby limiting the bias amplitude in the recording gap 58 at high bias amplitudes.

It should be noted that

pole pieces

46 and 47 made of a soft material, which is defined as a material which does not retain magnetism permanently but loses most of it when the magnetizing field is removed, may be employed for the magnetic recording head, in which case the magnetic coating on the band 31 or the backplate 4, i.e. the recording medium 49 would need to be of a high coercivity magnetic material having a rectangular hysteresis loop. Alternatively, both the

pole pieces

46 and 47 and the recording medium 49 may be made of a high coercivity magnetic material having a rectangular hysteresis loop, the main criterion being that the closed loop .magnetic path should have a rectangular hysteresis loop.

The A0. bias signal (-FIG. 15) applied by way of the

terminals

50 and 51 to the A.C. bias winding 41 is an oscillatory waveform having a frequency which is several times higher than the highest frequency of the signal waveform which is to be recorded on the recording medium 49. As can be seen from FIG. 15, the AG. bias signal gradually decays from a maximum value to a minimum value over a period of time which is determined by the period of time the signal to be recorded on the recording medium 49 appears across the magnetic recording head.

As can be seen from the drawing according to FIG. 14B, which shows a side elevation of a diagrammatical representation of the magnetic recording head shown in FIG. 14A, the bias flux formed by the A.C. bias signal follows a path having a reluctance which reduces gradually' from a maximum value at position A to a minimumv value at position C due to the positioning of the AC. bias winding 41 relative to the recording gap 58. By way of example, assuming that the

pole pieces

46 and 47 are of a soft material and the recording medium 49 is of a. magnetic material having a rectangular hysteresis loop, then when the AC. bias signal is at maximum amplitude i.e. position A ('FIG. 15), the bias field is sufiicient to sweep the rectangular hysteresis loop of the recording medium 49 i.e. cyclically magnetizing it over the whole length of the recording gap 58, i.e. between positions A and C. At half amplitude i.e. at position B (FIG. 2) on the AC. bias signal, the AC. bias is insuflicient to sweep the rectangular hysteresis loop of the recording medium 49 between the positions A and B but is still sufficient to sweep the rectangular hysteresis loop of the recording medium 49 between the positions B and C. The AJC. bias threshold, i.e. the AC. bias signal amplitude which is only just sufiicient to sweep the rectangular hysteresis loop of the recording medium 49 to cyclically magnetize it, is thus at position B. At minimum A.C. bias signal amplitude i.e. at position C on the A.C. bias signal, the AC. bias is barely sufficient to sweep the rectangular hysteresis loop of the recording medium 49 at position C.

It can therefore be seen from the above that as the amplitude of the AC. bias signal is varied, the bias threshold mentioned in the preceding paragraph can be moved anywhere along the magnetic recording head between the position A and C. In order to understand the mechanism involved consider only a small strip of the recording medium 49 positioned anywhere between the positions A and C of the magnetic recording head shown in the drawing according to FIG. 14B and assume that the signal which is to be recorded is absent from the signal winding 48. At a time I when the magnetizing force due to the A.C. bias signal does not exceed the coercivity value of the small strip of the recording medium 49, then the state of the small strip of the recording medium 49 hereinafter referred to as a domain, will be unchanged. However, at a time t which is later than the time t the domain is swept through its rectangular hysteresis loop by the AC. bias signal, which affects it with diminishing amplitude, i.e. the domain is cycled through a succession of minor hysteresis loops as the amplitude of the AC. bias signal decreases until finally the domain settles out at position 0 on the rectangular hysteresis loop 54 shown in the drawing according to FIG. 16, which is the neutral state, at a time t which is later than the time t At and after the time t the domain will be left in a demagnetized state until the next A.C. bias signal is applied thereto.

Successive domains of the recording medium 49 across the width of the magnetic recording head i.e. between the positions A and C will similarly be treated as the A.C. bias signal passes therethrough with diminishing effect.

The maximum amplitude of the signal to be recorded, which would in practice he a serial signal waveform, must be below a level at which any recording could take place i.e., as shown in FIG. 16, the magnetizng force H 1 due to the maximum signal to be recorded should never exceed the coercivity value for the recording medium 49 (it must be kept below the value of the knee 52 of the rectangular hysteresis loop 54 shown in the drawing according to FIG. 16).

If the states of the domain or domains mentioned in a preceding paragraph are now considered when the signal to be recorded is present in the signal winding 48, then at'the time t the domain will only be influenced by the signal waveform, which as previously stated is incapable of changing its state of magnetization. However at and after the time t the domain will be left in a state of magnetization corresponding with the instantaneous signal level which prevailed at the time 2 for example positio 55 shown in the drawing according to FIG. 16, and will remain unaffected by subsequent changes in the incoming signal but will be erased by the next A.C. bias signal at the time t Again, successive domains of the recording medium 49 across the width of the magnetic recording head, i.e. between the positions A and C will similarly be magnetized to correspond with the signal condition which prevailed as the A.C. bias signal passes therethrough with diminishing effect.

The following detailed analysis of the recording process will give a greater appreciation of the mechanism involved. Consider the hysteresis loop 54 shown in FIG. 16 for a given domain and assume that i+H1 and -H1 are the magnetizing forces which are equivalent to mar and space signal inputs respectively, neither being of sufficient amplitude to change the state of the domain in question.

The passage of the scanning A.C. bias signal waveform shown in the drawing according to FIG. 15 will subject the domain in question to a succession of alternations of magnetizing force, decreasing progressively from a maximum value towards zero. It is permissible for the peak amplitude of the A.C. bias signal waveform to be very much greater than that required to traverse the rectangular hysteresis loop. This is of no significance, however, since it has no effect upon the final recorded condition.

The important moment is that at which the amplitude of the scanning A.C. bias signal waveform has fallen to a level which is just insufficient to carry the domain past the knee of its hysteresis loop. In the absence of any signal input, the first scanning half-cycle of the A.C. bias signal which fails to reach an amplitude in excess of the value H1 will also fail to change the state of the domain. An ideal magnetic material, having the switching regions of its hysteresis loo strictly vertical, could thus only adopt one or other of its saturated conditions, depending upon the polarity of the last half-cycle of sufficient amplitude to change its state.

In a practical case, where this is a finite slope between the knee 5 2 of the hysteresis loop and saturation, the situation is somewhat different. Scanning half-cycles of the A.C. bias signal immediately preceding the critical one might have amplitudes such as to switch the domain only partially. If there are several such half-cycles, i.e. if the decay of the A.C. bias signal waveforms is relativelyslow over this region, the final condition of the domain will be at B=O which is, of course, the conventional process of demagnetization.

Although potentially capable of exploitation, the demagnetized condition, resulting from zero signal input, will be disregarded. Instead, it will now be assumed that a mark signal input +Hl appears immediately before the critical half-cycle of the A.C. bias signal arrives. The alternating field due to the A.C. bias signal will now have added to it the DC. (relatively) component due to the signal. Preceding half-cycles of the A.C. bias signal of greater amplitude will still sweep the domain through its rectangular hysteresis loop, but the critical half-cycle new becomes the first-cycle to have an amplitude not exceeding 2H1. The resulting magnetising force therefore becomes equal to H1, which is insufficient to return the domain towards negative saturation. Any subsequent half-cycles, which will be of lower amplitude, will also be incapable of removing the domain from its condition of positive saturation, which is of course, the desired mark condition +Bl. It will be evident that a space signal input will, by the identical process, bring the domain to the condition of negative saturation B1.

The definition (elements per unit distance) of the signal, as recorded transversely across the storage medium,

is not inherently defined by any mechanical or magnetic discontinuity in the magnetic recording head structure. To a first order the definition will be determined by the rate of change of the applied signal in relation to the velocity of the scanning A.C. bias signal waveform, for example, at a scanning velocity of 8 inches per millisecond a 400 kc./s. signal waveform should be recorded with its successive half-cycles each 0.01 inch wide.

However, the recorded definition will also be governed by the frequency of the scanning A.C. bias signal wave form. It has been shown in a preceding paragraph that a transition to the mark condition can only coincide with a positive-going half-cycle of the A.C. bias signal waveform. Similarly the space transitions are confined to the negative-going half-cycles. Hence the true positions of the recorded signal transitions will be distorted to coincide with the phase of the scanning A.C. bias signal waveform, the possible error lying within the spatial equivalent of plus or minus one half-cycle of this waveform. The significance of this effect can however be minimized by ensuring that the frequency of the scanning A.C. bias signal waveform is made several times higher than the highest frequency of the signal to be recorded.

While the magnetic recording head shown in FIGS. 14A and 14B utilizes the same gap, i.e. the recording gap 58, for both the signal which is to be recorded and the A.C. bias signal, this need not be the case. A third pole piece could be used to provide another gap for the A.C. bias signal, for example as shown in the drawing according to FIGS. 17A and 17B. However, the arrangement shown in FIGS. 17A and 17B operates in exactly the same manner as the arrangement shown in FIGS. 14A and 14B.

Referring to FIGS. 17A and 17B a cross-sectioned end elevation and a side elevation of another arrangement of the magnetic recording head shown in FIGS. 14A and 14B are respectively diagrammatically illustrated. In this arrangement the recording gap 58 is provided in a pole piece 53, which together with a pole piece 61 forms another gap 57 for the A.C. bias signal (shown in FIG. 15) applied by way of the A.C. bias winding 41 which is wound on a former 42 and which is terminated at each end thereof at the

terminals

50 and 51.

The pole pieces 53 and 61 support the former 42, the flanges 43 of which are spaced from the pole pieces 53 and 61 by means of the spacing members 44, and the signal winding 48 which is shown as a single wire, passes as shown in FIGS. 17A and 17B through an aperture 56 which is provided in the pole piece 53.

In side elevation the magnetic recording heads shown in FIGS. 14B and 17B are generally the same in that the A.C. bias winding assembly is positioned relative to the recording gap 58 and the gap 57 such that the bias flux formed by the A.C. bias signal follows a path having a reluctance which reduces gradually from a maximum value at position A to a minimum value at position C.

As is the case with the arrangement shown in FIGS. 14A and 14B, the pole pieces 53 and 61 may be of either a soft material, in which case the recording medium 49 would need to be of a high coercivity magnetic material having a rectangular hysteresis loop, or a high coercivity magnetlc material having a rectangular hysteresis loop. Alternatively, both the pole pieces 53 and 61 and the recording medium 49 may be made of a high coercivity magnetic material having a rectangular hysteresis loop, the main criterion being that the closed loop magnetic path should have a rectangular hysteresis loop similar to the one shown in FIG. 16.

Thus in operation the serial signal waveform to be magnetically recorded on the length of the recording medium 49 immediately below the recording gap 58 of the magnetic recording head is fed through the signal winding 48, the direction of propagation being from position A to position C, and the individual elements of the serial signal waveform are magnetically recorded at different positions along this length due to the A.C. bias signal as its amplitude progressively decreases. At the instant the A.C. bias signal reaches the position C either the band 31 of magnetic material is moved by appropriate means relative to the magnetic recording head or the recording head is moved by appropriate means relative to the recording medium over a selected area thereof such that the next length of the recording medium 49 onto which the next serial signal waveform is to be recorded is positioned beneath the recording gap 58 of the magnetic recording head. When the magnetic recording head or recording medium is relocated, the next serial signal waveform may be applied to the signal winding 48 and the next A.C. bias signal may be simultaneously applied to for example the magnetic recording head shown in FIGS. 14A and 14B.

Alternatively, the band 31 of the magnetic recording head could be moved continually instead of intermittently. In either case a synchronizing pulse interposed between any two serial signal waveforms which are to be recorded on adjacent strips of the band 31 causes the A.C. bias to return to maximum amplitude at the correct instant and ensures that the previous recording is not erased.

It will be evident from the preceding description that the recording process is not dependent upon a critical amplitude of the serial signal waveform, since the recording takes place when the A.C. bias signal decays towards zero. It is, however, important to ensure that when the position A on the magnetic recording head is reached, the amplitude of the leading cycle is still suflicient to sweep the recording medium 49 through its major rectangular hysteresis loop despite attenuation effects due to the fiux path formed by the side members 45.

Although this means that, at the beginning of the scan, the peak amplitude of the A.C. bias signal will be much greater than necessary, this presents no problems. The initial cycles of a very high amplitude A.C. bias signal will drive the recording medium 49 well beyond saturation, but since any given domain will be subjected to successive cycles of diminishing amplitude there will always be an instant at which this domain is seeing the proper conditions for recording to take place.

Referring to FIG. 18A the magnetic recording head shown diagrammatically therein in a pictorial view basically comprises a main body 66 having formed therein a tapered magnetic recording gap 72 which in practice could have a contour which varied in accordance with any desired law, for example, linear, exponential, sinusoidal or logarithmic, and an aperture 70 to provide a former 71 around which a signal winding 73 is wound which is terminated at each end thereof at the terminals 74 and 75.

As can be seen from the drawing according to FIG. 18B, which is a. cross-sectional end elevation of the magnetic recording head shown in the drawing according to FIG. 18A, the main body 66 is formed by a generally square tube member of a soft material; the

pole pieces

67 and 68 on either side of the tapered magnetic recording gap 72, which have chamfered edges 76, are formed by adjacent sides of the main body 66 and the former 71 is formed by one of the other sides of the main body 66. The cross-section of the main body 66 may be of any desired shape, the main criterion being that the main body should have a minimum of three sides, i.e.two adjacent sides for the

pole pieces

67 and 68 and the third side for the former, which may or may not be formed as an integral structure.

FIG. 19 illustrates the scanning action of the magnetic recording head shown in the drawings according to FIGS. 18A and 18B, and for the purposes of the subsequent description it is assumed that the contour of the tapered magnetic recording gap 72 varies linearly and that either the band 31, having the magnetic coating thereon with a rectangular hysteresis loop similar to the one shown in the drawing according to FIG. 16, is moved relative to the tapered magnetic recording gap 72 in the direction of arrow F or the tapered magnetic recording gap 72 is moved relative to the backplate 4 or the band 31 over a selected length thereof in a direction opposite to that of arrow F, therefore the pole piece 68 will be the leading pole piece and the pole piece 67 will be the trailing pole piece.

14 Now M.M.F H: L (1) where:

=l6- (Gilberts) wherein N=the number of times the signal winding 73 links a magnetic path and I=the current flowing in the signal winding 73.

For a given critical magnetic field H, which is sufficient to drive the recording medium 69 (FIG. 20), i.e. the magnetic coating on the band 31 or the backplate 4, to a saturated state of the rectangular hysteresis loop 54 shown in the drawing according to FIG. 16, and a given M.M.F., it is evident from

Equations

1 and 2 that there will be a critical magnetic recording gap length L For L L then H H and for L L then H H Thus by varying the amplitude, i.e. the ampere turns (N l in Equation 2, the position along the Width of the magnetic recording head of the critical magnetic recording gap length L will vary.

The operating principles of the magnetic recording head shown in the drawings according to FIGS. 18A and 18B are illustrated in the drawing according to FIG. 20, wherein the direction of movement of the magnetic recording medium 69 relative to the tapered magnetic recording gap 72 is represented as previously stated by the arrow F and wherein the arrow G indicates the direction of the original recording medium magnetization. It is assumed that the recording medium 69 has been driven or otherwise caused to be in the saturated state 0 of the rectangular hysteresis loop 54 shown in the drawing according to FIG. 16 and allowed to relax to the magnetized condition B2. The shaded area 77 (FIG. 20) represents the zone of the recording medium 69 which is influenced by the trailing pole piece 67 as the recording medium 69 is moved relative to the magnetic recording head or as the magnetic recording head is moved relative to the recording medium 69.

If the signal winding 73 is energized with a current of say 10 units at a time 1 this current being sufiiciently high to give rise to the position M.M.F. shown in the drawing according to FIG. 21 which causes the critical magnetic field intensity -|-H between the

pole pieces

68 and 67 to appear at the position LMAX, (FIG. 20), the shaded area 77 between the positions L to L will be driven to the saturated state 1 of the rectangular hysteresis loop 54 shown in the drawing according to FIG. 16. Upon removal of the current of 10 units, the shaded area 77 between the positions L to L will relax to a magnetized condition +B2 on the B curve shown in the drawing according to FIG. 16. If at a time t which is later than the time I a current of 9 units which is of reverse direction to the current of 10 units, i.e. 9 units is then applied to the signal winding 73, the negative M.M.F. (see FIG. 20) due to this current will cause the critical magnetic field intensity H to appear at the position L thereby causing the remanent state of the shaded area 77 between the positions L to L to be reversed i.e. driven to the saturated state 0 of the rectangular hysteresis loop 54. Hence upon removal of this current of 9 units the shaded area 77 between the positions L to L will be left in the state 1, i.e. at the magnetized condition +B2, and the shaded area 77 between the positions L to L will relax to a magnetized condition -B2.

This process may be repeated by applying the positive M.M.F. as shown in FIG. 21 to cause the critical magnetic field intensity +H to appear at the position L then apply the negative M.M.F. as shown in FIG. 21 to cause the critical magnetic field intensity -H to appearat the position L thereby leaving the shaded area 77 between the positions L and L to be left in the magnetized condition +B2.

The process may be repeated several times by applying alternate current pulses of opposite sense and decreasing amplitude to the signal winding 73 thereby progressively leaving the desired magnetized pattern i.e. discrete elements of the shaded area 77 in the state 1 on the surface of the band 1 between the positions L to L If the band 1 is moved past the magnetic recording gap 72 in the direction of the arrow F then it will be possible to cover its entire surface area with whatever pattern is required by appropriate control of the waveform shown in the drawing according to FIG. 21.

Thus in operation a current waveform which gives rise to a driving waveform similar to the waveform shown in the drawing according to FIG. 21, which comprises a series of pulses, alternate pulses being of opposite polarity and the magnitude of each pulse being less than the magnitude of the preceding one by an amount which is equivalent to a length of the band 1 below the magnetic recording gap 72 which is to be left in a magnetized state, is applied to the signal winding 73, and after a predetermined time a series of discrete elements of the recording medium surface are left in a magnetized state. At the instant the current waveform has caused the necessary series of discrete elements of the recording medium surface below the magnetic recording gap 72 between the positions L to LMAX- to be left in a magnetized state, i.e. the condition +B2, either the band 31 is moved by appropriate means relative to the magnetic recording head in the direction of the arrow F or the magnetic recording head is moved by appropriate means relative to the backplate 4 or a selected length of the band 31 in a direction opposite to that of arrow P such that the next length of the recording medium onto which the next magnetic pattern is to be recorded is positioned beneath the magnetic recording gap 72, i.e. it occupies the shaded area 77. When the magnetic recording head or recording medium is relocated, the next current waveform is applied to the signal winding 73. Alternatively, the band 31 or the magnetic recording head could be moved continually instead of intermittently in which case the necessary current waveform would also be continually applied to the signal winding 73.

It can be seen from the above description with reference to the drawing according to FIG. that it is necessary to effect precise control of the driving (or ampere turns) in order to achieve high resolution. In order to achieve a definition of the order of 160 zones per inch, a total of 320 levels would be needed. As magnetic recording head structures of several inches in width are envisaged and as practical ratios of LMAX, to L are likely to be limited to approximately 10:1 (implying a ratio of M.M.F. to M.M.F. of 10:1 also) it will necessary to use a multiplicity of windings in place of the single signal winding 73 shown in the drawing according to FIG. 18A.

The multiplicity of windings may taken many forms, for example it may take the form of a binary type of winding arrangement, i.e. of turns which increase in the manner of a binary code, i.e. (l, 2, 4, 8, 16, 32 etc.). Then, by providing a few levels of current only and applying them to the multiple winding, the appropriate windings may be switched into circuit at the appropriate time to give the desired result, i.e. a multilevel waveform.

It may be that the two magnetomotive force pulses, which define any one of the discrete magnetized elements on the recording medium, are displaced relative to the magnetomotive force pulses associated with an adjacent preceding magnetized element by a fairly long time interval; and the practical problems involved in the con struction of an input signal circuit to generate and couple the necessary current waveform to the signal winding 73 to give rise to these magnetomotive force pulses may be somewhat simplified if the input signal circuit were arranged to generate a current waveform giving rise to a series of magnetomotive force pulses of one polarity and decreasing amplitude, the write-in pulse being included when required between any two of these pulses whose position coincided with the write-in position.

In order to ensure that the discrete elements of the shaded area 77 which are driven to the state 1 are of the same length at any point along the width of the tapered magnetic recording gap 72, it may be necessary to either arrange that the leading pole piece is dropped back from the surface of the recording medium 69, cut teeth in the gap edge of either or both of the

pole pieces

68 and 67 or arrange for either or both of the

pole pieces

68 and 67 to be made from a high-coercivity magnetic material having a rectangular hysteresis loop as shown in the drawing according to FIG. 16.

Referring to FIG. 22, a plan view of a diagrammatical representation of a magnetic recording head is shown and comprises a main body 81 having formed therein a single magnetic recording gap 83 which may in practice extend the full width of the recording medium 79, i.e. the magnetic coating on the band 31 of the backplate 4 having a rectangular hysteresis loop similar to the one shown in the drawing according to FIG. 16, and an aperture 86 to provide a former "82 around which a signal winding 84 is wound which is terminated at each end thereof at the

terminals

65 and 78.

The pole pieces and 90 on either side of the single magnetic recording gap 83 which have chamfered edges 85 as shown in the drawing according to FIG. 23 are in practice of a soft material, but they may be made of a high-coercivity magnetic material having a rectangular hysteresis loop 54 as shown in the drawing according to FIG. 16.

As can be seen from the drawing according to FIG. 22, due to the physical shape of the pole pieces 80 and and the location of the signal winding 84 relative to the single magnetic recording gap 83, the magnetic flux formed by an electrical signal applied to the winding 84 will follow a path having a reluctance which reduces gradually from a maximum value at position C to a minimum value at position A.

If the signal winding 84 is energized with a gradually increasing current entering at the terminal 65, the pole piece 80 will become a magnetic North Pole and the pole piece 90 will become a magnetic South Pole. As the current increases, the magnetic field in the single magnetic recording gap 83 at position A will reach a value such as will completely saturate the surface of the recording medium 79 situated immediately below position A, i.e. the surface of the recording medium 79 will be driven to state 0 on the rectangular hysteresis loop 54 shown in the drawing according to FIG. 16. As the applied drive current is further increased, the surface of the recording medium 79 situated immediately below position B and then ultimately below position C will be similarly driven to state 0 on the rectangular hysteresis loop 54, thereby causing that strip of the surface of the recording medium 79 immediately below the single magnetic recording gap 83, i.e. between positions A and C to be driven to state 0. Similarly a current applied to the signal winding 84 in the reverse direction, i.e. entering at the terminal 78, would set that strip of the surface of the recording medium 79 immediately below the single magnetic recording gap 83, i.e. between position A and C to be driven to state 1 on the rectangular hysteresis loop 54 shown in the drawing according to FIG. 16.

If there is relative movement between the recording medium and the magnetic recording head shown in the drawings according to FIGS. 22 and 23 then the entire surface of the recording medium 'will be caused to be driven to the same remanent condition provided the value of the electrical signal applied to the signal winding '84 is sufficiently high to effect this state i.e. sufficiently high to cause the surface of the recording medium 79 between positions A and C to be driven to either state or state 1. Under this condition there would be insufficient emergent flux from the surface of the recording medium for the formation of the two dimensional visible image.

However, consider the situation where the entire surface of the recording medium has been driven or otherwise caused to be in state 0 and allowed to relax to the magnetized condition B2 shown in the drawing according to FIG. 16. Then on the application of a current of say units to the signal winding 84 at a time t this current being sufficiently high to drive the surface of the recording medium 79 situated immediately below the single magnetic recording gap 83 between positions A and B to state 1, the surface of the recording medium 79 between positions A and B will be driven to this state. Upon removal of the current of 10 units, the surface of the recording medium 79 situated immediately below the single magnetic recording gap 83 between positions A and B will relax to a magnetized condition +B2 whereas between positions B and C the surface of the recording medium 79 will remain undistributed in the magnetized condition B2. If at a time t which is later than the time t a current of 9 units which, is of reverse direction, i.e. -9 units, is then applied to the signal winding 84, the single magnetic recording gap fiux will cause the surface remanent state to be reversed along part of that strip of the recording medium 79 between positions A and B. Hence upon removal of this current of 9 units a short strip of the surface of the recording medium 79 near position B will be left in state 1, i.e. at the magnetized condition -l-B2.

This process may be repeated, normally starting at position C and then applying alternate current pulses of opposite sense and decreasing amplitude thereby progressively leaving the desired magnetized pattern, i.e. discrete elements of the recording surface in the state 1 on the surface of the band 31 or the backplate 4 between the positions A and C.

If there is relative movement between the recording medium and the single magnetic recording gap 83, it will be possible to cover the entire surface area of the recording medium with whatever pattern is required by appropriate control of the current waveform.

Thus in operation the current waveform, which comprises a series of curret pulses, alternate pulses being of opposite polarity and the magnitude of each pulse being less than the magnitude of the preceding one by an amount which is equivalent to a length of recording surface immediately below the single magnetic recording gap 83 which is to be left in a magnetized state is applied to the signal winding 84 and after a predetermined time a series of discrete elements are left in a magnetized state. At the instant the current waveform has caused the necessary discrete elements of the recording surface immediately below the single magnetic recording gap between positions A and C to be left in a magnetized state, i.e. condition +B2, either the band 31 is moved by appropriate means relative to the magnetic recording head or the magnetic recording head is moved by appropriate means relative to the backplate 4 or a selected length of the band 31, such that the next length of the recording medium onto which the next magnetic pattern is to be recorded is positioned beneath the single magnetic recording gap 83 of the magnetic recording head. When the magnetic recording head or recording medium is relocated, the next current waveform is applied to the signal winding 84. Alternatively, the band 31 of the magnetic recording head could be moved continually instead of intermittently.

It may be that the two current pulses which define any one of the discrete magnetized elements on the recording medium are displaced relative to the current pulses associated with an adjacent preceding magnetized element by a fairly long time interval, and the practical problems involved in the construction of an input signal circuit to generate and couple these current pulses to the signal winding 84 may be somewhat simplified if the input signal circuit was arranged to generate a series of pulses of one polarity and decreasing amplitude and to include the write-in pulse when required between any two of these pulses whose position coincided with the write-in position.

In order to control the relationship between the current amplitude and its limiting point of influence along the length of the single magnetic recording gap 83, it may be necessary to shape the pole pieces and to a specific contour, for example as shown by the chain dotted lines 87 shown in the drawing according to FIG. 22 or as shown by the chain dotted line 89 in the drawing according to FIG. 24, which figure shows a crosssectional side elevation of the magnetic recording head shown in the drawing according to FIG. 22 taken along the line XX. In either case the purpose of the contouring is to gradually reduce the cross-sectional area of each of the

pole pieces

80 and 90 in accordance with a desired law thereby presenting to the magnetic flux formed by the electrical signal applied to the signal winding 84 an increasing reluctance path which will tend to control the relationship between the current amplitude and the length of the single magnetic recording gap 83 influenced thereby.

The magnetizing regions in the

pole pieces

80 and 90 may be more sharply defined by use of laminations which would need to be shaped in accordance with the pole piece cross-section shown in the drawing according to FIG. 23 and which would be spaced apart from one another by non-ferromagnetic spacers. Either pole piece or both may be laminated in this manner. Alternatively the magnetising regions may be more clearly defined by cutting teeth in the gap edge of either or both of the

pole pieces

80 and 90.

It is to be understood that the arrangement of the magnetic recording head as shown in the drawings according FIGS. 22 and 24 is purely diagrammatic. The magnetic recording head may take many forms and be constructed in a variety of ways in order to achieve the same result; the main criteria involved being that the magnetic flux, formed by an electrical signal in coupling means which couple a signal winding assembly, to which the electrical signal is applied, to a single magnetic recording gap (which may extend the full width of a recording medium) should, follow a path having a reluctance which reduces gradually from a maximum value at one end of the recording gap to a minimum value at the other end of the recording gap, and that the electrical signal applied at each position of the recording medium should comprise a series of current pulses of one polarity and decreasing amplitude each one of said pulses being followed by at least one other pulse which is of the opposite polarity and lesser in magnitude.

In order to obtain the series of current pulsses outlined in the preceding paragraphs it may be necessary to use, as was the case with be recording heads shown in FIGS. 18A and 18B, a multiplicity of windings in place of the single winding 84 shown in the drawing according to FIGS. 22 and 24. The multiplicity of windings may take many forms. For example, as previously stated with reference to FIGS. 18A and 18B, it may take the form of a binary type of winding arrangement, i.e. of turns which increase in the manner of a binary code, for example, 1, 2, 4, =8, 16, 32 etc. Then by providing a few levels of current only and applying them to the mul-