CA2072964C - Magnetic visual display - Google Patents

Abstract

An apparatus and method for providing a magnetic display in which a magnetic fie ld (17) produce visual patterns upon exposure to the apparatus. The apparatus comprises an enclosure (50) which conta ins magnetically active flakes (16) held within a dispersion medium (14) which holds the magnetically active flakes in suspensio n, yet allows alignment of the flakes along the flux lines (18) of the magnetic field when the flakes are exposed to the locus o f the magnetic field.

Description

W~9l/07287 ~ I PCT/US90/0595l -1- 207296~

TIC VISUAL DISPLAY
Back~,ound of the Invention The present invention relates to a magnetic visual display that uses magnetic force to orient magnetically S active flakes contained within a dispersion medium to allow - light to pass therethrough.
The existing techniques of forming a visual display through magnetic means generally comprise applying a magnetic field to fine ~agnetic particles dispersed within a viscous liquid. The particles migrate to the magnetic field and accumulate along the locus of the field, thereby creating an image comprising an accumulation of the particles along the locus of the magnetic field.
The attractability of these particles may be defined as an additive process, that is, prior to drawing, the entire field of visible bach~.ound is ,~ne~dlly void of any magnetic particlee. When a magnetic field is displayed to the liquid, the magnetic particles are drawn up from the bottom of the liquid to the top of the liquid, thus proA~lcing a visible image at the top surface.
~ r~-~er, after attraction, the particles tend to precipitate away from the surface of the liquid, making it difficult to retain the image over an exte~ period of time. Additionally, since the magnetic particles within the influence of the magnetic field are attracted to the field, magnetic particles follow the locus of the magnetic field and are carried away from the desired area of demarcation;
thus forming a Aiscontinuo~e line with reduce~ cor.L~st and ` resolution.
. 30 ~he prior art has dealt with co... L.ast and resolution difficulties in a number of ways. For instance, the patent ,. to Murata, et al. ~U.S. Patent No. 4,643,684), discloses ; the use of a magnetic display panel having a dispersing medium having a yield value of 5 dyne/cm2 or more, the medium comprising an inorganic thickener, fine magnetic .'~;' .

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particles, and a colorant Murata discloses the use of a multi-cell structure which confines the ~ispersing medium within each cell, the ~tructure assisting in limiting the migration of the medium and the magnetic particles from one cell into the next during the application of a magnetic field to the particles However, regardless of the precautions taken by the prior art, the action of the magnetic field on the magnetic ~ particles ~jsEt~rsed within the liquid of t~e prior magnetic ; 10 mar~ing devices produceE a number of inherent difficulties For example, during movement of the magnetic field across the magnetic particle containing liguid, the magnetic particles move through the liquid, from the bottom of the liquid to the top of the liquid, to the magnetic field This localized movement of particles through the liquid creates a ~oid of particles within the liquid This void is created when the particles are pulled through to and along the top layer of the .~bs~te by their attraction to ~-~ the magnetic fiela When the ~agnetic f$eld i8 moved, as when the device iB used for drawing puL~es~s, the attracted part$cles are pulled along the locus of the ~agnetic field, throughout the substrate, creating ~n incomplete di~tribution of particles Additionally, a magnetic field i~ reguired to erase the imag- pro~uced by these prior art devices The erasing magnet .~E~sitions the magnetic particles ~fter magnetic field attraction Thus, when the cle~ning or erasure of a ^plry is desir-d, a magnetic field is applied to the bottom of the device to draw the magnetic particles from the top of the lig~d to their original position at the bottom of the liguid, thus eliminating the ir-~ p~olucing particles from the top of the liguid lewevcr, there exist a number o~ limitations of this techni~le of erasure For instance, incomplete or ~on~niform application of the 3S magnetic field across the bottom of the liguid pro~t~C
~` loc~lized areas of particle accumulation after rasure, thus e ~ing the ~s~quent drawing of a true line during . .

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W~gl/07287 ~ ~ PcTtus9o/osssl 207296~

application of the ~agnetic field to the top of the liquid due to the i~ lete di6tribution of particles throu~hout the liquid. Additionally, after repeated use and erasure by magnetic means, it hec~ extremely difficult to redisperse the particles to attain uniformity throughout the liguid due to the magnetically attractive properties of the partieles.
Thus, there exists a need for an apparatus and method for producing a magnetic display which eliminates the drawing and erasure difficulties inherent in the additi-~e processes used in the prior art magnetic display devices.
The ples~r~ invention provide6 _ magnetic visual display which is true, uniform, and of high resolution and n~ast. ~he present invention alco provides a method and apparatus for producing an image by orienting magnetically - ~ 15 active flakes contained within a ~i~p~rsion medium such that when a magnetic field is displayed to the flakes within the ~is~ersion ~edium, the magnetically active fl a~ 5 are oriented to rha- e the light transmission characterictics of the A ~ 6ion medium. The orient_tion of the magne~jrally active flake~ of the pkE~nt invention o~. without gros6 translation of the flakes wlthin the Aj ~ersion medium, thus providing a uniform, consistent ~isp~rsion of the flakes thro~ho~t the medium.
~y~marY of the Invention A magnetic marking appar_tus is described herein, the ~- ~ apparatus comprising an enclosure having at least one ~ transparent or translucent surface area; a dispersion ;~ ~ med$um which ha6 a plur_lity of magnetically active flakes cont~ai t ' within it; and a m_gnet comprising a magnetic field. The`aagnetic fi-ld has a plurality of flux lines.
When the magnetic field and its flux lines are ~i6pl-ayed to ~-~ the magnetically active fl a~s, the f1a~s align along the flux lin-s of the magnet, thus changing t~e light trans~ission characteristic~ of the di ~ersion ~ediu~ to ~ 35 ~oduce an image. The magnetically active flakes may ; comprise nir~el flakes, and the translucent or transparent :-:

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W091~07~7 - PcT/us~o/~s5l 20729~4 _4_ surface area of the enclosure may be deformable to the touch, to provide complete or discrete erasure capability, A magnetic display panel is also disclosed, comprising an enclosure having a front and a rear panel, forming a liquid sealing space with at least one of the front or rear panels having a transparent or translucent area. The panel also contains a dispersion medium comprising a plurality of magnetically active flakes, the ~isp~rsion medium ~ealed in a liquid sealing space formed between the front and the rear panels. The display panel also comprises a magnet comprising a magnetic field, the magnetic field comprising a plurality of flux lines. When the magnetic field is displayed to the flakes, the flakes align along the flux lines of the magnetic field, thus changing the light lS transmission characteristics of the dispersion medium.
A method for orienting magnetically active flakes -is also disclosed, the method comprising the steps of mixing magnetically active flakes within a ~ispersion medium, distributing the medium uniformly within a container, the container having at least one tr~nsparent or translucent areas: displaying an oriented magnetic field to the container, the field having a plurality of flux lines; and changing the light transmission characteristics of the medium by aligning the flakes along the flux lines.
Brief DescriDtion of the Drawin~s Figure 1 is a perspective view of the magnetically active flakes of the pr~ent invention ~ispersed within the ~i~persion medium, with a magnet suspended above the medium, yet not infl~ncing the flakes.
Figure 2 is a pe~pe.~ive view of the ~e~^nt invention, the magnetic flux lines ext~nding into the spersion medium and influencing the flakes.
Figure 3 is a plan view of a preferred æ~ iment of the apparatus of the present invention.
Figure 4 is a fragmentary cross-sectional view of a preferred r hsAiment of the apparatus of Figure 3, taken ~ along line 4-4.

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WO9l/07287 ~ ) PCT/US90/05951 Detailed Description of ~he Preferred F~bodime~t In the magnetic display of the p~ r~ invention, an image is formed by aligning magnetically active flakes contained within a dispersion medium along the flux lines of a magnetic field. Alignment of tbe flakes provides a change in ligh~ transmission through the dispersion medium, thereby creating a visible image.
When a magnetic field is applied from a permanent magnet, for instance, those comprised of iron nickel alloy lo composition or an amorphous magnet of iron nickel boron composition, magnetically active particles tend to be attracted to the magnetic field o the magnet and accumulate at the locus of the field.
This phenomenon of induced magnet$sm in magnetically active particles may also be observed by ~i-cpersing the magnetically active particles within a vieco~c liguid. By dispersing the particles in a visco~c liguid, the viscosity of the liguid slows down the magnetic alignment of the particles by the counter foroe of friction. Thus, these magne~cAlly active particles are c~e~ to flow through the liguid to a ~_gnetic field ~ Qd to the external surface of the liquid, thus forming an accumulation of magn-tically _ctive particles along the surface of the ` liguid at the iocus of the magnetic field.
The thi~ness of the layer of these magnetically active particles will be some function of the .~ r.~ration of particles in the liguid and may range from a monolayer to a multi-tiered layer, ~ep-n~;ng on the number and ,~ .
~- density of magnetically active particles and th- ar a and density of the magnetic field.
It has been observed that the overall geometry of each of these magnet~c~lly active particles exhibiting this attraction phenomenon whic~ travel through the vi~co~c liguid to the magnetic field have a g-ometry which is ~en~ally spherical. In fact, it has been observed that as these magnetically active particles become more spherical in shape, the travel of the particles through the vio~ouc .
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207296~ -6-liquid to the applied magnetic field occurs with greater frequency and becomes more apparent. However, as the configuration of the magnetically active particles becomes less spherical and more flattened or flake-like, these particles tend to align along the flux lines of the magnetic field and not travel through the viscous liquid to the locus of the magnetic field, remaining relatively stationary. Thus, the ability of the particles to form an image in the present invention is ~e~en~ent on the geometry of the maynetically active particles.
One measure of the geometry of parti~le is the ratio .
of a particle's length to width to height. For convenience, this ratio is defined as the aspect ratio of the particle. Determination of the aspect ratio of a magnetic particle provides a measurement in absolute terms of the qeometry-of a magnetic particle. Calculation of the aspect ratio thus provides a standard for selecting metallic particles for use in the present invention which have the desired alignment characteristics along the flux lines of the applied magnetic field.
In a ~pherical particle, the aspect ratio i6 1:1:1, or unity. Particles with an aspect ratio approximating unity generally do not align along the flux lines of the magnetic field when cont~ined in a viscous liquid, but exhibit the attraction and movement phenomenon as described above, traveling through the liquid and accumulating at the locus of the magnetic field.
For instance, commercially available metal particles such as Inco Nickel Powder Type 123, have a particle size approximating four microns with the particles having a dendritic geometry. rc~e~ due to the small, irregular size of the particles, it is difficult to determine which is the l~ e~L axis for determination of an aspect ratio of t~e particles. Nonetheless, these particular particles behave like spherical particles having an aspect ratio of unity when they are DYpose~ to a magnetic field. In like manner, spherical nickel particles, such as those ~`' ' .

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W09l/07~7 ~ ' ~ PCT/US~/05951 _7_ 20729~
co ercially available from Novamet, Inc , (Novamet 4SP), an eight-micron diameter sphere with an aspect ratio of unity, will travel through a di~p~r6ion medium when attracted to a magnetic field and not align along the flux S lines of the magnetic field (Commercially available ferrous powders, such _s 325 mesh and lOo mesh by Hoeg~n~es, also exhibit the _ttraction ph~nr -rsn ) It is when the aspect ratio of the particles varies from that of unity that the particles tend to line up with their lon~e5L axis in the direction of the flux lines of an applied magnetic field, providing the alignment and change in light transmission characteristics of the present invention Magnetically active particles, including metallic and non-metallic particles having an aspect ratio greater than unity which xhibit the _lignment phenomenon along the flux lines of an applied magnetic field, Are hereinafter referr d to as magneticaliy active fl_kes Magne~ically _cti~e fl al-~ e are thu~ defin-d as ~etallic particl-s exhibiting the _lignment char_ct-ristics which provide the ~ha~, e in the light tr_nsmi~slon ch_racteristics of the ; ~ispersion medium of the p~s~t invention For inst_nce, flakes that are 15 microns in length and width and 1 micron in he$qht have an aspect r_tio of 15 15 1 With an aspect ratio o~ 5 15 1, these flakes exhibit the- al$gnment phenomenon aIong the flux lines of a magnetic field Also, L~ e of the ild~c~l ~agnetic field properties of the flak s _fter exposure to the magnetic field, the fla~es exhibit both attraction and repulsion characteristics which assist in ~L6J~ci~ _nd maint~ini~g flake ~lignment ~nd ~;~ resist transl_tio~A~ movement of the fl_kes The alignment ~; of the f2~e along the m~gnetic flux lines coupled with their attraction and repulsion ~Lv~eLLies relative to ea¢h other when aligned provide the desired ~h~-, e in light transmission characteristics in the ~ispersion medium Anoth-r example of a magnetically active flake exhibiting the _spect ratio phenomenon which provides the .
-, .-WO91/07287 ~ : I PCT/US90/05951 desired alignment properties in the present invention are magnetic fine cylindrical fibers. For instance, when seven-micron diameter nickel-coated graphite fibers are cut to 50-micron lengths, these fibers have an aspect ratio of 50:7:7 and exhibit the desired alignment characteristics within the dispersion medium of the present invention during exposure to the flux lines of a magnetic field.
Preferably, complete alignment of the flakes will occur in the present invention when the flakes are exposed to the magnetic field, ass~ i~g that each of the flakes has the proper geometry or aspect ratio to align itself with the flux lines of the magnetic field. However, differences in the aspect ratios between individual flakes used in the present invention may produce an incomplete alignment of each flake in the system when a magnetic field is introduced thereto. However, the alignment effect is most prono~nce~
as the average aspect ratio increases within a given population of magnetic flakes.
A pop~ tion of magnetically active flake6 with ~n aspect ratio having at least two of the height, length or width measurements of preferably ayp~o~imately about 5:1 or greater, or, most preferably, a~p~oximately about 10:1 or greater is preferred to overcome most effects of varying flake ~ize. Magnetically active flakes having aspect ratios in these ranges have been observed to provide the desired change in light transmission in the ~isp~rsion medium during flake alignment. ~weve" in the event irrQg~larly-shaped flakes (which ~,e~r.~ true measurement of absolute length, width or height) are used in the present invention the measurements u ed to calculate the aspect ratio preferably oo.~syol)(l to the l~ s~ linear measurement along the geometry of the flake, the other aspect ratio meaaur~ - ~s taken perpendicular thereto.
The relative density of the flux lines o~ a magnetic field can be taken as a measure of the field strength of the magnet or magnetic field source. Thus, magnetic field strength or flux line density varies both according to ~he W09l/07287 ~ ) PCTtUS90105951 relative strength of the magnetic field and to the configuration of the magnet or magnetic field source.
Therefore, the strength of the magnet and density of the flux lines is an important factor to consider in inducing the flake alignment phen~ -non of the present invention.
The relative density of the flux lines, particularly around the outer portions of the magnetic field and the extent to which they extend outwardly along the edges of the magnetic field also determine the extent to which the magnetically active flakes line up along the lines of flux.
-Referring to the Figures, Figure 1 show6 a magnet 10 suspended above a dispersion medium 14 within whic~ are susp~n~ a plurality of magnetically ~ctive flakes 16 in a random position 40. Separating the dispersion medium 14 from the magnet lo is ~ surface 26. The surface 26 preferably comprises a tr~nCparent or translucent area which allows observation of the flake alignment phenomenon th~u~h it, as will be ~ec~ t` in detail hereinafter.
; The magnet 10 has a positive pole 20 and a negative pole 22, the magnet having a magnetic field 17 comprising a plurality of flux lines 18 radiating around its circumference.
Referring to Figure 2, the magnet 10 is shown interacting with the ~isp~rsion medium 14. As the flux lines 18 of the magnetic field 17 ~scen~ into the dispersion medium 14 past the surface 26, the magnetically act~ve fl~es 16 orient t~eelves along the flux lines 18.
In this particular embodiment of the present invention, a variety of alignment zones are observed. With the magnet 10 having flux lines 18 extenAing therefrom in a manner depicted as in Figures 1 and 2, the magnetically active flakes 16 exhibit the alignment phenomenon in the areas where the flux lines 18 extend into the ~is~ersion medium 14.
35The alignment zone 30 shows two layers of ~agnetically active flakes 16 aligned along the lines of flux, with the phenomenon of in~llced magnetism producing magnetic charges WO9lt07287 ~ ; PCT/US90/05951 207296~

upon the flakes, indieated as (+) and (-) 50 The ~nAuce~
magnetism of the magnetic flakes 16 not only assists in the alignment p~en~menon by sta~ing the flakes 16 60 that their positive (+) and negative (-) poles are attracted to S each other, thus providing the columnar alignment, but the charges 50 also provide lateral repulsion characteristics so that the aligned flakes 16 also remain in formation, and are not attracted or additionally Ai ~ersed throughout the ~ispersion medium 14 When a cylindrical magnet lo having flux lines 18 such as that depicted applies its flux lines 18 to the ~ispersion medium 14, a slight void zone 33 may occur where some of the flakes 16 directly beneath the magnetic field 17 and~not directly inflv~n~e~ by the flux lines 18 remain in the random orientation, yet, those flakes 16 in the periphery of the void zone 33 translate to and are attracted by the flux lines 18 to the alignment zone 30 It has also been observ d that at the periphery of the alignment zone 30, the flakes 16, when ~ to the flux lin-s 18 of th- ~agnet 10 as ~ t- herein, tend to aove out of their random orientation and ~ a ~om-what V-shaped orientation, the open part of the V ~ein~ the magnet 10, the closed part of the V f~ng aw~y from the magnet The V-sh~pe~ aliqnment of the flakes 16 in the V-zone 37 also r~a ~e the light transmission characteristicsof the ~isFersion medium 14 to some extent, as th- V-sbAre~
. ~,, orientation of the flakes 16 tends to relatively ~c _ease ;;~ transmission of liqht through the V-zone of the ~edium 14 and r-flect light ~x~ d to the ~urfac- of the ~i ,ersion -medium 14, thus providing a ~halo~ effect along the edges of the alignment zone 30 which result~ in ev n greater ;~ con~st for the image proA~lce~ by the pL~s~nt invention ;~ At tbe outer pe~ y of the V-zone 37, the flakes 16 `~ remain ~;nfl~t ;e~ by the flux lines 18 of the magnet 10 and remain in the random position 40 ~ t will be apparent to those skilled in the art that this alignment phenomenon, along w~th the number of zones .~ .

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Wo91/07~7 ~ ~ PCT/~S90/05951 of influence of the magnetically active flakes 16, may vary depen~ing upon the type and strength of magnet u~ed, along with the orientation and geometry of the flux lines 18.
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For instance, it has been observed that when a bar magnet s lo such as that depicted in Figures 1 and 2 is placed on its side, i.e. rotated 90 de~.ees, and introA~1ce~ to t~e medium, the void zone 33 is generally not o~served and the flakes 16 tend to completely align throl~ghout the area of the dispersion medium 14 influence~ by the flux lines 18 of the maqnetic field 17. Additionally, the polarities of the magnet 10 and the induced magnetic charges 50 of the flakes 16 may vary from that depicted herein, as can be appreciated by those skilled in the art.
The factors which govern the flake alignment phenomenon include: composition of the A; cper6ion medium;
~ ~ strength- of the magnetic field; diameter of thé magnetic - field: density and orientation of flux lines; aspect ratio , of the magnetically active flakes, preferably with at least two of the relative measurements of length, width and ~; 20 height of the flakes h_ving a relative ratio of at l-ast about 5:1, and mo~t preferably, a r~tio of at l-ast about 10:1; density of the ~1 a~eS relative to that of the -- ~ AiFpersion medium; and mass of the flakes.
;~ Dispersion Medi~
The Ai~persion medium preferably compri6es particular densities, ViSCositiQs, and thixotropies which, in conjunction with the partic~ r magnetically active flakes us-d, keep the magnetically active flr~.e~ evenly -~s~ J^~
`~ th.v.. J~o ~ the ~persion medium and a66i~t in providing ~`~ 30 the alignment and change $n light trancmi~sion characteristics of the ~æEert invention.
Any suitable ~isF~r~ion medium for the magnetically i~ .
} ~ ~ctive flr~es can be employed in conjunction with the .2 -nt invention. The ~isper6ion medium should be capable of Du~4~nAing the magnetically active flakes ~o a6 to allow them to ch~ orientation and align along the flux lines of an applied magnetic field.
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W o 91/07287 ~ ` PC~r/US90/05951 207296~ -12-The suspended magnetically active fl~kes in the ~;Sp~rSion medium of the present invention preferably have a density such that the fla~es will ~ sp~n~e~
therein in a generally uniform layer without a great tendency to either sink or float Therefore, the density of the dispersion medium ~ho~ be approximately the same as that of the magnetically active flakes 80 that the Slakes àre supported substantially at equilibrium without rising or s j ~k i ~ .
The viscosities and/or thixo~L~ies of the di~pers medium should be such that the interaction of t magnetically active fla~es to each other and to the magnetic field are properly cG ~lolled Therefore, the dispersion medium preferably comprises viscosities and/or thixotropies such that ~ certain minimum force must be ~` applied by the magnetic field on the magnetically active flakes in order to align the magnetically active flakes, ~; yet overcome the vi~col~ and thixo~o~ic propertie~s of the p-rsion medium, ~nd provide ~ degree of stability to the ~ 20 y-tem by minimizing unwanted disorientation of the ; magnetically activ- flakes Dens ~s, viscositi-s, and ;~ thix~ i-s arc imparted by the d ~rsion m-dium itself, or mi~L~ s of medium, ~6 ~vell ~s _y the i ~..,duction of ; agents providing desired densities, vi w ositi-s, and/or thixo~lopies -Th- magnetically ~ct$ve flakes are preferably substantially immobilized within the A i~persion medium when at r st, yet exhibit the ability to align the~sQlves in the ` ~ A; ~ersion mediu~ along the flux lin-s of a m~gnetic fi-ld ; ~ 30 where the field is _x~cse to the fl~e~, yet not travel thLo~glo~,~ the medium to the locu8 of the magnetic field Thus, there is an interrelation 1-~w an density, viscosity, nd thixot~ in selecting the ~o~c~ comp~ of the ersion medium 35Thixot~o~ic agents h~e the ~ Ferty, ~hen ~ispersed ~` in suitable medium, of exhibiting a variable ~iscosity ; which ~epends on the shear st~s applied to the flakes .

WO91/07287 ~ PCT/US90/05951 -13- 2~72964 cont~; n~ in the medium. At low ~hear stresses, or at rest, thixotropic ~i~p~rsions have high viscosities in the - nature of elastic solids, while at high shear stresses theyhave low viscosities. Thixotropic liguids are non-S Newtonian, whereas non-thixotropic liquids are Newtonian liquids, i.e., thixotropic liquids behave like elastic solids at low shear, or at rest, and behave like liquids at high shear. Therefore, they are fundamentally different from viscous non-thi~o~,opic liquids which behave like liquids both at rest and under low and high ~hear.
By controlling the thixoL~o~r of the dispersion medium, the self-adjustments of the thixotropic system preferably impart proper variable viscosities under stress and static conditions. The maqnetically ~ctive flakes of the present invention are thus limited from interacting and clumping when thixotropic liquids (i.e., which behave like ~olids at rest or low shear) are employed in the dispersion medium.
Typical thixotropic agents includ- inorganic ~ubstar-es such as montmorillonite cl_y (a tetraalkyl ~mmonium smectite), _ttapulgus clay (a crystall~r E hydrated ; magnesium aluminum ~ilicate), ~ilicon AiQ~ , organic ~ thickeners such as ~occæsed derivatives of castor oil, ; polysaccharides, guar gum, starch, organic polymers CUch asca~Lo~r~inyl polymers, cellulose derivatives ~nd emulsions.
Emulsions are defined as a heteLG~enous system consisting of ; at least one immiscible liquid ~ispersed in another liquid ~ wherein at least one liquid will be water or ~n aqueous `~ solution and the other liguid generally described ac an oil - 30 phase. Metallic soaps, which are metàl salts combined with high molecular weight, organic acid (fatty acid6) such as ~tearic, lauric, oleic and behenic are also contemplated for use. The major metals used in this ~ystem include zinc, calcium, aluminum, magnesium and lithium. Organic soaps consisting of high molecular weight organic acids combined with organic alkyl salts are also contemplated.

W091/0~28~ ~ ) PCT/US90/05951 207296~ -14-A dispersion medium having thixotropic properties preferably enc~ses the magnetically active flakes firmly and securely when at rest. Yet, where the flakes are placed under the influence of a magnetic field where S movement of the flakes to align them along the lines of flux is desired, the thixotropic dispersion medium surro~n~ing the magnetically active flakes liquifies when subjected to the stress from the movement of the flakes due to the influence of the magnetic field, thereby allowing movement of the flakes to align with the flux lines of the magnetic field.
The relationship between the mass and density of the magnetically active flakes with the viscosity and density ~; of the dispersion medium is also important. It is desirable that the flakes be held within the disper ion medium in buoyant 6-~epeneion and not travel throughout the medium when subjected to the magnetic field. Unless the spersion medium is-quite vi-co~e, the magnetic~lly ~ctive flakes, if lighter than the ~rp-rsion medium, will rise and break out to the surface of the medium, or, if ~e~~~r, will fall to the bottom of the medium.
A wide variety of materials which have these characteristics can be employed in preparing the ~ispersion medium. These materials may preferably comprise both organic and inorganic thickeners, including both natur~l and synthetic polymers or mi~u~es of both natural and synthetic ; polymers.
~ hus, an important a-,e_t of the ~se~ invention relates to the choice of ~ispersion medium composition with specific dénsities, viscosities and th~xotropies in conjunction with the choice of magnetically ~ctive fl~kes.
The properties of the A-spersion medium combine to limit displacement and travel of the magnetically active flakes throug~out the ~i ~ersion medium both at rest and when influence~ by a magnetic field. Preferably, the uniform distribution of the magnetically active flakes throughout the medium and the ability of the flakes to ~lign along the ~ ~ .
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W~91/07287 ) PCT/US90/05951 flux lines to change the light transmission properties of the ~ispersion medium is maintained throughout repeated and rigorous use of the present invention.
The dispersion medium of the present invention also S preferably comprises non-electrostatic properties which give the medium the ability to disperse ele~LLons produced by electrostatic movement of the flakes through the dispersion medium, preventing the accumulation of ele_~losLatic areas within the medium whic~ may retard or ~_ver~ hsequent proper _lignment or distribution of the magnetically active flakes.
It is quite evident that other ~ispersion medium, gels and emulsion systems: other s~1spen~in~ or carrier fluids permitting mobility, including thixotropic agents; other lS magnetically active flakes or magnetically induced ~ particles or flakes: other types of magnets or magnetic fields; etc., are known or will be developed contin~ y which could be used in thic invention. It i~, therefore, i~possibl- to attempt a comp~ cive catAlo~ of _uch components. To attempt to describe the invention in its broader ~ ~e ~s in terms of _pecific co~ponent_ which could be used would be too voluminous and ~ rs~e sary since one ~; ~ skilled in the art could, by following the description of the invention herein, select useful ~irpersion medium, thixotropic and viscous agents, m_gnetic fields and magnet;cAlly active flakes for the present invention. From the description in this specificAtion, and with the knowledge of one kill-d in the art, one will know or deduce with confids :e the applic_bility of Epecific ~; 30 com~ponents ~uitable in this invention.
Thus, the examples given herein are inte.~e~ to be illustrative, and ~arious modifications and rhA ,e6 in the materi_ls, DL~U~L~L~S and compositions may be _pparent to those skill-d in the art without departing from the spirit of this invention.

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Examples of Thixotropic and Viscous Aqents A. Carboxyl Vinyl Polymers B. Cellulose Derivatives 1. Sodium Carboxymethycellulose 2. Hydroxyrthylcellulose 3. Hydroxypropylcellulose C. Polysaccharides 1. Xanthan Gum D. Natural Thick~erc 1. Algin 2. Guar Gum 3. Starch 4. Tragacanth 5. Locust Bean Gum E. Polyvinylpyrrolidone (PUP) 1. PVP/Vinyl Acetate Co-Polymers Exammles of DisDersion Medium:
The following are examples of the dispersion medium O1 the present in~ention. The following examples in which all proportions are given in parts by weight, unles~ otherwise indicated, will serve to illustrate, but not limit, the present invention.

A. Oil-Based Medium Co~ponents 1. Mineral Oil 40 parts ethylene glycol monostearate 5 parts ;~` calimulse PRS 5 parts (Pilot Chemical, 0 Santa Fe Springs, C~) 2. propylene glycol S parts petrolatum 6 parts water 39 parts Procedure: ~elt and mix the components of group 1 at 160-F, add the co~onents of group 2 to the mixture of group 1 with mixing at 160-~, slowly cool (add additional WOgl/07287 ~ ~ ~ PCT/US90/05951 water if necessary to proper viscosity). Finally, add 2 parts by weight of nickel flakes.

B. Micellar Gels Micelles are ay~e~ated units of molecules of a surface active material (surfactants), formed as a result of tbe thermodynamics of the interaction between the solvent (usually water) and lyophobic (or hydrophobic) portions of the molecule.
A micellar gel is a term used to describe the irreversible union of two or more surfactant-forming ingredients, one of which consists of a water-immiscible hydrophobic, saturated, or unsaturated fatty acid (oleic, stearic, palmitic, etc.) or alkyl benzene ~uch as ;~; 15 docylbenzene sulfuric acid, in addition to an alkali hydrophilic salt such as triethanolamine, monoethanolamine, - isopropanolamine or sodium hyd~o~ide. The gel described herein i~ formed by the CGn~ olled addition and agitation of the proper amount of the alkali constituent to the ~cid constituent to form a gel. The6e gels can be modified by the addition of a non-ionic surfactant prior to the addition of the hyd~ophobic ingredients. The addition of non-ionic surfactant allows water to be added -in small amounts in order to control the viscosity of the gel.
com~Gncn~s:
Oleic acid 7 parts Non-ionic al~yl phenylpolyether i0 parts ethanol TriethonQlamine 2 parts Water 50 parts PlGced~re: The Oleic acid i5 added with mixing to the non-ionic alkyl phenylpolyether ethanol. The triethanolamine is then slowly mixed to form a gel. Add water to adjust to proper viscosity. Finally, add 2% by weight to the total gel formula of nicXel flakes.

W091~07287 ~ ~ ~ PCT/US90/05951 2072964 -18- ~
C. Fmulsions:
Co~nents:
Triton X-100 10.0 parts (Rohm & Haas) Mineral Oil 51.0 parts Oleic Acid 4.0 parts Stearic Acid 3.0 parts Sodium Hydroxide.S parts Water 31.5 parts '~' 10 .u ed-~e: Triton X-loo, 6tearic acid and oleic acid are added to the mineral oil and agitated until homoqenous. To ease the solution of the stearic acid, heat the mineral oil to 160-F. Make a concentrate from the sodium hydroxide in part of the water and add to the above mixture. Continue subsurface agitation until uniform. Slowly add the remainder of the water and stir until smooth. The final .odu~L is a white o~a~l~ paste. The viscosity can be lowered or raised by the addition of increments of water or mineral oil. To the ~bove, add 3 parts by weight of 6tainless steel flakes.

D. Tn~ra~nic Thickeners:
. -:
- ÇQE~onenLs:
bentone 5 parts vegetable oil 90 parts non-ionic surfactant5 parts .~
.~ c: Add the bentone to the vegetable oil with high ~ ~30 shear agit~tion. A medium with a gel-like ~on~i~tency will ;~form slowly; next add the non-ionic surfactant. Blend in 2.5 parts by weight of nickel flakes at moderate speed.
~ .
E. Or~anic Thickeners:
Com~v.. ~n~s:
xanthan gum 3 parts glycerin 5 parts ',' ~
.

wosl/o7287 1 PCT/US90/05951 non-ionic surfactant 2 parts water 90 parts ~ ocedure: Dissolve and thoroughly mix xanthan gu~ and S water. Add the glycerin and thè non-ionic surfactant to the xanthan/water gel slowly. Allow the above to settle for at least 24 hours to expel the air bubbles. Finally, `~ add 3 parts by weight of nickel flakes with moderate agitation.
,~ 10 ~ F. Water-Soluble Resins:
- C ~:nar.~s:
carboxymethylcellulose 2 parts ;~ propylene glycol 10 parts ~j 15 water - 88 parts non-ionic surfactant S parts ','~ , ;~ PL~ n~ L: Add the propylene glycol to the w_ter. With ~ery low speed _gitation, add the carhQ~ymethyl cellulose to form a slurry. G~ lly incre_se agitation until _ cle_r g-l has been formed. Add the non-ionic curfactant to the gel. Finally, with moderate _gitation, add three parts by weight of ~tainless steel flakes.
,.
2S E~ les of Ma~neticall~ Active Fiak~
Ex~mples ~of suitable magnetically ~ctive flakes which can be used in this invention include flakes comprising m_gnetic ~etal materials made of alloys b_sed on, for xampl-, iron, cobalt, or nickel and granulated forms of these ~_teri_ls. If neces-~ry, the fl_kes may be _djusted for their color tone. However, any _ppropriate magnetic~ally active flakes as known to those killed in the _rt _re contemplated for use in t~e p~ invention. The ~following are examples of fl_kes having char~cteristics desirable for use in the ~sent invention.

~ ' ''~'' ' ' :~

, , wo91/07~7 ' ) PCT/US90/05951 Apparent Thickness Screen spec. Grav. Density inAnalvsis(C) ~
~/cm3 (a) g/cm3 (b) lmicrons)~Q l325 -325 5 Nickel Leafing (a) 6.69 1.39 0.37 2.3 3.4 94.3 Nickel ~eafing (b) 7.60 1.19 0.47 2.5 3.8 93.7 ' Stainless Steel (a) 6.53 1.03 0.88 0.8 21.4 7.8 Stainless - -Steel ~' (b) 6.68 1.52 0.83 1.6 12.7 85.7 , 20 Stainless, ~; Steel "
(c)6.,99 1.22 l.oO 69.2 18.2 12.6 ~:, ' 25 Stainless Steel ~i (d) 7.14 1.07 1.00 45.0 43.6 11.4 '~ 30 'a' As determined by ASTM Standard l329- ' " 'b As determined by Scott Volunteer lasTM Standard B 329).
'c, U.S. Standard Service.
Nfckel 99.9% Ni) ~Stainless Steel 68% Fe, 17% Cr, 13% Ni, 2% M0) cw~r, it will be appreciated by those skilled in the art that a variety of the non-metallic flakes having magnetically active properties may be used with the present invention. For instance, polymeric substAnces having 40 magnetically active coatings are contemplated for use in the p~nt invention.
The amount of flakes added to the ~ispersion medium may vary according to a number of factors, the factors including: the composition of the flakes: the nize of the 4S flakes: the amount of display cor.L~ast desired; the strength of the magnet; and the composition of the dispersion medium. However, it is contemplated that t~e percent weight of the magnetically active flakes in W091/07287 ~ ) PCT~US90/05951 relation to the weight of the dispersion medium may preferably comprise between about 0.25% by weight to about 10% by weight of the dispersion medium, and, most preferably, between about 1% by weight to about 5% by weight. However, those skilled in the art will appreciate that these ranges may be varied beyond those presently indicated, depDn~;ng upon the particular application of the present invention and the composition of the dispersion medium.
Colorants In addition to the special benefit of the flake confi~uration as to its magnetic attraction, the magnetically active flakes of the present invention may preferably comprise a high specular reflectance. Thus, the flakes used in the present invention preferably comprise flat surfaces which reflect light and produce a smooth-looking coating when distributed randomly within the dispersion medium and viewed from a transparent or translucent surface. The magnetically active flakes can be further coated with a metallic substance such as silver or gold, or with a ceramic or other ap~Lopliate coating or colorant to enhAnce the cohLLast or provide a particular color in conjunction with ~pecific uses of the present invention.
If desired, the addition of colorants to the ~ispersion medium are al~o contemplated for use with the present invention. Dark-colored pigments or dyes that are soluble in the ~i~persion medium are preferred for use with the present invention, providing in appropriate inst~nces increased CO~ st between those areas of the ~ispersion medium containing aligned flakes, and adjacent areas where the flakes are randomly distributed.

DisDlaY ADr~Aratu~;
The apparatus of the present invention preferably comprises an enclosure into which tbe dispersion medium is placed, the enclosure comprising at least one transparent w091/07~7 ) PCT/US90tO5~1 207296~ -22-or translucent surface area. In a preferred embo~i -nt depicted in Figures 3 and 4, the enclosure 50 of the present invention comprises two spaced planar eurfaces 53, 55 having interposed therebetween the dispersion medium 14 S in a liquid sealing space 51, the medium 14 bearing in sucp~nsion the magnetically active flakes 16.
In a preferred embodiment, the epace 51 between the two surfaces 53, S5 comprising the enclosure SO may be varied according to the specific application of the display apparatus. To provide a ~harp display with high contrast and good erasure capability, the surfaces may be spaced by a distance of from about 5 to about Soo mm, preferably from about 5 to about 25 mm. The front surface 53 from which the display is read preferably comprises a transparent material, but, dependent on the particular application, it may comprise a translucent material. In either case, a variety of different plastics and glass can be employed.
The other, or re_r, surface 55 need not necess~rily be m~de of a traneparent material and, hence, a wide variety of plastics, glass, and met_ls c_n be used. '~w_~er, in a preferred D~hodiment, both the front 53 and rear S5 surface comprise an area comprising a trAnsparent or translucent material capable of providing an observation of the change of the light transmission characteristics of the ~ispersion medium 14.
In inst~nce6 where both the front 53 and rear 55 - ~urfaces compri~e a tra~p~rent or translucent material, the apparatus may be configured such that the display of a ~- magnetic field to one eide of the apparatus will align the flakes 16 t~ou-l~o~t the ~ispersion mediu~ 14 between the surf_ces 53, 55 euch that light is allowed to be transmitt-d through both of the 6urfaces 53, 55 and the ~isper~ion mediu~ 14 in areas of flake alignment.
In another preferred embodiment, the apparatus may be configured such that images may be p~ol~cc~ separately on the opposing sides of the enclosure 50, 6uch that they are eeparately viewable through the oppos~ng ~urfaces 53, 55 of WO 91/0~287 ~ ' ) PCr/US90/059~1 -23- 207296~
the enclosure 50. In inst~nces where two or more different images are to be separately prod~ce~ to be viewed on opposing surfaces 53, Ss of the enclosure 50, special consideration should be given to a variety of factors including: the thickness of the ~ispersion medium between the surfaces; the thic~ne6s of the ~urfaces: and the strength of the magnetic field. Those skilled in the art will appreciate that these factors, among others, determine - whether the alignment of the flakes 16 pr~duces an image in the dispersion medium 14 throug~o~t the space 51 between the surfaces 53, 55 when the flux lines 18 of the magnetic field 17 are ~Ypose~ to only one surface, 53 or 55; or whether the alignment of the flakes 16 produces an image in the dispersion medium 14 only o~servable through the 15 surface 53, 55 to which the magnetic field 17 is exposed.
Alignment of the flakes 16 in the second instance preferably allows the enclosure 50 to have separate images proAuce~ along and visible th,ough opposing surfaces 53, 55, the images preferably not interfering with each other.
If manual redistribution and orientation of the magnetically active flakes is desired to p~o~e image erasure, one or both or the ~urfaces 53, 55 preferably comprises a flexible material which can be deformed by the user to physically re-orient the magnetically active flakes 16 to a random orientation within the ~ispersion medium 14, thus restoring the original light transmission characteristics of the medium 14.
The thir~n~ss of the surfaces 53, 55 i8 important.
The thic~ne~s of the surfaces 53, 55 is preferably from about 0.5 to about 1.0 mm; if the thic~ness goes beyond 1.0 ~m, the image may have less con~ ast due to the reduction of the relative ~trength of the magnetic field 17 as the magnet 10 is displaced further away from the flakes 16 within the ~ispersion medium 14. The front 53 and rear surfaces may be formed of one continuouc piece by pLocæ~l~Les known in the art such as by conventional molding W09l/07287 , 1 PCT/US9OtO5951 207296~ -24-techniques, or the surfaces ~ay al~o be hQ~ed together by, for instance, heat-sealants or adhesives A preferred embodiment of the enclosure 50 of the present invention comprises the surfaces 53, 55 comprising S Polyvinylchloride (PVC) or Copolymer cont~ining Vinyl Chloride, Polyethylene ~erephthalate (PET), polycarbonates, acetates, or other appropriate polymeric material The front surface 53 may be affixed to the rear ~urface 55 by means of an adhesive over the peripheral edges of the surfaces The edges 59 of the surfaces 53, 55 can also be secured together by the use of high-frequency welding, ultrasonics, or similar ploctsses familiar to those of ordinary skill in the art One of the surfaces .
may preferably be recessc~ in part to provide a chamber between the surfaces in which is located the dispersion medium 14 Uc~e~er, it will be apparent to those skilled in the art that the enclosure 50 of the present invention ~ay al80 compri~e surfaces which are non-planar, the enclo-ur~ 50 comprising ~urfac-~ which ~ e a thr~e-~- 20 dimensional configuration of the enclo~ure, the~e configurations including ~ s, cubes and cyl~
In ~e~a~ion, the Slux lines 18 of the magnetic field 17 ~re displayed to and pass th~ou~l a surface 53, 55 of the enclosure 50, c~using the magnetically active flakes mixed within the ~isFersion medium to orient themselves and align along the flux lines of the magnetic field, creating an i~age It i~ this alignment of the magnetically active flake~ which _a ~ an image to take place as a result of a h~ in the transmission of light through and into the ~i ~ersion ~edium 14 Thus, when the flux lines 18 of the magnetic field 17 are in~ _ce~ to the flakes 16 as depictQd in Figure 1, the flA~es 16 align with the longit~ n~l axi~ of each of the flakes 16 becoming orient-d such that they are pr-f-r~bly J~ lly align-d along and generally parallel to the flux lines 18 of the magnetic field 1~ which infl~~r:es the ar-a of the ~ispersion medium 14 in which the flakes 16 are ~ispersed .
. ~

WO9l/07287 ~_ ) PCT/US90/05951 While lined up along the flux lines 18, the magnetically active fla~es 16 change the light transmission characteristics of the dispersion medium 14, thus producing an image.
S In a preferred hoAiment, the image produced by the magnetic display of the present invention is effected by a magnet 10. The magnetic field 17 of the magnet 10 acts upon the suspended magnetically active flakes 16 in an area ; ~d;acent to the locus of the magnet tip. Moving the magnet tip over the enclosure 50 cause6 the flakes 16 in an area adjacent to the 6urface of the enclosure 50 to be oriented from a random position to another position essentially vertical to the tip of the magnet 10, the fla~es 16 aligned ~ along the flux lines of the magnetic field 17 as previously `~ 15 described. To the observer, this re-orientation of flakes 16 produces a black image, in contrast to the metallic sheen of -the remainder of the essentially non-aligned, randomly distributed magnetically active flakes 16 unaffected by the magnetic field 17.
Me~h~Ac of FrA~.-re an important ~ ~e_~ of the ~ r~ invention is the ability of the user to ~electively or completely erase the image p~uce~ by non-magnetic means.
After an image ig formed, it may be desirable to erase the image such that the original light transmission characteristics of the ~i~persion medium 14 in the areas of flake alignment are recalled. Erasure, as defined in the present invention, preferably comprises ,~-u~..ing the flakes 16 from their aligned position to their random state exist~ng prior to the ~,vdu~Lion of the image within the ~;sper6ion medium 14, the erasure of the image discretely or completely. Non-magnetic erasure means are preferably employed to effect the erasure of an image.
Examples of applicable era~ure means include: (1) applying pressure to the surface of the enclosure, such that the ~urface is deformed and contacts the ~ispersion medium, redistributing the dispersion medi~m 14 in the area .' ' :~ ~ . ... ..

WO9l/07287 1 i ~ PCT/US90/05951 ` 2072964 -26-of deformation to randomly orient the fla~es 16, thus providing complete or selective erasure o the image previously pro~u~e~; (2) sliding or moving one of the surfaces of an apparatus having opposing surfaces laterally in relation to the opposite surface, or, alternatively, sliding or moving an erasure means, preferably comprising a separate surface, panel or roller located between or out6ide the &urfaces of a planar apparatus or a three dimensional enclosure, such that the surface or erasure means contacts the ~s~ersion ~edium 14 and causes the medium 14 to redistribute and thus randomly orient the flakes 16; and ~3) ~hA~ng the entire ~agnetic display device, manually or mechanically, to cause the dispersion medium 14, and thus the flakes 16, to redistribute to a random orientation - The ~anuai or mechanical erasure as described in ~3) is particlllArly effic~cjo~c when the ~pparatus of the ~L~ ~r L invention comprises ~n nclosure having a three-dimensional ~j~pl~y area, such a~ that of a bottle Thi6 means of erasur- can be used to erase i~ages ~L'e ~ in a dispersion ~edium 14 which fill- an enclosure or, alternati~ely, in a medium 14 distributed as ~ coating on the interior of an enclosure which contacts and covers the ~` inside of the enclosure, yet does not fill the enclosure Referring to Figures 3 and 4, an erasure means comprising a erasure panel 60 is shown in conjunction with a pr-f-rr-d mbodiment of the ~enQnt invention The era~ur panel 60 is ~ se ~~t~c_n the ~UL f~c~ 53, 55, ;~ within th~ A ~e-l~n~ p~ce 51 ~nd defines ~ first imag- ar-a 63 and a ~c~ image area 66 lGc~t~ h~ er the panel 60 and the surfaces S3, 55 The A~ ~persion ~edium 14 is located within the image areas 63, 66, ~nd is prefer~bly in fluid communication with the panel 60 and the surfac-s 53, 55 The erasure panel 60 i~ re ted to a 35 handle 69 located outside the enclosure 50, the handle 69 co~ ed to the panel 60 by ~ ~r-F ~ing rod ~0 To ensure a fluid-tight ceal throughout the nclosure 50, the ;.
. .

;.
. ,~ .

W091/07287 ~ ) PCT/US~/05951 207296~

connecting rod 70 is inserted through a gasket 72 which extends through the edge 59 of the surfaces 53, 55.
In use, the handle 69 is translated 60 that the connecting rod 70, surrounded by gasket 72, moves the . 5 erasure panel 60 laterally. The panel 60 contacts the dispersion medium 14 located in the image areas 63, 66, moving the medium 14 between the surfaces 5~, 55 and the panel 60, thus causing the medium 14 to redistribute in the areas 63, 66.
lQ With each of the above-described erasure methods, the object is to physically orient the flakes 16 away from their aligned position and r~ ly orient the flakes 16 so that the light transmission characteristics of the ~isp~rsion medium 14 LeLuL,. to the random state existing prior to production of the image. However, other methods of erasure or distribution of the flakes 16 apparent to those skilled in--the art are contemplated for use in the ~e~e~L invention.
While particular embodiments of the invention have been described in detail, it will be apparent to those skilled in the art that the disclosed embodiments may be modified. Therefore, the foregoing description is to be considered exemplary, rather than limiting, and the true scope of the invention is that defined in the following claims.

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Claims (10)

CLAIMS:

1. A magnetic marking apparatus, comprising:
an enclosure defining a liquid sealing space and having a plurality of transparent or translucent surfaces through which said liquid sealing space is viewable;
a dispersion medium contained within said liquid sealing space having a plurality of magnetically active particles; and a magnetic field outside said enclosure selectively applicable to a portion of said dispersion medium sufficient to cause movement of said magnetically active particles in said portion of said dispersion medium.

2. A magnetic marking apparatus, comprising:
an enclosure having a plurality of transparent or translucent surfaces;
a dispersion medium contained within said enclosure having a plurality of magnetically active flakes; and a magnetic field outside said enclosure selectively applicable to a portion of said dispersion medium sufficient to cause alignment of said flakes in said portion of said dispersion medium so as to form an image viewable through one of said surfaces.

3. A magnetic marking apparatus, comprising:
first and second planar, parallel non-opaque surfaces.;
a housing supporting said non-opaque surfaces and, along with said non-opaque surfaces, defining a sealed interior space;
a dispersion medium sealed in said interior space;
a plurality of magnetically active particles dispersed within said dispersion medium; and a permanent drawing magnet outside said interior space having a magnetic field; wherein said non-opaque surfaces are spaced close enough so that the application of said drawing magnet to one of said surfaces causes movement of said particles throughout the dispersion medium proximate said drawing magnet and between said first and second non-opaque surfaces, so as to create an image on said first and second non-opaque surfaces.

4. A magnetic marking apparatus, comprising:
an enclosure having a transparent or translucent surface and forming a single liquid sealing space;
a dispersion medium having a plurality of randomly oriented, magnetically active flakes contained within said liquid sealing space; and a magnet outside said enclosure having a magnetic field, said flakes aligning when said magnetic field is displayed to said flakes, said dispersion medium containing aligned flakes having light transmission characteristics different from said dispersion medium containing said randomly oriented, magnetically active flakes.

5. The apparatus of Claim 3, wherein said flakes have an aspect ratio having at least two of the height, length or width measurements of about 5:1 or greater.

6. The apparatus of Claim 3, wherein said flakes have an aspect ratio having at least two of the height, length or width measurements of about 10:1 or greater.

7. A magnetic marking apparatus, comprising:
an enclosure having a front and rear surface, with at least one of said front surface and said rear surface having a transparent or translucent area, said rear surface spaced from said front surface to form a liquid sealing space;
a dispersion medium sealed within said liquid sealing space, said dispersion medium having disposed therein a plurality of randomly oriented, magnetically active flakes;
and a magnet outside said enclosure having a magnetic field, said flakes aligning when said magnetic field is displayed to said flakes, said alignment changing the light transmission characteristics of said dispersion medium.

8. A magnetic marking apparatus, comprising:
an enclosure having a transparent or translucent surface;
a dispersion medium contained within said enclosure, having a plurality of randomly oriented, magnetically active flakes; and a magnetic field selectively applicable to a portion of said dispersion medium sufficient to cause alignment of said flakes in said portion of said dispersion medium so as to form an image viewable through said surface, said image being erasable by manually shaking said apparatus.

9. A magnetic marking apparatus, comprising:
an enclosure having a planar transparent or translucent surface and forming a single cavity, the interior of said cavity being visible through said transparent or translucent surface;
a dispersion medium disposed within said cavity;
a plurality of randomly oriented, magnetically active flakes dispersed within said dispersion medium; and a magnet outside said enclosure for selectively aligning said flakes to change the light transmission characteristics of the dispersion medium containing said selectively aligned flakes, the difference in light transmission characteristics between the dispersion medium containing said selectively aligned flakes and the dispersion medium containing said randomly oriented flakes creating an image viewable through said transparent or translucent surface.

10. A method of forming an image, comprising the steps of:
providing a dispersion medium having a thixotropic agent;
providing a plurality of magnetically active flakes with an aspect ratio having at least one of the height, length or width measurements of about 50:7 or greater;
suspending said flakes in said dispersion medium in a random orientation;

providing an enclosure having a transparent or translucent surface and a single liquid sealing space adjacent thereto;
sealing said dispersion medium in said liquid sealing space;
applying to said dispersion medium from outside said enclosure a magnetic field; and aligning said flakes proximate said magnetic field, whereby said alignment changes the light transmission characteristics of said dispersion medium containing the aligned flakes so that the contrast between the dispersion medium containing the aligned flakes and the remainder of the dispersion medium produces an image visible through said transparent or translucent surface.