U.S. patent number 3,909,258 [Application Number 05/430,044] was granted by the patent office on 1975-09-30 for electrographic development process.
This patent grant is currently assigned to Minnesota Mining and Manufacturing Company. Invention is credited to Arthur R. Kotz.
United States Patent |
3,909,258 |
Kotz |
September 30, 1975 |
Electrographic development process
Abstract
An electrographic process involving the development of
electrical potential patterns on a recording medium surface, for
example electrostatic charge patterns on a photoconductive element,
with electronically conductive, magnetically attractable toner
material utilizing electrical forces generated by the electrical
potential in the image areas of the recording medium to overcome a
threshold magnetic counterforce exerted on the toner material by
means of an electrical potential biased toner material applicator.
The electrical potential pattern may be supplemented by a
corresponding electronic conductivity pattern on the recording
medium surface.
Inventors: |
Kotz; Arthur R. (White Bear
Lake, MN) |
Assignee: |
Minnesota Mining and Manufacturing
Company (St. Paul, MN)
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Family
ID: |
26928274 |
Appl.
No.: |
05/430,044 |
Filed: |
January 2, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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234778 |
Mar 15, 1972 |
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Current U.S.
Class: |
430/122.8;
430/31; 430/101; 118/638; 430/97; 430/103; 430/111.41; 347/112 |
Current CPC
Class: |
G03G
13/09 (20130101); B82Y 15/00 (20130101); G03G
15/0914 (20130101) |
Current International
Class: |
G03G
15/09 (20060101); G03G 13/06 (20060101); G03G
13/09 (20060101); G03g 013/08 (); G03g
013/22 () |
Field of
Search: |
;96/1R,1SD ;117/17.5
;118/637 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Schaffert, "Electrophotography," 1965, Focal Press, pp. 30, 31,
362-366..
|
Primary Examiner: Martin, Jr.; Roland E.
Attorney, Agent or Firm: Alexander, Sell, Steldt &
Delahunt
Parent Case Text
This is a continuation of application Ser. No. 234,778 filed Mar.
15, 1972.
Claims
What is claimed is:
1. A process for applying toner material selectively to
predetermined areas of one surface of a layer of material
comprising:
1. providing one surface of a layer of material with areas thereof
having an electrical potential with respect to the other surface of
said layer in a range defining image areas and other areas thereof
having an electrical potential with respect to the other surface of
said layer in a range defining non-image areas, said areas defining
an electrical potential pattern corresponding to the pattern to be
produced,
2. providing a cylindrical electrically conductive support
electrically connected to said other surface with said support
presenting a uniform quantity of one-component magnetically
attractable, electronically conductive toner material bound to said
support by a magnetic force of attraction which is uniform along
the axial length of said cylindrical support,
3. arranging said electrically conductive support adjacent to and
in spaced relation to said one surface at a uniform distance
therefrom of at least twice the major dimension of the largest of
said toner material whereby said toner material provides a
electronically conductive path between said support and said one
surface,
4. establishing said support at an electrical potential of a
magnitude and polarity such that the difference in electrical
potential between said support and said one surface, when the toner
is presented in accordance with steps 2 and 3, induces a transient
electrical transfer force on said toner material in said
electronically conductive path, which for a period of time is
greater than and opposed to said magnetic force of attraction in
said image areas and less than said magnetic force of attraction in
said non-image areas, and
5. progressively presenting said layer to said toner by providing
unidirectional relative movement between said layer and said
support with the axis of said support normal to the direction of
the relative movement, said relative movement being at a rate such
that said electronically conductive path
a. is maintained for a period sufficient to allow said transient
electrical transfer force which is greater than and opposed to said
magnetic force of attraction in said image areas to be induced,
and
b. is discontinued while such level of said transient electrical
force exists whereby said toner material is selectively deposited
on said image areas of said surface.
2. The process of claim 1 wherein said image and non-image areas
are electrically insulating while said electronically conductive
path is present.
3. The process of claim 1 wherein at least said image areas are
electrically insulating while said electronically conductive path
is present.
4. The process of claim 1 wherein said image areas are electrically
insulating and said non-image areas are electrically conductive
while said electronically conductive path is present.
5. The process of claim 1 wherein there is a conductivity pattern
corresponding to said electrical potential pattern while said
electronically conductive path is present, said conductivity
pattern being defined by relatively conductive areas in the
non-image areas and relatively insulating areas in the image
areas.
6. The process of claim 1 wherein said electrical potential pattern
is provided by electrostatic charges.
7. The process of claim 1 wherein said one surface comprises
photoconductive zinc oxide disposed in an insulating binder.
8. The process of claim 1 wherein said one surface comprises
photoconductive selenium.
9. The process of claim 1 wherein said non-image areas are at an
electrical potential of about ground.
10. The process of claim 1 wherein the difference in potential
between image and non-image areas is at least 20 volts in
magnitude.
11. The process of claim 1 wherein said one surface comprises a
dielectric layer overlying a photoconductive layer..
12. The process of claim 1 wherein said electrical potential
pattern is provided in accordance with an imagewise pattern of
differing dielectric constant at said one surface.
13. The process of claim 1 wherein said magnetic force of
attraction is at least 10.sup.-.sup.5 dynes.
14. The process of claim 1 wherein said non-image areas and said
support are at about equal electrical potential.
15. The process of claim 1 wherein said non-image areas and said
support are at about ground potential.
16. The process of claim 1 wherein said one surface comprises
photoconductive zinc oxide disposed in an insulating binder and
said non-image areas and said support are at about equal electrical
potential.
17. The process of claim 1 further comprising fixing said toner
material to said one surface.
18. The process of claim 1 further comprising imagewise
transferring said toner material to a second surface.
19. The process of claim 1 further comprising imagewise
transferring said toner material to a second surface and fixing
said toner material on said second surface.
Description
This invention relates to the electrographic development of latent
images with toner or marking material, and particularly, to the
development of latent images in the form of an electrical potential
pattern under carefully controlled conditions to produce excellent
quality likenesses of a desired configuration on a recording
medium.
A vast majority of the electrographic copying processes in use
today involve creation on a suitable recording medium of an
electrostatic charge pattern corresponding to a pattern of light
and shadow to be reproduced and the development of that pattern by
deposition of marking material on the recording medium according to
forces generated by such electrical potential pattern. Xerography
is the most widely known of these techniques. The substrate may be
photoconductive, such as in the case of selenium as taught in
Carlson's U.S. Pat. No. 2,297,691, or may be a conventional
insulating substrate overlying a photoconductive layer, as
described in Watanabe, U.S. Pat. No. 3,536,483, to name a few
examples.
After creation, the electrical potential pattern is generally
developed by means of a finely divided developer powder thus giving
form to the hitherto latent electrostatic image. In a common
technique a fine, insulating, electroscopic powder is cascaded over
the electrical potential pattern bearing member. The powder is, in
the conventional use, triboelectrically charged to a definite
polarity and deposits preferentially in regions of the surface
where there is a preponderance of charge of the opposite polarity.
The triboelectric charging is caused by presence of carrier beads
in the powder mix. This technique of development is called cascade
development.
In another form of cascade development, called magnetic brush
development, magnetic carriers or magnetic toners are employed as
in U.S. Pat. No. 2,846,333 to Wilson. In this technique a magnetic
force is used to provide adherence of the toner-carrier mixture to
a support member which is then presented to the image bearing
member. While less efficient than conventional cascade development,
magnetic brush development fills in solid areas better, is more
compact, and does not depend on gravity to present the toner to the
surface, a factor which allows freedom in locating the developer
station.
In yet another form of electrostatic charge pattern development, a
conductive one-component toner is used by bringing a conductive
support member bearing a layer of fine conductive toner powder into
contact with the charge pattern bearing member, as in U.S. Pat. No.
3,166,432 to Gundlach. In this case the toner is held to the
support member by van der Waal's forces and the conductive support
member is held at a bias potential during development. This
technique fills in solid areas and requires only one component in
the developer material.
A further method of developing an electrostatic charge pattern is
to employ an electroscopic toner suspended in a liquid. With the
proper choice of materials, the toner becomes charged to a definite
polarity when dispersed in the liquid. When the electrostatic
charge pattern bearing member is brought into contact with the
liquid suspension, the toners deposit where there is a
preponderance of charge of the opposite polarity as in cascade
development.
While all of the above techniques have certain advantages in
particular situations, each one suffers from disadvantages which
impair their utility in actual machines.
In the conventional cascade development technique the toner-carrier
combination has a definite charge polarity and is not reversible
without changing the toner or the carrier. Thus, positive and
negative developed images cannot easily be made. Also the images
are hollow and solid areas are not filled in resulting in
low-fidelity development compared to the original charge pattern.
The triboelectric properties of the toner, while necessary to
development, cause severe problems. Uneven charging of the toners
causes backgrounding as do the uneven forces between carrier and
toner result in varying threshold levels from toner to toner. Also,
since the toner retains its charge for long periods of time, during
cascading some toners escape the development region and enter other
parts of the apparatus causing mechanical problems familiar to
those skilled in the art. These problems, coupled with the inherent
problems of using a two-component system where only one component
is depleted, definitely limit the utility of such techniques.
The magnetic brush development, being a form of cascade
development, suffers from some of the above mentioned disadvantages
although it overcomes others. As mentioned above, this technique is
less efficient but helps to fill in solid areas. However, it still
requires triboelectric toners, usually with two-components, which
have the concomitant problems mentioned above. Also, due to the
mechanical brushing action and other electrical characteristics,
this technique usually results in high background depositin and
poor machine latitude.
The process described in Gundlach, U.S. Pat. No. 3,166,432, has
many advantages over the above mentioned cascade type techniques.
However, it suffers from severe drawbacks which limit its
applicability. The van der Waal's forces, which act to adhere the
toner onto the conductive support member, are a counterforce to the
image producing electric force generated by the electrostatic
charge pattern, and as such must be selectively overcome to have
toner deposited. The van der Waal's forces are weak and non-uniform
from one toner to the next. This small, poorly controlled
counterforce gives high background deposition and poor machine
latitude. See Orr, Particulate Technology, MacMillan Co., N.Y.,
1966, p. 406. Also high contrast is more difficult to achieve. The
fact that the van der Waal's forces are not under direct control
but subject largely to the surface properties of the materials
involved makes the system highly susceptible to alteration of
development properties upon wearing of the involved surfaces or
variations in ambient conditions of temperature and humidity. It is
also difficult to obtain a uniform, smooth layer on the conductive
support member. All of these facts limit the utility of such a
system.
In a liquid development technique most of the problems of cascade
development are present in addition to other unique to a liquid
system. The technique requires triboelectric charging, making image
reversal difficult as explained above. Also, as in the case of
cascade development, the charge on a given toner is not well
controlled, resulting in high background deposition, poor machine
latitude, and a characteristic splotchiness in large dark or grey
areas. The inherent problems of handling liquids, usually solvents,
in a machine are also present.
In the present invention, two-component marking systems, the use of
liquids, reliance or van der Waal's forces, and other detractive
aspects of known electrographic techniques for developing
electrostatic charge patterns are eliminated. It should be noted,
moreover, that the present invention is applicable to the
development of electrical potential patterns in general, regardless
of whether provided by electrostatic charge as in conventional
xerography or by some other equivalent means. The advantages
accruing therefrom will be discussed in detail hereinafter.
In accordance with the present invention, a process is provided for
applying toner material selectively to predetermined areas of a
surface comprising:
1. providing a surface with areas thereof having an electrical
potential in a range defining image areas and other areas thereof
having an electrical potential in a range defining non-image areas,
said areas defining an electrical potential pattern corresponding
to the pattern to be produced,
2. contacting said surface with an electrically conductive support
bearing a uniform quantity of magnetically attractable,
electronically conductive toner material bound to said support by a
magnetic force of attraction, such contacting providing an
electronically conductive path between said surface and said
support through said toner material, said support being at a direct
current electrical potential of a magnitude and polarity such that
the difference in electrical potential between said support and
said surface results in a transient electrical transfer force on
said toner material greater than and opposed to said magnetic force
of attraction in said image areas and less than said magnetic force
of attraction in said nonimage areas,
3. maintaining said electronically conductive path for a period
sufficient to allow said transient electrical transfer force to be
produced, and
4. discontinuing said electronically conductive path while said
transient electrical force exists whereby said toner material is
selectively deposited on said image areas of said surface.
As can be seen from the foregoing statement of the invention, a
surface is provided with an electrical potential pattern which
defines the areas which will utilimately receive the toner material
(image areas) and those areas whic will not (the non-image or
background areas). The technique for providing this electrical
potential pattern may be any of the wide variety of known
techniques.
For purposes of this invention, the surface upon which is formed
the electrical potential pattern may be classified as one of two
types, depending upon whether or not there exists in addition to
the electrical potential pattern a coincident pattern of electrical
conductivity corresponding in location to the electrical potential
pattern. Certain surfaces do not provide a coincident electrical
conductivity pattern; others do. Both types provide the excellent
quality of recorded image characteristic of the present invention.
As will be seen, however, certain distinctive advantages accrue
from the type of surface employed.
An example of a surface which does not provide a coincident
electrical conductivity pattern is photoconductive selenium such as
is employed in many xerographic processes. The selenium is
generally coated in 1 to 100 micron thicknesses on a conductive
substrate, e.g., aluminum or other metal. The electrical potential
pattern is typically provided by applying to the selenium surface a
uniform electrostatic charge by a corona discharge device and then
exposing the thus charged surface to a light pattern which results
in charge loss in the light struck areas.
Another surface of this type is provided by a transparent
electrically insulating film overlying a photoconductive layer, an
example of which is a polyester film overlying a layer composed of
photoconductive cadmium sulfide disposed in an insulating binder.
Beneath the photoconductive layer is an electrically conductive
substrate. Such constructions are described in U.S. Pat. Nos.
3,457,070 and 3,536,483. The transparent insulating film and the
underlying photoconductive layer generally range in thickness from
10 to 60 micrometers. In these constructions, the electrical
potential pattern is provided by applying a uniform electrostatic
charge to the surface by a first corona discharge device, then
simultaneously applying to the charged surface an electrostatic
charge of the opposite polarity to the first charge, exposing the
surface to a light pattern, and finally uniformly exposing the
surface to light whereby the potential pattern is created. The
electrically conductive substrate is at ground potential during
both charging steps.
Still another example of this type, and not involving a
photoconductive material, is an insulating layer such as a
polyester film which is provided with an electrical potential
pattern by selectively electrostatically charging certain areas of
the surface by means of electrically conductive pins or styli. The
dielectric layer typically covers an electrically conductive
substrate. An electrical potential of at least about 250 volts
relative to the conductive substrate is applied to the styli
whereby electrostatic charges are placed on the dielectric surface
in an imagewise pattern. Such electrostatic stylus recorders
generally involve cascade electrostatic development of the charge
pattern by triboelectrically charged toner powders as in U.S. Pat.
No. 2,932,690, or liquid electrostatic development as in U.S. Pat.
No. 3,540,412. Such electrostatic charge patterns on dielectric
layers can easily be developed according to the present
invention.
A further example of a suitable electrical potential pattern
bearing surface is the imagewise electrostatically charged
dielectric layer overlying a conductive substrate resulting from
the imagewise ion projection of charged gas ions through an
imagewise electrostatically charged screen according to the
teachings of U.S. Pat. No. 3,582,206. Here, the original light
image is projected onto an electrostatically charged
photoconductive-coated screen. The final result, before
development, is an imagewise charged dielectric layer (see Col. 5,
lines 53-58) in one example, which would provide a suitable
imagewise potential pattern for development by the teachings of the
present invention.
Other surfaces of this type are provided by thin solid evaporated
films including amorphous arsenic triselenide and amorphous arsenic
trisulphide, and various coated organic photoconductors such as
polyvinylcarbazole, poly-N-vinylcarbazole and others described in
U.S. Pat. Nos. 3,287,120 and 3,484,237, and British Pat. 990/368.
Employing surfaces of the type not providing a coincident
electrical conductivity pattern at the time the electronically
conductive path through the powder is present in the operation of
the process of this invention, the image and non-image areas of the
surface have about the same electrical conductivity, which is
preferably electrically insulating as defined herein.
An example of a surface which does provide an electronic
conductivity pattern coincident with the electrical potential
pattern is a layer comprising photoconductive zinc oxide disposed
in an insulating binder, generally an insulating resin binder. This
layer may overlie an electrically conductive substrate or there may
be insulating layer between the photoconductive layer and the
electrically conductive substrate. See U.S. Pat. No. 3,563,734 and
U.S. application Ser. No. 820,062 now U.S. Pat. 3,764,313 for
examples of such constructions. It should be noted that due to the
sensitivity and controllability provided by the process of this
invention, the photoconductive zinc oxide layer may be present in
significantly reduced amounts relative to present constructions;
less than 3 grams per square foot dry weight and generally less
than 2.5 grams per square foot. This is advantageous from a cost
and aesthetic standpoint, the latter because such a zinc oxide
coated paper construction more closely approximates the feel of
conventional bond paper. Other surfaces of this type are provided
by a layer of photoconductive cadmium sulfide dispersed in an
insulating resin binder and photoconductive titanium dioxide again
dispersed in an insulating resin binder, each overlying an
electrically conductive substrate.
A suitable technique for providing the electrical potential pattern
employing surfaces of the type here under discussion is the
application of a uniform electrostatic charge followed by exposure
to a light pattern. At the time the electronically conductive path
is present in the operation of the process of this invention, the
surfaces of this type are defined by image areas which are
relatively electrically insulating and non-image areas which are
relatively electrically conductive as defined herein.
The photosensitive surfaces which do not provide a coincident
conductivity pattern return to the dark, relatively insulating
state in a brief time relative to the time between the exposure and
development steps, the latter usually being about one second.
Photosensitive surfaces exhibiting persistent electrical
conductivity require times in excess of the time between exposure
and development to return to the dark, insulating state.
As will be apparent hereinafter, various forces are at work in the
operation of this process, all contributing to a carefully
controlled set of conditions to provide maximum control over the
application of toner material to the electrical potential bearing
surface.
The following drawings are provided depicting the competing nature
of these forces to the end that a sound understanding of the
invention is provided. In these drawings:
FIG. 1 is a side elevational view of a recording medium bearing a
potential pattern;
FIG. 2 is a schematic view showing development of the potential
pattern on the recording medium of FIG. 1;
FIG. 3 depicts the influence of magnetic lines of force on the
toner material during development;
FIG. 4 is a detailed illustration of the electrical forces present
during development in accordance with the process of this
invention; and
FIG. 5 is a graph of a plot of the electrical force on the toner
material versus development time in the process of this
invention.
Referring to FIG. 1, recording element 1 includes layer 3, which
may but need not be a photoconductive layer as conventionally used
in xerography, backed by a conductive layer 4 which is grounded. A
potential pattern exists on the surface. In area 5 the charge has
been dissipated whereas in area 6 the charge, along with the image
charges 7, remains.
In FIG. 2, a development roller 8 is depicted including a long
cylindrical magnetically permeable shaft 9 on which are mounted
four long cylindrical sector-shaped magnet sections 10 coaxial to
the shaft 9. The number of magnet sectors is chosen as four here
only for purposes of convenience and illustration. The number may
be more or less than this so long as toner transports smoothly
around the shell 11. These sectors consist of a permanent magnetic
material such as is commercially available under the tradename
Plastiform. The magnets are magnetized uniformly along their length
as indicated by the N,S designations in the diagram. Coaxial to,
and surrounding, the magnet sections 10 is an electrically
conductive hollow cylindrical shell 11 extending axially relative
to said shaft and provided with a means not shown for connecting
the shell to a unidirectional direct current electric potential or
to ground.
A finely divided, magnetically attractable, relatively
electronically conductive toner material 13, such as disclosed in
U.S. Pat. No. 3,639,245 to Nelson, is placed in a reservoir support
member 14, adjacent to but not touching the surface of shell 11. As
shell 11 rotates (counterclockwise in FIG. 2) the toner material 13
is smoothly and uniformly dispensed onto the surface of the shell
11, being held in adherence thereto by the magnetic forces arising
from the magnet sections 10. Developer roll may rotate in a
clockwise direction if desired by dispensing the toner material 13
from a side opposite to that shown. The amount of toner 13 on the
shell 11 can be controlled by the distance between the reservoir
edge 15 and the surface of the shell 11. It has been found that,
instead of rotating the shell 11, the shaft 9 and the magnet
sections 10 attached thereto can be rotated while the shell 11
remains stationary. In the case illustrated, magnets 10 and shaft 9
rotate clockwise to transport the toner material 13 around the
stationary shell 11 in a counterclockwise direction. Both
techniques are applicable to this invention and work equally well
in dispensing a smooth, uniform and well regulated supply of toner
13 from the reservoir 14. For the sake of definiteness we shall, in
this embodiment, illustrate the former case wherein the shell 11
rotates while the shaft 9 remains stationary.
In operation, the development roller 8 is placed above the
potential pattern bearing layer 3 of recording element 1 such that
the axis of the development roller 8 is parallel to the plane of
the potential pattern bearing layer 3 and placed at such a height
above such layer that the uniform toner layer on the development
roller 8 makes physical contact with layer 3 forming a well defined
nip region 16. The development roller 8 is moved relative to the
potential pattern bearing layer 3 in the direction shown while
maintaining a uniform distance between shell 11 and layer 3 to
provide a uniform electronically conductive path therebetween by
means of the conductive toner material 13. In this way, development
of the potential pattern proceeds in time from one side of the
recording element 1 to the other.
Due to the presence of the magnetic field the magnetic toner 13 in
the nip region 16 forms into small chainlike groups 17 which follow
the lines of magnetic force 18 between the shell 11 and the layer 3
as in FIG. 3. These chainlike groups 17 become small electronic
circuits between the shell 11 and layer 3. The circuits are
connected at the moment of physical contact of the toner 13 with
layer 3 (19 in FIG. 3) and are disconnected when the contact is
terminated (20 in FIG. 3). The formation of these chains has been
observed by using a microscope focused on the nip region. Thus the
magnetic sections 10 serve many purposes: to transport, uniformly
and controllably, toner material around the conductive shell of the
development roller, to create chain-like electronic circuits in the
nip region, and to provide an uniform counterforce to the
electrical development force.
To better understand the process by which development takes place
reference is made to FIG. 4. This description is idealized and
simplified for the purposes of clarity, but illustrates the
substantial phenomena about which this invention is concerned.
Further details, extensions, and generalizations of this
description will be apparent to those skilled in the art. FIG. 4 is
a detailed illustration of the nip roller 16 during actual
development. The recording element 1 is moving from right to left.
The shell 11 is connected electrically to ground. The surface of
layer 3 prior to development as at point 21 is uniformly charged to
a surface potential V.sub.s. The chain-like formations of toners
22, 23, 24, 25 and 26 represent progressive times or stages in the
development process, 22 being the earliest and 26 being the latest.
In actual practice there are many more toner chains set up in the
nip region but they have been reduced, as have the number of toner
particles in a chain, for purposes of illustration. The shell 11,
here shown as rotating counterclockwise, continuously presents
fresh chains of toner material to the potential bearing surface. At
chain 22 the above mentioned electronic circuit has not as yet been
completed. However, due to the presence of the surface charge 27,
opposite polarity charge 28 is induced in the conductive shell 11.
This induced charge, chosen as negative here for purposes of
illustration, immediately begins flowing through the chain towards
the positive surface charge. This process takes place even after
the toner chain contacts the charge bearing surface as in chain 23.
In chain 23 most of the negative charge has reached the end of the
toner chain. At this stage, due to its opposite charges on the
toner and the surface 3 there is an electrical force on the toner
particle adjacent to the surface, such force being directed from
the toner particle downward toward the charge bearing layer 3.
However, at a small time interval later, at the stage illustrated
by chain 24, another process begins to occur.
Since the toner is relatively conducting, some positive charge from
the potential bearing surface 3 begin to leak onto the toner chain
across the interface 29. Equivalently, one can say negative charge
leaks from the toner down onto the photoconductive surface. Either
case leads to the same results but for the sake of this
illustration the former is chosen. As this leakage occurs, the
charge on the toner adjacent to the photoconductive surface begins
to be neutralized and hence the electric force tending to pull the
toner towards the said surface is diminished in time. Such charge
leakage continues at a rate governed substantially by the
electronic conductivity of the toner and the nature of the surface
of layer 3. The toner adjacent to the surface and the surface layer
of the recording element itself form an interface region in which
this charge transfer takes place. The rate of flow of charge
(current) from surface to toner is determined by the effective
capacitance and resistance of this interface. In general, the more
conductive the interface region the faster will be the leakage
across the interface.
At the next stage, illustrated by chain 25, the toner chain is just
ready to be pulled up by the magnetic counterforce, thereby
breaking the aformentioned electronic circuit. At this stage there
are two substantial forces acting on the toner 30; one, the
electric force due to the charge difference between the toner and
the adjacent surface, the other, the uniform magnetic counterforce
due to magnet sections 10. The uniform magnetic counterforce acts
as a threshold since all toners in which the electric force is
greater than the magnetic counterforce will remain on the recording
element surface and those in which the magnetic counterforce is
greater than the electric force will be pulled up toward the magnet
and not deposited on the recording element. The former condition is
depicted here and toner material is thus deposited on layer 3. The
magnetic counterforce may vary spatially as one traverses the nip
region due to the cylindrical or other geometry of the magnet
structure, but the important point is that a definite and
controllable counterforce exists everywhere in the nip and at the
position of the separation point establishing a threshold
counterforce to deposition at this separation point. Since the
powder transport and nip region is well controlled, this is a
constant and uniform threshold counterforce in time.
Since the electrical force on the toners adjacent to the recording
element surface becomes greater as more charges of opposite
polarities are present at the interface between the toner and
surface of layer 3, the more initial charge 27 on the surface of
layer 3 that is present the larger will be the electrical forces on
these toners. Hence, the more toners will remain on the surface of
layer 3 after the developer roll assembly has passed by. Since a
charge on the surface of layer 3 before development is usually
related to a surface voltage it has been observed that as the
initial surface voltage of the layer 3 increases, the amount of
toner deposited also increases. When there is no initial surface
voltage present or the surface voltage results in an electrical
transfer force less than the magnetic counterforce, no toner
material is deposited.
The time interval in which this process takes place, from the
initial formation of the circuit until its termination, is about
10.sup..sup.-3 seconds to about 1 second depending on the size of
the nip region and the linear relative speed o the potential
pattern bearing surface and the developer assembly.
In the above manner, high contrast, low background images are
developed in which the solid areas are filled. The developed image
may be fixed directly to the recording element or it may be
transferred by conventional means to another substrate. Means for
doing this are well known to those skilled in the art. The
development technique described in this embodiment is as efficient
as the best previous techniques and allows for unusually high
machine latitude.
The technique described in the above embodiment has been
necessarily specific for purposes of illustration. Alterations,
extensions and modifications of this technique would be obvious to
those skilled in the art.
In this process, as can be ascertained from the above embodiment,
the time in which the powder resides in the well-defined nip region
in which an electronic circuit or path is formed is very important
to the quality of development of the potential pattern. If the time
is too short, the induced charges from the grounded conducting
shell will not have time to reach the toners immediately adjacent
to the recording element surface. If the time is too long, all of
the charge on the toners will be neutralized by leakage of charge
through the toner-recording element interface. This situation can
be better understood by reference to the graph in FIG. 5. Here the
electrical force (EF) on the toners adjacent to the potential
bearing surface is plotted against the time since the formation of
the chain-like circuit of which those toners are members. The
uniform magnetic counterforce, which is approximately constant for
this period of time, is superimposed but is directed opposite to
the electronic force. For the toners to be deposited at the moment
when the chain is pulled back to the developer roll, the electrical
force must be greater than the magnetic force. Thus, in the
instance graphed in FIG. 5, the nip time should be between t.sub.1
and t.sub.2.
The above embodiment can also be used to explain another advantage
of this invention. By varying the electric potential of the
conductive shell 11 in FIG. 4, variations in density in charged and
uncharged areas may be accomplished. As the potential of the shell
11 (called a bias potential) is moved away from ground and towards
the surface potential of the undeveloped surface of the recording
element, the amount of toner deposited in those areas will diminish
until, when the bias potential is at about the surface potential,
no toner will deposit. However, in the undercharged areas, or those
nearer ground potential, the higher the bias is raised the larger
will be the potential difference between the developer roll and the
recording element surface and, therefore, the more toner material
will be deposited. This will lead to a reversed image.
In the following paragraphs we describe an embodiment of this
invention employing a surface providing an electrical conductivity
pattern coincident with an electrical potential pattern. FIG. 4
also applies to this case except that now, in addition to the
surface of the recording element carrying a potential pattern, a
conductivity pattern, conforming to the potential pattern, is also
present. In this particular case, the non-image area is more
electrically conductive, such as is provided by light exposure of a
photoconductive surface, where there is little or no surface
potential, and more insulating in the image areas where there is a
high surface potential, such as is provided by non-light exposure
of a photoconductive surface. It will be seen that the presence of
this corresponding conductivity pattern, in addition to the
potential pattern, acts to enhance the contrast and dramatically
reduce background deposition.
In the case of a photoconductive surface, in dark areas the process
is the same as in the first embodiment above. But in the light (and
grey) areas another effect occurs which tends to reduce the amount
of toner powder deposited there. Since, in these areas, the rate of
charge leakage across the toner-surface interface is greater in
these conducting areas, less electrical transfer force is built up.
Thus, in these areas a higher surface potential is needed to
develop the same amount of toner as in the first case. Or,
likewise, for the same surface potential, less toner is deposited
for the same nip time, toner electronic conductivity, and uniform
magnetic counterforce. This conductivity pattern need not be
present throughout the thickness of the photoconductor. All that is
required is a surface conductivity pattern present at the time of
development.
The same processes that occurred in the first embodiment also occur
in this case but the leakage rate across the interface between the
toner and the recording element surface is different. In a
conductive area, the leakage and neutralization of the charge which
is responsible for the electric force is much faster and thus for a
given nip time (which is equivalent to a given development speed)
less toner deposits than would deposit in the case where a
conductivity pattern is absent.
In both of the above embodiments the conductive shell 11 and the
conductive layer underlying the recording element surface layer
have been electrically grounded through a connector. In the
operation of this invention it is not necessary that such a ground
connection be made as long as there is enough coupling to ground,
either AC or DC, so that the currents described above will flow.
The coupling may be accomplished, for example, by capacitive
coupling or leakage conductance of the developer assembly support
materials.
A suitable developer roll for supplying toner material to the
electrical potential bearing surface is described in U.S. Pat. No.
3,455,276. Either the outer shell providing the support for the
toner material or the enclosed magnetic force generating members
may rotate. The magnetic counterforce is generally at least about
10.sup.-.sup.5 dynes in magnitude. This is to be contrasted with
the far weaker van der Waal's forces relied upon in the process
described in Gundlach, U.S. Pat. No. 3,166,432 (see especially Col.
7, lines 20-53).
According to Gundlach's teachings, one would expect that an
applicator which applies a substantial counterforce to the
deposition of toner particles on an electrostatic image would give
inferior results. Surprisingly, according to the present invention
a magnetic toner applicator used with magnetic, and simultaneously
electronically conducting toner powder, gives improved quality
copies with low background optical density due to unwanted toner
deposition in background areas. Further benefits which result from
utilizing this type of applicator are (1) the magnetic toner is
easy to transport and contain without undue contamination of the
machine interior due to weakly-bound or electrostatically charged
toner particles floating about; (2) the development gap, that is
the distance from the powder applicator electrode surface to the
developable surface, can be quite large (many toner particle
diameters) affording uncritical mechanical components. The magnetic
applicator acting on the magnetically attractable toner causes the
powder to stand up in chains and assures electrical contact from
the applicator electrode to the developable surface. The gap should
be between about 25 .times. 10.sup.-.sup.4 cm and 50 .times.
10.sup.-.sup.2 cm. In all cases, it should be at least equal to
twice and preferably five times the dimension of the largest
particles.
Further, this type of applicator and toner affords precise metering
of a predetermined quantity of toner powder onto the applicator
surface. For example, a doctor blade situated a fixed distance from
the surface of a rotating cylindrical applicator, as in U.S. Pat.
No. 3,455,276, meters out a constant supply of toner material onto
the applicator surface. This assures a well-controlled contact nip
between the powder and the developable surface as the two move
relative to one another, and consequently assures a well-controlled
development time, which is the time that a unit area of the
developable surface is contacted by the above-mentioned nip of
powder. The precise metering also assures a constant and
well-controlled magnetic counterforce to be exerted on the powder
particles.
In the practice of the present invention, the contact time, i.e.
the duration of contact, between the toner-filled applicator
electrode and the developer surface is very important. It must be
long enough for the electrical transfer forces in image areas
opposing the magnetic counterforce to build up sufficiently. It
further must be short enough so they have not decayed below the
threshold counterforce in these areas in the case where a decaying
force is involved. This build up and decay of the electrical
imaging forces is a function of the electrical conductivity of the
toner material.
The magnetic applicator and associated toner metering devices,
along with the well-controlled toner chains result in a
well-controlled and reproducible conductivity of the toner powder
in each incremental area of the nip region. The contact time, or
duration, is also well controlled and reproducible. Typical nip
widths (contact region) vary from about 0.1 cm to about 5 cm, and
preferrably from about 0.2 cm to about 1 cm for cylindrical roll
applicators. With the linear development speeds varying from about
0.5 cm/sec to about 200 cm/sec., and preferrably from about 1
cm/sec to about 100 cm/sec, these nip widths result in a contact
duration time of from about 10.sup.-.sup.3 seconds to about 1
second and preferrably from about 10.sup.-.sup.2 second to about
1/2 second.
In Gundlach, U.S. Pat. No. 3,166,432, van der Waals forces are the
preferred forces to hold the toner to the applicator electrode.
These forces are quite weak, strongly dependent upon the distance
between the toner powder and the applicator surface, and also
strongly dependent upon the size of the toner particle. The van der
Waals forces also vary considerably, being dependent upon the
material and surface condition of the applicator surface as well as
the toner. Furthermore, van der Waals forces do not lend themselves
to uniform metering of controlled amounts of toner powder onto the
applicator electrode surface. The weak forces and small particles
(less than 20 microns and preferrably less than 5 microns, Column
7, lines 9 through 20) result in only a very thin layer of
particles on the applicator surface. To contact the developable
surface everywhere requires extremely well controlled mechanical
tolerances to be maintained between the applicator electrode
surface and the developable surface.
The substantial counterforce of the present invention results in a
distinct threshold which must be overcome by imaging forces. There
are at least two significant consequences of this not achieved by a
process relying upon little or no counterforce. One is that
background areas are cleaner, not having as many toner particles
deposited due to mechanical forces, or even van der Waals forces
between the toner particles and the developable surface, and the
second is that the uniform counterforce assures uniform development
of gray and black areas which would not be realized with no
counterforce, or with a nonuniform or spurious counterforce.
A suitable magnetically attractable, electronically conductive
toner material for use in the present invention is described in
Nelson, U.S. Pat. No. 3,639,245. The toner material may have a
static conductivity, as determined according to the technique
described at column 3, line 54 - column 4, line 47 of U.S. Pat. No.
3,639,245, in the range of from 10.sup.-.sup.13 to 10.sup.-.sup.4
mhos/cm and preferably from 10.sup.-.sup.12 to 10.sup.-.sup.6
mhos/cm at an electric field of 100 volts/cm. Preferably, the
conductivity of the toner material is electric field dependent and
monotonically increasing with electric field in the range of 10
volts/cm. to 10.sup.4 volts/cm. Magnetic attractability is provided
by inclusion in the toner material particle of a finely divided
magnetically attractable material such as magnetite. The major
dimension of the toner material particles may suitably range from
about 0.5 micrometers to about 100 micrometers, preferably from
about 2 to about 30 micrometers. Spherical shaped particles are
preferred. Particles whose size is below 2 micrometers have been
found to be subject to unpredictable and uncontrollable
electrostatic and van der Waal's forces, resulting in higher
background deposition and thus reduced quality. Particles above 30
micrometers limit resolution. Toner material which exhibits
electric field dependency is highly conductive under developing
field conditions when electrical current flow is desirable to
create imaging forces and less conductive prior to and after
development when the electric fields are substantially reduced and
current flow is not desired.
The powder conductivity should be such that at high electric
fields, as in image areas of the developable surface, it permits a
relatively large current flow from the applicator electrode to the
developable surface. However, the powder should not be so
conductive that after one layer is deposited on said surface it
thereafter electrically shields subsequent layers of powder from
said surface, accepting their charge but preventing their
deposition as would happen with a highly conductive powder.
Additionally, at low or zero electric field, the conductivity
should be considerably smaller so the powder which was deposited on
the developable surface retains its charge for a time period
sufficient to permit transfer of the powder from said surface to a
receptor sheet.
Surfaces employed in the present invention which provide an
electrical conductivity pattern have relatively conductive
non-image areas and relatively insulating image areas as described
in Shely, U.S. Pat. No. 3,563,734, incorporated herein by
reference. The conductivity of the non-image areas, the
light-struck areas when the surface is a photoconductive layer,
ranges from about 10.sup.-.sup.17 mhos/cm. to about 10.sup.-.sup.7
mhos/cm. and the conductivity in the image areas, the non-light
struck areas when the surface is a photoconductive layer, ranges
from about 10.sup.-.sup.18 mhos/cm. to 10.sup.-.sup.9 mhos/cm.,
provided the non-image areas are at least twice, and preferably 100
times, as conductive as the image areas. The conductivity of the
toner material should be at least 10, and preferably at least 100
times as conductive as the image areas of the electrical potential
bearing surface to be developed. It is further desirable, but not
necessary, that the toner also be more conductive than the
non-image areas.
The intensity of the light used to expose photosensitive surfaces
in the practice of this invention will vary depending on many
factors including the type of photosensitive element employed. A
typical exposure range is from 0.05 to 20 foot-candle-seconds.
The electrical potential pattern to be developed includes areas
which will provide a transient electrical force less than the
magnet counterforce exerted by the toner material support
(non-image areas) and areas which will provide a transient
electrical force greater than such magnetic counterforce (image
areas). The electrical potential difference between image and
non-image areas depends upon the particular application and may be
as small as 20 volts for some applications. A difference in
potential of 200 volts is desirable in the case where the recording
medium is a conventional photoconductor. Typically, the non-image
area, in this case, is at a voltage of from a few volts to 50 volts
and the image area is at a voltage of from 200 to 300 volts.
In the latter case the support bearing the toner material is biased
to a potential within about 20 volts of the non-image area
potential and at least about 20 volts different from the image
areas at least in the case where the surface does not provide a
coincident conductivity pattern. Preferably, in all cases, the
applicator and the non-image areas are at substantially equal
potential, and most preferably that is ground potential. The
difference of potential between the applicator and a non-image area
can be much larger in the case when the non-image area is
conductive, i.e., when a conductivity pattern is present. The
difference may be as much as several hundred volts since no powder
will be deposited in a conductive region of the surface unless very
much larger voltages are present. See Shely, U.S. Pat. No.
3,563,734. From a practical standpoint, this translates to greater
processing latitude and sensitivity for the embodiment wherein a
coincident conductivity pattern is present.
Most photoconductors are more sensitive while in the presence of an
electric field and thus a photoconductor initially charged to 1000
volts would be more sensitive than the same photoconductor charged
initially to only 500 volts. The same light exposure would give a
difference of potential between light struck areas and dark areas
of greater amount in the former case than in the latter. However,
in the former case the final potential in the illuminated areas
would not be about ground but at some potential other than ground
even though the difference of potential between an unilluminated
area and an illuminated area is greater for the same amount of
light. The preferred potential for the non-image areas and the
applicator is about ground potential and for the image areas about
200 volts or more. In some instances the non-image areas will not
be at ground potential and in these cases the applicator is biased
to a potential within about 20 volts of the non-image areas by a
direct current power supply.
While overall, certain advantages are enjoyed by the embodiment of
this invention wherein a coincident electronic conductivity pattern
is provided corresponding to the ever present electrical potential
pattern, there is an attractive feature distinctive to the other
embodiment. That feature is the capability of producing either
positive or negative images by varying the direct current
electrical potential bias on the toner material support (developer
roll). To illustrate, assume a potential pattern wherein certain
areas of the potential bearing surface are at ground or zero
potential and other areas are at a potential of +200 volts. In the
case where the surface is a photoconductive surface, the areas at
ground potential constitute light struck areas where an
electrostatic charge has been dissipated and the areas at +200
volts constitute non-light struck areas. Of course, in a real
situation, the surface potentials may vary over a wide range
representing areas which have received varying amounts of light.
Each potential will generate its own electrical transfer force, and
depending on its magnitude relative to the magentic counterforce,
such area will or will not receive transferred toner material.
Positive images will be developed by biasing the toner material
support to the potential of the light-struck areas, which in this
hypothetical situation means holding the support at ground
potential or within about 20 volts thereof in accordance with the
above discussion. By the expedient of biasing the support to the
non-light struck areas, however, the light struck areas of the
potential pattern bearing surface may be developed, producing
negative images.
Another method by which a potential pattern suitable for
development by this invention involves uniformly precharging the
outermost surface of a ferroelectric layer such as barium titanate
by means of a corona discharge device. The layer is then
selectively heated in an imagewise pattern to a temperature at
which the dielectric constant increases substantially in the heated
areas. This results in a potential pattern in which the differences
in potential are caused by differing dielectric constant. The layer
can then be developed by the means described in the above
embodiments. Other techniques, based on similar concepts, are known
to those skilled in the art.
The electrical potential bearing surface which is developed in
accordance with this invention may constitute the ultimate record
of the pattern to be produced or it may be an intermediate record
wherein the developed image is transferred to another substrate.
The imagewise deposited toner material may be fixed on a recording
medium by any of the variety of conventional techniques. Toner
material having a thermoplastic resin matrix is preferably fixed by
conventional heat fusion, typical resins include B-stage phenol
aldehyde polymers, polyvinyl acetate, and epoxy resins.
Examples of suitable insulating binders for the photoconductive
materials employed in this invention include styrene-butadiene
resins such as that sold under the tradename Pliolite S-7,
polyethylene resin, chlorinated polyethylene, polyvinyl acetate,
and Lexan (tradename) polycarbonate. In addition, a photoconductive
layer may include various additives such as sensitizers, humidity
control agents, and the like. The layer providing the electrical
potential bearing surface may be applied to a variety of substrates
including conductive paper, metals, paper-metal foil laminates, and
metal coated resin films, or constructions of the above substrate
including an insulating dielectric layer adjacent to the
substrate.
The invention is further illustrated by the following examples
wherein parts and percentages are by weight unless otherwise
stated.
EXAMPLE 1
This is an example wherein a potential pattern not having a
coincident conductivity pattern is developed. The photoconductor
consists of a 15 micrometers thick layer of evaporated amorphous
selenium having a conductivity of about 10.sup.-.sup.16 mhos per
centimeter coated on a conductive aluminum backing. In the dark,
the selenium surface is electrostatically charged to an electric
potential of about +500 volts with respect to the conductive
backing. The charging is accomplished by means of a corona
discharge device drawn over the surface of the selenium layer.
Thereafter the charged selenium surface is exposed to an imagewise
pattern of light and dark; exposure in the light struck areas being
about 0.5 foot-candle-seconds. The potential of the surface in the
light struck areas is reduced to a potential of about +50 volts or
less. Since, in the dark areas, the potential remains approximately
the same, this exposure step results in the creation of a latent
electrostatic image on the surface of the photoconductive selenium
layer.
The potential pattern bearing surface is then moved past the
developing station described and illustrated above. The distance
between the surface of the conductive shell and the potential
pattern bearing surface is about 0.07 centimeters and is uniform
from end to end. The toner developing material is a thermoplastic,
magnetically attractable, electronically conductive powder of the
type described in U.S. Pat. No. 3,639,245. The static electrical
conductivity of the toner material is about 10.sup.-.sup.11
mhos/centimeter at an electric field of 100 volt/cm. The size range
of said toner material is from about 5 micrometers to 21
micrometers in diameter with an average size of about 13
micrometers. The magnets inside the conductive shell of the
developer assembly are rotated at a speed of about 300 revolutions
per minute and exert an average magnetic counterforce of about
10.sup.-.sup.4 dynes. The potential pattern bearing surface is
moved past the developer assembly at a linear speed of about 15
cm./sec. The electrical potential of the conductive shell is held
at about ground potential.
The resulting developed image pattern has toner selectively
deposited on the above mentioned non-light struck areas whereas in
the light struck areas where the potential is close to ground (+50
volts) no toner powder is deposited. Thus in this case the image
areas are the high potential, non-light struck regions of the
potential pattern bearing surface and the non-image areas are the
low potential, light struck areas of the surface. The resulting
image is of good quality, having high density in the image areas,
low background in the non-image areas, and uniformly filled solid
image areas. Also the continuous tone (grey scale) areas are well
reproduced.
EXAMPLE 2
In this example, the bias potential of the conductive shell of the
developing assembly is fixed such that negative images with respect
to Example 1 are obtained. The procedure follows that of Example 1
except that during the actual development of the potential pattern
bearing surface the electric potential of the conductive shell of
the developer roll is fixed at a value about equal to the potential
in the dark, non-light struck areas, i.e., about +500 volts. Thus
the potential difference between the dark areas of the potential
pattern bearing surface and the conductive shell is zero whereas in
the light struck areas the difference is about -450 volts. The
developing assembly is then moved across the selenium surface just
as in Example 1. The resulting toner image pattern is a negative of
the developed image pattern obtained in Example 1. Again the
resulting toner image is of good quality with high density in the
image areas and low background in the non-image areas, with solid
areas being uniformly filled.
EXAMPLE 3
This example illustrates development of a potential patterned
surface also providing an electronic conductivity pattern.
The photoconductor layer consists of zinc oxide dispersed in an
organic resin binder and coated on a sheet of paper. The zinc
oxide-resin layer consists of about 75% by weight of French process
zinc oxide, about 15% by weight of an acrylic resin available under
the tradename Arotap 3211, about 8.15% by weight of soy alkyd
resin, and about 1.85% by weight of a mixture of sensitizing dyes
consisting of about 30% by weight of Bromophenol Blue, 50% by
weight of Sodium Fluorscein, and 20% by weight of Euchrysine GGNX.
The photoconductive slurry is then coated out of a solution of
toluene and methanol, onto 45 lb. paper bearing the trade
designation CC base G rawstock (Weyerhauser). The layer of zinc
oxide-resin has a dry weight of about 2.5 gm./sq. foot. The dark
conductivity of the zinc oxide-resin layer is about 10.sup.-.sup.16
mhos/cm.
After dark adapting, the paper-zinc oxide-resin construction is
electrostatically charged by a conventional corona device to an
electric potential of about -500 volts relative to an
electronically conductive backing plate contacting the underside of
the paper substrate. The photoconductive surface of this
photoconductive element is then exposed to an image pattern of
light and dark; exposure in the light struck areas being about 10
foot-candle-seconds. This reduces the potential in the light struck
areas to about -25 volts whereas the potential in the dark areas
remains at about -500 volts.
The thus formed potential pattern is then developed by means of the
developer assembly described and illustrated above. The potential
bearing surface is moved past the developer assembly at a linear
rate of about 3 inches/second; the distance between the surface of
the developer assembly and the potential pattern bearing surface
being about 0.085 cm. The toner developer material is the same as
that employed in Examples 1 and 2. The developer assembly is held
at an electric potential of about ground.
This process results in a developed image pattern on the pattern
bearing surface in which toner is selectively deposited in the dark
areas and no toner is deposited in the light struck areas. The
toner is then fused onto the zinc oxide-resin surface by means of
an infrared oven. The resulting copies have unusually low
background in the light struck (non-image) areas and high density
in the dark (image) areas. The solid image areas are faithfully
filled, and the continuous tone areas (grey scale) are well
reproduced.
EXAMPLE 4
A sheet of 0.0005 inch thick polyester film available under the
tradename Mylar is coated on one surface with a thin film of
electrically conductive aluminum and wrapped around the periphery
of a 4 inch diameter cylindrical aluminum drum with the aluminum
coated side against the drum. The film composite is then taped in
place. As the grounded drum rotates with a surface speed of about 5
inches/sec., a conductive copper wire stylus of about 0.01 inch
diameter contacts the insulating polyester surface. A voltage of
about +300 volts is applied to the wire stylus. A rotating shell,
fixed magnet, magnetic developing station as described above
contacts the polyester surface after the surface has been charged
by the wire stylus. The cylindrical development electrode shell has
its axis parallel to the aluminum drum axis and rotates at a
surface speed of about 1.5 inches/sec. The magnetic developer
powder is of the type described in U.S. Pat. No. 3,639,245 and has
a static conductivity of about 10.sup.-.sup.6 mhos/cm. at 100
volts/cm. The powder applicator shell is grounded. Dense black
lines are developed where the polyester was charged by the high
voltage stylus, and virtually no powder is deposited elsewhere.
This invention is applicable to the development of potential
patterns in general. We have illustrated this by electrostatic
charge patterns but the concept is not limited to such patterns.
Potential patterns may be created with uniformly charged (or
uncharged) surfaces by varying the capacitance of the member upon
which the potential pattern is to be produced. This can be seen by
noting that V = Q/C where V is the electrostatic potential, Q is
the charge present on C, the capacitance of the member. We see from
this relationship that V can be varied by varying either Q or C or
both. Whereas most of the present applications of potential
patterns involve varying the charge Q, varying the capacitance C is
equally effective for producing potential patterns suitable for
development.
* * * * *