US3674532A - Control for bias of magnetic brush and method - Google Patents

Abstract

IMPROVED DEVELOPEMENT OF ELECTROSTATIC CHARGE PATTERNS IS ACHIEVED BY THE USE OF THE DEVELOPMENT CONTROL DEVICE DISCLOSED WHICH ELECTRICALLY BIASES A MAGNETIC BRUSH DURING DEVELOPMENT TO A VOLTAGE INITIALLY SENSED BY THE BRUSH.

Description

July 4, 1972 MORSE 3,674,532

CONTROL FOR BIAS OF MAGNETIC BRUSH AND METHOD Filed July 25; 1970 THEODORE H. MORSE INVENTOR.

A TTOR/VEY United States Patent ()fice 3,674,532 Patented July 4, 1972 U.S. Cl. 117-17.5 6 Claims ABSTRACT OF THE DISCLOSURE Improved development of electrostatic charge patterns is achieved by the use of the development control device disclosed which electrically biases a magnetic brush during development to a voltage initially sensed by the brush.

This invention relates to the development of electrostatic charge patterns and to a novel method and apparatus for development of electrostatic charge patterns.

Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 and many others. Generally, such processes employ an electrostatically charged photoconductive insulating element which responds to imagewise exposure to electromagnetic radiation by selectively dissipating charges to form an electrostatic charge pattern. The electrostatic charge pattern is then rendered visible by contacting the charged surface of the photoconductive element with suitable developer marking particles.

One development method involves cascading xerographic developer across the image-bearing surface as described in U.S. Pat. No. 2,618,551. This cascade development system is adequate for ordinary line copies; however, it has limited application where solid area development is required. For solid area development, a magnetic brush development system has greater utility.

In a magnetic brush development system, a developer mix typically comprising ferromagnetic iron carrier granules together with colored resin toner particles is applied to an electrostatic charge pattern by means of an apparatus of the type described in U.S. Pat. No. 3,003,462. In such an apparatus, the iron particles are held by a magnet in a bristle-like formation resembling a brush with the toner particles adhering to the iron by electrostatic attraction. The bristles of iron particles are electrically conductive and contribute to the transfer of toner to the charge image-bearing surface.

A magnetic brush of this type is typically electrically grounded, that is, electrically connected to the machine to which it is mounted. This generally gives very satisfactory development of charge images in which the potential of the background or exposed areas is reduced to zero. In the development of charge images in which the background potential has not been reduced to zero, considerable density appears in background areas upon development.

This developed density is undesirable and has been overcome in the past by electrically biasing the magnetic brush at some fixed potential above ground. Such a system is generally satisfactory where the background density of the document to be reproduced is more or less constant. In many documents, however, the background may be of variable density, or even of several colors. Documents of this type do not reproduce correctly with uniform freedom of background density using a system of fixed bias. Consequently, there is a need in the art for a magnetic brush development system which overcomes these disadvantages, which is adapted to give uniformly good reproduction of documents of many types, and which gives freedom from background toning under a wide variety of conditions.

Copending patent application of A. P. Turner and H. A. Miller, Ser. No. 57,654, filed on July 23, 1970, entitled Autobiasing Development Electrode for Xerography discloses a method and apparatus which overcomes the aforementioned disadvantages. In that application, uniform development was accomplished by connecting a series resistance between the magnetic brush and a reference voltage, such as ground. In practice, this system is satisfactory except where the contact area between the brush and the surface of the print being developed is unavoidably re-' duced, such as at the trailing edge of the print. Since the average sensed potential is lower in these areas, the brush is not sufiiciently biased and a band of high density is developed in the background. This situation also arises when a dual magnetic brush is used for development, such as, for example, a brush of the type disclosed in copending patent application of I. B. Ville, Ser. No. 653,'934,-filed July 17, 1967, entitled Electrophotographic Developing Method and Apparatus. When both brushes are in contact with the surface of the print being developed, a given potential is induced. After the trailing edge of the print leaves the first brush, the contact area is cut in half, and the induced potential is reduced correspondingly.

It is an object of this invention to provide a development method and apparatus which will successfully reproduce documents having a wide range of image and background densities.

It is another object of this invention to provide a method and apparatus which will produce copies having little or no background density from documents having a wide variety of background colors.

It is still another object of this invention to provide a magnetic brush development method and apparatus which controls the background density of a xerographic reproduction in accordance with the surface potential of the electrostatic charge pattern sensed during development.

It is a further object of this invention to provide a magnetic brush development method and apparatus which substantially reduces over-response to variations in sensed surface potential, thereby permitting more nearly uniform development bias to be applied in accordance with the overall image potential.

The present invention provides, in a xerographic development process which utilizes a conductive development electrode and facing and closely spaced apart therefrom the charge image-bearing surface of an electrographic element, between which particles of a marking material are caused to move, which electrode derives its bias potential from a path having a predetermined resistance between itself and a voltage reference source, which may be ground, a method of preventing downward fluctuations away from an arbitrary voltage of the bias potential so derived from exceeding a predtermined magnitude. The arbitrary voltage most often used is derived from the surface potential of the element being developed. An apparatus is also provided which senses changes in the surface potential of the element, compares the potential so sensed with a stored potential, and supplies an additive corrective potential when needed.

In a preferred embodiment of this invention, the carrier for the marking particles and the development electrode are the same member, as in the well-known magnetic brush development system. Other types of electrode development systems are equally suited to the method of this invention, as will become apparent.

Reference is now made to the drawings, in which:

FIG. 1 is a partially schematic cross-sectional view of a development station suitable for use with the method and apparatus of this invention.

FIG. 2 schematically illustrates circuitry for preventing the magnitude of bias potential from dropping below a predetermined level during the development of an electrostatic image at the station shown in FIG. 1.

FIG. 3 illustrates schematically further switching whereby the circuitry in FIG. 2 is actuated.

Referring initially to FIG. 1, there is shown in cross section a magnetic brush assembly, generally designated 2, comprising a reservoir 4 closed at its ends with covers (not shown) and containing developer 6. Shaft 3 is rotatably secured in the end covers of reservoir 4 and a cyllindrical magnet 8, magnetized across a diameter thereof is concentrically mounted on shaft 3. The rotatable mounting of shaft 3 in the end covers permits orientation of the magnet for optimum brush formation during the development of electrostatic charge patterns. Also journalled on the shaft for rotation thereon are two circular end caps (not shown), between which is concentrically secured a rotatable cylindrical non-ferromagnetic shell 10. The surface of shell is preferably grooved parallel with its axis of rotation to facilitate carrying developer along on its surface as it rotates. The end caps and shell 10 completely enclose magnet 8 to prevent contact between the magnet and developer 8. The reservoir 4 is substantially filled with dry magnetic developer 6 suitable for use in magnetic brush development. Shaft 3 is so mounted in the end covers that approximately of the shell 10 extends above the average level of developer 6 in reservoir 4. Electrically connected to shaft 3 and reservoir 4 are wires 12 and 14,

respectively, which are brought to a common tie point at the point of electrical connection A to the circuit FIG. 2.

In operation, a member to be developed, generally designated 16, is moved in the direction of arrow 18. Member 16 may comprise a conductive support 24 bearing an insulating layer 20 on one surface. The surface of layer 20 carries the electrostatic charge pattern 22 which is to be developed. Electrically insulated layer 20 comprises a material not sensitive to activating radiation, such as an insulating polymeric material, which may additionally contain a substance which is normally insulating but which becomes conductive in the presence of activating radiation, such as a photoconductive material. Shell 10 is customarily caused to rotate in the direction of arrow 26, counter to the direction of translation of member 16. As it rotates, it carries along on its surface bristles of the developer 6 which are formed along the lines of magnetic flux connecting the N and S poles of magnet 8. As the bristles wipe across the charge pattern 22 on the surface of layer 20, marking particles (toner) are caused to be removed from the carrier particles of the developer in typical fashion and are deposited in accordance with the charge pattern. During development, an additional potential appears between the surface of shell 10 and the surface of layer 20. It is believed that one component of this potential is an induced voltage. One possible explanation of the cause of this voltage is now given. When the conductive magnetic brush is brought in close proximity to the charge image on the surface layer 20, an induced charge appears on the surface of the magnetic brush shell 10. Since there is a resistive element (40, 42) between the brush shell 10 and the reference voltage or ground, charges flow into the brush from the source of reference voltage, producing a voltage difference between the brush shell 10 and the reference voltage. The greater the charge density of the image on layer 20, the greater the induced charge, thus producing a greater current flow through resistive elements 40, 42. correspondingly, a greater bias voltage between the magnetic brush and the reference source is also produced. If the magnitude of the resistive element is increased, the bias voltage due to the current flow through this resistance will also increase. Conversely, if the resistance is decreased, a lower bias voltage will result. Other contributions to the bias potential may result from triboelectric effects and from the removal of toner from the brush to the image surface. Thus, when the conductive magnetic brush is brought into close proximity with the charge image on the surface of layer 20, toner is transferred from the brush to the surface. There then remains on the brush an excess of charge having a polarity opposite to that of the lost toner particles. Since there is a resistive element 40, 42 between the magnetic brush and the reference voltage or ground, a current flows between the brush and the reference source, and a voltage appears across the resistive element as a bias voltage. The greater the rate at which toner is removed from the brush, the greater is the voltage of the brush, and therefore the greater is the bias voltage. As the value of the resistance is increased, the charge produced on the brush is prevented from being neutralized as rapidly, and therefore the brush assumes a higher voltage. Conversely, as the value of the resistance is decreased, the charge produced on the brush is able to be neutralized more rapidly, and the brush therefore assumes a lower voltage.

Unlike bias from an external D.C. source, the potential spontaneously generated on the developer brush varies directly with the potential of the image area in contact. Thus, in areas representing maximum, intermediate, and minimum densities in the original, electrostatic-image potentials of 600, 300 and 75 volts, respectievly, might result in potentials of the order of 300,130 and 30* volts, respectively, in the corresponding local surface areas of the developer. Since a considerable area of the electrostatic image is in contact with the developer at any one time, the instantaneous average potential, determined, for example, by reading the potential drop across the resistors will represent a weighted average of all the separate potentials on the developer surface contacted at that moment. For instance, in a typical xerographic process in which an electrostatic image is developed by traversing a magnetic brush as hereinbefore described, the above range to 300 to 30 volts might results in measurements of potential differences across the external resistor in the range of from, 200 to volts during the developing period.

An advantage gained from the use of the method of the present invention for obtaining a bias potential is that the exposure latitude is increased and image-free areas remain free of unwanted density while reproduction of fine-lined etail is maintained.

Since the effective bias is not constant over the entire image area, but depends on the local instantaneous value of surface potential, the foregoing advantages can be realized.

Effective bias, as used herein, refers to the voltage difference or difference in potential between the brush and the surface of the element being developed. This is to be distinguished from the customary method of specifying bias as the voltage between the developing electrode or brush, and ground. Clearly, it is the effective bias which determines whether background density will be developed in a given area, or whether there will be produced a high enough voltage to cause sparking between the brush and the surface of the element.

A useable portion of the induced voltage appears at point A and is thus applied across resistors 40 and 42 in series. Proper choice of the value of resistors 40 and 32 enables preselection of a range of development conditions. When the combined resistance is relatively low, e.g., below about 10 ohms, the self-bias potential produced is relatively low and varies only slightly with variations in the surface potential of the local area being developed. Relatively narrow exposure latitude results together with good solid-area fill in of those areas having substantially uniform potential. When the resistance is relatively high, e.g., about 10 ohms, the self-bias potential produced is relatively high and highly dependent on the surface potentials of the local areas being developed. This reduces the spread of effective bias potential values, which causes a lowering of overall contrast and increase in exposure latitude. Fillin of extended solid areas is then reduced, and development is largely of the fringing type. Values of resistance intermediate these extreme conditions produce a continuous gradation and resulting compromise between solid-area fill-in with accompanying tendency toward producing background density should the exposure not be exactly correct and extreme fringing development with wide exposure latitude, freedom from background, and poor solid-area fill-in.

' Reference is now made to FIG. 2, which schematically illustrates a circuit to be used in conjunction with a magnetic brush for preventing the value of bias potential from becoming less in magnitude than a predetermined value. The circuit is connected to the magnetic brush-apparatus at point A.

The circuit of FIG. 2 functions in the hereinafter described manner. A predetermined fraction of an initial potential produced on the brush by contact with the charge-bearing surface is impressed on a voltage storage means. At a predetermined time, the circuit containing the voltage storage means is disconnected from the brush and the stored voltage is amplified through a high input impedance amplifier chain by the amount necessary to make the amplified voltage equal to the initial voltage. The amplified voltage is compared with the instantaneous voltage.

It should be noted that the device is not a closed loop, that is, it does not use feedback to achieve stabilization, since the input is disconnected from the brush when the circuit is in operation. This is necessary because the feedback necessary if a closed loop were used would be positive, as can be readily seen. If it is assumed that a positive transient with respect to the reference voltage, or ground, appears on the brush, a positive transient would be fed to the amplifier, amplified, and applied to the brush. If the gain of the amplifier chain is strictly maintained at unity or less, no problem should occur. However, it is very difficult to keep an amplifier employing positive feedback stable for more than a few seconds or minutes at the most if the gain of the feedback loop is unity or greater than unity. Consequently, an open loop amplifier is preferred.

The operation of the circuit will now be discussed in detail. The values of resistors 40 and 32 are selected from values which will provide a sample voltage when arranged to form a voltage divider between the magnetic brush and the voltage reference point 44, to sample a small fixed percentage of the voltage generated by the magnetic brush as it contacts charge pattern 22. When the polarity of the charge pattern is negative, the reference is ground, as will be seen hereinafter. As an example, if the percentage selected were one percent, the value of resistor 42 would be 1/100 of the value of resistor 40. This voltage is impressed on capacitor 48 through the normally closed contacts of section B of relay 46 which is shown in the nonactuated position. Relay 46 permits capacitor 48 to be electrically connectable to the voltage sampling junction of resistors 40 and 42. As can be seen by looking at the contacts of section C of relay 46, there is no input to amplifier 72 under these conditions, so that amplifier gives no effective response to the input signal.

Reference is now made to FIG. 3 in order to illustrate the operation of relay 46. A source of voltage 32 is connected to actuate relay 46 when switch 30 is closed. Switch 30 (FIG. 1) is conveniently a spring-return S.P.S.T. switch, normally open.

Reference is again made to FIG. 2, taken together with FIG. 1. Image support member 16 bearing a pattern of electrostatic charge 22 on insulating surface is caused to move in the direction of arrow 18 over the surface of rotating shell 10 of the magnetic brush. In so doing, member 16 contacts the thin layer of developer 6 borne on the magnetic brush surface, causing deposition of marking particles in accordance with the charge pattern. During development, a potential is produced between point A and reference level 44. At a predetermined time, such as when leading edge 28 of member 16 actuates switch 30, relay 46 is energized. The moveable relay blades are then caused to take the positions shown in dashed lines. Section B of relay 46 disconnects the input of amplifier 50 from the voltage divider formed by resistors 40 and 42, leaving it connected only to capacitor 48. Section A of relay 46 connects resistor 41 between the magnetic brush and reference level 44, to prevent unwanted build-up of voltage on the brush which would otherwise result from contact with an area of unusually high potential on surface 20. Section C of relay 46 closes the amplifier chain and permits the constant po tential stored in capacitor 48 to be impressed on the brush after being suitably amplified.

Amplifiers 50, 64 and 72 are what are known in the analog computer art as operational amplifiers. Such amplifiers are characterised in that they can perform addition and subtraction of continuous functions. Typically, they have two inputs and may have one or two outputs depending on the choice of phase of the output. The output obtained is the sum of the inputs, or the difference, depending on how the amplifier is connected into its associated circuit. Operational amplifiers are normally prepackaged, requiring only a power source to supply operating potentials and external resistors to control voltage gain and determine the operation to be performed. Gain is typically controlled by reducing the effective input voltage by feeding back a certain fraction of the output voltage out of phase with the input. Customarily, the gain is the ratio of the feedback resistance, such as 68 for amplifier 64, to the input resistance, or 66 for the same amplifier. If the resistances are the same, the gain is unity.

Initially, it is assumed that a negative polarity charge 22 is to be sensed on the surface of insulating layer 20. The four sections of switch 52, a polarity selecting switch, are then in the positions shown in solid lines, the positive output of amplifier 72 is thereby grounded, and amplifier 64 is switched into the circuit. When relay 46 is actuated as just described, capacitor 48 is disconnected from its input source by the opening of relay contacts 46B. The input of amplifier 50 is then held at a constant potential equal to 1% of the potential sensed by the brush. Amplifier 50 must have a high input impedance to prevent it from discharging capacitor 48 excessively during the interval of operation. In general, its input impedance must be in excess of about 10 ohms in most situations. The output potential is adjusted by proper selection of the values of resistances 58 and 62 and by proper adjustment of variable resistor 60, as well as by appropriate choice of values of +LV and LV. The supply voltages for these latter two may conveniently be made between and --10 volts to and -50 volts, with and 15 to 25 volts being preferred in the specific circuit described. The voltage gain of amplifier 50 is controlled by adjustment of feedback resistance 54. Amplifier 50 is a noninverting amplifier and thus does not change the phase of the signal.

The output level of amplifier 50 is then fed to the input of phase-inverting amplifier 64, which is adjusted to have a gain of unity. Amplifier 64 need not have a high input impedance, as it is not located in the circuit in such a place as to discharge any voltage storage means. The output of amplifier 64 is then fed through switch section 52B and variable resistor 70 to the input of amplifier 72. Amplifier 72 is chosen to have a voltage gain of up to approximately 200, in order that an output of 200 volts may be obtained with an input of a volt or two. The particular amplifier shown in the examples has an output capability of up to 2000 volts, so resistor 70 is included to attenuate the input signal sufiiciently to limit the output to about 500 volts. Amplifier 72 has two outputs, one of which is positive-going and the other of which is negative-going. Either output can be grounded and the other used as the signal output, depending on the output polarity desired. For negative output, the positive out is grounded. The negative-going output then appears at the cathode of diode 74. When the magnitude of the potential at point A drops, that is, it becomes less negative, diode 74 conducts, permitting amplifier 72 to restore the proper bias. This happens when, for example, the trailing edge of the element 16 passes over the brush so that part of the area sensed carries no charge, and the average potential sensed is lower than required bias. The amplifier holds the bias at the correct level until the trailing edge 34 of member 20 rides past switch 30, releasing it and with it relay 46. By this time, element 16 is well past the surface of the developing brush 10, so there is no further need to hold the bias automatically.

In the event a positive initial polarity is to be sensed on the surface of insulating layer 20, switch 52 is thrown to the other position, that is, the position indicated in dashed lines. Amplifier 64 is thus removed from the circuit, and a different gain-adjusting resistor 56 for amplifier 50 switched on. This is done because a different effective gain may be needed for this operation. The negative output of amplifier 62 is grounded, and the positive output fed to the anode of diode 76. A drop in positive potential sensed by the surface of the brush reduces the potential of the cathode of diode 76, causing it to conduct. Amplifier 72 then restores the correct bias by the same mechanism described in connection with the use of a negative surface potential.

Each of diodes 74 and 76 must be capable of withstanding a high peak inverse voltage (p.i.v.) that is, with the cathode positive with respect to the anode. The p.i.v. must be at least equal to the maximum surface potential expected to be encountered, since, before relay 46 is actuated, the brush is at a high potential whereas the amplifier output is essentially at ground potential. It must also have a high back resistance preferably in excess of ohms. Silicon diodes generally meet both requirements adequately.

The operation of the invention will now be described by reference to certain preferred embodiments thereof.

EXAMPLE 1 Negative surface potential Tabulated below are the values selected for proper operation of the circuit with the particular amplifiers chosen.

Resistors:

40l0 ohms 41-10 ohms 4210 ohms 54-50,000 ohm potentiometer 56-50,000 ohm potentiometer 5847,000 ohms 6010,000 ohm potentiometer 6247,000 ohms 66-l0,000 ohms 68l0,000 ohms 7050,000 ohm variable Capacitor: 48-0.0068 microfarad Diodes:

742-1N629 diodes in series 76Same as 74 Amplifiers:

50--Operational amplifier Model 141A, made by Analog Devices, Inc., Cambridge, Mass. 64-Model 111, same manufacturer 72High voltage operational amplifier (Kepco Model OPS-2000, made by Kepco, Inc., Flushing, NY.)

Power supplies:

The values selected for +LV and LV are respectively and volts Other operating potentials as specified by manufacturers Gain control resistance 70 is initially set so that the output potential of amplifier 72 is 500 volts with a 2-volt input. Gain control resistance 54 is then adjusted to give a potential of 500 volts at the output of amplifier 72 when the input potential of amplifier 50 is 5 volts, that is, the total gain of the amplifier chain exactly compensates for the attenuation of the sensed potential produced by the voltage divider comprising resistors 40 and 42. When an insulating surface bearing a pattern of negative charge corresponding to that produced by exposure to a photographic positive is developed with the brush, the circuit is found to hold the bias potential constant over the entire interval of development. This is true for a surface potential of any value in the interval between 150 volts and 600 volts.

EXAMPLE 2 Positive surface potential The components used are identical to those used in sensing a negative surface potential, except, as previously noted, amplifier 64 and its associated resistances are switched out of the circuit. The apparatus is used to develop a pattern of positive electrostatic charge corresponding to that produced by exposure to a photographic negative, that is, one in which the background areas are charged instead of discharged. It is found that, using the same gain as is used in connection with Example 1, the bias potential generated is only about one-half of that required for optimum development. To compensate for this, gain control resistance 56 is adjusted to give an overall gain of 200 or double that required in Example 1. That means that the output of amplifier 72 is 500 volts when the input to amplifier 50 is 2.5 volts instead of 5 volts. The circuit is found to hold the bias potential constant for sensed potentials in a range of from about volts to about 500 volts or higher.

From the foregoing, it will be apparent that the method of bias of this invention is not limited to use with magnetic brush development. It is equally Well suited for use with any method in which there is provided an electrode spaced in close proximity to a charge-bearing surface. Equally contemplated, therefore are such methods of development as liquid development, aerosol development, powder cloud development, cascade development, development with fur brushes including metallized fur brushes, and the like. For example, a support having attached thereto a plurality of non-metallic filaments such as natural or synthetic fur materials which in turn may optionally bear a thin, adherent layer of metal may be used to form a development brush. The method of manufacture of such materials is disclosed in copending Miller application U.S. No. 9,457, filed Feb. 6', 1970, entitled Metallized Fur Materials. Their use in making development brushes is disclosed in copending Ville application U.S. Ser. No. 9,224, filed Feb. 6, 1970, entitled Development Process and Apparatus. A preferred brush arrangement utilizes two such brushes in tandem, one brush having low electrical conductivity and the other high conductivity. This arrangement is more fully disclosed in copending Miller application U.S. Ser. No. 9,225, filed Feb. 6, 1970, entitled Development Apparatus and Process. A particularly preferred arrangement utilizes a double magnetic brush, such as for example, one of the type disclosed in copending Ville U.S. application Ser. No. 653,- 934, filed July 17, 1967, entitled Electrophotographic Developing Method and Apparatus.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

I claim:

1. In an electrographic reproduction process wherein an electrostatic charge pattern on a surface of a photoconductive insulating layer carried by an electrically grounded conductive support is developed by moving such surface relative to and in contact with a toner-carrying, electrically conductive, development brush which is grounded through a resistive path so that the charge produced on the brush during development of the charge pattern acts as the instantaneous bias potential for the brush, the improvement comprising a method of stabilizing the bias potential on the brush during development of the charge pattern, said method comprising the steps of:

storing an electrical signal which is proportional to the instantaneous bias potential after a portion of the charge pattern has been developed;

applying a bias potential on the brush which is proportional to the stored electrical signal to maintain the bias potential on the brush above a minimum level during development of the remaining portion of the charge pattern; and

reducing the resistance of said resistive path to such a level as to prevent any substantial increase in the brush potential above that potential applied to the brush by said applying means during the development of the remaining portion of the charge pattern.

2. In an electrographic apparatus comprising means for developing an electrostatic charge pattern formed on a dielectric surface, such developing means including a development electrode and means for applying toner to such surface while the development electrode is in close proximity to such surface, the development electrode being connected to ground potential through a resistive path so that the charge produced on the electrode during the development of the charge pattern acts as the instantaneous bias potential for the development electrode, the improvement comprising means for stabilizing the bias potential on the development electrode after a predetermined portion of the charge pattern has been developed, said stabilizing means comprising:

means operatively connected to the resistive path for storing a charge proportional to the instantaneous charge produced on the development electrode during development of the charge pattern;

means for disconnecting said storing means from said path after a predetermined portion of the charge pattern has been developed;

means for applying a bias potential to the development electrode which is proportional to the level of charge stored by said storing means immediately prior to being disconnected from said resistive path, whereby the bias potential on the electrode is maintained above a minimum level during development of the remaining portion of the charge pattern; and

means for reducing the resistance of said resistive path upon disconnecting said storing means from said path to a level such as to prevent any substantial increase in the electrode potential above that potential applied to the development electrode by said bias potential-applying means during the development of the remaining portion of the charge pattern.

3. The invention according to claim 2 wherein said storing means comprises a capacitor.

4. The invention according to claim 2 wherein said development electrode comprises an electrically conductive electrographic development brush.

5. The invention according to claim 4 wherein said brush comprises a magnet.

6. The invention according to claim 4 wherein said brush comprises a metallized fur.

References Cited UNITED STATES PATENTS 2,956,487 10/1960 Giaimo 1l717.5 X 3,554,161 1/1971 Blanchette ll88 3,452,185 6/1969 Hanson 32319 X 3,399,338 8/1968 Burgert et a1 32322 X 3,509,448 4/1970 Bland 32322 X 3,037,478 6/1962 Lace l18-637 3,599,605 8/1971 RolSton et a1. ll717.5 3,611,982 10/1971 COriale 1184 WILLIAM D. MA-RTIN, Primary Examiner M. SOFOCLEOUS, Assistant Examiner US. Cl. X.R.

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