US3492476A - Electrostatic charging device utilizing both a.c. and d.c. fields - Google Patents

Description

Jan. 27, 1970 J. E. GERMANOS ELECTROSTATIC CHARGING DEVICE UTILIZING BOT A.C AND D.C FIELDS 2 Sheets-Sheet 1 Filed March 18, 1968 E/ ZO INVENTOR JOHN E. GER ANOS ATTORNEYS Jan. 27, 1970 J. E. GERMANOS 3,492,476

ELECTROSTATIC CHARGING DEVICE UTILIZING BOTH A.C AND D.C. FIELDS Filed March 18, 1968 2 Sheets-Sheet 2 m is 5% 22; w 00 2 0:: 82 E o m :1: 2 v u. o o I o 0 l- 88 2 M w v a '3 8 I b E 9 o Ed m 3 5 o z 2' 2 lco 9 '3" 0 9:1. 05 88 law L I l 1 l l l l I I l PLATE CURRENT INVENTOR JOHN E. GERMANOS A7" TOR/VEYS United States Patent 3,492,476 ELECTROSTATIC CHARGING DEVICE UTILIZ- ING BOTH A.C. AND D.C. FIELDS John E. Germanos, Rochester, N.Y., assignor to Xerox gorporation, Rochester, N.Y., a corporation of New ork Filed Mar. 18, 1968, Ser. No. 713,965

Int. Cl. H011 37/26 Us. or. 250-495 ABSTRACT OF THE DISCLOSURE 21 Claims BACKGROUND OF THE INVENTION This invention relates to a corona discharge device and system for the uniform charging of a charge-retaining surface. More particularly, this invention relates to the field of xerography and to a charging apparatus for placing a uniform electrostatic charge, either positive or negative, by corona discharge on the photoconductive insulating surface of a xerographic member. Additionally, this invention relates to a corona discharge device suitable for transferring a developed image from the photoconductive insulating surface of a xerographic member to a transfer member.

In the process of xerography as described, for example, in Carlson US. Patent No. 2,297,691, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing member is given a uniform electrostatic charge over its surface and then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity which reaches them thereby creating an electrostatic latent image suitable for development. Development is effected with a finely divided material, such as an electroscopic powder, which is brought into contact with the latent image-bearing photoconductive insulating surface and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the powder image is transferred to a suitable base, such as paper, and is fixed thereon by any suitable means. After transfer, any toner powder remaining on the xerographic plate is removed.

By present techniques, the charging of the xerographic member in preparation for the exposure step is accomplished by means of a corona generating device whereby an electrostatic charge on the order of 500 to 600 volts is applied to the xerographic member. In general, corona charging may be accomplished in a variety of ways, the particular technique chosen being that which is most compatible with the requirements of the particular application. For example, the corona discharge device may comprise a single wire or a series of parallel wires that are fed from a high voltage source, relative to which the xerographic member may be moved at a uniform rate of speed to place an electrostatic charge thereon. Conversely, the xerographic member may be held stationary and the corona discharge unit may be moved relative 3,492,476 Patented Jan. 27, 1970 thereto to deposit a charge thereon. For other applications, it is expedient to hold both the xerographic member and the corona discharge device stationary during the charging operation. In all instances, however, it is essential that the electrostatic charge placed on the xerographic member the uniform throughout in order to provide good quality in the finish PY, and that the charge be on a predetermined magnitude and polarity, depending upon the nature of the photoconductive insulating material utilized and the type of copy to be made.

Typical of the corona generating devices employed heretofore are those described in Walkup US. Patent No. 2,777,957 and in Vyverberg US. Patent No. 2,836,- 725, each constructed generally of an electrode wire or wires supported relatively close to the surface to be charged. A grounded metallic shield generally surrounds the electrode, except for an opening through which charge is emitted, the shield being adapted to attract surplus emission emanating therefrom. Such corona generating devices are particularly suitable for use in automatic equipment employing the principles of xerography which utilize a xerographic plate in the form of a cylindrical drum which is continuously rotated through a cycle of sequential operations including charging, exposure, development and transfer.

As discussed in Carlson, photoconductive insulating coatings comprises anthracene, sulphur, or various mixtures of these materials such as sulphur with selenium, etc. to thereby form uniform amphorous coatings on the conductive base material. These materials have a sensitivity largely limited to the shorter wavelengths and a further limitation of being only slightly light sensitive. Consequently, there has been a continuing effort to produce improved photoconductive insulating materials on xerographic plates. The discovery of the photoconductive insulating properties of highly purified vitreous selenium has resulted in this material becoming the standard in commercial xerography. The photographic speed of this material is many times that of the prior art photoconductive insulating materials. Such a plate is characterized as being capable of receiving a satisfactory electrostatic charge and selectively dissipating such a charge when exposed to a pattern of light and shadow. Under conditions of optimum use, a vitreous selenium plate can be used to prepare 100,000 or even more copies before it deteriorates to the point of unsatisfactory image formation.

It is known that such selenium layers conduct both electrons and holes but that the mobility for holes is approximately 10 times that for electrons. Vitreous selenium, then, can be considered as a p-type semiconductor. Accordingly, in the conventional xerographic process wherein selenium is utilized as the photoconductive insulating material, a uniform positive electrostatic charge is applied to the selenium surface prior to exposure. During exposure to actinic radiation, hole-electron pairs are created in the selenium layer, the holes moving through the selenium layer to the conductive backing while the electrons discharge adjacent portions of positive surface charge in accordance with the intensity of the imaging radiation. Because of its short range for electrons, vitreous selenium is not usually used with negative electrostatic charging. Though the holes can move through the selenium layer to discharge the surface negative charge in light exposed areas, the electrons, because of their limited mobility, cannot reach the conductive backing member and, therefore, a trapped bulk space charge is created which reduces image quality.

The development of suitable photoconductive insulating materials has not been limited, however, to vitreous selenium. It has been proposed that various two component materials be used as photoconductive insulating layers for xerographic plates. Exemplary two component xerographic plates are those disclosed by Middleton et al. in US. 3,121,006 wherein particulate photoconductive mate rials are dispersed in an insulating organic resinous binder. The present preferred photoconductive material is zinc oxide which, as is known, is an n-type semiconductor, that is, its mobility for electrons is greater than its mobility for holes. Accordingly, when used in a xerographic mode, the zinc oxide binder plate, for example in the form of zinc oxide coated paper is given negative electrostatic charge over its exposed surface and then exposed to an actinic radiation pattern of light and shadow to thereby create a developable latent electrostatic image thereon. There is thus established a definite need for a charging device which can deposit a negative electrostatic charge on a charge-retaining surface, such as the exposed surface of a xerographic member.

In the art of xerography it has been established that consistently high quality reproductions can best be effected when a uniform potential is applied to the xerographic member to prepare the member for the exposure step. This has not proven to be a problem with positive charging since positive corona discharge from a wire generally occurs in a volume in the form of a continuous uniform sheath surrounding the wire. With negative corona charging, however, the negative corona has a tendency to concentrate itself at discrete points along the wire with the result that negative charge deposits on the charge-retaining member in a non-uniform pattern corresponding to the non-uniformity of the corona discharge itself. This is undesirable because it leads to unsatisfactory reproductions.

OBJECTS OF THE INVENTION It is, therefore, an object of this invention to provide a corona charging apparatus which deposits a charge of uniform density on a charge-retaining surface.

It is a further object of the present invention to provide a negative corona charging apparatus for depositing a uniform negative electrostatic charge on a charge-retaining surface.

A further object of the present invention i to provide a negative corona charging apparatus for use in the xerograp-hic mode wherein a uniform negative electrostatic charge is deposited on a photosensitive surface.

Still a further object of the present invention is to provide a corona charging apparatus for depositing a uniform positive electrostatic charge on a charge-retaining surface.

Yet a still further object of this invention is to provide a corona charging apparatus suitable for use in the xerographic mode as a means to transfer the developed image from the photoconductive insulating surface of a xerographic member to an adjacently positioned transfer member.

The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed disclosure of specific exemplary embodiments of the invention.

SUMMARY OF THE INVENTION The above and still further objects of the invention may be accomplished in accordance with the present invention by providing a chargeable layer in contact with a conductive backing member or layer, the exposed surface of said chargeable layer being considered the chargeretaining surface, and a corona charging device having at least one corona wire in parallel closely spaced relationship to the surface to be charged, a high voltage direct current potential source connected between the corona wire or wires and the conductive backing member or layer, and a conductive shield. The advantageous results of the present invention are achieved by biasing the conductive shield with a high voltage alternating current signal with respect to the conductive backing layer. It has been found that the improvement is most pronounced when the effect of the alternating current field felt by the corona generating means is comparable to the effect of the direct current field. As the effect of the AC field is both distance and voltage dependent, no strict generalization can be made as to the magnitude of the AC voltage, etc.', however, when the spacings between the corona wire and the conductive shield are substantially equal (as in the example to be given later in the specification) the improvement is most pronounced when the peak-to-peak alternating current voltage is substantially equal to or higher than the direct current potential applied to the corona wire or wires. In fact, with the arrangement given in the example, more uniform charge density was achieved as the peak-to-peak alternating current voltage was increased and, as can be seen in FIGURE 4, substantial uniformity was obtained when the absolute value of the peakto-peak AC voltage was greater than the DC potential applied to the corona wire. It should be understood, however, that this improvement in uniformity is a gradual change which increases with increasing AC voltage.

As can be seen from FIGURE 4, with such an arrangement as given in the example, the charge density deposited on the charge retaining surface with negative corona is substantially more uniform than the charge density obtained with conventional negative corona charging techniques.

The invention is also applicable to positive corona charging merely by reversing the connections between the direct current potential source and (l) the corona generating means and (2) the conductive backing layer.

Besides being used in a charging mode, the device of the present invention is equally applicable for use in a discharging mode wherein a previously charged surface is discharged by a uniformly deposited charge of opposite polarity. Additionally, the device can be used in a transfer mode wherein the side of a transfer member opposite from the developed image on a supporting surface (e.g. a xerographic member) is uniformly charged to induce the transfer of the developed image to the transfer member.

BRIEF DESCRIPTION OF THE DRAWINGS The nature of the invention will more easily be understood when it is considered in conjunction with the accompanying drawings wherein:

FIGURE 1 is a schematic diagram of the corona charging device of the present invention and the electrical circuit therefor for depositing uniform negative corona on a charge-retaining surface;

FIGURE 2 is a schematic illustration of an automatic xerographic machine, incorporating the corona generating device of the present invention;

FIGURE 3 illustrates the various forms of conductive shields which are suitable for use with the corona generating device of the present invention; and

FIGURE 4 is a chart representing the charge density distribution on a conductive plate for various negative corona charging techniques, the most uniform density shown therein being obtained with the technique of the present invention.

Referring to FIGURE 1, there is seen a xerographic member 10 having a photosensitive layer 12 and a conductive backing member 14. An exemplary photosensitive layer 12 is a dispersion of zinc oxide throughout an organic resinous binder. Pbotosensitive layer 12 has a charge-retaining surface 16. In closely parallel spaced relationship to surface 16 is a corona wire 18 which is electrically connected to the negative terminal of direct current potential source 20. The positive terminal of potential source 20 is connected to conductive backing member or layer 14. Spaced on the opposite side of wire 18 from photosensitive member 10 is a conductive shield 22 which is connected to conductive backing member 14 through a high voltage alternating current source 24.

For a general understanding of the xerographic processing system in which the invention may be incorporated, reference is made to FIGURE 2 in which the various system components are schematically illustrated. As in all xerographic systems based on the concept disclosed in the aforementioned Carlson patent, a radiation image of copy to be reproduced is projected onto the photosensitive sur face of a xerographic member to form an electrostatic latent image thereon. Thereafter, the latent image is usually developed with an oppositely charged developing material to form a xerographic powder image, corresponding to the latent image on the xerographic member surface. The powder image is then electrostatically transferred to a support surface to which it may be fused by any suitable form of fusing device whereby the powder image is caused to permanently adhere to the support surface.

The xerographic apparatus described herein typically may be of a type disclosed in Cerasani et al. U. S. Patent No. 3,076,392. As in the apparatus thereof, opaque copy to be reproduced is placed on a support tray 30 from which it is fed onto a transport mechanism generally designated as 31. Suitable drive means are provided for the transport mechanism from motor 32 to endless belt 33 whereby the copy is moved past the optical axis of projection lens system 34 that is illuminated by a projection lamp LMP-l. The image of the copy is reflected by mirror 35 through an adjustable lens 36 and then refiected by mirror 37 downwardly through a Variable slit aperture assembly 38 and on to the photosensitive surface of a xerographic member in the form of drum 39.

Xerographic drum 39 includes a cylindrical member mounted in suitable hearings in the frame of the machine and is driven in a clockwise direction by motor 42 at a constant rate that is proportional to the transport rate of the copy whereby the peripheral rate of the drum surface is identical to the rate of movement of the reflected light image. The drum includes a surface comprised of a layer of photoconductive material 42 on a conductive backing 44 that is sensitized (i.e. electrostatically charged) prior to exposure by means of corona generating device 10, constructed in accordance with the present invention. As shown, corona generating device has two corona wires as opposed to the corona device of FIGURE 1 which has only one.

The exposure of the drum to the light image discharges the photoconductive layer in the area struck by light whereby there remains on the drum a latent electrostatic image in image configuration corresponding to the light image projected from the copy. As the drum surface continues its movement, the electrostatic latent image passes through a developing station 46 in which a twocomponent developing material 47, which may be of the type disclosed in Walkup US. Patent No. 2,638,416, 1s cascaded over the drum surface by means of developing apparatus 48 which may be of the type disclosed in copending application Ser. No. 393,058, filed Nov. 19, 1953, now abandoned, in the names of C. R. Mayo et al.

In the developing apparatus developing material is carried up by conveyor 49 driven by suitable drive means from motor 50 and is released onto chute 51 wherefrom it cascades down over the drum surface. Toner component 52 of the developer that is consumed during development is stored in dispenser 53 and is released therefrom in amounts controlled by gate 54.

Thereafter, the powder image passes through an image transfer station 62 at which the powder image is electrostatically transferred to a support surface web 63 by means of a second corona generating device 64, also constructed in accordance with the present invention. Corona means 64 charges the back of web 63 more negatively than the potential remaining on the surface of xerographic drum 39 so that the positive charge is transferred to web 63 in image configuration.

The support surface to which the powder image is transferred may be of any convenient type, such as paper, and is obtained from a supply roll 65 and fed over guide rolls 66 and 67 and over suitable tensioning rolls into surface contact with the drum in the immediate vicinity of transfer corona generating device 64. After transfer, the support is separated from the drum surface and guided through a suitable fusing apparatus 68 which may be an adaptation of the type disclosed in Crumrine US. Patent No. 2,852,651 whereby the powder image is permanently aflixed to the support surface. Thereafter, the support surface is fed over a further system of guide and tensioning rolls and onto a take-up roll 72 that is driven by motor 73.

After separation of the support surface from the drum, a third corona generating device 74 directs electrostatic charge to the residual powder image on the drum surface. Corona device 74, which may also be constructed in accordance with this invention reduces the electrostatic attraction of the residual toner particles for the underlying xerographic drum. Thereafter, the xerographic drum surface passes through a cleaning station 75 wherein the drum surface is brushed by a cleaning brush assembly 76, rotated by a motor 77, whereby residual developing material remaining on the drum is removed. The drum surface then passes through a discharge station 78 at which it is illuminated by a fluorescent lamp LMP-Z whereby the drum surface in this region is completely flooded with light to remove any electrostatic charge that may remain thereon. Suitable light traps are provided in the system to permit any light rays from reaching the drum surface, other than the projected image, during the period of drum travel immediately prior to electrostatic charging by corona generating device 10 until after the drum surface has completely passed through the developing station 46 In the xerographic system just described, the basic processing steps include the electrostatic charging of the xerographic drum, the exposing thereof to a radiation pattern, the developing of the latent electrostatic image with a suitable developing material, the transfer of the powder image to a support surface and the fixing thereon, and the cleaning of the xerographic drum to make it suitable for use in the preparation of the next reproduction. The corona charging device of the present invention is also suitable for use in a xerographic system wherein zinc oxide coated paper, commonly known as Electrofax, is utilized as the xerographic member. Such a system differs from the system described in FIGURE 2 in that it eliminates the transfer and cleaning operations. After formation of the latent electrostatic image on the zinc oxide coated paper, the image is developed with a suitable powdered material and then aflixed to the paper without transfer to a further support surface. The copy is then ejected from the apparatus whereby the steps of charging, exposing, developing, and fixing can be repeated on the next zinc oxide coated copy sheet which passes through the system. The corona generating device of the present invention has particular applicability to this latter system since zinc oxide, as previously indicated, is an n-type semiconductor and receives a negative electrostatic charge over its exposed surface prior to the exposure thereof to an actinic radiation pattern. The corona generating device herein disclosed also is suitable for use in uniformly negatively charging an insulator surface whereon a latent electrostatic image, suitable for development, is formed by non-optical, non-photoconductive techniques (e.g. by contact with a patterned conductive member).

The corona device herein disclosed can also be utilized for the uniform positive charging of a charge-retaining surface, e.g., amorphous selenium, paper, etc.

As previously indicated, a conductive shield is one element of the corona generating device of the present linvention. FIGURE 3 illustrates in cross-section the widely variant forms that the shield may take on, all of which are operable in the process of the present invention. In each figure the conductive shield is labeled 90 and the corona generating wire is designated 91. In FIG- URE 3A, the conductive shield is a flat plate as shown in FIGURE 1. In FIGURE 3B, the conductive shield is a rectangular trough open at the lower portion for emanation of negative ions to the surface to be charged. The conductive shield of FIGURE 3C is similar to the shield of FIGURE 3B except that the side walls perpendicular to the surface to be charged have converging portions which more clearly define a longitudinal slit through which negative ions generated by the wire are emitted from the assembly. In FIGURE 3D, the conductive shield comprises a semi-cylindrical trough. In FIGURE 3E, the conductive shield comprises a cylindrical trough of greater than 180 are open at the lower end thereof to permit negative ions generated at the wire to be emitted therefrom towards the surface to be charged. The conductive shield should be parallel lengthwise both to the corona wire or wires and the charge-retaining surface and should extend sufiiciently far about the corona wire or wires as to prevent corona discharge past the edges of the conductive shield. The shield may be a continuous conductive surface, an apertured conductive grid or a continuous insulating surface having parallel spaced conductors embedded therein.

DESCRIPTION OF A SPECIFIC EMBODIMENT The following example is given to enable those skilled in the art to more clearly understand and practice the invention. It should not be considered as a limitation upon the scope of the invention but merely as being illustrative thereof.

The charging device comprising a 3.5 mil Inconel wire approximately 8 inches in length is connected to a grounded 11" x 11" x aluminum plate through a direct current potential source. The aluminum plate is connected to a fiat aluminum shield of same dimension through a 60 c.p.s. alternating current potential source. The wire to plate distance is 1.0 cm. and the wire to shield distance is 1.5 cm.

The foregoing charging device was tested under three conditions, the results of which can be seen in FIGURE 4. In the first condition, representing a conventional corona charging technique, 5000 volts was placed on the corona wire and the shield grounded as in the normal practice. The non-uniform charge distribution is shown as curve 1 in FIGURE 4.

In the second condition, 5000 volts was placed on the corona wire and 2000 volts peak-to-peak alternating current voltage was placed across the plate and shield. The non-uniform distribution obtained with this condition is shown as curve 2 in FIGURE 4.

-In accordance with the teachings of this invention, -5000 volts were placed on the corona wire and 6000 volts peak-to-peak alternating current voltage was placed across the plate and shield. The substantially more uniform charge distribution obtained on the aluminum plate is shown as curve 3 in FIGURE 4.

The charging structure described in the example is a suitable device for analyzing the uniformity of charge distribution from a corona wire. The disposition of a charge-retaining surface, whether it be photoconductive or non-photoconductive, on the surface of the conductive plate most closely adjacent the corona wire or wires would result in the deposition thereon of the uniform charge described in the preceding example.

While the present invention has been described with reference to a conductive shield or grid placed on the opposite side of the corona generating means from the charge-retaining surface, it is also possible to position a conductive shield or grid between the corona generating means and the charge-retaining layer. As previously indicated, the improvement in uniformity is achieved when the effect of the AC field felt by the corona generating means is comparable to the effect of the DC field between the corona generating means and the conductive backing member supporting the charge-retaining layer.

As is well known, the corona threshold potential and the corona current from an energized wire are functions of the wire diameter, i.e., the corona threshold increases and the corona current for any given potential decreases as the wire diameter is increased. Variations in the potential applied to corona wires of a given diameter will cause relatively large changes in corona with corresponding variations in the charging rate. In addition, the corona threshold potential and corona current are also affected directly by deposits of dust that may accumulate on the wire, by atmospheric conditions such as humidity, temperature and pressure, and by variations of movement and ionized conditions of the air sheath surrounding the wire. Thus when operating at the corona threshold, minute differences in wire diameter, slight accumulations of dust on the wire, and variations in air current, atmospheric conditions and the spacing between the wire and the xerographic plate drastically affect the corona gen erating otential of the wire and cause a non-uniform electrostatic charge to be deposited on the xerographic plate. In any corona generating device, appropriate provisions must be made for the minimization of these deleterious effects.

While the invention has been described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teaching of the invention without departing from its essential teachings.

What is claimed is:

1. A device for imparting electrostatic charges to a charge-retaining member comprising corona generating means operatively connected through a direct current potential source to the charge-retaining member, and means positioned adjacent said corona generating means and operatively connected to the charge-retaining member through an alternating current potential source for impressing an alternating curent field upon said corona generating means.

2. The device of claim 1 wherein said means for impressing an alternating current field upon said corona generating means comprises a conductive shield or grid.

3. The device of claim 2 wherein the conductive shield or grid is positioned on the opposite side of said corona generating means from the charge-retaining member.

4. The device of claim 1 wherein the charge-retaining member comprises an insulating layer adjacent said corona generating means and having an underlying conductive member.

5. The device of claim 1 wherein the charge-retaining member comprises a xerographic plate having a photoconductive insulating layer adjacent said corona generating means and an underlying conductive support.

6. The device of claim 1 wherein the effect of the alternating current field impressed upon said corona generating means is comparable to the effect of the direct current corona-generating field.

7. The device of claim 1 wherein the peak-to-peak alternating current voltage is substantially equal to or higher than the direct current potential applied to the corona generating means.

8. The device of claim 1 wherein the corona generating means is connected to the negative terminal of said direct current potential source and the charge-retaining member is connected to the positive terminal.

9. The device of claim 1 wherein said corona gener ating means is connected to the positive terminal of said direct current potential source and the charge-retaining member is connected to the negative terminal.

10. The device for imparting electrostatic charges to a charge-retaining member comprising corona generating means positioned in spaced relationship to the chargeretaining member, a corona-generating direct current potential source operatively connected between said corona generating means and the charge-retaining member, means positioned adjacent said corona generating means for impressing an alternating current field upon said corona generating means, and a high voltage alternating current potential source operatively connected between said impressing means and said charge-retaining member.

11. The device of claim 10 wherein said corona generating means is connected to the negative terminal of said corona-generating direct current potential source.

12. The device of claim 10 wherein said impressing means comprises a shield or grid positioned on the opposite side of said corona generating means from the charge-retaining member.

13. The device of claim 10 wherein the effect of the alternating current field impressed upon said corona generating means is comparable to the effect of the direct current corona-generating field.

14. The corona generating device for depositing negative charge of uniform density onto a charge-retaining member having a charge-retaining layer overlying a grounded conductive backing member comprising corona generating means in spaced relationship to the chargeretaining layer, a corona-generating direct current potential source the negative terminal of which is connected to said corona generating means and the positive terminal of which is connected to the conductive backing member, a conductive shield or grid adjacent said corona generating means and an alternating current potential source connected between said conductive shield or grid and the conductive backing member.

.15. The device of claim 14 wherein the effect of the alternating current field impressed upon said corona generating means by said conductive grid or shield and said alternating current potential source is comparable to the effect of the direct current corona generating field.

16. The device of claim 10 wherein the peak-to-peak alternating current voltage is substantially equal to or higher than the direct current potential applied to the corona generating means.

17. A process of electrostatically charging a chargeretaining member comprising providing a charge-retaining member in contact with a conductive backing member, applying a direct current corona generating potential to a corona discharge electrode adjacent said charge-retaining member and simultaneously therewith impressing an alternating current field upon said corona discharge electrode by applying a high voltage alternating current potential between said conductive backing member and a conductive shield or grid positioned on the opposite side of said corona discharge electrode from said chargeretaining member.

18. The method of claim 17 wherein the effect of the alternating current field impressed upon said corona discharge electrode is comparable to the effect of the direct current corona-generating field.

19. The method of claim 17 wherein the peak-to-peak alternating current voltage is substantially equal to or higher than the direct current potential applied to the corona discharge electrode.

20. The method of claim 17 wherein the charge-retaining member has positive electrostatic charge deposited thereon.

21. The method of claim 17 wherein the charge-retaining member has a negative electrostatic charge of uniform density deposited thereon.

References Cited UNITED STATES PATENTS 3,433,948 3/1969 Gallo 250-495 RALPH G. NILSON, Primary Examiner S. C. SHEAR, Assistant Examiner

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