Electrographic process of imaging by modulation of ions

An electrographic process of producing, on a dielectric coated record medium, an electrostatic copy latent image by modulating a flow of corona ions with the aid of an electrostatic latent image which has been produced on a photoconductive photosensitive screen having a number of openings. The process is characterized by making a ratio K of a maximum surface potential V volts of the electrostatic copy latent image produced on the dielectric coated record medium to an intensity of the electric field E volts/mm established between the photoconductive photosensitive screen and the dielectric coated record medium, i.e. K=V/E smaller than about 0.18 for the purpose of preventing enlargement of dots of the copy picture image.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an electrographic process of producing, on a 
dielectric coated record medium, an electrostatic copy latent image by 
modulating a flow of corona ions with the aid of an electrostatic latent 
image which has been produced on a photoconductive photosensitive screen 
having a number of openings. 
2. Description of the Prior Art 
Various kinds of the above described electrographic processes have been 
well known. In such conventional electrographic process, the electrostatic 
copy latent image produced on the dielectric coated record medium is 
developed to obtain a copy picture image or the developed image is 
transferred to another record medium to obtain a copy picture image. The 
copy picture image thus obtained is decomposed into picture elements due 
to the openings of the photoconductive photosensitive screen. As a result, 
the conventional electrographic process has the drawbacks that the 
resolving power of the copy picture image becomes degraded, that the 
concentration of a thin line-shaped picture image becomes smaller than 
that of a thick line-shaped picture image, and that the contour of the 
peripheral portions of the picture image becomes unclear. 
The above described drawbacks are caused by a phenomenon that the electric 
field established between the photoconductive photosensitive screen and 
the dielectric coated record medium is disturbed by the electrostatic copy 
latent image produced on the dielectric coated record sheet. 
Experimental tests have demonstrated the result that the above mentioned 
phenomenon is associated with an intensity of the electric field 
established between the photoconductive photosensitive screen and the 
dielectric coated record medium, a maximum surface potential of the 
electrostatic copy latent image produced on the dielectric coated record 
medium, an electrostatic capacity of the dielectric coated record medium, 
etc. 
SUMMARY OF THE INVENTION 
An object of the invention, therefore, is to provide an electrographic 
process which can obtain a copy picture image excellent in concentration 
and picture quality by suitably selecting the above mentioned various 
conditions and combining them in a correct manner. 
A feature of the invention is the provision of an electrographic process of 
producing, on a dielectric coated record medium, an electrostatic copy 
latent image by modulating a flow of corona ions with the aid of an 
electrostatic latent image which has been produced on a photoconductive 
photosensitive screen, characterized by making a ratio K of a maximum 
surface potential V volts of the electrostatic copy latent image produced 
on said dielectric coated record medium to an intensity of the electric 
field E volts/mm established between said photoconductive photosensitive 
screen and said dielectric coated record medium, i.e. K=V/E smaller than 
about 0.18 for the purpose of preventing enlargement of dots of the copy 
picture image. 
Another feature of the invention is the provision of an electrographic 
process wherein said dielectric coated record medium has an electrostatic 
capacity of larger than about 500 pF per 1 cm.sup.2. 
A further feature of the invention is the provision of an electrographic 
process wherein said intensity of the electric field E volts/mm 
established between said photoconductive photosensitive screen and said 
dielectric coated record medium is about 500 to 1,000 volts/mm. 
A still further feature of the invention is the provision of an 
electrographic process wherein as said dielectric coated record medium use 
is made of an electrostatic record sheet composed of a substrate having 
upper and lower electrically conductive surfaces and a surface insulating 
layer coated on one side of said upper and lower electrically conductive 
surfaces, the other electrically conductive surface of said substrate 
having a surface specific resistance of about 2.times.10.sup.6 to 
2.times.10.sup.9 .OMEGA. and said electrostatic record sheet having an 
electrostatic capacity of about 500 to 1,500 pF per 1 cm.sup.2. 
Another feature of the invention is the provision of an electrographic 
process wherein as said dielectric coated record medium use is made of an 
electrostatic record sheet composed of a substrate having upper and lower 
electrically conductive surfaces and a thickness of about 50 to 100 .mu. 
and a surface insulating layer coated on one side of said upper and lower 
electrically conductive surfaces and having a thickness of about 5 .mu., 
said surface insulating layer containing a matting agent such as an 
insulating resin, metal oxide or the like, the other electrically 
conductive surface of said substrate having a surface specific resistance 
of about 2.times.10.sup.6 to 2.times.10.sup.9 .OMEGA. and said 
electrostatic record sheet having an electrostatic capacity of about 500 
to 1,500 pF per 1 cm.sup.2. 
That is, in the present invention, the ratio K of a maximum surface 
potential V volts of the electrostatic copy latent image produced on the 
dielectric coated record medium to the intensity of the electric field E 
volts/mm established between the photoconductive photosensitive screen and 
the dielectric coated record medium, i.e. K=V/E is made smaller than about 
0.18.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will now be described in greater detail with reference to the 
accompanying drawings and experimental examples to be described later. 
FIG. 1 is a schematic cross-sectional view illustrating a step of 
producing, on a dielectric coated record medium, an electrostatic copy 
latent image according to the invention. In the present embodiment, a 
photoconductive photosensitive screen 1 is composed of an electrically 
conductive mesh 2 having 100 to 300 meshes, an insulating layer 3 coated 
on one side of the mesh 2, an electrically conductive layer 4 coated on 
the insulating layer 3, and a photoconductive layer 5 coated on the other 
side of the mesh 2. The photoconductive layer 5 of the photoconductive 
photosensitive screen 1 is uniformly charged with a positive polarity by 
means of a corona discharge device, then exposed to an optical image so as 
to produce thereon an electrostatic latent image corresponding to the 
optical image and subsequently a step of producing, on a dielectric coated 
record medium, an electrostatic copy latent image is effected. 
In the step of producing, on the dielectric coated record medium, an 
electrostatic copy latent image shown in FIG. 1, a dielectric coated 
record medium 7 disposed on a field electrode 6 is opposed to and 
separated from the photoconductive layer 5 of the photoconductive 
photosensitive screen 1 bearing an electrostatic latent image. In the 
present embodiment, the dielectric coated record medium 7 is composed of 
an electrically conductive substrate 9, a dielectric layer 8 coated on one 
side of the substrate 9 and a field electrode 6 coated on the other side 
of the substrate 9. Above the electrically conductive layer 4 of the 
photoconductive photosensitive screen 1 is arranged a corona discharge 
device 10 which functions to direct a flow of corona ions having a 
polarity which is opposite to that of electric charge of the electrostatic 
latent image produced on the photoconductive layer 5 and which is a 
negative polarity in the present embodiment toward the electrically 
conductive layer 4. Between the electrically conductive mesh 2 of the 
photoconductive photosensitive screen 1 and the electrically conductive 
layer 4 is connected a screen bias source 11 and between the electrically 
conductive mesh 2 and the field electrode 6 is connected a field electrode 
bias source 12. 
The bias voltage supplied from the screen bias source 11 functions to 
establish an electric field which prevents the flow of corona ions having 
the negative polarity and directed from the corona discharge device 10 
from passing through openings of the photoconductive photosensitive screen 
1 at its imagewise exposed area, that is, that part of the photoconductive 
layer 5 on which the latent image electric charge is absent. The bias 
voltage supplied from the field electrode bias source 12 functions to 
establish an electric field which causes the flow of corona ions directed 
from the corona discharge device 10 and having the negative polarity and 
passed through the openings of the photoconductive photosensitive screen 1 
at the imagewise dark area thereof, that is, at that part of the 
photoconductive layer 5 at which the latent image electric charge is 
present to reach the dielectric coated record medium 7 without diffusing. 
As a result, the flow of corona ions having the negative polarity and 
uniformly directed from the corona discharge device 10 is modulated by the 
electrostatic latent image which has been introduced on the 
photoconductive photosensitive screen 1 to produce, on the dielectric 
coated record medium 7, an electrostatic copy latent image corresponding 
to the electrostatic latent image on the photoconductive photosensitive 
screen 1 and composed of the electric charge having the negative polarity. 
The electrostatic copy latent image is made visible by a toner charged 
with a positive polarity, for example, and then subjected to a fixing step 
to provide a final copy picture image. 
FIG. 2 is a schematic cross-sectional view illustrating a phenomenon of 
distorting the electric field of the field electrode by the electric 
charge on the dielectric coated record medium 7 in the step of producing, 
on the dielectric coated record medium 7, an electrostatic copy latent 
image and of enlarging the dots of the electrostatic copy latent image 
thus produced. 
In FIG. 2, equipotential lines are shown by a full line located between the 
photoconductive photosensitive screen 1 and the dielectric coated record 
medium 7. These equipotential lines are rectilinear and not distorted when 
the electrostatic copy latent image is not produced on the dielectric 
coated record medium 7 or when the electrostatic copy latent image is 
produced on the dielectric coated record medium 7, but the potential 
thereof is low. 
If the flow of corona ions passed through the openings of the 
photoconductive photosensitive screen 1 arrives at the dielectric coated 
record medium 7 and the potential of the electrostatic copy latent image 
is raised up in the negative direction, the potential at this portion 
becomes changed toward the potential of the photoconductive photosensitive 
screen 1. As a result, the equipotential lines are distorted toward the 
electrostatic copy latent image. This is because of the fact that the 
field electrode 6 is applied with a voltage having a polarity of directing 
the flow of corona ions for producing the electrostatic copy latent image 
toward the dielectric coated record medium 7, that is, a voltage having a 
positive polarity which is opposite to that of the flow of corona ions, 
and that the voltage is cancelled by the flow of corona ions arrived at 
the dielectric coated record medium 7. 
In FIG. 2, electric lines of force are shown by dotted lines drawn 
perpendicular to the equipotential line. The flow of corona ions moves 
along the electric lines of force. As a result, if the equipotential line 
is distorted, the electric lines of force become also distorted and hence 
the flow of corona ions moves along the distorted electric lines of force. 
This phenomenon occurs particularly at the edge portion of the 
electrostatic copy latent image to enlarge the dots at the edge portion, 
thereby inducing fading of the copy picture image. 
In order to prevent the dots from enlarging due to distortion of the 
electric lines of force, it has been proposed to determine the field 
electrode bias voltage such that the intensity of the electric field of 
the field electrode is higher than 500 v/mm so as to restrict the corona 
charging width. Experimental tests under such condition have demonstrated 
the result that excellent copy picture images are not always obtained 
owing to the following reasons. 
FIGS. 3a, 3b, 3e and 3d are enlarged schematic cross-sectional views 
illustrating mode of distorting the electric field of the field electrode 
by the electrostatic copy latent image. In this case, the intensity of the 
electric field of the field electrode is made 1,000 v/mm which is higher 
than the above mentioned 500 v/mm. 
As shown in FIG. 3a, if the electrostatic copy latent image is absent on 
the dielectric coated record medium 7, equipotential lines of 100 v 
equally spaced apart from each other by 0.1 mm extend in the direction 
from the dielectric coated record medium 7 toward the photoconductive 
photosensitive screen (not shown). 
Let it be assumed that a flow of corona ions having a diameter of 0.12 mm 
is directed toward the dielectric coated record medium 7. At first, the 
flow of corona ions is directed as it is toward the dielectric record 
medium 7. If the flow of corona ions arrived at the dielectric coated 
record medium 7 causes the electrostatic copy latent image to obtain a 
potential of -50 v, an equipotential line of -100 v becomes distorted in a 
direction toward the electrostatic copy latent image, but it does not 
reach the surface of the dielectric coated record medium 7 as shown in 
FIG. 3b. In this case, respective equipotential lines of -200 v, -300 v . 
. . are also distorted, but the degree of distortion is gradually 
decreased in the order as mentioned above. 
If the electrostatic copy latent image potential reaches -100 v, the 
equipotential line of -100 v completely passes through the surface of the 
dielectric coated record medium 7 and the degree of distortion of 
respective equipotential lines of -200 v, -300 v . . . becomes increased 
as shown in FIG. 3c. 
If the electrostatic copy latent image potential is further raised up to 
-200 v, the equipotential line of -200 v passes through the surface of the 
dielectric coated record medium 7 and the equipotential line of -100 v is 
penetrated into the dielectric coated record medium 7 through 
substantially outside the electrostatic copy latent image. It is a matter 
of course that respective equipotential lines of -300 v, -400 v . . . are 
further distorted as shown in FIG. 3d. As a result, the electric lines of 
force shown by dotted lines, that is, the flow of corona ions is further 
distorted, thereby considerably enlarging the dots. 
In practice, the above described enlarging action of the dot is 
continuously increased by the arrival of the flow of corona ions, so that 
the dot has a peak point at its center and is gradually inclined toward 
its periphery so as to form a mountain-shaped dot. 
This dot enlarging action is inversely proportional to the intensity of the 
electric field of the field electrode established between the 
photoconductive photosensitive screen and the dielectric coated record 
medium. That is, the higher the intensity of the electric field of the 
field electrode the lesser the dot enlarging action. In practice, use is 
made of the intensity of the electric field of the field electrode of 
about 500 to 1,000 v/mm, preferably about 800 v/mm. 
If the intensity of the electric field of the field electrode exceeds 1,000 
v/mm, it becomes near the dielectric breakdown voltage of air and there is 
a risk of the corona discharge or spark discharge being produced between 
the photoconductive photosensitive screen and the dielectric coated record 
medium. 
As can be seen from the above description with reference to FIG. 3, the dot 
enlarging action is influenced by not only the intensity of the electric 
field of the field electrode but also by the maximum surface potential of 
the electrostatic copy latent image produced on the dielectric coated 
record medium. 
As a result, it is not always possible to obtain satisfactory copy of the 
picture images under the above described conditions of the intensity of 
the electric field of the field electrode and corona charging width as 
proposed by the conventional electrographic process. 
In addition, the action of the maximum surface potential of the 
electrostatic copy latent image produced on the dielectric record medium 
exerted on the electric field of the field electrode is somewhat charged 
by the field electrode bias voltage value. For example, let it be assumed 
that the intensity of the electric field of the field electrode is 
constant. If a distance between the field electrode 6 or the electrically 
conductive substrate 9 of the dielectric coated record medium 7 (FIG. 1) 
on the one hand and the photoconductive photosensitive screen 1 on the 
other hand is made short and the field electrode bias voltage value is 
made small, the amount of fading, that is, the dot enlarging action 
becomes somewhat increased, even when the electrostatic copy latent image 
potential is the same, if compare with the case when the above mentioned 
distance is long. 
In the present invention, the maximum surface potential of the 
electrostatic copy latent image produced on the dielectric coated record 
medium is determined to a value lower than 100 v to 150 v and hence the 
field electrode bias voltage value becomes for higher than the 
electrostatic copy latent image potential, so that it is possible to 
disregard the dot enlarging action due to the higher or lower field 
electrode bias voltage value. As a result, it is conceivable that the dot 
enlarging action, that is, amount of fading the copy picture image is 
determined by a ratio of the maximum surface potential of the 
electrostatic copy latent image produced on the dielectric coated record 
medium to the intensity of the electric field established between the 
photoconductive photosensitive screen and the dielectric coated record 
medium. 
The above described dot enlargement results in degradation of the resolving 
power of the copy picture image, of the concentration of the thin 
line-shaped copy picture image, of the contrast of the copy picture image, 
etc. and hence exerts an important influence upon the picture quality of 
the copy picture image. That is, if the dot is enlarged, that portion of 
the copy picture image at which lines of a letter, for example, are close 
with each other becomes deformed into a black point or thin line-shaped 
picture image becomes further thin and difficult to be discerned. 
FIGS. 4a and 4b show thin line-shaped and thick line-shaped copy picture 
image obtained when the dot is not enlarged, respectively. In these cases, 
the picture images are formed of clear dots, respectively. 
FIGS. 4c and 4d show thin line-shaped and thick line-shaped copy picture 
images obtained when the maximum surface potential of the electrostatic 
copy latent image produced on the dielectric record medium is increased so 
as to enlarge the dots, respectively. As described above with reference to 
FIGS. 2 and 3a to 3d, the dot is usually enlarged at the distorted part of 
the equipotential line and an extremely small distortion of the 
equipotential line occurs at the center part of the picture image having a 
relatively thick line and wide area, and hence the dot is not so much 
enlarged at such part. As a result, at the center part of the thick 
line-shaped picture image shown in FIG. 4d, the dots are enlarged and such 
enlarging action is ceased when the dots are brought closer together. But, 
at the periphery of the thick line-shaped image shown in FIG. 4d and the 
thin line-shaped image shown in FIG. 4c, the dots become enlarged in 
response to the increase of the maximum surface potential of the 
electrostatic copy latent image produced on the dielectric coated record 
medium. As a result, in the thin line-shaped image shown in FIG. 4c, the 
enlargement of the dots, that is, the dispersion enlargement of the 
electric charge results in a decrease of the density of the electric 
charge, and as a result, the thin line-shaped image shown in FIG. 4c 
becomes a faded line image which is lower in concentration than the thin 
line-shaped image shown in FIG. 4a. On the contrary, in the thick 
line-shaped image or wide area image shown in FIG. 4d, the dots are 
considerably enlarged at their periphery and brought closer together at 
their center, so that the electric charge density is not so much 
decreased. As a result, the picture image other than the periphery thereof 
becomes higher in concentration than the thick line-shaped image shown in 
FIG. 4c, but the picture image at the periphery becomes faded and lowered 
in concentration. In this way, the enlargement of the dots causes the 
quality of the copy picture image to considerably deteriorated, thereby 
rendering difficult to discern the copy picture image. 
As described above, the enlargement of the dots is determined by the 
maximum surface potential of the electrostatic copy latent image produced 
on the dielectric coated record medium and the intensity of the electric 
field established between the photoconductive photosensitive screen and 
the dielectric coated record medium. As a result, let the maximum surface 
potential of the electrostatic copy latent image produced on the 
dielectric coated record medium be V volt and let the intensity of the 
electric field established between the photoconductive photosensitive 
screen and the dielectric coated record medium be E volt/mm, the amount of 
enlargement of the dots K is given by K=V/E. 
It is preferable to make the value of K small. For this purpose, the 
intensity of the electric field of the field electrode E volts/mm must be 
made high. But, the intensity of the electric field of the field electrode 
E must be restricted as above described. In practice, the upper limit of 
the intensity of the electric field of the field electrode E is made 500 
to 1,000 volt/mm and the maximum surface potential V volts of the 
electrostatic copying latent image produced on the dielectric coated 
record medium is made small. 
The maximum surface potential V volts of the electrostatic copying latent 
image produced on the dielectric coated record medium is given by V=Q/C, 
where C is an electrostatic capacity of the dielectric coated record 
medium and Q is an amount of electric charge for producing the 
electrostatic copy latent image on the dielectric coated record medium. As 
a result, in order to make V small, it is necessary to make Q small or 
make C large. However, it is not desirous to make Q small in order to 
practically ensure the concentration of the picture image due to the 
development. In addition, the large and small amount of Q means the large 
and small amount by the flow of corona ions for producing the 
electrostatic copy latent image. As a result, in order to provide a high 
speed electrographic apparatus, it is impossible to make the value of C so 
much large. It is also conceivable to make C of the dielectric coated 
record medium large so as to increase the amount of the flow of corona 
ions. The use of such measures, however, is an obstacle to provide a high 
speed electrographic apparatus comprising a photoconductive photosensitive 
screen or a drum-shaped dielectric coated record medium. 
Concerning the concentration of the copy picture image, in order to obtain 
the maximum concentration of the picture image by means of the same amount 
of electric charge Q, it is necessary to make C small so as to make the 
maximum surface potential V volts of the electrostatic copy latent image 
large. But, as described above, it is not desirous to make V large in 
association with the enlargement of dots and the intensity of the electric 
field of the field electrode E volts/mm. 
The above described analysis of various points of view can be summarized as 
follows. 
(1) In order to prevent the enlargement of dots, a value given by K=V/E is 
made smaller than a certain value. 
(2) The maximum surface potential V volts of the electrostatic copy latent 
image allowable for 500 to 1,000 volts/mm which is a value of the 
practically allowable intensity of the field electrode electric field E 
has its upper limit. 
(3) In order to satisfy the condition required for the above mentioned 
maximum surface potential V volts of the electrostatic copy latent image 
and to ensure the practical development concentration, the amount of 
electric charge Q for producing the electrostatic copy latent image has 
its lower limit. 
(4) In order to provide a high speed electrographic apparatus, the amount 
of electric charge Q for producing the electrostatic copy latent image has 
its upper limit. 
In order to determine the above mentioned limit values, the inventors have 
effected experimental tests of producing copy picture images by using 
various kinds of electrostatic record sheets as the dielectric coated 
record medium. In the experimental tests, use was made of a drum-shaped 
photoconductive photosensitive screen having a diameter of 205 mm and 
required 3.64 seconds for its one rotation. The distance between the 
drum-shaped photoconductive photosensitive screen and the field electrode 
was 4.8 mm. The field electrode bias voltage was 4 kv. The electrostatic 
record sheet was fed at a speed of 160 mm/sec. The corona discharge device 
for directing the flow of corona ions for producing the electrostatic copy 
latent image had an area of directing the flow of corona ions of 40 
mm.times.280 mm. The amount of corona electric current passing through the 
openings of the drum-shaped photoconductive photosensitive screen and 
directed toward the field electrode was 46 .mu.A when the total surface of 
the picture image was dark. The apparatus had an ability of producing a 
copy picture image of 27.94 cm.times.43.18 cm (11 inch.times.17 inch) at a 
speed of 16.5 sheets/min. As a development system, use was made of a wet 
type development system. 
The experimental tests on the copy picture image concentration effected by 
the above mentioned apparatus with the aid of the various kinds of 
electrostatic record sheets have demonstrated the result shown in the 
following Table 1. 
TABLE 1 
______________________________________ 
Electrostatic 
Electrostatic Copy picture 
Sample record sheet 
copy latent Picture 
image con- 
number # image potential 
quality 
concentration 
______________________________________ 
No. 1 #1 .circleincircle.280V 
1 1.08 
No. 2 " .circle.100V 
3 0.49 
No. 3 #2 .circleincircle.210V 
1 0.94 
No. 4 " .circle.100V 
3 0.53 
No. 5 #3 .circleincircle.210V 
2 1.0 
No. 6 " .circle.100V 
3 0.68 
No. 7 #4 .circleincircle.180V 
2 1.12 
No. 8 " .circle.100V 
3 0.85 
No. 9 #5 .circleincircle.170V 
3 1.07 
No. 10 " .circle.100V 
4 0.81 
No. 11 #6 .circleincircle.160V 
3 0.94 
No. 12 " .circle.100V 
5 0.71 
No. 13 #7 .circleincircle.150V 
2 0.72 
No. 14 " .circle.100V 
3 0.69 
No. 15 #8 .circleincircle.135V 
3 0.82 
No. 16 " .circle.100V 
3 0.72 
No. 17 #9 .circleincircle.115V 
3 0.91 
No. 18 #10 .circleincircle.110V 
5 0.94 
No. 19 " .circle.140V 
4 1.12 
No. 20 #11 .circleincircle.80V 
5 0.77 
No. 21 " .circle.140V 
4 0.88 
No. 22 #12 .circleincircle.80V 
4 0.65 
No. 23 " .circle.100V 
4 0.87 
No. 24 #13 .circleincircle.80V 
4 0.79 
No. 25 " .circle.100V 
4 0.93 
No. 26 #14 .circleincircle.60V 
5 0.72 
No. 27 " .circle.80V 5 0.96 
______________________________________ 
In the above Table 1, the electrostatic record sheet #1 to #14 include the 
conventional electrostatic record sheets and the electrostatic record 
sheets adapted to be used in the electrographic method according to the 
invention. Data designated by a symbol in the electrostatic copy latent 
image potential are obtained under the above mentioned condition of the 
standard amount of corona electric current. Data designated by a symbol 
are obtained by intentionally changing the condition of the amount of 
corona electric current. The picture quality was judged by functional 
tests on the copy picture images obtained with respect to sharpness, 
resolving power, thin line-shaped picture image concentration or the like. 
Data 5 and 4 represent a practically good picture quality. 3 represents a 
picture quality which exhibits some drawback in the case of reproducing 
small Japanese letters or the like, but has no problem in the case of 
reproducing letters having usual size. 
FIG. 5 shows a graph illustrating the experimental test result listed in 
the Table 1 and showing the relation between the electrostatic copy latent 
image potential plotted on the abscissa and the picture quality plotted on 
the ordinate. In FIG. 5, the judgement of the picture quality and the 
meaning of the symbols and are the same as those described with 
reference to the Table 1. 
As can be seen from FIG. 5, if the intensity of the electric field E 
volts/mm established between the photoconductive photosensitive screen and 
the electrostatic record sheet is given by E=4 kv/4.8 mm=833 v/mm, the 
allowable electrostatic copy latent image potential V should be lower than 
about 150 volts. The picture quality becomes different depending on the 
kind of the electrostatic record sheet. Even though such picture quality 
difference is taken into consideration, the copy picture image having a 
good picture quality could not be obtained if the electrostatic copy 
latent image potential V is higher than 200 volts. In general, a good 
picture quality is obtained when the electrostatic copy latent image 
potential V is lower than 100 volts. In the case of an electrostatic 
record sheet considerably suitable for the electrographic process 
according to the invention, a good picture quality is occasionally 
obtained even when the above potential is about 150 volts. 
FIG. 6 is a graph showing the relation between the electrostatic copy 
latent image potential V plotted on the abscissa and the copy picture 
image concentration plotted on the ordinate. In FIG. 6, the meaning of the 
symbols , is the same as that described with reference to the Table 1. 
A symbol .cndot. designates a picture quality larger than 4 in the Table 1 
and a symbol designates a picture quality smaller than 3. As can be seen 
from FIG. 6, the copy picture image concentration in the standard amount 
of corona electric current () tends to gradually increase at those 
electrostatic copy latent image potentials which are higher than about 150 
volts, but tends to suddenly decrease at those electrostatic copy latent 
image potentials which are lower than about 150 volts. In the 
electrostatic copy latent image potential higher than 150 volts, if the 
amount of corona electric current is decreased to 100 volts, the copy 
picture image concentration becomes low and the copy picture image having 
bad picture quality only is obtained. 
In the electrostatic record sheet which makes the electrostatic copy latent 
image potential low and makes the copy picture image concentration low 
under the standard corona electric current condition (Table 1, #10 to 
#14), if the amount of corona electric current is increased to increase 
the amount of charge, it is possible to obtain a copy picture image having 
a high concentration and good picture quality. As described with reference 
to FIG. 5, even in the above described case, it is desirous to make the 
copy picture image concentration sufficiently high within that range of 
the electrostatic copy latent image potential which is lower than 150 
volts. 
The electrostatic capacity of the electrostatic record sheet which can 
obtain the copy picture image having a high concentration and good picture 
quality will now roughly be estimated by taking the above mentioned facts 
into consideration. In the first place, the amount of charge Q per 1 
cm.sup.2 of the electrostatic record sheet is given by 
##EQU1## 
where I.times.t is a total amount of electric charge flowing during one 
rotation of the drum-shaped photoconductive photosensitive screen and S is 
an area of the electrostatic record sheet to be scanned during one 
rotation of the drum-shaped photoconductive photosensitive screen. The 
above amount of Q is obtained under the standard corona electric current 
condition. Under this condition, the electrostatic capacity C of the 
electrostatic record sheet for obtaining the electrostatic copy latent 
image potential of 200 volts is given by 
##EQU2## 
Similarly, if the electrostatic copy latent image potential is made 150 
volts, the electrostatic capacity C of the electrostatic record sheet 
becomes 620 pF and if the electrostatic copy latent image potential is 
made 100 volts, the electrostatic capacity C of the electrostatic record 
sheet becomes 930 pF. As a result, the electrostatic capacity C of the 
electrostatic record sheet required for obtaining a copy picture image 
having a high concentration and good picture quality is about 500 to 1,500 
pF/1 cm.sup.2. 
The electrostatic capacity of the electrostatic record sheet occasionally 
shows values different from each other depending on the method of applying 
the electric charge, charging time and the electric charge density. 
In the electrographic process according to the invention, it is preferable 
to direct a flow of corona ions having a Q of 0.2.times.10.sup.-7 to 
5.times.10.sup.-7 coulombs per 1 cm.sup.2 of the electrostatic record 
sheet toward the drum-shaped photoconductive photosensitive screen for 
0.05 to 0.5 second with the intensity of the electric field of 500 to 
1,000 volts/mm established between the drum-shaped photoconductive 
photosensitive screen and the dielectric coated record medium. 
The above described condition required for the electrostatic capacity of 
the electrostatic record sheet is similarly applicable to an intermediate 
transfer member used as the dielectric coated record medium. In this case, 
on the intermediate transfer member is produced the electrostatic copy 
latent image which is then developed into a toner image and this toner 
image is transferred onto a usual paper to obtain a copy picture image. 
The electrostatic capacity C of the intermediate transfer member is 
determined in association with the material of the dielectric layer and 
thickness thereof and given by C=.epsilon..sub.o .epsilon..sub.s S/d, 
where .epsilon..sub.o =8.854.times.10.sup.-12 F/m, .epsilon..sub.s is a 
specific inductive capacity of the dielectric layer, S is an area of the 
dielectric layer and d is a thickness of the dielectric layer. 
Now, the thickness d of the dielectric layer satisfying the above described 
electrostatic capacity condition and formed of synthetic resin is given by 
##EQU3## 
where .epsilon..sub.s is usually 2 to 5, but is represented by 3 and C is 
1,000 pF per 1 cm.sup.2 and hence 1.times.10.sup.7 pF per 1 cm.sup.2. 
Similarly, the thickness d of the dielectric layer having an electrostatic 
capacity C of 500 pF per 1 cm.sup.2 is 5.32.mu.. 
If the dielectric layer is formed of a glass containing lead oxide and 
having a low melting point, its specific inductive capacity of the 
dielectric layer is about 20. In this case, the thickness d of the 
dielectric layer having an electrostatic capacity C of 1,000 pF/1 cm.sup.2 
is 17.7.mu. and the thickness d of the dielectric layer having an 
electrostatic capacity C of 500 pF/1 cm.sup.2 is 35.5.mu.. 
If the dielectric layer is formed of aluminum oxide, its specific inductive 
capacity .epsilon..sub.s is 8.6 to 10.55 and can be represented by 
.epsilon..sub.s =10. In this case, the thickness d of a dielectric layer 
having an electrostatic capacity C of 1,000 pF/1 cm.sup.2 must be 9.mu. 
and the thickness d of a dielectric layer having an electrostatic capacity 
C of 500 pF/1 cm.sup.2 must be 18.mu.. 
In the above described intermediate transfer member, the dielectric layer 
formed of synthetic resin may be formed by a spraying, dipping into resin 
solution, vapor phase polymerization process or the like. In this case, 
the synthetic resin layer is relatively thin in thickness, so that there 
is a risk of the synthetic resin layer being peeled off the electrically 
conductive substrate for each of adhering force therebetween. In order to 
eliminate such drawback, the synthetic resin layer is coated on another 
synthetic resin layer made electrically conductive by adding carbon 
powders, metal powders, etc. 
When the dielectric layer is formed of glass having a high dielectric 
constant, such dielectric layer may be formed by dipping the substrate 
into a glass solution. The dielectric material for forming the dielectric 
layer by vacuum deposition method is MgF.sub.2, ZnS, CeO.sub.2, SiO, 
SiO.sub.2, etc. Each of these materials may be formed into a dielectric 
layer having a thickness corresponding to each dielectric constant of 
these materials. 
In the above described intermediate transfer member, the thickness of the 
dielectric layer having the electrostatic capacity of 1,000 pF/1 cm.sup.2 
to 500 pF/1 cm.sup.2 has been calculated. The thickness of the dielectric 
layer having the electrostatic capacity of 700 pF/1 cm.sup.2, which is 
intermediate between 1,000 pF/1 cm.sup.2 and 500 pF/1 cm.sup.2 and 
preferable in practice is 3.8.mu. when the specific inductive capacity 
.epsilon..sub.s =3, 13.mu. when the specific inductive capacity 
.epsilon..sub.s =10 and 26.mu. when the specific inductive capacity 
.epsilon..sub.s =20. 
The results investigated in detail as above described can be summarized as 
follows. 
(1) The condition given by K=V/E should be smaller than 0.18 is determined 
by the conditions that the electrostatic copy latent image potential V 
allowable for the purpose of eliminating the fading in the copy picture 
image should be lower than about 150 volts and that the intensity of the 
electric field E of the field electrode is 4 kv/4.8 mm.perspectiveto.830 
volts/mm. 
(2) Under the above condition (1), in practice the intensity of the 
electric field of the field electrode E is determined to a value within a 
range between 500 volts/mm and 1,000 volts/mm. 
(3) Under the above described condition required for the electrostatic copy 
latent image potential, the dielectric coated record medium is required to 
cause the copy picture image concentration to increase sufficiently high. 
For this purpose, as described with reference to FIG. 6, the amount of 
charge per 1 cm.sup.2 of the electrostatic record sheet is required to be 
at least 0.2.times.10.sup.-7 coulomb/cm.sup.2, preferably at least 
1.times.10.sup.-7 coulomb/cm.sup.2. In this case, the electrostatic copy 
latent image potential V should be lower than about 150 volts. 
(4) It is preferable that the intensity of the electric field of the field 
electrode E is a value within a range between 700 volts/mm and 900 
volts/mm and the electrostatic capacity of the dielectric coated record 
medium is at least 500 pF/1 cm.sup.2. In the case of a high speed 
recording, use is made of a dielectric coated record medium having an 
electrostatic capacity of about 500 to 1,500 pF/1 cm.sup.2 and the above 
mentioned K=V/E has a value of smaller than 0.18. 
(5) The above described conditions (1) to (4) are effectively applicable to 
the electrographic process which makes use of the electrostatic record 
sheet as the dielectric coated record medium. 
(6) The above described conditions (1) to (4) are effectively applicable to 
an electrographic process which makes use of an intermediate transfer 
member such as an intermediate transfer drum or belt, etc. which functions 
as a dielectric coated record medium for producing an electrostatic copy 
latent image thereon and in which the electrostatic copy latent image is 
developed into a toner image which is then transferred onto a usual paper. 
(7) The electrostatic record sheet having an electrostatic capacity of 500 
to 1,500 pF/1 cm.sup.2 is effectively applicable to the electrographic 
process according to the invention. 
(8) The intermediate transfer member having an electrostatic capacity of 
500 to 1,500 pF/1 cm.sup.2 is also effectively applicable to the 
electrographic process according to the invention. 
(9) The intermediate transfer member described in the condition (8) is 
composed of an electrically conductive substrate and a film coated thereon 
and formed of synthetic resin, aluminum oxide, various kinds of inorganic 
dielectrics such as MgF.sub.2, ZnS, CeO.sub.2, SiO, SiO.sub.2, etc. to be 
vapor deposited under vacuum, glass, ceramics, etc. 
FIG. 7 is a schematic cross-sectional view of one embodiment of the 
apparatus for carrying out the electrographic process according to the 
invention. In the present embodiment, use is made of a drum-shaped 
photoconductive photosensitive screen 13 having a diameter of 20.5 cm and 
effective recording width in the axial direction of 28 cm. The drum-shaped 
photoconductive photosensitive screen 13 is rotated in a direction shown 
by an arrow at a rate of one rotation per 3.64 seconds. A manuscript 
carriage 14 moves in a direction shown by an arrow in synchronism with the 
rotation of the drum-shaped photoconductive photosensitive screen 13. A 
manuscript (not shown) disposed on the manuscript carriage 14 is exposed 
to light emitted from an illumination lamp 15 and reflected by a light 
source mirror 16. A light reflected by the manuscript passes through an 
optical image mirror 17, projection lens 18 and light exposure window 19 
and is incident on the drum-shaped photoconductive photosensitive screen 
13. The light exposure window 19 is provided in a shield member 20 for 
surrounding the drum-shaped photoconductive photosensitive screen 13 
except the window 19 and for preventing the screen 13 from adhering with 
dust, etc. The drum-shaped photoconductive photosensitive screen 13 is 
uniformly charged with a positive polarity by means of a first corona 
discharge device 21 arranged inside the drum and then exposed to an 
optical image, thereby producing thereon an electrostatic latent image 
corresponding to the optical image. 
A roll-shaped electrostatic record sheet 22 is enlarged in a roll sheet 
cassette 23 and cut into a given length of segment by means of a cutter 24 
in synchronism with the rotation of the drum-shaped photoconductive 
photosensitive screen 13. The electrostatic record sheet segment is fed 
through a paper guide 25 onto a vacuum suction conveyor belt 26. It should 
be noted that the electrostatic record sheet 22 must satisfy the above 
mentioned electrostatic capacity condition. The roll sheet cassette 23 
functions to shield the electrostatic record sheet 22 from the open air so 
as to maintain its stabilized ability and prevent the front end of the 
record sheet from getting clogged. The vacuum suction conveyor belt 26 
functions also as a field electrode and is arranged around a vacuum 
suction box 27 which functions to suck the electrostatic record sheet 
segment onto the vacuum suction conveyor belt 26. The distance between the 
vacuum suction conveyor belt 26 and the drum-shaped photoconductive 
photosensitive screen 13 is adjusted to 4.8 mm and between which is 
applied a bias voltage of 4 kv. A second corona discharge device 28 is 
arranged inside the drum-shaped photoconductive photosensitive screen 13 
and located at a position opposed to the vacuum suction conveyor belt 26. 
If the electrostatic record sheet segment arrives at the vacuum suction 
conveyor belt 26, the second corona discharge device 28 functions to 
direct a flow of corona ions toward the electrostatic record sheet 22. The 
flow of corona ions is modulated by the electrostatic latent image which 
has been produced on the drum-shaped photoconductive photosensitive screen 
13 to produce, on the electrostatic record sheet 22, an electrostatic copy 
latent image corresponding to the electrostatic latent image which has 
been produced on the drum-shaped photoconductive photosensitive screen 13. 
To the corona discharge wire of the second corona discharge device is 
applied a high direct current voltage of 10 to 11 kv which is negative 
with respect to the drum-shaped photoconductive photosensitive screen 13 
from a voltage source 39 shown in FIG. 8. A shield member 29 of the corona 
discharge device is provided at the inner periphery of its opening through 
which the flow of corona ions passes with an electrically conductive 
member 30 for controlling the width of the flow of corona ions. The 
electrically conductive member 30 is connected through a resistor 40 whose 
resistance value is 100 M.OMEGA. to the drum-shaped photoconductive 
photosensitive screen 13. The electrically conductive member 30 is held at 
-3 to -4 kv due to its self-biasing action and functions to prevent 
occurrence of spark between the corona discharge wire and the electrically 
conductive member 30. The amount of the corona electric current of the 
second corona discharge device 28 is determined such that about 46 .mu.A 
of electric current flows from the second corona discharge device 28 
through the drum-shaped photoconductive photosensitive screen 13 to the 
electrostatic record sheet corresponding to the picture image of totally 
black surface. 
The electrostatic copy latent image produced on the electrostatic record 
sheet 22 is fed to a liquid developing device 31 and made visible by the 
latter. The liquid developing device 31 comprises three pairs of 
electrically conductive rollers 32a, 32b, 32c and four pairs of stationary 
developing electrodes 33 which also function as paper guides. The 
electrostatic record sheet whose electrostatic copy latent image has been 
developed into the visible image passes through a pair of squeeze rollers 
34, a pair of suction rollers 35 and drying rollers 36 and becomes 
completely dried. The dry electrostatic record sheet then passes through a 
pair of delivery rollers 37 and is superimposed upon a tray 38. 
In the case of obtaining a plurality of copy picture images from one 
manuscript, the step of producing the electrostatic copy latent image only 
is carried out for desired number of times. In this case, it is possible 
to obtain 16.5 copy picture images of 27.94 .times. 43.18 cm (11 inch 
.times. 17 inch) every 1 minute. After a desired number of copy picture 
images have been obtained, the drum-shaped photoconductive photosensitive 
screen 13 is uniformly exposed to light, etc. in the rear of the first 
corona discharge device 21 viewed in the direction of rotation of the 
drum-shaped photoconductive photosensitive screen 13 shown by the arrow. 
As a result, it is possible to delete the electrostatic latent image 
remained on the screen 13, thereby completing the preparation of producing 
visible copies of the next manuscript. 
The above described apparatus satisfies all of the conditions required for 
the electrographic process according to the invention and hence can obtain 
the copy picture images which are high in concentration and excellent in 
picture quality. 
In the embodiment shown in FIG. 7, the second corona discharge device 28 is 
provided with one corona discharge wire. But, if the amount of the corona 
electric current must be increased in order to obtain copies at a high 
speed or to use an electrostatic record sheet having a high electrostatic 
capacity, provision may be made of two corona discharge wires as shown in 
FIG. 8. In this case, the amount of corona electric current becomes about 
1.5 times larger than that of the corona discharge wire shown in FIG. 7. 
But, it is not preferable to provide a number of corona discharge wires as 
they considerably increase the corona charging width. If the copying speed 
is made low, the amount of electric charge given to the electrostatic 
record sheet is increased and hence use may be made of an electrostatic 
record sheet whose capacity is higher than 1,500 pF/1 cm.sup.2. 
Various conditions required for the electrographic process according to the 
invention have been described. The fundamental construction of the 
electrostatic record sheet for use in the electrographic process according 
to the invention which can satisfy the above mentioned conditions, which 
is stable against the change of the outside temperature and humid and 
which can be produced in a commercial scale will now be described. 
FIGS. 9a, 9b and 9c are cross-sectional views of conventional electrostatic 
record sheets, while FIG. 9d is a cross-sectional view of one embodiment 
of an electrostatic record sheet for use in an electrographic process 
according to the invention. The conventional electrostatic record sheet 41 
shown in FIG. 9a is composed of an electrically conductive substrate 42 
impregnated with a low resistance agent 43 and a surface insulating layer 
44. The conventional electrostatic record sheet 41 shown in FIG. 4b is 
composed of a substrate 42, a low resistance agent layer 43 coated on one 
side of the substrate 42 and a surface insulating layer 44 coated on the 
low resistance agent layer 23. The conventional electrostatic record sheet 
41 shown in FIG. 9c is composed of a substrate 42, two low resistance 
agent layers 43, 43 coated on both sides of the substrate 42, 
respectively, and a surface insulating layer 44 coated on one side of the 
two low resistance agent layers 43, 43. 
The electrostatic record sheet 41 for use in the electrographic process 
according to the invention and shown in FIG. 9d is composed of a substrate 
42, two low resistance agent layers 43, 43 coated on both sides of the 
substrate 42, respectively, the low resistance agent 43 being partly 
impregnated into the substrate 42, so as to decrease the effective 
thickness of the substrate 42, and a surface insulating layer 44 coated on 
one side of the two low resistance agent layers 43, 43. 
Even if the conventional electrostatic record sheet is applied to the 
electrographic process according to the invention, it is impossible to 
obtain a good picture image. It is conceivable that this is because of the 
fact that the principle and process of producing, on the conventional 
electrostatic record sheet, the electrostatic copy latent image are 
different from those of the electrographic process according to the 
invention. 
For example, in a process of producing an electrostatic latent image by 
applying a pulse voltage to an array of pin electrodes in succession, a 
relatively large current is applied to a minute area for an extremely 
short time. The electric potential of the electrostatic latent image 
produced in this case is determined by the applied voltage, an electrical 
resistance against the transfer of electric charge toward the lower layer 
of the surface insulating layer, signal source impedance, tan .delta. 
characteristic of the material for forming the surface insulating layer, 
etc. on the one hand and by the electrostatic capacity of the surface 
insulating layer on the other hand. In addition, the electric potential of 
the electrostatic latent image is influenced by the roughness of the 
surface insulating layer and the configuration of the pin electrodes. 
In the transfer of Electrostatic Latent Image (TESI) process in which an 
electrostatic record sheet is closely brought into contact with the 
electrostatic latent image produced on a photoconductor and the 
electrostatic latent image is transferred from the photoconductor to the 
electrostatic record sheet, the photoconductor and the electrostatic 
record sheet are spaced apart by a distance through which the electric 
charge can be transferred through an air layer sandwiched therebetween. As 
a result, the electric charge or the electrostatic latent image is 
transferred within an extremely short time. Similar to the above described 
process of using the pin electrodes, even in the TESI process, and 
electric charge having a reverse polarity and corresponding to the charge 
to be transferred to the surface insulating layer of the electrostatic 
record sheet must be supplied within a relatively short time. As a result, 
provision must be made of a semiconductive layer beneath the surface 
insulating layer. In the TESI process, the electrostatic capacity of the 
surface insulating layer is determined in the way such that the charge on 
the photoconductor is finally capacitively divided into a charge on the 
photoconductor and a charge on the surface insulating layer and that the 
discharge ceasing potential difference is present across these two 
members. 
That is, the electrostatic capacity of the surface insulating layer is 
determined by such elements as the electrostatic capacity of the 
photoconductor, electric potential of the electrostatic latent image, 
electrostatic capacity of the electrostatic record sheet inclusive of the 
surface insulating layer, tan .delta. characteristic of the material for 
forming the surface insulating layer, resistance against the transfer of 
the electric charge of the substrate for constructing the electrostatic 
record sheet, etc. 
In the above described electrographic process according to the invention, 
the electrostatic copy latent image is formed of dots one of which is 
supplied with substantially uniform charge for a long time of the order of 
several tens to several hundreds milliseconds. That is, the amount of 
charge supplied to the electrostatic copy latent image is determined 
entirely independently of that supplied to the electrostatic record sheet. 
The charge having the reverse polarity and corresponding to the charge 
(electrostatic copy latent image) on the surface insulating layer of the 
electrostatic record sheet can be supplied for a sufficiently long time. 
As a result, the tan .delta. characteristic of the surface insulating 
layer and the absolute value of the resistance of the lower layer of the 
surface insulating layer cause no trouble. In the electrostatic record 
sheet for use in the electrographic process according to the invention, 
therefore, the electrostatic capacity of the surface insulating layer or 
the composite electrostatic capacity of the surface insulating layer and 
of the substrate, becomes important. 
But, in the conventional electrostatic record sheet 41 shown in FIGS. 9a, 
9b and 9c, the surface insulating layer 44 is composed of an insulating 
resin material and a white pigment usually formed of metal oxide, etc. and 
added for the purpose of exhibiting a writing property, ink absorptivity, 
natural feeling, etc. The surface insulating layer 44 has a thickness 
which is generally of the order of 5.mu. order that the surface insulating 
layer 44 can hold the charge supplied when necessary, that the surface 
insulating layer 44 has a dielectric strength which can resist the applied 
voltage, and that the surface insulating layer 44 gives impression similar 
to the usual paper without inducing curling, etc. The electrostatic 
capacity of the surface insulating layer 44 only is considered to be 1,500 
to 3,000 pF per 1 cm.sup.2 owing to the fact that the dielectric constant 
of the above mentioned white pigment is generally high. As a result, such 
conventional electrostatic record sheet 41 could not be used for obtaining 
copies at a high speed. On the contrary, the substrate 42 has generally a 
thickness of 60 to 100.mu. and a considerably small electrostatic 
capacity. 
As described above, the electrostatic record sheet for use in the 
electrographic process according to the invention has an electrostatic 
capacity of at least 500 pF per 1 cm.sup.2, preferably 500 to 1,500 pF per 
1 cm.sup.2. As a result, the conventional electrostatic record sheet 41 
shown in FIG. 9a whose electrostatic capacity is determined by the surface 
insulating layer 44 could not be used for the electrographic process 
according to the invention. 
Experimental tests have demonstrated the result that a copy picture image 
obtained by the conventional electrostatic record sheet 41 constructed as 
shown in FIG. 9b could not be used in practice. In addition, the 
conventional electrostatic record sheet 41 constructed as shown in FIG. 9c 
could not be used for the electrographic process according to the 
invention since even though the surface insulating layer 44 only has a 
large electrostatic capacity, the electrostatic capacity of the record 
sheet 41 as a whole is remarkably small. 
FIG. 10 shows an equivalent circuit of the conventional electrostatic 
record sheet 41 constructed as shown in FIG. 9c and also of the 
electrostatic record sheet 41 constructed as shown in FIG. 9d according to 
the invention. In the equivalent circuit shown in FIG. 10, the flow of 
corona ions directed from the photoconductive photosensitive screen is a 
constant current source represented by an electric source 45 and a 
resistor 46 connected in series. 
The electrostatic record sheet is represented by a series circuit 
consisting of a capacity C.sub.S of the surface insulating layer and an 
electrostatic capacity C.sub.B of the substrate. This series circuit is 
connected to the above mentioned constant current source by means of a 
switch 47. In the electrographic process according to the invention, the 
electrostatic record sheet is charged for a relatively long time, so that 
it is possible to omit a resistor component from the electrostatic record 
sheet. In the equivalent circuit shown in FIG. 10, the electrostatic copy 
latent image potential produced on the electrostatic record sheet is a 
charging voltage V.sub.S +V.sub.B across two terminals of the series 
connected C.sub.S and C.sub.B. 
In the conventional electrostatic record sheet 41 constructed as shown in 
FIG. 9c, C.sub.B is considerably smaller than C.sub.S, so that its 
composite electrostatic capacity C is given by 
##EQU4## 
That is, the composite electrostatic capacity C is substantially 
determined by the electrostatic capacity C.sub.B of the substrate 42. 
Let the specific inductive capacity .epsilon..sub.s be 3, then the 
composite electrostatic capacity C per 1 cm.sup.2 is given by 
##EQU5## 
This composite electrostatic capacity C per 1 cm.sup.2 is considerably 
smaller than the electrostatic capacity of the electrostatic record sheet 
for use in the electrographic process according to the invention, i.e. 500 
to 1,500 pF per 1 cm.sup.2. 
On the contrary, in the electrostatic record sheet 41 for use in the 
electrographic process according to the invention and shown in FIG. 9d, 
the low resistance agent 43 coated on the both side surfaces of the 
substrate sheet 42 are partly impregnated into the substrate 41 so as to 
decrease the effective thickness of the substrate 41. As a result, the 
composite electrostatic capacity C consisting of the electrostatic 
capacity C.sub.S of the surface insulating layer 44 and the electrostatic 
capacity C.sub.B of the substrate sheet 42 connected in series becomes 
increased. The composite electrostatic capacity C can easily be changed 
without adjusting the thickness and composition of the surface insulating 
layer 44. 
That is, the composite electrostatic capacity C can easily be changed by 
adjusting the degree of impregnation of the low resistance agent 43 into 
the substrate 42. Thus, the electrostatic record sheet 41 shown in FIG. 9d 
is far advantageous in manufacture if compared with the conventional 
electrostatic record sheets 41 shown in FIGS. 9a, 9b, 9c. A practical 
example of an electrostatic record sheet for use in an electrographic 
process according to the invention and shown in FIG. 9d will now be 
described. 
EXAMPLE 
Pulp composed of 60 parts of NBKP and 40 parts of LBKP was adjusted to a 
degree of heating of 60.degree. SR. The pulp thus adjusted was added with 
clay to obtain a sheet having a thickness of about 80.mu.. The sheet was 
passed through 12 stages calender roll to prepare a substrate having a 
density of 0.95, size degree of 12 seconds and thickness of about 56.mu.. 
The low resistance agent was prepared by adding 5 parts of methyl alcohol 
to 100 parts of low resistance solution mainly consisting of polyvinyl 
benzine trimethyl ammonium chloride and mixed at various ratios with 
polyvinyl alcohol as a resistance adjusting material. The methyl alcohol 
was added for the purpose of penetrating the low resistance solution into 
the substrate. Addition of too much amount of the methyl alcohol causes 
the low resistance solution to penetrate into the entire part of the 
substrate sheet. Several low resistance agents were coated on both side 
surfaces of the substrate with amounts variable from about 4 g/m.sup.2 to 
provide several kinds of low resistance substrates. On one side surface of 
each of such low resistance substrates was coated a surface insulating 
layer consisting of the following compositions 
______________________________________ 
Butyral 65 parts 
Styrene resin 5 parts 
Calcium carbonate 20 parts 
Titanium oxide 10 parts 
______________________________________ 
and having a dry weight of about 6 g/m.sup.2 (thickness of about 5.mu.) to 
provide several kinds of electrostatic record sheets. On 16 electrostatic 
record sheets prepared as above described were produced respective 
electrostatic copy latent images by means of the apparatus for carrying 
out the electrographic process according to the invention and then these 
latent images were developed to obtain the copy picture image. The 
electrostatic copy latent image potential, picture image concentration and 
picture quality of the copy picture image thus obtained are shown in the 
following Table 2. 
TABLE 2(a) 
__________________________________________________________________________ 
Surface Specific 
Resistance of Copy 
Substrate Electrostatic 
Electrostatic 
Picture 
Electrostatic 
(.OMEGA.) Capacity Copy Latent 
Image 
Record Sheet 
Back Surface 
Intermediate 
(pF/1 cm .sup.2) 
Image Potential 
Concen- 
Picture 
No. Layer Layer Substrate 
Whole 
(volt) tration 
Quality 
__________________________________________________________________________ 
1 1.9 .times. 10.sup.6 
1.9 .times. 10.sup.6 
about 2,000 
1,000 
85 0.65 5 
2 " 1.6 .times. 10.sup.7 
about 2,000 
-- -- 0.75 5 
3 " 1.3 .times. 10.sup.8 
about 1,900 
-- -- 0.70 5 
4 " 1.8 .times. 10.sup.9 
about 1,800 
-- -- 0.80 5 
5 1.6 .times. 10.sup.7 
1.9 .times. 10.sup.6 
about 1,500 
850 
100 0.85 5 
6 " 1.6 .times. 10.sup.7 
about 1,500 
-- -- 0.80 5 
7 " 1.3 .times. 10.sup.8 
about 1,400 
-- -- 0.85 5 
8 " 1.8 .times. 10.sup.9 
about 1,300 
-- -- 0.90 4 
__________________________________________________________________________ 
TABLE 2(b) 
__________________________________________________________________________ 
Surface Specific 
Resistance of Copy 
Substrate Electrostatic 
Electrostatic 
Picture 
Electrostatic 
(.OMEGA.) Capacity Copy Latent 
Image 
Record Sheet 
Back surface 
Intermediate 
(pF/1 cm.sup.2) 
Image Potential 
Concen- 
Picture 
No. Layer Layer Substrate 
Whole 
(volt) tration 
Quality 
__________________________________________________________________________ 
9 1.4 .times. 10.sup.8 
1.9 .times. 10.sup.6 
about 1,000 
650 130 0.95 4 
10 " 1.6 .times. 10.sup.7 
about 1,000 
-- -- 0.95 4 
11 " 1.3 .times. 10.sup.8 
about 900 
500 170 1.15 2 
12 " 1.8 .times. 10.sup.9 
about 800 
-- -- 1.10 3 
13 2.0 .times. 10.sup.9 
1.9 .times. 10.sup.6 
about 500 
400 210 1.25 1 
14 " 1.6 .times. 10.sup.7 
about 500 
-- -- 1.30 1 
15 " 1.3 .times. 10.sup.8 
about 500 
-- -- 1.20 1 
16 " 1.8 .times. 10.sup.9 
about 400 
-- -- 1.25 1 
__________________________________________________________________________ 
In the above Table 2, the surface specific resistance and electrostatic 
capacity of each substrate were measured in the above mentioned step of 
manufacturing the electrostatic record sheet. The surface specific 
resistance of the substrate was measured after adjusting humidity for 1 
hour at 20.degree. C. in atmosphere of 65% RH by applying a voltage of 100 
V with the aid of "Room Temperature Measuring Box D-601" and "Ultra 
Insulation Meta-30" made by Kawaguchi Denki K.K. in Japan. The 
electrostatic capacity of the substrate was measured with the aid of a 
high voltage Schering bridge. The electrostatic capacity of each 
electrostatic record sheet as a whole was calculated on the basis of the 
electrostatic copy latent image potential and amount of flow of corona 
ions at the time of producing, on each electrostatic record sheet, the 
electrostatic copy latent image by the apparatus shown in FIG. 7. The 
picture quality of the copy picture image was evaluated with the same 
standard as that of the Table 1 with respect to the reproduction of the 
minute parts of the copy picture image. 
As can be seen from the Table 2, the electrostatic capacity of the 
substrate becomes increased as the surface specific resistance thereof is 
decreased and particularly becomes considerably changed by the resistance 
value of the back surface layer which is opposed to the surface insulating 
layer. In addition, the electrostatic capacity of the electrostatic record 
sheet as a whole tends to increase in response to increase of the 
electrostatic capacity of the substrate and becomes saturated to a limit 
value of the electrostatic capacity of the surface insulating layer. 
Strictly speaking, the depth of impregnation, etc. of the low resistance 
agent is considered to be an element other than the surface specific 
resistance. But, the above described measuring method can obtain the 
result inclusive of such element. As a result, the evaluation of 
manufacture, management, etc. can be effected by the above mentioned value 
in practice. It is not fully elucidated that the electric potential 
applied in the case of development is V.sub.S or V.sub.S +V.sub.B shown in 
FIG. 10. But, in the Table 2, the electrostatic copy latent image 
potential corresponding to the electrostatic capacity of the electrostatic 
record sheet as a whole corresponds to the copy picture image 
concentration in a relatively good manner. As a result, it is proper to 
consider that the electric potential V.sub.S +V.sub.B from the back 
surface of the electrostatic record sheet acts in substantially effective 
manner. 
As can be seen from the above Table 2, a good copy picture image is 
obtained when the electrostatic capacity of the electrostatic record sheet 
as a whole is larger than 650 pF per 1 cm.sup.2. In addition, the 
electrostatic capacity of the electrostatic record sheet as a whole should 
not be smaller than 500 pF per 1 cm.sup.2 even if the selection of the 
surface insulating layer material, change of the substrate sheet 
composition, etc. are taken into consideration as already described with 
reference to the Table 1 and FIGS. 5 and 6. 
As described above, in order to ensure the practical copying speed, the 
electrostatic capacity of the electrostatic record sheet should be smaller 
than 1,500 pF per 1 cm.sup.2. The electrostatic record sheet for 
satisfying such condition can easily be obtained by constructing it as 
shown in FIG. 9d and listed in the Table 2. 
In order to obtain the above described desired electrostatic capacity of 
the electrostatic record sheet, the degree of low resistance treatment of 
the substrate is substantially determined by the surface specific 
resistance of the back surface layer of the substrate listed in the Table 
2. That is, the desired electrostatic capacity of the electrostatic record 
sheet can be obtained by determining the surface specific resistance of 
the back surface layer of the substrate sheet to the order of about 
2.times.10.sup.6 .OMEGA. to 1.4.times.10.sup.8 .OMEGA.. Even if the 
thickness or material of the surface insulating layer is changed, it is 
possible to provide an electrostatic record sheet which can satisfy the 
above mentioned range of the electrostatic capacity by selecting the above 
mentioned surface specific resistance to the order of about 
2.times.10.sup.6 .OMEGA. to 2.times.10.sup.9 .OMEGA.. 
As stated hereinbefore, the electrostatic record sheet adapted to be 
effectively used for the electrographic process according to the invention 
is composed of a substrate made electrically conductive and having a 
thickness of 50 to 100.mu., a surface insulating layer coated on one side 
surface of the substrate and having a thickness of about 5.mu., the 
surface insulating layer being formed of an insulating resin mixed with or 
without an inorganic oxide, etc., the electrostatic capacity as calculated 
from the amount of flow of corona ions received through a photoconductive 
photosensitive screen and from charging potential being 500 to 1,500 pF 
per 1 cm.sup.2 and the surface specific resistance of the other side 
surface of the base sheet being 2.times.10.sup.6 .OMEGA. to 
2.times.10.sup.9 .OMEGA.. The electrostatic record sheet thus constructed 
can obtain the copy picture images which are extremely excellent in 
resolving power, thin line contrast, concentration and picture quality. 
As stated hereinbefore, experimental tests have demonstrated the result 
that the dots of the electrostatic copy latent image produced on the 
dielectric coated record medium become enlarged in dependence with the 
relation between the intensity of the electric field of the field 
electrode E and the maximum surface potential V of the electrostatic copy 
latent image and that if K=V/E is made smaller than 0.18, it is possible 
to effectively prevent the dots from enlarging. As a result, if the above 
condition is satisfied, it is possible to effectively prevent the 
degradation of the resolving power of the copy picture image, decrease of 
the concentration of the thin line-shaped picture image and fading, etc., 
thereby obtaining the copy picture images which are high in concentration 
and good in picture quality. In addition, experimental tests have yielded 
the result that if the electrostatic capacity of the dielectric coated 
record medium for use in the electrographic process according to the 
invention is at least 500 pF per 1 cm.sup.2, preferably be 500 to 1,500 pF 
per 1 cm.sup.2, then it is possible to effectively satisfy the above 
mentioned condition that K=V/E is smaller than 0.18. As a result, if use 
is made of the dielectric coated record medium which can satisfy the 
condition required for the electrostatic capacity, it is possible to 
obtain the copy picture images which are extremely excellent in 
concentration and picture quality. In addition, a high speed copying 
apparatus may be realized in an extremely efficient manner. 
The invention is not limited to the above described embodiments, but 
various alternations and modifications are possible. For example, in the 
above described example and the apparatus shown in FIG. 7, the 
electrostatic copy latent image produced on the electrostatic record sheet 
has been developed into a visible image by the humid type developing 
system. Instead of such humid type developing system, use may be made of 
well known dry type developing system, developing system in which 
thermoplastic resin is heated, etc. In addition, instead of the 
drum-shaped photoconductive photosensitive screen, use may be made of 
sheet-shaped or belt-shaped photoconductive photosensitive screen. The 
photoconductive photosensitive screen may be modified such that the 
photoconductive layer may be coated so as to cover the electrically 
conductive member or the photoconductive layer and the insulating layer 
may be coated so as to expose a part of the electrically conductive 
member.