US3688107A

US3688107A - Electrostatographic charging apparatus

Info

Publication number: US3688107A
Application number: US84005A
Authority: US
Inventors: John M Schneider; John Lennon
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1970-10-26
Filing date: 1970-10-26
Publication date: 1972-08-29
Anticipated expiration: 1989-08-29
Also published as: DE2153288B2; GB1365130A; DE2153288A1; JPS5333855B1; DE2153288C3; CA938660A

Abstract

A corona generating apparatus is provided comprising a back-up plate and screen wires, at least one corona discharge wire positioned therebetween, adapted to be mounted in closely spaced relation with an electrostatographic plate to uniformly apply an electrostatic charge onto said plate, a corona-generating potential source connected to said corona discharge wire adapted to generate a corona discharge from said corona wire, and a second potential source adapted to be connected to said screen wires and impress a time or position varying voltage upon said screen wires permitting high initial charging current to flow thereby rapidly charging said plate while limiting the ultimate charge applied thereto.

Description

United States Patent Schneider et al.

[ Aug. 29, 1972 [54] ELECTROSTATOGRAPHIC CHARGING APPARATUS [73] Assignee: Xerox Corporation, Rochester, NY.

[22] Filed: Oct. 26, 1970 [2l] Appl. No.: 84,005

[56] g References Cited UNITED STATES PATENTS 9/1970 Culhane ..250/49.5 ZC 2/1970 Cunningham ..250/49.5ZC

Primary ExaminerArchie R. Borchelt Assistant Examiner-C. E. Church AttorneyJames J. Ralabate, William Kaufman and Barry Kramer ABSTRACT A corona generating apparatus is provided comprising a back-up plate and screen wires, at least one corona discharge wire positioned therebetween, adapted to be mounted in closely spaced relation with an electrostatographic plate to uniformly apply an electrostatic charge onto said plate, a corona-generating potential source connected to said corona discharge wire adapted to generate a corona discharge from said corona wire, and a second potential source adapted to be connected to said screen wires and impress a time or position varying voltage upon said screen wires permitting high initial charging current to flow thereby rapidly charging said plate while limiting the ultimate charge applied thereto.

9 Claims, 2 Drawing Figures PATENTEflnuczs m2 SHEET 1 [IF 2 Illa.

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2 or 2 wws ELECTROSTATOGRAPHIC CHARGING APPARATUS This invention relates to electrostatography. More particularly this invention relates to improved electrical circuitry for controlling a corona generating device for rapidly and uniformally applying electrostatic charge onto an electrostatographic plate.

The basic electrostatographic process is disclosed in the Carlson U.S. Pat. No. 2,297,691. In this process an electrostatographic plate comprising a photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is 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 and thereby creates an electrostatic latent image on or in the plate coating which may then be developed with an electroscopic material which electrostatically clings to the plate in a visual pattern corresponding to the electrostatic image. Thereafter the developed image is usually transferred to a support material to which it may be fixed by any suitable means.

The charging of the electrostatographic plate in preparation for the exposure step can be accomplished by means of a corona generating device whereby electrostatic charge is applied to the electrostatographic plate to raise it to a potential of approximately 500 to 600 volts. One form of corona generating device for this purpose is disclosed in the Walkup U.S. Pat. No. 2,777,957, wherein a plurality'of parallel wires are connected in series to a high voltage source and are supported in a conductive shield that is arranged in closely spaced relation to the surface to be charged. When the wires are energized, corona is generated along the surface of the wires and ions are caused to be deposited on the adjacent photoconductive surface. Suitable means are usually provided to effect relative movement of the surface to be charged and the corona generating device. A biased wire shield placed between the corona wires and the electrostatographic plate permits energizing the corona wires to a potential well above the corona .threshold potential thereof without causing damage to the electrostatographic plate because the excess of corona current over that required for proper charging of the plate is drained off by the biased shield. This type of corona generating device is referred to in the art as a scorotron.

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 current 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 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 or in air pressure drastically affect the corona generating potential of the wire causing nonuniform electrostatic charge to be deposited on the electrostatographic plate.

It has heretofore been established that consistent high quality reproductions can best be obtained when uniform potential is applied to the electrostatographic plate in preparation of the plate for the exposure step. If the electrostatographic plate is not charged to a sufficient potential, the electrostatic latent image obtained upon exposure will be relatively weak and the resulting deposition of an electroscopic material thereon will be correspondingly small. If, however, the electrostatographic plate is overcharged, the converse will occur and if overcharged sufficiently, the photoconductive layer of the electrostatographic plate can be permanently damaged.

Since the contrast value, comparable to the contrast values obtained from silver halide papers, of the electrostatic latent image may be related directly to the potential charge on the electrostatographic plate before exposure, it is apparent that if the plate is not uniformly charged over its entire area, the contrast value of the electrostatic latent image obtained upon exposure will vary in different areas on the plate, and a mottled effect will be visible on the image when developed.

An improved scorotron device whereby a uniform electrostatic charge can be deposited on the electrostatographic plate is disclosed in the Codichini U.S. Pat. No. 3,062,956. The scorotron device described therein consists of a backup plate, corona generating wires called the coronode, and screen wires. The coronode fires, by corona discharge, charging the photoconductive surface of the electrostatographic plate. The potential applied to the plate surface is varied by changing the screen potential. To ensure a constant charging current during operation at any given contrast setting, the charging circuit contains a current stabilizer and a regulated direct current power supply. The current stabilizer is a device for controlling charging current by automatically adjusting the screen potential when a current change is sensed.

In operation, any change in the charging current from the coronodes to the electrostatographic plate produces a change in the applied voltage to the grid of a control tube, for example, a high gain pentode, the output of said control tube being applied to the screen wires of the scorotron device. In this manner, any change in the charging current from the coronodes to the electrostatographic surface results in a change in the control tube resistance which, in turn, produces a change in the screen potential. With this circuit, as a decrease in charging current occurs, the resistance of the control tube increases thereby increasing the screen voltage to permit the charging current to increase back to its desired value and, of course, the converse is true as the charging current increases above a desired value. In this manner, the charging current can be maintained at a constant value and is not influenced by any of the normal variables such as geometry of the scorotron, voltage variations, dirty wires, atmospheric changes, etc., which ordinarily affects charging current.

Although scorotron devices of this type permit the obtainment of high quality reproductions having the desired contrast value on a continuous and automatic basis, recent improvements in the electrostatographic process enabling high speed reproductions to be obtained give rise to the requirement of a faster charging process. The charging time required with current scorotron and other charging devices presents a limiting factor in the wide spread introduction and use of high speed reproduction techniques.

It is therefore the principle object of this invention to provide electrical configurations for a corona generating devicewhereby a uniform electrostatic charge may be rapidly deposited on an electrostatographic plate.

It is another object of the present invention to provide a method for accelerating the charging process of a scorotron device by applying a time varying voltage gradient to the scorotron screen.

It is still another object of the present invention to provide methods for obtaining the necessary voltage gradients for application to the scorotron screen.

These as well as other objects are accomplished by the present invention which provides corona generating apparatus comprising a back-up plate and screen wires, at least one corona discharge wire positioned therebetween, adapted to be mounted in closely spaced relation with an electrostatographic plate to uniformly apply an electrostatic charge onto said plate, a coronagenerating potential source connected to said corona discharge wire adapted to generate a corona discharge from said corona wire, and means for applying potential to said screen wires adapted to impress a voltage gradient upon said screen wires permitting high initial charging currents to flow thereby rapidly charging said plate while limiting the ultimate charge applied thereto.

In one embodiment, the corona generating apparatus contains a high voltage transient-producing circuit adapted to be connected in series with the screen wires, said circuit comprising a capacitor, means for charging said capacitor and switching means for connecting said capacitor in series with the screen wires whereby the capacitor is charged, connected to said screen wires and discharged simultaneously with the applied potential to said screen wires from a potential source to provide a transient increase in the screen potential enabling rapid uniform charging of the electrostatographic plate. 1

In an alternate embodiment, a time varying voltage gradient is obtained upon relative movement between the corona generating apparatus and the electrostatographic plate by imposing a voltage gradient upon the screen wires decreasing in the direction of rotation of the electrostatographic plate. In this manner, the plate is initially subjected to a high voltage which tapers down to the desired voltage as the plate rotates past the corona charging device. The voltage gradient is conveniently obtained by connecting each of the screen wires to taps contacting a plurality of potential dividers in series with a high voltage D. C. source, such as a battery. The leading screen wire in the direction of rotation is connected to the potential divider at the high potential end of the series of potential dividers. Each subsequent screen wire is adapted to pick off a lower voltage with the terminal screen wire being adapted to pick off the ultimately desired screen voltage from the potential divider at the low potential end of the series of potential dividers.

For a better understanding of this invention, together to the following detailed description of the invention to be read in connection with the accompanying drawing wherein:

FIG. 1 is a schematic diagram of a scorotron circuit as modified by the time varying voltage gradientproducing circuit of the present invention.

FIG. 2 is a schematic diagram of a scorotron circuit as modified by an alternate position varying voltage gradient-producing circuit of the present invention.

The circuit illustrated in FIG. 1 is applicable for use with corotron charging devices used in electrostatographic processes wherein there is no relative movement between the charging device and the electrostatographic plate during charging. In the circuit illustrated in FIG. 1, the primary 10 of a stepup transformer 12 is connected to a source of alternating current, such as a commercial outlet of volts, 60 cycle alternating current. One end of the high voltage secondary winding 14 is connected to the anode 16 of a high voltage rectifying tube 18. The other end of secondary 14 is connected through a voltage dividing resistor or potentiometer 20 to form a rectifying circuit so that pulsating direct current appears across the resistor 20. A smoothing capacitor 22 is connected across the resistor 20 to smooth out the pulsations and yield a more constant direct current. The high potential end of the potential divider 20, i.e., the end connected to the cathode 24 of the diode rectifier 18 is connected by conductor 26 to the electric connector terminal 28 of corona discharge wires 30. The low potential end of the potential divider is preferably grounded, as shown, and is also connected by conductor 32 to electrical connector terminal 33 of the electrostatographic plate 34.

A secondary winding 36 forming a secondary of transformer 29 is provided to heat the cathode 24, although other energizing means could be provided.

Terminal 38 of control electrode or screen wires 40 is connected by conductor 42 to a tap 44 on the potential divider 20 to charge electrode 40 to a predetermined potential below the voltage of the corona wires 30. It is obvious that another voltage source, such as a battery, can be used, if desired, to obtain this potential. Terminal 46 is employed to connect shield 48 to ground or a low potential.

A potential of at least about 4,000 volts between the corona electrode and the nearest conductors or electrodes is usually required in order to generate a useful corona discharge and it is preferable that the voltage be less than 10,000 volts in order to avoid sparking or excessive space charges in structures of practical dimensions. Either positive or negative charge can be deposited on the electrostatographic plate depending on the polarity of the power supplies. FIG. 1 indicates the correct polarity for positive charging. The D. C. potential can be either constant or it can be pulsating, such as that obtained by half-wave rectification of alternating current. An alternating potential can also be used since ions of both polarities are then made available in alternate half-cycles.

The voltage applied between the control electrode or screen wires 40 and the electrostatographic plate 34 is generally a constant D. C. voltage or a pulsating voltage which is synchronized with a pulsating corona voltage.

With D. C. corona, the potential of. the control electrode or screen wires can be intermediate to the potentials of the corona electrode and the conductive backing electrode of the electrostatographic plate or the same as the potential of the conductive backing. In any event, the voltage between the control electrode and the conductive backing of the electrostatographic plate is kept below the breakdown voltage of the insulating layer to be charged. With electrostatographic plates, this voltage usually will not exceed 1,000 volts.

' With D. C. corona, the polarity of charge applied to the plate will be the same as the polarity of the corona electrode regardless of the polarity of the control electrode. With A. C. corona the polarity of the charge applied to the insulating layer will be the same as that of the control electrode.

In accordance with the present invention, it has been found that the electrostatographic plate current is generally proportional to the potential of the screen wires or control electrode. Thus, the charging current can be considered as being proportional to the difference between the scorotron screen potential and the potential on the surface of the electrostatographic plate. Through the present invention, a time varying potential gradient is imparted to the electrostatographic plate by means of the scorotron screen, enabling more rapid charging of the electrostatographic plate. In the embodiment shown in FIG. 1, the time varying potential is obtained by connecting a charged capacitor 50 in series with the supply voltage to the scorotron screen. A high voltage transientproducing circuit comprises capacitor 50, switch 52 and a high voltage D. C. power source, such as battery 54. The capacitor is charged when the switch is in position a. In this position, the scorotron acts in the conventional manner. However, when extra charging speed is required, switch 52 is switched into position b in an appropriate time sequence with the application of the supply voltage to the screen. In this manner, the magnitude of the increased screen voltage is determined by the potential of battery 54 and the duration of the increased screen potential is determined by the time constant which is equal to RC. Preferably, the product of the screen current and the resistance R is small as compared to the high voltage to afford accuracy in setting the screen voltage.

In operation, for example, if it is desired to charge a sample to -300 volts, the conventional screen setting would be 300 volts. According to the present invention, however, a high voltage transient of, for example, 800 volts could be impressed upon the screen by allowing the capacitor 50 to discharge in series with the supply voltage thereby imparting a high voltage transient for the duration of the time constant. Thereafter the screen would return to its conventional 300 volts. In this manner, larger initial charging currents are permitted to flow and therefore charge the sample more rapidly.

The embodiment shown in FIG. 2 is suitable for use with automatic recyclable electrostatographic copying machines.

The automatic electrostatographic reproducing apparatus generally comprises an electrostatographic plate 100 including a photoconductive layer or light receiving surface on a conductive backing, formed in the shape of a drum, which is mounted on a shaft journaled in a frame to rotate in the direction indicated by the arrow to cause the drum surface sequentially to pass a plurality of electrostatographic processing stationsduring the reproduction cycle.

The scorotron charging device generally designated is situated above the electrostatographic plate and is adapted to deposit a uniform electrostatic charge thereon upon rotation of the plate past the charging device.

In the circuit illustrated in FIG. 2, a high voltage is applied to the corona discharge wires 112 in the same manner as described in reference to FIG. 1. The backing shield 114 and the electrostatographic plate 100 are connected to ground or a low potential. The screen wires shown representatively as 116a, l16b and 1160 are respectively connected via conductors 118, 120 and 122 to

taps

124, 126 and 128 on a plurality of potential dividers 130, 132 and 134 connected in series with a high voltage D. C. source such as battery 136. The low potential end of the series of potential dividers is preferably grounded as shown. In this manner, a multiplicity of voltages is conveniently impressed upon the screen wires. Upon relative movement between the screen wires and the electrostatographic plate, a time varying voltage gradient is generated at each point on the electrostatographic plate in the direction of rotation enabling rapid uniform charging of the plate.

In operation, if it is desired, for example, to charge a sample to 300 volts, the conventional screen setting would be 300 volts. According to this embodiment of the present invention, however, a multiplicity of voltages is imposed upon the screen wires, for example, by connecting the lead screen wire 116a in the direction of rotation of the plate 100 to the potential divider 130 at the high potential end of the series of potential dividers. The tap 124 can be set to draw 800 volts, for example, from potential divider 130. The next screen wire l16b in the direction of rotation of the plate can be connected to potential divider 132 through tap 126 set, for example, at an intermediate value of 550 volts. The last screen wire ll6c in the direction of rotation can be connected to the potential divider 134 at the low potential end of the series via tap 128 set at the desired sample voltage of -300 volts. In this manner, upon relative movement between the charging device and the rotating plate, every point on the plate surface is exposed to a time varying voltage gradient permitting larger initial charging current to flow thereby charging the sample more rapidly.

While the invention has been described with reference to the circuits disclosed herein, it is not confined to the detail set forth since it is apparent that certain electrical equivalent components may be substituted for components of the preferred circuits without departing from the scope of the invention. It is readily apparent, for example, that although power sources 54 in FIG. 1 and 136 in FIG. 2 are shown as being independent power sources, they need not be. The required potential can easily be obtained by appropriate tapping of the main power supply such as by tapping the potential divider 20 in FIG. 1. This invention is therefore intended to cover such modifications or changes as may come within the purposes of the invention as defined by the following claims.

What is claimed is:

1. Corona generating apparatus comprising a backup plate and screen wires, at least one corona discharge wire positioned therebetween, adapted to be mounted in closely spaced relation with an electrostatographic plate to uniformly apply an electrostatic charge onto said plate, a corona-generating potential source means connected to said corona discharge wire adapted to generate a corona discharge from said corona wire, and a second potentialsource means coupled to said screen wires for applying to said screen wires a potential having a waveform which continuously varies in amplitude for causing an initially high charging current to flow to said plate and to vary in amplitude from a high initial amplitude to a lower amplitude thereby rapidly charging said plate while limiting the ultimate charge applied thereto.

2. Corona generating apparatus comprising a backup plate and screen wires, with at least one corona discharge wire positioned therebetween, adapted to be mounted in closely spaced relation to an electrostatographic plate to uniformly apply an electrostatic charge onto said plate; a corona-generating potential source connected to said corona discharge wire adapted to generate a corona discharge from said corona wire, a second potential source connected to said screen wires adapted to limit the potential applied to said electrographic plate by said corona discharge, a high potential transient-producing circuit adapted to be connected in series with the screen wires, said transient-producing circuit comprising a capacitor, means for charging said capacitor and switching means for connecting said capacitor in series with the screen wires whereby the capacitor is charged, connected to said screen wires and discharged in sequence with the application of potential to said screen wires to provide a transient increase in the screen potential enabling rapid charging of the electrostatographic plate.

3. Circuitry for generating a corona discharge adjacent to an electrostatographic plate for rapidly and uniformly applying an electrostatic charge onto said plate including a source of direct current having a potential divider in said source and a common return,

at least one corona discharge wire positioned in close proximity to said electrostatographic plate adapted to apply an electrostatic charge onto said plate,

means coupling said corona discharge wire to said direct current at a relatively high potential,

at least one screen wire positioned between said corona discharge wire and said electrostatographic plate and said screen wire coupled to said direct current, at a potential reduced in amplitude with respect to said relatively high potential,

a back-up plate positioned adjacent said corona discharge wire on the opposite side thereof from said screen wire, said back-up plate coupled to said common return,

means coupling said electrostatographic plate to said common return,

means for applying a potential which varies continuously in amplitude, with respect to said common return during a charging interval, to said screen wire in series with said reduced potential for regulating the charge being applied to said elec- 5ili $1fi ?$%m% nai?si3l32? ifl%%i% tial, such that a corona discharge having a high initial value, and a lower terminating value is applied to the electrostatographic plate, thereby enabling the rapid and uniform charging of said plate to said lower terminating value.

4. Circuitry for generating a corona discharge asin claim 3 and in which said means for applying a varying potential includes an R-C charging circuit and a multi-position switching means arranged for coupling the R-C charging circuit to a charging potential, in one switch position and in another switch position for coupling a capacitor of the R-C charging circuit to said screen wire for discharging said capacitor through said screen wire circuit thereby varying the potential applied to said screen wire during discharge of said capacitor.

5. A fast charging corona generating apparatus for charging an electrostatographic plate comprising:

a discharge assembly having a back-up plate, a screen grid, and at least one corona discharge wire positioned between said plate and grid;

said assembly adapted to be mounted in closely spaced relation with respect to an electrostatographic plate for uniformly establishing an electro static charge of potential V on said plate;

a source of corona generating potential coupled to said corona discharge wire for establishing a corona discharge from said wire; and,

circuit means coupled to said screen grid for applying to said screen grid a potential which varies continuously in amplitude during at least a portion of a charging interval from an amplitude V which causes the flow of an initially high charging current to said plate, to an amplitude V 6. The apparatus of claim 5 wherein said circuit means is adapted for applying to said screen grid a D. C. potential of amplitude V and for applying in series therewith a potential which varies continuously between the amplitudes V and V 7. The apparatus of claim 5 wherein said circuit means provides a screen grid potential which varies exponentially between the amplitudes V and V,.

8. The apparatus of claim 7 wherein said circuit means for providing an exponentially varying amplitude includes an RC. discharge circuit.

9. The apparatus of claim 8 wherein switching means are provided for alternatively coupling and decoupling said R.C. circuit in series with the potential V applied to said screen grid.

Claims

1. Corona generating apparatus comprising a back-up plate and screen wires, at least one corona discharge wire positioned therebetween, adapted to be mounted in closely spaced relation with an electrostatographic plate to uniformly apply an electrostatic charge onto said plate, a corona-generating potential source means connected to said corona discharge wire adapted to generate a corona discharge from said corona wire, and a second potential source means coupled to said screen wires for applying to said screen wires a potential having a waveform which continuously varies in amplitude for causing an initially high charging current to flow to said plate and to vary in amplitude from a high initial amplitude to a lower amplitude thereby rapidly charging said plate while limiting the ultimate charge applied thereto.

2. Corona generating apparatus comprising a back-up plate and screen wires, with at least one corona discharge wire positioned therebetween, adapted to be mounted in closely spaced relation to an electrostatographic plate to uniformly apply an electrostatic charge onto said plate; a corona-generating potential source connected to said corona discharge wire adapted to generate a corona discharge from said corona wire, a second potential source connected to said screen wires adapted to limit the potential applied to said electrographic plate by said corona discharge, a high potential transient-producing circuit adapted to be connected in series with the screen wires, said transient-producing circuit comprising a capacitor, means for charging said capacitor and switching means for connecting said capacitor in series with the screen wires whereby the capacitor is charged, connected to said screen wires and discharged in sequence with the application of potential to said screen wires to provide a transient increase in the screen potential enabling rapid charging of the electrostatographic plate.

3. Circuitry for generating a corona discharge adjacent to an electrostatographic plate for rapidly and uniformly applying an electrostatic charge onto said plate including a source of direct current having a potential divider in said source and a common return, at least one corona discharge wire positioned in close proximity to said electrostatographic plate adapted to apply an eLectrostatic charge onto said plate, means coupling said corona discharge wire to said direct current at a relatively high potential, at least one screen wire positioned between said corona discharge wire and said electrostatographic plate and said screen wire coupled to said direct current, at a potential reduced in amplitude with respect to said relatively high potential, a back-up plate positioned adjacent said corona discharge wire on the opposite side thereof from said screen wire, said back-up plate coupled to said common return, means coupling said electrostatographic plate to said common return, means for applying a potential which varies continuously in amplitude, with respect to said common return during a charging interval, to said screen wire in series with said reduced potential for regulating the charge being applied to said electrostatographic plate by said corona discharge wire to a value proportional to said varying potential, such that a corona discharge having a high initial value, and a lower terminating value is applied to the electrostatographic plate, thereby enabling the rapid and uniform charging of said plate to said lower terminating value.

4. Circuitry for generating a corona discharge as in claim 3 and in which said means for applying a varying potential includes an R-C charging circuit and a multi-position switching means arranged for coupling the R-C charging circuit to a charging potential, in one switch position and in another switch position for coupling a capacitor of the R-C charging circuit to said screen wire for discharging said capacitor through said screen wire circuit thereby varying the potential applied to said screen wire during discharge of said capacitor.

5. A fast charging corona generating apparatus for charging an electrostatographic plate comprising: a discharge assembly having a back-up plate, a screen grid, and at least one corona discharge wire positioned between said plate and grid; said assembly adapted to be mounted in closely spaced relation with respect to an electrostatographic plate for uniformly establishing an electrostatic charge of potential V1 on said plate; a source of corona generating potential coupled to said corona discharge wire for establishing a corona discharge from said wire; and, circuit means coupled to said screen grid for applying to said screen grid a potential which varies continuously in amplitude during at least a portion of a charging interval from an amplitude V2, which causes the flow of an initially high charging current to said plate, to an amplitude V1.

6. The apparatus of claim 5 wherein said circuit means is adapted for applying to said screen grid a D. C. potential of amplitude V1 and for applying in series therewith a potential which varies continuously between the amplitudes V2 and V1.

7. The apparatus of claim 5 wherein said circuit means provides a screen grid potential which varies exponentially between the amplitudes V2 and V1.

8. The apparatus of claim 7 wherein said circuit means for providing an exponentially varying amplitude includes an R.C. discharge circuit.

9. The apparatus of claim 8 wherein switching means are provided for alternatively coupling and decoupling said R.C. circuit in series with the potential V1 applied to said screen grid.