US2925467A - Electrographic transmitter - Google Patents

Description

F. K. BECKER 2,925,467

E-LECTROGRAPHIC TRANSMITTER Feb. 16, 1960 Filed Oct. 31, 1958 2 Sheets-Sheet 1 F I6. I

22 20 UTILIZATION so'uncs DEW-CE 0F EXC/TA TION y 36 FIG. 2 i o a 1 LENGTH b 27 a f DISPLAY DEVICE 34 35 age. 3 f

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L 52 F l G 54 4|. I EX 1 I TRANSMISSION Mon CHAS/NE; EL

MOD.

FIG. 4

VOLTAGE sou/ms LAW N INVENTOR RES/STANCE E K- BECKER BY 1 LENGTH x A T TORNE Y Feb. 16,1960 F. K. BECKER 2,925,467

YELECTIROGRAPHIC TRANSMITTER Filed Oct. 31, 1958 2 Sheets-Sheet 2 n0. 5 FIG. 6

SOURCE OF E XC TA T/ON HYBRID m0 CIRCUIT TRANSMISSION LINE 5 TYL US OUTPUT INVENTOR E K. BECKER Y By amkwpy ATTORNEY Unite. see OT This invention relates to theelectrical transmission of graphic material from one station"to' another distant sta-' in the writing surface at the'discretion" of an operator.

. Since field distortions are minimized, the entire area, from Floyd K. Becker, Summit, NJ., assignor to'BellTele- V borderto borden may be used." a V The coordinate identifying potentials, developed as the V stylus traces out a written message or other cursive design on the writing surface, are transmitted conveniently by conventional means, to a, distant receivin'g station and there employed to generate, instantaneously, a r'eplicaof the-message. Since a record 'ofthe message is immediate ly"available at both stations, a two-way telewriting conversation is readily accomplished. Although any form tion and the instantaneous reproduction ofthe material at the distant station. More particularly, it'relates to a,

two-parameter electrographicsignal generatorfor instan taneously converting cursive writing and the like into n electrical signal, and has for its principal object the im* provement of the faithfulness withwhichthe conversion is'eftected. .1: V

Fully electrical telewriting generators,' which have been developed to replace, the mechanical telautog'raph with of unrestricted conductive stylus is. provided to establish an electrically conductive path between selected-points on' the resistive surface and a reference potential thereby to generate a signal whose magnitude is a function of the coordinates of position of thevselected pointsfi-Iovvever', extreme nonlinearity of the electrical fields developed in the writing surface, .particularly near';,eachiedgefof the surface, often reduces the usable writing'area to a fraction of the total writing surface. If, on the onehand,

the writing surface is energized by voltages" applied to of .reproduc'er is suitable for use in thepractice of the invention, mechanical recorders'on the 'one' han'd and cathode ray display devices well suited to this use.

.. Theinvention willbe fully apprenhended from'the' following detailed description of a preferred illustrative embodiment thereof taken connectionwi pended drawings in which: 1 Y I Fig: 1 is a simplified'diagrammatic representation of w an electrographic transmission system inaccordance with its pantograph and captive stylus arrangementfgenerally employ a resistive writing surfacetoisupporttime or frequency. separated orthogonal electric. fields." Some form i surface illustrated'in Figj'3; I

the present invention;

i 1 s I p 1... Fig. '2 is a diagram; partially in block. schematic for-1n,-- illustrating one form of energized resistive writing'surface which may be employed in a practice ofthe inven n; Y V Fig. 3 is adiagram, partially in block schematic forrmsurfaceuseful-in the practice of the invention;

illustrating another form of energized resistive writing -Fig. 4 illustrates the voltage and resistance relation ships which hold along-oneof the bounding electrodes and-within the adjacentportion of the resistive writing Fig; 5 is a graphic illustration. helpful in 'theexplanation=of the embodimentof the invention shown in Fig. 3; Fig. "6 a diagrarn showing the" structural details of another 1 form of resistive" writing. surface in accordance 1 with the invention;

elongated electrodes located at the edges of the surface,

the resulting equipotential lines produced-"within the surface are uniformly distributedonly in the center por tionof the writing surface and are nonuniformly distributed'at or near the edges.' Consequently, theusable writing area is greatly reduced. If, on the other "hand, the surface is energized at a number of independent'terminals positioned along the periphery of thesurface, and

suitable field correcting voltages are employed to energize the surface, the nonlinearity is, to some extent reducted,- but the overall useful area is, nevertheless, still somewhat smaller than desirable.

It is another object of the present invention to improve substantially'the-linearity of fields developed within. a writing surface of an electrographic;transducer thereby substantially'to increase the usable writing area. 1:

In accordance with the present invention, substantially V linear fields are developed in 'a plane resistive writing surface by applying field generating potentials. toiielecv art andare.known commercial1y as resistance t cardsf? Typically the writingsurface has'a resistivity of "ap-* 'ing the invention. s I a t .Referring now .to. Fig. 1', there is shown an electro- Fig. 7' illustratesan alternative form of electrographi'c transmitter employing a resistive writing. surface according to the-invention; and

Fig. 8 illustrates a two-way telewriting system e'ntlbody -v graphic sig'nal generato'r which includes a-base. member 10 for supporting a resistive writing tablet 11. The'writ ing tablet llmay comprise asheet orlayer of finely= divided carbon granules-in a. suitable binder deposited on. a fiberorother insulating backing material. Resistive sheets of this sort which, exhibit a relatively. high, and

uniform resistance perunitjarea are welljknown in the proximatelyTSOO ohms per unit area, ;-The resistiYQsheet 11 may be physically supported, if desired, between two 7 sheets of an insula'tingmaterial, a base member-8 and-a 1 frame member'9 which together form base ltl. e-The writtrodes which terminate the edges of the surface throughout their entire lengths and which support the same potential gradients as those. developed in'the surface itself. As a ingsurfac'e is supplied :with suitable exeitationsignals derived, for example, from generator .20 to establish-a result, 'equipotential lines arevdevelope'd within the surface which are uniformly distributed in; straight line paths extending-to the extreme edges of thefw'riting. surface. For use in a telewritinglsystem as described above, a writing instrument,

potential, is employed to derive-from points in the writing surface," potentials whose magnitudes are functions of the coordinate positions of-thelwriting instrument. "Preferably, the stylus is completely-independent of mechanical couplings and linkages andis tree to contact any. point which may. be. a stylus or contactele-' ment adapted to provide an electrically conductive path between a point on the writing surface and a reference ,7

pair of mutually perpendicular fields within the resistive surface. 'The fields maybe separated either in: time, frequency or}. in *space. I be a stylus or contact member 21 adapted to provide an electrically conductive path between a point on the writing surface andta reference potential point, is provided to derive from. the writing surface potentials whose mag: nitudes are functions of the instantaneous coordinate position of the writing instrument. These potentials are supplied to a utilization device 22 :which may, for example,

include terminal equipment fortransforming. the signals into a form suitable for transmission to a distant stationi Since the 'potential at everylpo'int in the-interior of 2,925,467 V I P F b.

on the other are particularly A'writing instrument, which may jplaneuniform resistance element is determined by the geometrical configuration ofthe edges and by thema'g nitude of the potential supplied to the edges of the surface, uniformly distributed potentialv gradients ex.-

tending throughout the entire area of the surface are secured by effectively 'terminatingthe. edges of the surface with aninfinite number of terminals supplied with potentials which correspond exactly with those to be developed within the surface. This is accomplished in accordance with the present invention by completely bounding or framing the oppositesides of the resistive writingvtablet 11 with parallel elongated electrodes. The resistivity per unit length of the electrodes is selected to be substantially lower than that of the writing surface. In particular, if the high resistance writing surface is selected to have a: resistance of, for example,

approximately 500 ohms per unit area, the electrodes.

which may be formed from Nichrome resistance wire, shouldhave a resistance of approximately 50 ohms per unit. length, i.e., the ratio of resistances should be on the order of ten to one.

By applying a voltage between the corner extremes of the; electrodes bounding the opposite X dimension edgesof the surface, the two electrodes bounding the opposite Y dimension edges are maintained at uniform potentials, throughout their entire lengths. Thus, the IR. drop in each ofthe individual Y. dimension electrodes isazero and the two electrodes are held at different potentials corresponding respectively to the potentials. of the opposite ends ofthe X dimension electrodes. Hence,- a field is produced within the surface in the X direction. The potential of the tabletconsequently is tapered m the. x direction and parallel equipotential lines. extend: in the Y. direction of the surface. The electrodesbounding the X. dimension edges, to the contrary, being connected in series: with. the potential source; supportv a potential gradient identical. to the one". produced. in. the writing, surface. Consequently, edgeidistortion or scalloping of the equipotential lines is avoided and a. virtually. distortionless field ofconstant force is produced in which linearity of the equipotential lines-:extend to the extreme edges. ofthe surface. In like fashion, a Y direction field within the surface and congruent potential gradients in the Y dimension electrodesare produced by applying a voltage between the corner extremes of the electrodes boundingtthe'op-y p'ositeY dimension edges-of the surface.

Byrapidly alternating the connections outlinedabove,

' thetwo pairsofparallel electrodes bounding the opposite-sides of the writing surface alternately act as constant potential edges and as supports for a gradient potential congruent with the developed field to establish mutually perpendicular voltage gradients in the surface. Since-the resistivity of each electrode is low as comparedwiththesurface resistance, eachpair of" elec.

trodes-may assume both roles in the alternating cycles of operation. Accordingly, the ends of adjoining 6166-, trodes are connected electrically together and to suitable electrical terminals. Excitation signals are applied to the terminals. 7

Referring once again to Fig; 1, the surface 11 is bounded or framed by low resistance electrodes 12, 13,

1'4: and 15 which are-firmly bonded together at each corner. The corners of the electrode frame are connected to terminals 16, 17, 18 and19 which are conveniently mounted on the insulating surface of base 10. Equipotential lines within the surface extending in the X direction are produced by connecting a-- source 1 of potential, for'example, abattery, between terminals 16 and 19 and between terminals 17 and 18. With these connections, a potential gradient identical to the one supported by the writing surface is'developedwith in the lengths of'the. electrodes 12 and 13. In like fashion, equipotentiallines' extending in the Y direction are produced byconnecting the scum? l d ni l' tween terminals 16 and 17 and between terminals 18 and' 192 A more detailed schematic diagram of one form of the writing surface 11 and the means for energizing it are shown in Fig. 2. Frequency domain separation of the fields is illustrated by way of example. This form of excitation is, in many cases, preferred since the output signal derived from the surface by stylus 21 can be supplied immediately to a transmission line or the like without subsequent. modulation of a carrier wave. In this embodiment of the invention, the writingsurface 11, composed of a uniformly distributed high resistance material, isbounded by electrodes 12 through 15. Adjoining electrodes are electrically connected at the junctions a, b, c and d. Separate energizing signals for the X and Y directions are supplied by the constant current sources 23 and 24 which may comprise oscillators or the like operating at slightly different frequencies at any desired points in the spectrum. A convenient separation of two to three. hundred cycles has been found to be. desirable. Oscillator 23 operating at a first fre quency f is connected by'means of coupling device 34 between'the ends a and'c and between the ends b and d of the electrodes 12 and 13, respectively. Serially connected resistors 26-27 and 28-29 connected between terminals a and b and between terminals c and d may beutilized to facilitate the energizing of the electrodes. With these connections, energy from source 23 at a frequency f produces an electric field in the Y direction. The sec ond source of oscillation 24, operating at a' second frequency f iscoupled by means of device 35 to the mid-points of the serially connected resistors lid-31 and 32-33 to-produce, in the writing surface, an electric field in the X. direction. The fields are mutually exclusive. Since the electrodes effectively frame the writing surface, the voltages developed in the electrodes effective ground at theexact'geometric center of thesurface': and provides an indication of stylus. contact with :the surface.

Asthe stylus 21- contacts any point of the surface 11, including the portions contiguous to the bounding electrodes, three separate signals are derived which are. indicativeof the X andY coordinate positions of the stylus and the Z-axis'or. contact condition of the stylus; Since thevoltage gradients developed along;each

length of the surface are separated in frequency, the

amplitude-of the derived signals of both frequencies in dicate exactly the coordinate position of the stylus;

The three components may be transmitted over conventional facilities to a display device 40. In the device 40 simple selective filterscentered about the frequencies f f and f and associated envelope detectors are sufiicient'to detect the signals. The signals arepreferably displayed on a directviewstorage tube. although they may, of course, be used to energize a mechanical telautograph writer or an xy recorder.

Spatialseparation may also. be used to define accu rately the coordinates of position :ofa stylus'inaccorda ancewith the invention. Fig. 3 illustrates a forrnlof electrographic generator which employs this 'form'of separation. In accordance-with this :embodiment ofthe'inr' I vention electric'gradients; proportional to the. distance froma-common neutral point positioned 'Within the writing surface 41'are .formedialong theXand Y. axes; Al: though there are-severa'hwaysof accomplishing this it has been found; convenienti to. employ a writing, tablet whose resistance ten-he rty taper d, together with bond ingelectrodes formedfto exhibit the same resistance charactei'istic. Thus, for example, the electrodes 57,' 58, 59

and 60 maybe physically tapered so that their resistance per unit length is a linear function of'the length of the electrodes. Alternatively, bonding electrodes whose resistance per unit length is a linear function of distance may be employed. As before, adjoining electrodes are electrically connected to each other and to external terminals. Physical'tapering of the resistivesurface 41 itself has been found to be a convenient way of providing'the necessary tapering of the resistance of the surface.

With resistive tapering ofthis sort, a voltage proportional to the square of the distance of thestylus from theefiective neutral point indicates the coordinate position of the stylus. Battery 51 connected to thesurface through resistive dividers 42- -43 and 44-45,.and'ba-ttery 52 connected to the surface through dividers 46-47 and 48-,49 produce simultaneously, orthogonal electric fields within the surface. .A multitipped stylus .53is utilized to detect the difference in potential between closely spaced points within the surface in both directions. This is equivalent to taking a first order derivative of the potential gradient at the point of contact with the surface. Higher order derivatives may also be employed,

of course, to define the coordinates of the stylus position; This information, which unambiguously defines the coordinates of position 'of. the stylus, is applied, for example, to modulators'54 and 55 wherein the amplitude or frequency characteristicof suitablecarrier waves is altered: .The modulatedwaves are combined in a mixing device 56 for transmission overa conver tional channel.

,"Fig.j4 illustrates thevoltageandresistance relationships existent in the writing surface 41 and its. framing paper is'pressed into contact with the resistive Lsurface electrodes 57 through 60'. -'As shown,'the resistance is a linear function of distance while the voltage developed" within the surface asquare-lawl a V I 1 Fig. 5' illustrates two alternative configurations for the tips ofthe multitipp'ed stylus :53; The simple triangular array, shown at 5A, is sufficient toiproduce between a and the bonding electrodes follows both the X and in the Y directions. Thefour-tipped stylus arrangement, shown in Fig. 5B, is sufficient to produce two completely separated voltage components written message.

' whichextendfin the 'x direction: Linearityfofthefequii potential lines in theY direction isthus insured, and at the same time X direction fidelity is improved; A'further improvement may be made by providing additional electrodes 67 extending in the Y direction. A firm electrical bond is provided at each intersection'of the electrodes within the matrix.

An electrode matrix which use with commercially availableresistance, cards or the 'like, for improving the linearity of thec ard' when-used in telewriting applications, may be formed from so-called wire cloth or from conductive screen material. If this I material is used, a frame and support arrangement" of the sort illustrated in Fig. 1 is employed to provide a pressure contact at the outer edges of the writing-sure facebetween "the wire cloth matrix and the resistance sheet; Alternatively,.the matrix may be formed from individual lowre'sistance wires bonded at each -int ersece tion or from individual'conductive elements deposited together with theresistive granules on a printed circuit board. Conductive paper, placed over the wire matrix, provides both the electrical contact between the stylus and the'resistance sheet and a permanent record of the Fig.7 illustrates'a telewriting generator in accordance with the invention which isparticularly suitable for use in those systems in which a permanent record of the message'is required. In this embodmient of the inven tion, writing surface 71 is provided with an electrode, matrix of the sort described above in connection" with Fig. o. I-tis supported in a base 70 in a fashion'such' that the writing surface is exposed through a window 76. ,A sheet of conductive paper 72 drawn from a roll 73 and passed through a slot 74 extending throughoutithe length of the base 70 covers thewriting surface 71.; .The

. 7-l both by the edges of'the window 76'and'bythestylu's 75 at the point of contact of- 'thetwop Although any form of conductive paper may be used, it has been found that' absorbent paper of the type kriown commercially as' 'Ieledeltos is particularly well suited foruse in this' application. If this material orthe like is used, thestylus common corner tip the. otherftips', addressvoltag'esin representative of the X and Yfco'ordinates of position of.

the stylus.

Commercially available resistive cards, employed as 'the writing surface in accordance with the invention,

ordinarily possess reasonably, good resistance uniformity in'both coordinate directions? By means of the unique electrode framing arrangement employed to energize the card, linearity ofthe resulting equipotentiallineswithin the card is insured toits'very edges. However,"if extremely .largewriting surface areas are employed, ,islighti non-uniformities of the resistance card are m'ore"pfronounced and may give rise to serious distortions in the j surface fields. This difficulty is circumvented in accordance with the presentinvention by subdividing .the area or the resistive surface'into a number. of small subele-- rnents, each of which effectively. is a separate resistive card bounded on all four sides by low resistance conductive electrodes. Thus, thejsurface'iframing mechanism may beextendedx to roim; rrenve1 ;a matrix of contiguous elemental writing surfaces, each independently energized. Consequently, small corrections arem'ade as writing takes place and cumulativeerrors are avoided.

, tion and thesubscriber atthe otherfterminal may simul: 1

.Fig. 6 illustrates one form of, electrodegm-atrix suitable for energizing a resistance .card ina manner to insure uniformity throughout the surface. The card 61 ishonded at all four edges by the primaryelectrode, 62,63, 64 and 65, and is'energized at its: corners .inthe'fashion heretofore explained. v Within thelconfinesiof the outer bounding electrodes, parallel. electrodes '66' are provided face.

the writing surface. The used portion may, of course,

be removed and preserved as a' permanent inkedrecord of the correspondence;

A complete system employing theirnproved writing surface of the invention is, illustrated in Fig. .8. Pro vision is made in this embodiment for a two-way-tele. writing conversation. Accordingly, the subscriberat each endof a transmission circuit is supplied with the equipment illustrated in Fig. 8. LEachsubscribermay visu-' ally observe both his own writing and an incoming signal at one and the same time. For examplemhe subscriber at one terminal may start a sentence, drawing, or equa-v taneously, contribute to the final writing.

ln the apparatusof. Fig. 8, a res1stive .writingi,jsurf face 81, energized from excitation source 82; in'any of v the fashions described heretofore'or, indeed, by any other field separating means well known inthe art, is mounted in a suitable base 80. 'Signals derived by' a stylus '85 and indicative of 7 its coordinates of position are sup plied by means of hybrid circuit 94 to ,a transmission line 100.. The stylus" output signals are .simulta neously supplied locally to a sto-ra ge'display tube 84. The visual 'displayappea ring on the surface, of thetube 84 ispro' jected-by means of a half-silvered; mirrors}, iso thatjit appears to originate on the resistive writing;surfacejsii 5 The details of operationiof the storage.tubeand the pro:

is eminently suitable for ieetion-system-are-nct illustrated inasmuch as' they are welltkno'wn inthe respective arts. Assuming frequency separated coordinate" signals as an example, multi-frequen'cy signals defining the position of the stylus are separated by applying them to band pass filters 86, 87 and 88, tuned respectively to the X, Y, and Z signals. Electronic switches 89, 9t) and 91 transfer these signals to the X, Y and Z input terminals of the tube 8-4. I

Incoming signals from transmission line 100 are routed by means of hybrid circuit 94 to the band pass filters 95, 96 and: 97, which correspond to filters as, 87 and 88, to efiect their separation. Electronic switches 89, 90 and 91' supply these signals to the corresponding input terminals of the tube 84 in alternation with the locally generated signals. Effective dual beam operation of the storage" tube 84 is thus accomplished in a fashion well known: in the communications art. Hence, the two signals, i.e., the locally generated signal and the remotely generated signal, are projected simultaneously on to the writing surface 81 to produce a display which appears toal viewer as-a superimposition of the two individual signals.

To establish an initial alignment of the storage tube and to insure exact coincidence of both projected signals,

attenuators 92 and 93 are connected in the X and Y signal circuits of the local monitor, and attenuators 98 and 99 are connected in the X and Y circuits of the receiving circuit. Assuming that the optical system is adjusted to produce a satisfactory image of locally generated material, electrical alignment is effected by touching. the styliat both stations to the extreme corner, or other suitable reference point on the respective writing s'i'irfa'ces.v Under this condition the attenuators at both stations are adjusted to produce a coincidence of the locally generated and incoming line signals. The usual means for controlling the cathode writing beam of the storage tubes is also adjusted to produce the proper amplitude, orientation and intensity of the signals appearin'gl onithe surfacesof the-tubes. With this procedure, exact superimposition of the twosignals at both stations iste'stablishedbefore the initial transmission. Occasional readjustment will insure exact coincidence during subsequent transmissions.

Once thewritten conversation has been terminated, the messages persist on both storage tube screens, and hence appear onboth writing surfaces, until locally generated erase: signals (not shown) are supplied to the storage tubes. Conventional x-y recorders or photographic means and the like may be employed to provide a permanent record of the message.

Although-the invention has been described as relating to specific embodiments, the invention should not be deemed limited to the embodiments illustrated, since variousmodifications' and other embodiments will readily occur to-one skilled in the art.

What'is claimed is:-

1 An electrographic writing surface which comprises a substantially plane. conductive tablet having a relatively high resistance per unit area, means for producing a flow of current through said surface in one coordinate direc tionto'develop alpotential gradient between the oppositeedges of said surface, said gradient being represented by substantially straight equipotential linesextendingfrom-edge to edge, said means including a first pair of conductive electrodes having relatively low resistance per unit length connected in electrically conductiverelationship with the opposite sides of said tablet, a second pair of conductive electrodeshaving relatively low'resistance per unit length connected in electrically conductiverelationship with the other edges of said tablet, and means for applying excitation signals between the ends o'f each' of said first pair of electrodes thereby to develop within said pair of electro'des' potential gradients congruent. withv said" gradientdeveloped within said sur tace, and for applying; said excitation signals between 8 said second pair of electrodes to maintain each of said second pair respectively at uniform potentials. 2. An electrographic writing surface which comprises. a conductive tablet having. a relatively high resistance per unit area, means for producing a flow of current through said surface in at leasttwo coordinate directions to develop potential gradients therein between. opposite parallel edges of said surface, said gradients being.

represented by substantially straight equipot'entiallines extending from edge to edge, said means including a plurality of electrically conductive electrodes having relatively low resistance per unit length connected in continuous electrically conductive relationship with opposite parallel sides of said tablet, means for electrically connecting adjoining ones of said electrodes to one another and to corresponding electrical terminals, and means for applying separately distinguishable excitation signals between the terminal ends of pairs of opposite parallel electrodes. i

3 An electrographic writing surface as defined in claim 2 wherein said conductive tablet comprises a thin l'ayeri of carbon granules deposited on an insulating base and said electrodes comprise lengths of Nichrome resistance wire and'wherein the ratio of the resistance of said tablet to said electrodes is approximately ten to one.

4. An electrographic transmitter comprising a writing surface having uniform resistance per unit area, means for producing within said surface at least two mutually perpendicular potential gradients, said means comprising, a source of excitation, a conductive border electrode in continuous electrical contact witheach edge of said surface, each of said electrodes being electricallyconnected at each end thereof with the ends of adjacent electrodes and having uniform resistance per unit length, said' resistance being low with respect to the corresponding re sistance of said surface, means for connecting said excitation signals to the junctions of adjacent bars, thereby to develop within each of said electrodes potential gradients congruent with the corresponding gradients within said surface, electrical conductive means for contacting discrete points in said surface, and means for translating each point of contact into potentials whose amplitudesare proportional to the coordinates of position of said electrical conductive means.-

5. An electrographic transmitter as defined in claim 4, wherein said electrical conductive means comprises a freely'movable conducting stylus connected to said translating means.

6. An electrographic transmitter as defined in claim 4 wherein said source of excitation comprises a source of direct'current potential and said excitation signal connection means includes means for alternately applying said signal. to first one pair of opposite electrodes and thento the other.

7. An electrographic transmitter as defined in claim-4- wherein said source of excitationcomprises first, second and third constant current signal generatorsoperating. respectively at frequencies f f and f and wherein said excitation signal connection means includes means for applying. signals at frequency f continuously to one pair or" oppositely disposed electrodes, means for applying signals at frequency f continuously to the other pair of oppositely disposed electrodes, and means for applying signals at frequency f between a reference potential point and the effective geometrical center of said writ: ing surface area.

8. An electrographic writing surface which comprises a conductive tablet having a relatively high resistance'per unit area, the resistance of said tablet being tapered from edge to edge in atle'ast two coordinate directions, means for roducing a flowo'f current throughs'aid sur face in at le'a'st'two' ooordinate'directions'to'develop non linear potential gradients between opposit'eparallel edgesof said surface, said means including'a plurality of' ele'c tric'ally conductive electrodes having relatively low re";-

V combination, a writing surface having linearly tapered resistance per unit area, means for producing within said I surface at least two mutually perpendicular nonlinear potential gradients, said means comprising a source of potential, a conductive border strip in electrical contact with each edge of said surface,- each of said border strips being electrically connected at each end thereof with the ends of adjacent strips, the resistance of said strips being low with respect to the corresponding resistance of said surface and opposite ones of said strips being tapered linearly to support a nonlinear potential gradient identical with the corresponding gradient produced within said surface, means for connecting selected components of said source of potential to the junctions of adjacent bars, and electrically conductive means for determining the relative potential difference between spatially separated points along each ofsaid mutually perpendicular directions in said surface. r I i 10. An electrographic transmitteras defined in claim 9 wherein saidelectrically conductive determining means comprises an insulating support member provided with j V sage signals, said electrographic communication means a plurality of spatially distributed conductive styli, each electrically insulated one from another and connected to an external circuit.

11. An electrographic transmitter as defined in claim 10 wherein said support member contains one stylus connected in common with at leasttwo independent circuits and at least oneother stylus connected in each of said two independent circuits, said other styli being disposed along the locus of a pair of perpendicular lines intersecting said common stylus.

12. An electrographic transmitter as defined in claim 10 wherein said support member contains at least two pairs of styli, the styli of each pair being separated along mutually perpendicular loci, and in which each pair of styli is connected, respectively, to a separate circuit.

13. An electrographic writing surface which comprises a conductive tablet having a relatively high resistance per unit area, meansfor producing orthogonal electric v fields within said tablet, said fields being represented by potential gradients within said. tablet extending from edge to edge, and means including a closed frame of low resistance electrodes connected in continuous electrical conductive relationship with the perimeter. of said tablet, a plurality of low resistance electrodes electrically connected between one pair of opposite sides of said frame, said plurality of electrodes being spaced apart from and positioned substantially parallel with one another, and means for applying separately distinguishable excitation signals to opposite pairs of electrodes whereby said potential gradients become substantially uniform from edge to edge.

-14. An electrographic writing surface which comprises a conductive tablet having a relatively high resistance per unit area, means for producing orthogonal electric fields within said tablet, said fields being represented by 10 potential gradients within said tablet extending from edge to edge, and means including a closed frame of low resistance electrodes connected in continuous electrical conductive relationship with the perimeter of said tablet, a'plurality of low resistance electrodes electrically connected between both pairs of opposite sides of said frame,

said plurality of electrodes between both pairs of sides being respectively spaced apart from and positioned substantially parallel with one another to form a matrix of crossed electrodes, the points of intersection of 'said crossed electrodes being electrically connected together, and means for applying separately distinguishable excitation signals to opposite pairs of said frame electrodes, whereby said potential gradients become substantially uniform from edge to edge.

15. An electrographic writing surface as defined in claim 14 in combination with electrically conductive means operatively associated with said surface for contacting discrete points in said surface, and means for translating each point of contact into potentials proportional to the coordinates of position of said electrically conductive means.

16. An electrographic writing surface as defined in claim 15 wherein said electrically conductive means comprises a sheet of electrically conductive paper juxtaposed with said surface, and stylus means provided with a reservoir of conductive ink for establishing an electrical contact between'said conductive paper and said-translating means.

17. In. a two-way communicationsystem including a; ffirst and a second substation, electrographic communication means at each substation for providing subscribers with a graphic display 'of instantaneous mesclosed frame of low resistance electrodes connected in 1 continuous conductive relationship around the perimeter of said tablet, means for applying separately distinguishable excitation signals between the endterminals of opposite pairs of the electrodes of said frame, and electrically conductive means for detecting the potentials appearing at selected discrete points in said tablet, the potentials so derived being representative of the coordinates of position of said selected points, means including said transmission link for transmitting said detected potentials to said second substation, means for separating said excitation signals detected by said electrical conductive means to produce a first setof control signals representative of the points selected by the subscriber at said first substation, means for separating the distinguishable signal components of message signals received from said second substation to produce a second set of control signals repre- ,sentative of the points selected by the subscriber at said second substation, a direct view storage tube, means for applying to said storage tubein alternation said first and said second sets of control signals to produce a cornposite visual displayon said storage tube corresponding to the sequences of points selected by the subscribers at both said first and said second substations, and optical .means for projecting said composite display on to said conductive tablet.

No references cited.

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