US3623123A

US3623123A - Electrostatic printer

Info

Publication number: US3623123A
Application number: US806782A
Authority: US
Inventors: Alex E Jvirblis
Original assignee: Singer Co
Current assignee: Singer Co
Priority date: 1969-03-10
Filing date: 1969-03-10
Publication date: 1971-11-23
Anticipated expiration: 1988-11-23
Also published as: NL7003122A; GB1279532A; DE2011038A1; FR2037859A5

Abstract

An electrostatic apparatus for alphanumeric character generation is disclosed. Character generation is controlled by selectively switching the potential on selected elements of a multielement grid positioned between the corona source and the receptor. For increased writing speed, there is disclosed a structure having a corona source and a multielement grid on each side of the receptor.

Description

United States Patent 2,944,147 7/1960 Bolton 101/1 X 3,266,046 8/ 1966 Boyd 346/74 3,277,818 10/1966 Cone 346/74 X 3,298,030 1/1967 Lewis et a1 346/74 X 3,306,193 2/1967 Rarey et al.... 101/1 X 3,358.289 12/1967 Lee 346/74 Primary Examiner-Terrell W. Fears Assistant Examiner-Gary M. Hofiman Attorneys-Charles R. Lepchinsky, Patrick .lfSchlesinger. R.

Perry Shipman and Jay M. Cantor ABSTRACT: An electrostatic apparatus for alphanumeric character generation is disclosed. Character generation is controlled by selectively switching the potential on selected elements of a multielement grid positioned between the corona source and the receptor. For increased writing speed, there is disclosed a structure having a corona source and a multielement grid on each side of the receptor.

PATENTEDunv 23 can 3.623.123

SHEET 1 0r 2 Fig. 2

INVIJN'IUR.

ALEX EUGENE JVIRBLIS Maw PATENTEnunv 23 Ian SHEEI'E or 2 Fig. 3

IN VIiNTOR.

ALEX EUGENE JVIRBLIS BACKGROUND OF THE INVENTION Electrostatic printing is generally accomplished by positioning a suitable receptor between a pair of electrodes which are closely spaced and connected to opposite poles of a potential source. It is customary, in the prior art, to have at least one of the electrodes shaped in the form of the desired character. In addition, various means have been devised to produce a uniform corona source and/or to focus the ions from a corona source on the receptor. The receptor usually comprises a dielectric web or sheet which is moved relative to the electrodes as successive characters are presented to the receptor. A charge pattern which in effect is an electrostatic latent image is formed on the receptor and subsequently developed by known techniques to produce a suitable visual image.

Prior an electrostatic printers have employed a multielement electrode which is divided into a matrix of elements which are selectively energized to form individual characters.

In order to produce the ions that are required in an electro static printer, it is frequently necessary to employ voltage supplies of several thousand volts; and to produce successive characters, it is necessary to switch such potentials off and on. Switching potentials of this magnitude requires specialized, bulky, and expensive equipment.

OBJECTS 'static printer in which the ion field is selectively controlled by means of a control grid which is switched at a relatively low potential.

It is another object of this invention to provide an electrostatic printer in which character selection is controlled by means of a multielement grid. i

In accordance with the present invention, there is provided an electrostatic printer having astable corona source, a grid, and a plate. The printer deposits a controlled electrostatic charge pattern on a suitable receptor surface which is disposed in an ion path. The deposited charge pattern is subsequently developed by applying powdered inks or toners as in prior art electrographic processes. A multielement control grid is disposed in the ion path and close to the receptor. An

enabling or inhibiting potential is selectively applied to one or more elements of the grid to thereby enable or inhibit, respectively, the deposition of an electrostatic charge on the receptor in a controlled area. The grid may take a variety of forms and may, for example, comprise an array of orthogonal wires for character generation by, points; or the grid may be a set of plates having bar and/or are type slits which, when used in various selected combinations, form the desired character.

Further objects, advantages, and features of the invention which characterize the invention will become apparent as the following description proceeds, and the features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this application.

BRIEF DESCRIPTION OF THE DRAWINGS reference may 2 DESCRIPTION OF THE INVENTION It should be understood that to avoid obscuring the invention only those details have been shown which are required for a full and complete understanding of the invention. A given part is given the same identifying number in all views, although similar but nonidentical parts may be distinguished in two views by using primed numbers and/or letter postscripts in one view.

FIG. 1 presents a simplified representation of an apparatus embodying the invention. The apparatus includes an enclosing metallic member 10 which may include a small transparent area to permit inspection and observation. A corona generating member 20 having one or more pointed ends or needles 21 is connected by a wire 25 to a first terminal 31 of a first potential source 30. The second terminal 32 of said potential source 30 is electrically connected to a metallic platen 40. Positioned in close proximity to the platen 40 is a suitable receptor 50. The receptor 50 may take any of various forms which are well known in the art. Quite typically, the receptor 50 may comprise a mylar film which is metallized on the side facing the platen 40. Depending upon the application and other factors, the receptor 50 may move from a supply reel 51 to a takeup reel 52 somewhat in the manner of tape in a tape recorder; or the receptor 50 may be arranged in a closed loop and be reused afler being erased by suitable means. The receptor 50 may also comprise specially fonnulated or treated paper. The

form and choice of the receptor 50, and the means of transporting or developing the same do not form a part of this invention and, therefore, details relating to these features are not disclosed herein as such inclusion would only tend to obscure the actual invention.

Between the needles 21 and the receptor 50 there is positioned a grid 60 which is electrically connected to the swinger 71 of a transfer contact. The back contact 72 is connected to the second terminal 32 of potential source 30 and also to the second terminal 82 of another potential source 80. The front contact 73 of the transfer contact is connected to the first terminal 81 of potential source 80. It should be noted that

terminals

32 and 82 of

potential sources

30 and 80, respectively, are of the same polarity, and may be either positive or negative. Thus, the

corona generating members

20 and 21 may be either positive or negative with respect to the platen 40. But in either case, as already stated, the

terminals

32 and 82 are of the same polarity. The potential sources 30 and are of unequal magnitude with source 30 being of considerably higher potential. The potential 80 may be of the order of volts. The actual potential for source 80 is a function of several factors, including: the intensity of the ion source, the duration of a voltage pulse (to be described later), the grid and receptor dimensions, and the development conditions.

Examination of the circuit of FIG. 1 will show that the grid 60 may be selectively switched from the potential of terminal 82, which is the same as the potential of the platen 40, to the potential of terminal 81 which thereby places the grid at a potential between that of the platen 40 and that of the needles 21. To avoid the possibility of shorting the potential source 80, it is necessary that the transfer

contacts comprising contacts

71, 72, and 73 be 'of the break-before-make variety; or if desired, protective resistors or any other suitable means could be used. The purpose of shifting the grid 60 from one potential to the other will be shown hereinbelow.

Ions are pulled out of the corona field near needles 21 because of the potential of the grid 60 and/or the platen 40. The ions may be beamed and focused by means disclosed in the prior art and familiar to those having experience in the electrostatic printer an. If the grid 60 is at the platen potential, all available ions will be attracted to the grid and none will penetrate to the receptor 50 or platen 40. However, if the grid 60 is at a potential between that of the platen 40 and the needles 21, ions will be enabled to penetrate the grid and an electrostatic image of the ion field will be formed on the receptor 50. Accordingly, if the grid 60 has a predetermined shape, it will be effective to permit the passage of ions past the grid in a controlled pattern and thereby deposit a charge on the receptor in a corresponding predetermined pattern. To obtain a variety of patterns or characters, it is obviously necessary to change the grid pattern.

The grid pattern can be altered by using a multielement grid with each element of the grid separately controlled or switched between the two terminals of potential source 80. Consider, for example, the grid structure of FIG. 2. The grid 60 of FIG. 2 is composed of a plurality of separate elements 61, each of which are electrically insulated from each other element thereof. Each grid element has a slot 62 therein and, as may be seen from an examination of FIG. 2, the slots 62 are arranged in a familiar figure 8" pattern so that any of the digits may be constructed by using selected combinations of the slots 62. All spaces 63 between the individual grid elements 61 are filled with insulating material. Each grid element 61 is electrically connected to the swinger 71a to 713 of an individual transfer contact and each transfer contact may be independently operated to connect the corresponding grid element through front contacts 73a to 733 to terminal 81; or through back contacts 72a to 72g to tenninal 82 of potential source 80. Any grid element 61 which is connected to terminal 82 is at the same potential as platen 40 and will attract all ions in that area and prevent any ions from passing through opening 62 of grid element 61 to the receptor 50. On the other hand, any grid elements 6i which are connected to terminal 81 will enable ions to pass through their respective slots 62 and produce a corresponding charge pattern on receptor 50. Accordingly, with the grid shown in FIG. 2, it is possible to cause electrostatic images of any desired digit to be formed on the receptor 50.

The

circuit comprising contacts

710, 72a, and 73a, etc. to 71g, 72g, and 73g may actually be any suitable form of logic circuitry and be controlled by any suitable means for selecting the desired character. For example, signals could be read from perforated tape or cards or magnetic tape for individually controlling the plurality of contacts 71a to 73g. The specific control means do not form a part of this invention and, therefore, are not illustrated or described herein as such means is a matter of engineering choice using principles which are familiar to those having experience in the appropriate arts.

If desired, a more complicated grid 60 could be constructed with a greater plurality of elements 61 having bar and are openings 62 for forming alpha characters as well as numerics. It is not necessary that all grid elements 61 occupy the same plane.

The grid 60 may also take an entirely different form as shown in FIG. 3 and comprise an array of orthogonal wires. For convenience, the horizontal array (as viewed in FIG. 3) may be designated an X array and individual grid elements identified as 6lXa, 6IXb, etc., to 61Xn. In a similar manner, the vertical array (as viewed in FIG. 3)may be designated the Y array and individual grid elements identified as 61Ya, 6lYb, etc. to 6lYn. Use of the grid of FIG. 3 permits printing and character formation in a manner similar to that used in matrix printers such as that disclosed in an application of E. O. BLodgett entitled "Serial Character Matrix Page Printer" filed Apr. 14, 1967, and issued Feb. 11, l969, as US. Pat. No. 3,426,880 and assigned to the same assignee as the present invention. In matrix printers, a group of wires having their ends normally in a single reference plane and as close to each other as practical is used to print a variety of characters. Character formation and selection is accomplished by causing selected wires to be projected out beyond the reference plane and the projected print wires are then caused to press a print ribbon against a document positioned on a platen. The resultant character is formed of a plurality of individual dots which when viewed at normal reading distance seem to visually merge together to produce a character. The use of the grid of FIG. 3 will cause the production of a similar character in that the character is formed of a plurality of elements or dots that make up the character. If one will imagine selected areas of the grid network of FIG. 3 being filled in, it is possible to imagine the creation of any desired character.

It should be observed that none of the grid wires 6IXa, 6IXb, 61Xn, 6IYa, etc., to 6IYn are permanently electrically connected to any other grid wires. However, each individual grid wire may be selectively and individually connected to either terminal 81 or 82 of potential source 80. Some of the individual contacts 71Xa to 73Yn for selectively connecting each of the plurality of grid wires of FIG. 3 to the required potential are shown. It should be understood that each of the plurality of grid wires is individually and selectively switched by individual contact sets or equivalents. By this means, it is possible to produce an enabling potential at any desired cross point by connecting the appropriate grid wires to potential terminal 81. For example, if wires 6IXb and 6lYb are connected through their respective contacts 7IXb to 73Xb and 7IYb to 73Yb to terminal 81, then cross point 69 will be enabled and ions from the corona field could pass the grid at that point and leave an electrostatic image on the receptor. It may well be asked why a dot is printed at cross point 69 and not two lines or bars that cross at point 69. The point is well taken and probably either no printing would be obtained under the conditions given or else the two lines or bars corresponding to grid wires 6IXb and 6IYb would be printed. It should be understood that if the ions are accurately focused onto the receptor 50 that the grid wires would interrupt the flow of ions and tend to cause a shadow in the printing. Thus, if an enabled grid wire tended to print a bar, it would print a bar with a thin unprinted centerline. In actual practice, the inhibiting effect of any grid wire connected to terminal 82 must have a band of inhibiting influence at least twice as wide as the spacing between parallel grid wires. This desired effect may be obtained by judicious grid design, and selection of potential of potential source 80. If the just stated requirements are met, it will be obvious that when grid wires 6IXb and 61Yb are enabled by being selectively connected to the potential at terminal 81 that there will be no printing at cross point 69 as the inhibiting influence of grid wires 6IXa, 6IXc, 6lYa, and 6lYc all provide a band of inhibiting influence which precludes the possibility of any ions being passed anywhere along the length of grid wires 6IXb or 6lYb. The actual and practical method by which the grid of FIG. 3 may be used for selectively controlling the printing of selected characters will next be described.

By way of review, the following facts should be kept in mind: (a) Any one or more of the grid wires 61Xa to 6lXn and 6lYa to 6lYn may be selectively connected to terminal 82 which is at substantially the same potential as terminal 32 and the platen 40. Any grid wire so connected will have a potential which attracts all ions in the area thereof and prevents such ions from reaching the receptor; (b) The inhibiting band width of a grid wire connected to the inhibiting potential is at least equal to twice the width between adjacent parallel wires (but little, if any, more); (c) Any one or more of the grid wires 61Xa to 61Xn and 6IYa to 61Yn may be selectively connected to terminal 81 which has a polarity in the same direction as terminal 31 but a greatly reduced potential. Any grid wire so connected will have a potential which tends to allow ions to pass to the receptor; and (d) No ions will pass to the receptor as a result of a single enabled grid wire as the inhibiting influence of parallel adjacent grid wires is too great.

In order to allow ions to pass the grid 60 and deposit an electrostatic image on the receptor 50, it is necessary to enable an adjacent pair of X" wires and an adjacent pair of "Y" wires. For example, consider that grid wires 6IXd, 6IXe, 61Yd, and 61Ye are all enabled and all other grid wires are at the inhibiting potential. Under the proposed conditions, the area designated 68 will be enabled and ions will pass therethrough to deposit an electrostatic image on the receptor 50. There is no passage of ions to the receptor 50 in any area outside the area 68 because of the inhibiting potential of parallel adjacent grid wires which provide an inhibiting influence at least twice as wide as the spacing between individual grid wires.

By enabling appropriate adjacent pairs of X" grid wires and appropriate adjacent pairs of Y" grid wires, it is possible to enable the passage of ions to any desired area. Multiple areas can be enabled simultaneously; but care must be exercised to assure that unwanted areas are not enabled as well as wanted areas. For example, if two pairs of X grid wires and two pairs of Y" grid wires were placed at enabling potential at the same time, there would be at least four separate areas in which printing could take place and it is quite possible that printing would be desired in a limited number of these areas in the formation of a given letter. Consider, for example, the grid wires that would have to be placed at enabling potential to form the letter 0." In this case, all grid wires would be enabled and if they were enabled simultaneously a solidly printed rectangle would be printed rather than the letter 0" Accordingly, it is necessary to provide a means for sequentially enabling selected grid wires in order to form the desired characters. While a given pair of Y (or "X") grid wires is enabled, it is possible to enable more than one pair of X" (or Y") grid wires, provided certain rules are followed. Specifically between each pair or pairs of enabled X" (or Y) grid wires, there must be at least one grid wire at aninhibiting potential. By observing this rule, it is possible to print in more than one box at a time and form a given character in less than half the time it would take if each box had to be printed independent of any other box.

The grid elements of FIG. 2 might be fabricated on a printed circuit board with the grid elements 61 printed on both sides of the board. In some applications, both sides of corresponding grid elements would be electrically connected together while in other applications, there might be a resistor in the circuit coupling two corresponding elements. In some applications, the resistor would help to create a focusing grid and improve the focusing of the ions on the receptor 50 and thereby provide an improved image. The grid elements 6lXa to 61Xn may be in one plane while the grid elements 61Ya to 61Yn may be in another parallel and closely spaced plane.

For some applications, even the inherent highspeed printing potential of an electrostatic printer is not sufficient. In such a situation, the printing rate may be increased by using the structure shown in FIG. 4. As will be immediately apparent, the top half of FIG. 4 is substantially a duplication of FIG. 1 except that no platen 40 is included in FIG. 4. The bottom half of FIG. 4 is substantially a mirror image of the top half. The most significant difference between the top and bottom halves of FIG. 4 resides in the reversed potentials of one with respect to the other. For convenience, parts of the top half of FIG. 4 which correspond with equivalent parts shown in FIG. I are given identical numbers. Elements in the bottom half of FIG. 4 which correspond with elements in the top half are given identical identification numbers except that they are primed.

The potential of source 30 is reversed with respect to source 30. Accordingly, needles 21 and 21' are at different potentials and one set of needles produces positive corona while the other set of needles produces negative corona. By the means shown in FIG. 4, the grids 60 and 60' can enable ions to be directed to the receptor 50 from both sides, thereby depositing a greater number of ions on the receptor in a given unit of time. By this means, an acceptable density of ion deposition can be obtained in a shorter time, thereby resulting in the equivalent of more rapid printing.

It should be noted that when the structure in accordance with FIG. 4 is used, the

grids

60 and 60 should be identical and should be switched in an identical manner at all times, although one grid is positive with respect to the other. That is,

terminals

81 and 31 are at potentials in opposite directions. The grids 60 and 60' may be of the form illustrated in FIGS. 2 or 3 or any other suitable form.

The structure of FIG. 4 does not have a platen similar to that shown as 40 in FIG. 1. However, if a receptor support is required, it would be possible to add a suitable edge support.

While there has been shown and described what is considered at present to be the preferred embodiment of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the embodiments shown and described, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

I. An electrostatic printer comprising:

a first potential source;

a corona source coupled to said first potential source for generating a substantially continuous supply of ions;

an electrically conductive platen coupled to a reference potential for attracting said ions theretoward;

a receptor means positioned between said platen and said corona source for recording an electrostatic image of the charge pattern closely adjacent said platen;

a second potential source having a magnitude substantially less than said first potential source and greater than said reference potential and a polarity of the same sign as said first potential source;

a grid structure disposed between said receptor means and said corona source for regulating the flow of said ions toward said platen to control the configuration of said charge pattern, said grid structure comprising a plurality of electrically conductive elements insulated from each other;

switching means for selectively coupling said grid structure to said second potential source and to said reference potential, said switching means including a plurality of individually actuatable switching devices for coupling each of said plurality of elements to either of said potential source and said reference potential, individual ones of said switching devices being associated with individual ones of said plurality of elements; and

each said switching device comprising a first member coupled to said reference potential, a second member coupled to said second potential source, a central member coupled to said associated grid element and means for coupling said central member to either of said first and second members.

2. The apparatus of claim I wherein the magnitude of said second potential source is sufficient to cause said ions to flow to said platen when said grid structure is coupled thereto, and the magnitude of said reference potential is sufficient to cause said ions to be trapped by said grid structure when said grid structure is coupled to said reference potential.

3. The apparatus of claim 1 wherein said platen and said grid structure define an image area and including means for moving said receptor means through said image area at a predetermined rate.

4. The apparatus of claim 1 wherein said grid structure comprises a plurality of electrically conductive plate segments mutually separated by an electrically nonconductive substance, each said plate segment having a slotted opening therein, said segment openings being arranged to form a master character stencil.

5. The apparatus of claim 4 wherein said master character stencil comprises a figure eight.

6. The apparatus of claim 1 wherein said grid structure comprises a substantially coplanar array of a plurality of electrically conductive wires,

a first subplurality of said wires being arranged in mutually parallel, laterally spaced relation,

a second subplurality of wires being arranged in mutually parallel, laterally spaced relation,

said second subplurality of wires being disposed substantially perpendicular to said first subplurality of wires.

7. The apparatus of claim 6 wherein said wires are spaced to provide a band of inhibiting influence for preventing the passage of said ions to said platen equal to twice the grid spacing for each said wire when coupled to said reference potential.

8. An electrostatic printer comprising:

a receptor means for recording an electrostatic image;

first and second potential sources having substantially equal magnitudes and opposite polarities;

first and second corona sources disposed on opposite sides of said receptor means and coupled respectively to said first and second potential sources for generating a substantially continuous supply of ions;

first and second grid structures disposed between said first and second corona sources, respectively, and said receptor means for regulating the flow of said ions toward said platen to control the configuration of said electrostatic image, said first and second grid structures each comprising a plurality of electrically conductive elements insulated from each other;

third and fourth potential sources having substantially equal magnitudes and opposite polarities, the magnitude of said third and fourth potential sources being substantially less than the magnitude of said first ans second potential sources, respectively, said third and fourth potential sources having the same polarity as said first and second potential sources, respectively;

a reference potential;

switching means for selectively coupling said first and second grid structures to said reference potential and said third and fourth potential sources, respectively, said switching means including first and second sets of individually actuatable switching devices for coupling corresponding ones of said plurality of elements of said first and second grid structures, respectively, to either of said third potential source and said reference potential, and either of said fourth potential and said reference potential, respectively;

individual ones of said switching devices of said first and second sets being associated with individual ones of said plurality of elements of said first and second grid structures, respectively;

each said switching device of said first and second sets comprising:

a first member coupled to said reference potential;

a second member, said second member of each said switching device of said first set being coupled to said third potential source, said second member of each said switching device of said second set being coupled to said fourth potential source;

a central member coupled to said associated element; and

means for coupling said central member of corresponding ones of said switching devices of said first and second sets to either of said first and second members.

9. The apparatus of claim 8 wherein said first and second grid structures define an image area and including means for moving said receptor means through said image area at a predetermined rate.

10. The apparatus of claim 8 wherein said first and second grid structures each comprise a plurality of electrically conductive plate segments mutually separated by an electrically nonconductive substance, each said plate segment having a slotted opening therein, said segment openings being arranged to form a master character stencil.

ll. The apparatus of claim 10 wherein said master character stencil comprises a figure eight.

12. The apparatus of claim 8 wherein said first and second grid structures each comprise a substantially coplanar array of a plurality of electrically conductive wires;

a first subplurality of said wires being arranged in mutually parallel, laterally spaced relation;

a second subplurality of wires being arranged in mutually parallel, laterally spaced relation;

13. The apparatus of claim 12 wherein said wires are spaced to provide a band of inhibiting influence for preventing the passage of said ions to said receptor means equal to twice the grid spacing for each said wire when coupled to said reference potential.

Claims

1. An electrostatic printer comprising: a first potential source; a corona source coupled to said first potential source for generating a substantially continuous supply of ions; an electrically conductive platen coupled to a reference potential for attracting said ions theretoward; a receptor means positioned between said platen and said corona source for recording an electrostatic image of the charge pattern closely adjacent said plaTen; a second potential source having a magnitude substantially less than said first potential source and greater than said reference potential and a polarity of the same sign as said first potential source; a grid structure disposed between said receptor means and said corona source for regulating the flow of said ions toward said platen to control the configuration of said charge pattern, said grid structure comprising a plurality of electrically conductive elements insulated from each other; switching means for selectively coupling said grid structure to said second potential source and to said reference potential, said switching means including a plurality of individually actuatable switching devices for coupling each of said plurality of elements to either of said potential source and said reference potential, individual ones of said switching devices being associated with individual ones of said plurality of elements; and each said switching device comprising a first member coupled to said reference potential, a second member coupled to said second potential source, a central member coupled to said associated grid element and means for coupling said central member to either of said first and second members.

2. The apparatus of claim 1 wherein the magnitude of said second potential source is sufficient to cause said ions to flow to said platen when said grid structure is coupled thereto, and the magnitude of said reference potential is sufficient to cause said ions to be trapped by said grid structure when said grid structure is coupled to said reference potential.

6. The apparatus of claim 1 wherein said grid structure comprises a substantially coplanar array of a plurality of electrically conductive wires, a first subplurality of said wires being arranged in mutually parallel, laterally spaced relation, a second subplurality of wires being arranged in mutually parallel, laterally spaced relation, said second subplurality of wires being disposed substantially perpendicular to said first subplurality of wires.

8. An electrostatic printer comprising: a receptor means for recording an electrostatic image; first and second potential sources having substantially equal magnitudes and opposite polarities; first and second corona sources disposed on opposite sides of said receptor means and coupled respectively to said first and second potential sources for generating a substantially continuous supply of ions; first and second grid structures disposed between said first and second corona sources, respectively, and said receptor means for regulating the flow of said ions toward said platen to control the configuration of said electrostatic image, said first and second grid structures each comprising a plurality of electrically conductive elements insulated from each other; third and fourth potential sources having substantially equal magnitudes and opposite polarities, the magnitude of said third and fourth potential sources being substantially less than the magnitude of said first ans second potential sources, respectively, said third and fourth potential sources havIng the same polarity as said first and second potential sources, respectively; a reference potential; switching means for selectively coupling said first and second grid structures to said reference potential and said third and fourth potential sources, respectively, said switching means including first and second sets of individually actuatable switching devices for coupling corresponding ones of said plurality of elements of said first and second grid structures, respectively, to either of said third potential source and said reference potential, and either of said fourth potential and said reference potential, respectively; individual ones of said switching devices of said first and second sets being associated with individual ones of said plurality of elements of said first and second grid structures, respectively; each said switching device of said first and second sets comprising: a first member coupled to said reference potential; a second member, said second member of each said switching device of said first set being coupled to said third potential source, said second member of each said switching device of said second set being coupled to said fourth potential source; a central member coupled to said associated element; and means for coupling said central member of corresponding ones of said switching devices of said first and second sets to either of said first and second members.

11. The apparatus of claim 10 wherein said master character stencil comprises a figure eight.

12. The apparatus of claim 8 wherein said first and second grid structures each comprise a substantially coplanar array of a plurality of electrically conductive wires; a first subplurality of said wires being arranged in mutually parallel, laterally spaced relation; a second subplurality of wires being arranged in mutually parallel, laterally spaced relation; said second subplurality of wires being disposed substantially perpendicular to said first subplurality of wires.