US3691375A

US3691375A - Charging device

Info

Publication number: US3691375A
Application number: US70512A
Authority: US
Inventors: Stanley A Gawron; Kristian L Helland
Original assignee: Multigraphics Inc
Current assignee: AB Dick Co
Priority date: 1970-09-08
Filing date: 1970-09-08
Publication date: 1972-09-12
Anticipated expiration: 1989-09-12

Abstract

Corona discharge electrodes for applying a sensitizing charge to a moving sheet of photoelectrostatic paper are equipped with paper guide elements which support the sheet as it moves along a path of travel so that the sheet is kept clear of the fine wire electrodes. The fine wire electrodes are stretched inside an elongated housing, one side of which is open so as to freely emit the corona discharge. The opening is provided with laterally extending, spaced apart guides having a narrow flat support attached to the lead edges of the opening of the housing so that the sheets of paper may bridge the opening without entering and at the same time leaving an uninterrupted path for the corona discharge to reach the paper.

Description

[ 1 Sept. 12, 1972 United States Patent Gawron et al.

[541 CHARGING DEVICE [72] Inventors: Stanley Ostensen 5/1968 Young................250/49.5 ZC

Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church Attorney-Sol L. Goldstein Corporation, Mount Prospect, Ill.

Sept. 8, 1970 ABSTRACT Corona discharge electrodes for applying a sensitizing [22] Filed:

[21] Appl' 70,512 charge to a moving sheet of photoelectrostatic paper are equipped with paper guide elements which support Related Apphcauon Data the sheet as it moves along a path of travel so that the [63] Continuation of Ser. No. 726,793, May 6, sheet is kept clear of the fine wire electrodes. The fine wire electrodes are stretched inside an elongated 1968, abandoned.

housing, one side of which is open so as to freely emit [52] US. Cl.....................250/49.5 ZC, 250/495 GC the corona scharge. The opening is provided with laterally extending, spaced apart guides having a nar- 5 rm. 15/00 [58] Field of Search.250/49.5 20, 49.5 oc, 49.5 TC

row flat support attached to the lead edges of the opening of the housing so that the sheets of paper may bridge the opening without entering and at the same [56] References Cited time leaving an uninterrupted path for the corona UNITED STATES PATENTS discharge to reach the paper.

3,483,372 12/1969 Benson...............250/49.5 ZC 10 Claims, 3 Drawing Figures SHEET 1 OF 2 TILUQIIIZOT' I sim j. yawum 0 PATENTEU I 2 99?? PATENTED SE? 12 I972 SHEET 2 OF 2 Z y fi WM WW CHARGING DEVICE This is a continuation of Ser. No. 726,793 filed May 6, 1968 and now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to corona discharge electrodes and more specifically relates to the construction of a guide element for supporting the photoelectrostatic member as it passes over the charging electrode so that the member is protected against entering the electrode proper and at the same time providing an uninterrupted areaway through which the corona discharge may pass from the electrode.

The construction of discharge electrodes of the type transverse the path of the photoelectrostatic copying sheet in order that the full width of the sheet receives the sensitizing charge. Understandably, the individual sheets passing over such an electrode arrangement would on occasion find their way into the opening rather than pass over it, becoming entangled and damaging the line wire electrode.

To protect against this occurrence and still maintain a sufficient opening in the electrode through which the ionic discharge might pass, the opening in the conductive shield in prior construction was protected with fine diameter monotilament lines, usually nylon or teflon, stretched in an obliquely arranged lacing with respect to the path of the sheet. In this manner, the lacing of monofilament lines across the opening provided a support and guide structure for the sheet so that the sheet did not become entangled in the wire.

Such a construction was not without disadvantages since the plastic lacings tended to deteriorate in the ozone atmosphere generated in the vicinity of the electrode, becoming fragile and breaking. The unnoticed presence of a snapped lacing created the precise hazard to be avoided, namely creating an obstacle course for the paper. This required a frequent servicing problem necessitating replacement of the broken lacings, as well as broken wire electrodes.

An even greater disadvantage in this type of a paper support guide was the interference presented by the lacings to the ionic discharge preventing the sheet from receiving a uniform electrostatic charge. Areas on the member will be charged well below the saturation level, and upon exposure to originals having indicia of low contrast value, these areas will become discharged givin g the appearance of no image. The ability to lay down a uniform charge is very important to obtaining quality copies. Hence, it is desirable in the construction of such corona discharge electrodes to provide a reliable, and easy to maintain charging apparatus.

BRIEF DESCRIPTION OF INVENTION The present invention involves a corona discharge electrode in which the use of lacings of monofilament material across the opening to the electrode has been eliminated. One of its important advantages is that the same degree of protection against the photoelectrostatic copy sheet becoming entangled in the corona wire is provided without any obstruction whatsoever to the path of the discharge emitted by the corona wire. Since the opening across the charging device is unobstructed, nothing interfers with the corona discharge and the sheet is uniformly charged.

However, the present invention provides further advantages in that the support or guide construction is extremely sturdy so that it obviates the need for frequent replacement in order to maintain the charging device operative. In the preferred embodiment of this invention, there is provided an elongated conductive shield or housing which encloses a fine wire supported on suitable insulators along the longitudinal direction of the housing. The enclosures around the wire are complete, save for one side, for the purpose of providing an exitway for the corona discharge.

In the usual application a pair of such electrodes are disposed in opposing relation transverse the direction of travel of the photoelectrostatic member. The photoelectrostatic member is required to pass between the oppositely facing openings of the electrodes receiving an electrostatic charge on each surface. The electrodes are respectively connected to the positive and negative terminals of a high potential source. The first corona discharge device connected to the negative terminal of a high voltage supply is adapted to apply a negative voltage to the wire electrode. Similarly, the second corona discharge device is connected to the positive terminal of a high voltage supply and is adapted to apply positive voltage to the corona wire. Thus, one surface of the photoelectrostatic member is exposed to a corona discharge of one polarity and the opposite surface is exposed to a corona discharge of the opposite polarity. The high voltage applied to the first corona discharge device results in a production of a negative corona discharge which in turn generates a discharge of negative ions directed to the photoconductive surface providing the sensitizing charge. In general practice the photoelectrostatic member will have one surface coated with a photoconductive layer, such as for example zinc oxide in a resin binder. Such a photoconductive layer is charged negatively.

It will be appreciated that, as the sheet moves between the two corona electrodes, it is necessary that it be supported along a predetermined path out of contact with the electrode wires so that a uniform charge can be deposited thereon. To accomplish this the instant invention provides a guide element extending upward from the edge'of the opening of the housing or shield so that it presents a support over which the photoelectrostatic member can slide. The support itself is a rigid, narrow slatlike configuration which presents a flat surface extending longitudinally along the opening guiding the sheet as it moves transversely over the opening. A second such guide support is affixed to the In the circumstance that several electrodes are aligned side by side, a series of such guide supports would be provided so that the natural stiffness of short spans of the sheet will be sufficient to bridge the gap between said guide supports.

It has been found advantageous to incline the surface of the guide elements in the direction of the movement of the sheet relative to the horizontal in the range between and 1.22 radians. This serves to guide the sheetupwards and away from the opening as it tends to flex downward by virtue of its own weight.

The gap between the guide supports must be sufficiently narrow in order to enable the sheet to move across the opening without entering therein and becoming entangled with the corona wire. A relationship has been worked out relating the stiffness of the sheet as measured on a conventional paper stiffness tester. It was found that to span a gap of 1 A inches it would require a Gurley stiffness value measured in the machine direction of at least 50 units.

Another factor which affects the ability of the sheet to be sustained in a given path is the rate at which it moves past the guide supports. Moving an a rate of at least feet per minute, a photoelectrostatic member having the aforementioned minimum stifiness would easily bridge the gap of 1 inches between the guide elements. In practice the speeds encountered in copiers can range anywhere from 10 to 30 feet per minute, the distance between the guide elements range between about wt to 1 rs inches, preferably nine-sixteenths to 1 inch, and the stiffness of the sheet is usually greater than 50 units. If a sheet has a lesser stiffness value than 50 units, it may be necessary to decrease the distance between the guide elements and vary the speed.

While it has been proposed heretofore to locate multiple strands of corona wire in a single shield, it has been found advantageous to separate the individual wires by the conductive partition so as to provide a sup 7 port for the guide element. The presence of the intervening conductive wall between the individual wires tend to give a greater efficiency of emission of the discharge by providing a more complete conductive shield around each wire. By comparison to the practice of having multiple wires inside one shield, the ionic discharge from the several wires tend to interfere with one another. In the present circumstance the ionic discharge is further controlled by the presence of the added conductive surface. As the electrodes, having the guide elements disposed as aforedescribed, are oppositely disposed transverse the path of the photoelectrostatic member, there is provided a series of converging throat portions through which the photoelectrostatic member must pass as it is moved between the charging electrodes. At its widest portion the throat may range from one-half inch to three-quarters inch wide, gradually narrowing to an opening of about oneeighth to three-eighths inch.

As the sheet is charged it is important to avoid contact with a conductive material since this would cause the charge to leak off. To avoid this problem, the guide elements are covered with a dielectric material so that the charge deposited on the sheet is not dissipated as it makes contact with the guide elements. Any dielectric material may be employed having a resistivity greater that lO' ohm-centimeters. Such materials as polytetratluoroethylene, polyamide or polyvinylchloride have been found to give excellent results. The requirement of a dielectric material has given rise to another advantage such as when using a material such as polytetrafluoroethylene or a polyamide type plastic in that they provide almost a frictionless contact over which the paper can easily slide.

In another embodiment of this invention the photoelectrostatic member may have a Gurley stiffness less than 50 units, that is, it may be a soft and flimsy sheet. Understandably, such a sheet will tend to respond more easily to gravitational forces and fall into the opening between the guide elements of the lower electrode. To support the sheet out of contact with the corona wire, the guide construction as described above is employed, and additionally there is provided a gentle stream of air to support the sheet along a predetermined path between the two electrodes. To accomplish this, the conductive shield is provided with air ports connected to a blower which sends a stream of air directed against the bottom surface of the photoconductive member providing a platform of air as it moves over the guide members past the opposing discharging electrodes.

It is the general object of this invention to provide an improved corona discharge electrode that uniformly charges a photoelectrostatic recording member as it is supported out of contact from the source of corona discharge by keeping open and uninterrupted the path of discharge to the photoelectrostatic member.

It is another object of this invention to provide an improved corona discharge electrode in which the discharge wire is partially surrounded by a conductive shield having an uninterrupted opening through which the corona discharge may pass.

It is a further object of this invention to provide a corona discharge electrode employing an air stream for supporting the photoelectrostatic member out of contact from the discharge wires.

It is a specific object to provide an improved corona discharge electrode which more efficiently utilizes the available ionic discharge produced by the corona source.

DESCRIPTION OF THE DRAWINGS Several embodiments of this invention have been illustrated diagrammatically in the accompanying drawings, but it is to be expressly understood that said drawings are for purposes of illustration only and are not to be taken as a definition of the limits of the invention, reference being had to the appended claims for this purpose. In the drawings,

FIG. 1 is a cross-sectional view of the preferred embodiment of this invention;

FIG. 2 is a plan view taken along line 2-2 of FIG. 1;

FIG. 3 is a cross-sectional view of another embodiment of this invention using an air stream to hold the photoelectrostatic member away from the charging electrode.

Referring now to the drawings, FIGS. 1 and 2 illustrate the preferred embodiment of this invention in which there is shown a corona charging assembly identified by the general reference numeral 10. The charging assembly 10 is comprised of an upper charging electrode 12 and a lower charging electrode 14 which are disposed in opposing relationship to one another forming a pathway as indicated by the arrow 16. The photoelectrostatic member 20 is fed along the pathway 16 being advanced by a set of nipping rollers 22, which moves the member between the upper and

lower electrodes

12 and 14, respectively. The photoelectrostatic member, for example, may be a zinc oxide type coated paper. As the photoelectrostatic member 20 exist the assembly it is received by a set of nipping rollers 24 which advances the sheet to the next station in the copying apparatus.

it should be pointed out that the upper and lower electrodes are identical with the exception that they are connected to oppositely poled voltage supplies. Accordingly, the description of the upper electrode will be fully applicable to the lower electrode. The source of corona discharge is a fine wire 26 (FIG. 2) which is stretched between insulating

binding posts

28, 30 and 32, 34. The wire electrode 26 is enclosed in a compartmented housing or conductive shield 36 so that each set of binding posts having stretched therebetween the corona wire is fully shielded by the

wall sections

38, 40 and 42 so as to form the compartments 44 and 46. Each compartment holds a single wire and has an opening therein indicated by numeral 50 and 52.

It will be appreciated at this point that the photoelectrostatic member 20 as it passes between the two

electrodes

12 and 14 must be protected from making contact with the conductive wall sections of the conductive shield 36 or otherwise the sensitizing charge being applied by the corona wire 26 will be dissipated. In accordance with one of the features of this invention there is provided the

support members

38a, 40a and 42a integrally formed on the ends of the

wall sections

38, 40 and 42, respectively, providing a surface over which the photoelectrostatic member 20 may slide and be supported bridging the openings 50 and 52 without entering the compartments 44 or 46 and thereby becoming entangled or ensnared with the corona wire 26.

It will be observed that each of the

support elements

38a, 40a and 42a provide a flat face portion which tends to support a narrow transverse segment of the advancing photoelectrostatic member 20. As shown in FIG. 2, the face portions extend laterally along the length of the housing so as to form a continuous transverse support for the member 20 on either side of the openings 50 and 52.

The support members or guide

elements

38a, 40a and 420 are covered with a dielectric material 54 so as to form an insulating cover over the conductive shield protecting the photoelectrostatic member from making contact with metal. As described earlier, the dielectric material may be any one of a number of plastic materials which also offer the advantage of decreasing the coefficient of friction between the paper and the support member so that it easily slides over the surface.

Referring again to FIG. 1, it will be seen that each of the

support members

38a, 40a and 42a are formed at a slight incline in the direction of movement of the photoelectrostatic member along the path 16. Considering the path 16 as a horizontal plane, the angle at which the support member is positioned with respect to the horizontal may range between 0 and 1.22 radians, preferably between 0.35 and 0.8 radians. By positioning each of the support members in this manner there is formed between the corresponding support members of the lower electrode 14 a funnel type guide element having a rather wide entryway 56 which narrows down to an exitway 58. This arrangement tends to guide the sheet more accurately along the particular path and any tendency of the sheet to deviate from the horizontal will find it engaging one of the surfaces of the support members tending to redirect it towards the path of movement along the arrow 16. It will be observed that the support members leave the openings 50 and 52 clear and unobstructed so that the discharge from the wire 26 can act upon the photoelectrostatic member uninterrupted. This assures uniform charging of the member 20.

As described earlier, the spacing between the

guide elements

380, 40a and 42a is a function of the stiffness and speed of the member 20. For example, a conventional zinc oxide/resin binder photoelectrostatic copy paper generally has a Gurley stiffness measured in the machine direction greater than 50 units. if such a sheet is moved between the

electrode

12 and 14 at a speed which ranges about 10 to 20 feet per minute, the

distance between the guide elements can range' between about 1% to l zjnches, preferably nine-sixteenth to 1 inch. The stiffness of the sheet holds it on the path defined by the arrow 16 and the support members on the lower electrode 14 tend to support the sheet against the force of gravity guiding it along the path so that it is uniformly charged.

It will be observed that each of the wire electrodes 26 are enclosed by the conductive shield 36 and in particular, the compartments 44 and 46 are separated by the wall section 40. This arrangement provides a shielding effect on three sides for each compartment so that the ionic discharge from each wire does not interfere with the next adjacent wire permitting a more efficient utilization of the ionic discharge from the wires through their respective openings 50 and 52.

Referring to FIG. 3, there is illustrated another embodiment of the invention showing the oppositely disposed

electrodes

60 and 61 in spaced apart relation providing a path 63 therebetween for the passage of a photoelectrostatic member 20. This embodiment is particularly suited to handle sheets that are rather flimsy having a Gurley stiffness measurement of less than 50 units. The lower portion of the assembly is constructed in a manner similar to either one of the electrodes described in conjunction with FIG. 1. The corona wire electrode 62 is fixed at each end to a binding post identified as 64 and 66, only one post being shown of each pair. The wire is enclosed in a housing or conductive shield 68 having a series of upright walls 70, 72 and 74 so as to form

separate compartments

76 and 78 for each electrode. Affixed to the edge portion of each of the walls is a support member a, 72a and 74a which support the member advanced through the assembly by the infeed roller set 80 and the outfeed roller set 82.

The

support members

70a, 72a and 74a are similar in construction to the support members described in connection with FIG. 1. Due to the flimsy character of the sheet it will tend to deviate from the path which is equidistant from the electrodes and to keep it supported out of contact with the electrodes air-blow means are employed to urge the sheet along a straight 7 path. The bottom wall portion 84 of the housing 68 is provided with air inlet ports 86 through which is forced air from a centrifugal type blower 88 connected to said ports by a suitable conduit 90. The air enters through the ports 86, passes through

compartments

76, 78 and thence through the

openings

92 and 94 formed between the upright walls 70, 72 and 74. The use of a gentle stream of air to retain the photoelectrostatic member along its path of movement as it' passes through the charging assembly renders less critical the shape, size, and spacing between the support members. While in the previous embodiment the general shape and spacing between the support members was of importance, the use of a positive air stream requires only that a generally flat surface be provided which could be parallel to the direction of the sheet. The use of an air stream, therefore, provides another technique for controlling the photoelectrostatic member in a manner which permits an uninterrupted and unobstructive path for the ionic discharge emitted by the corona wire to the photoconductive surface on the sheet.

While the invention has been specifically described and illustrated with respect to certain embodiments, it

is to be understood that the invention is not restricted to these embodiments and that reference should be had to the appended claims for a definition of the limits of the invention.

We claim:

1. A charging device for applying uniform electrostatic charges to a photoelectrostatic copy sheet having a Gurley stiffness of greater than 50 units and moving at a predetermined rate, comprising:

corona discharge means including discharge electrode means and conductive shield means comprising a pair of elongated conductive housing units each having a U-shaped cross section and being disposed in opposing relation with the open portions facing one another and through said open side of which is emitted an ionic discharge from said discharge electrode means, the legs of the U of said housing being of a predetermined length, dielectric guide elements formed at the free ends of the legs of the U-shaped housing at opposite sides of the opening providing an uninterrupted path for said discharge therebetween, said elements providing a series of laterally extending supports transverse the path of movement of said copy sheet which guide said moving copy sheet along the path past said opening at a predetermined distance from said discharge electrode means, thereby to permit said sheet to receive a uniform, unimpeded charge from said discharge electrode means, said guide elements extending parallel to each other and each said element is inclined in the direction of the movement of the sheet and spaced from each other a distance of if: to l A inches across said opening to prevent said copy sheet from entering said opening while moving thereacross, said distance between said guide elements being established in accordance with the stiffness of said copy sheet and the rate of movement thereof across said opening.

2. The device as claimed in claim 1 wherein the guide elements have an angle of inclination relative to the horizontal which ranges between about 0 to 1.22 radians.

3. The charging device as claimed in claim 1 wherein said discharge electrode means comprises a plurality of corona discharge wires and wherein said conductive shield is provided with conductive partitions enclosing respective corona discharge wires so as to form a plural-walled compartment.

4. The device as claimed in claim 1 wherein said guide elements comprise a dielectric material selected from the group consisting of polytetrafluoroethylene, polyamide, or polyvinylchloride.

5. A device for uniformly charging a flexible photoelectrostatic copy sheet which has a Gurley stiffness greater than 50 units and which is moving along a predetermined path at a predetermined rate comprismg:

corona discharge means including discharge electrode means and conductiveshield means comprising a pair of elongated conductive housing units each having a U-shaped cross section and being disposed in opposing relation on opposite sides of said predetermined path and through the open sides of which are emitted from said electrode means an ionic discharge against the copy sheet, said opening defined by laterally extending spaced apart dielectric guide means formed at the free ends of the leg portions of the U shaped housing said guide means being inclined in the direction of the movement of said copy sheet which support and guide the moving copy sheet past said opening, and being separated from each other in the direction of movement of said copy sheet a distance of /4 to l 1$ inches, sufficient to support said sheet having said stiffness at a predetermined distance from said discharge electrode means while bridging said opening without entering therein.

6. The device as claimed in claim 5 wherein the sheet is moving at a rate of from 10 to 30 feet per minute.

7. The device as claimed in claim 6 wherein the passing sheet moves along a generally horizontal path and the guide means are inclined in the direction of movement of the sheet.

8. The device as claimed in claim 7 wherein the guide means have an angle of inclination relative to the horizontal which ranges between about 0 to 1.22 radians.

9. A charging assembly for charging photoelectrostatic recording sheet of a Gurley stiffness of greater than 50 units moving along a predetermined path at a predetermined rate comprising opposing discharge electrode means in spaced apart relation arranged so that said path passes therebetween for applying charges of opposite polarity to the respective surfaces of said sheet, each said discharge electrode means including a corona discharge wire partially surrounded by an elongated conductive shield having a U-sliaped cross section with the open side thereof extending parallel to said wire and through which ionic discharge may flow onto a respective surface of said sheet, and laterally extending dielectric guide elements mounted on the free ends of the legs of said U-shaped shield at opposing Ion gitudinal edges of said opening, said guide elements including flat surfaces extending from the legs of said shield and inclined slightly toward said recording sheet in the direction of movement thereof, the guide elements of said respective opposing discharge electrode means being in spaced alignment with respect to each other, the distance between said guide elements being chosen in accordance with the stiffness of the recording sheet being charged and the rate of movement thereof, said distance being in the range of 1$ to l inches, thereby to guide said sheet in its movement along said path without permitting the entrance of said sheet into said openings, said guide elements leaving the path from said wires to said respective surfaces on said sheet unobstructed along the longitudinal extent of said openings for the free passage therethrough of said ionic discharges and maintaining said recording sheet at a location so that a uniform charge is applied to the opposite surfaces thereof.

10. A device for charging a moving flexible photoelectrostatic recording sheet, comprising:

fixedly mounted corona discharge means partially enclosed in an elongated conductive shield having a longitudinally extending upwardly facing opening therein through which said discharge may pass uninterrupted to said moving recording sheet and air inlet ports located in said conductive shield including air blow means to provide an air stream through said opening toward the passing recording sheet, said air stream acting against the surface of said photoelectrostatic sheet as it passes across said opening to support and maintain said sheet in spaced relation from said opening and on a predetermined path as it passes said corona discharge means.

Claims

1. A charging device for applying uniform electrostatic charges to a photoelectrostatic copy sheet having a Gurley stiffness of greater than 50 units and moving at a predetermined rate, comprising: corona discharge means including discharge electrode means and conductive shield means comprising a pair of elongated conductive housing units each having a U-shaped cross section and being disposed in opposing relation with the open portions facing one another and through said open side of which is emitted an ionic discharge from said discharge electrode means, the legs of the U of said housing being of a predetermined length, dielectric guide elements formed at the free ends of the legs of the U-shaped housing at opposite sides of the opening providing an uninterrupted path for said discharge therebetween, said elements providing a series of laterally extending supports transverse the path of movement of said copy sheet which guide said moving copy sheet along the path past said opening at a predetermined distance from said discharge electrode means, thereby to permit said sheet to receive a uniform, unimpeded charge from said discharge electrode means, said guide elements extending parallel to each other and each said element is inclined in the direction of the movement of the sheet and spaced from each other a distance of 1/2 to 1 1/2 inches across said opening to prevent said copy sheet from entering said opening while moving thereacross, said distance between said guide elements being established in accordance with the stiffness of said copy sheet and the rate of movement thereof across said opening.

5. A device for uniformly charging a flexible photoelectrostatic copy sheet which has a Gurley stiffness greater than 50 units and which is moving along a predetermined path at a predetermined rate comprising: corona discharge means including discharge electrode means and conductive shield means comprising a pair of elongated conductive housing units each having a U-shaped cross section and being disposed in opposing relation on opposite sides of said predetermined path and through the open sides of which are emitted from said electrode means an ionic discharge against the copy sheet, said opening defined by laterally extending spaced apart dielectric guide means formed at the free ends of the leg portions of the U shaped housing said guide means being inclined in the direction of the movement of said copy sheet which support and guide the moving copy sheet past said opening, and being separated from each other in the direction of movement of said copy sheet a distance of 1/2 to 1 1/2 inches, sufficient to support said sheet having said stiffness at a predetermined distance from said discharge electrode means while bridging said opening without entering therein.

9. A charging assembly for charging photoelectrostatic recording sheet of a Gurley stiffness of greater than 50 units moving along a predetermined path at a predetermined rate comprising opposing discharge electrode means in spaced apart relation arranged so that said path passes therebetween for applying charges of opposite polarity to the respective surfaces of said sheet, each said discharge electrode means including a corona discharge wire partially surrounded by an elongated conductive shield having a U-shaped cross section with the open side thereof extending parallel to said wire and through which ionic discharge may flow onto a respective surface of said sheet, and laterally extending dielectric guide elements mounted on the free ends of the legs of said U-shaped shield at opposing longitudinal edges of said opening, said guide elements including flat surfaces extending from the legs of said shield and inclined slightly toward said recording sheet in the direction of movement thereof, the guide elements of said respective opposing dischaRge electrode means being in spaced alignment with respect to each other, the distance between said guide elements being chosen in accordance with the stiffness of the recording sheet being charged and the rate of movement thereof, said distance being in the range of 1/2 to 1 1/2 inches, thereby to guide said sheet in its movement along said path without permitting the entrance of said sheet into said openings, said guide elements leaving the path from said wires to said respective surfaces on said sheet unobstructed along the longitudinal extent of said openings for the free passage therethrough of said ionic discharges and maintaining said recording sheet at a location so that a uniform charge is applied to the opposite surfaces thereof.

10. A device for charging a moving flexible photoelectrostatic recording sheet, comprising: fixedly mounted corona discharge means partially enclosed in an elongated conductive shield having a longitudinally extending upwardly facing opening therein through which said discharge may pass uninterrupted to said moving recording sheet and air inlet ports located in said conductive shield including air blow means to provide an air stream through said opening toward the passing recording sheet, said air stream acting against the surface of said photoelectrostatic sheet as it passes across said opening to support and maintain said sheet in spaced relation from said opening and on a predetermined path as it passes said corona discharge means.