This paper reports magnetography using a dry method of development.
In magnetography which is the method of obtaining a visible representation of a recorded signal on a magnetic tape or a sheet, the wet method, that is, to suspend magnetic particles in a liquid, has been employed to development the latent image. However a dry method in divelopment is needed for many graphical applications, such as oscillography, high speed printer, color printing etc.
Hitherto, a dry method has not been used usually since a good dry inking powder has not been obtained. But we succeeded to get dry inking powder after many experimental and theoretical works which has good characters desired for this powder. Spherical iron particles in order of 20∼40 microns give good results without smearing the paper in development and transfer. Even using this dry method, better resolution has been obtained: the degree of resolution over 16 lines per mm.
For application to faccimiles the magnetic head having V shaped gap has been deviced which is successful in obtaining rich half tone. In color printing, development powder having thin dye coating has been employed, however some improvement should be required.

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Some electrical properties of the dye films on the glass plate are described. The films were deposited from the ethylalcohol solutions of dyes, or evaporated in vacuum. From the experimental result, the following conclusions were obtained.
(1) The dyes, having the absorption bands splitted by interaction between the excited states, show a higher photoconductive response than those of the films which have no splitted absorption band.
(2) The film of Rose Bengal shows the properties given by the p-type conduction.
(3) The thermal activation energies for electrical conductivity relate to the transition energy between the ground state of the two splitted states.
(4) It seems that the dye sensitization of ZnO has little relation to the photo conductivity of the dye.

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There are many reports concerning the mechanism of dye Sensitizatlon, but it seems that there is no definite theory. The present author has measured, therefore, the adsorption of Rose Bengale on the surface of zine oxide. He found that ZnO thermal treated in vacuum adsorbed less dye than the sample non treated. He measured also the discoloring of dye from the surface of ZnO, when it was immersed in the water. The discoloration was least at the sample thermal treated in vacuum.

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In order to clarify the foundamental process of liquid development of electrophotography and electrostatic printing, the adsorption characteristics of carbon black fine powders dispersed in ligroine on the charged surface were studied, varying some experimental conditions. The sample plates were made from coating the VYHH resin (vinyl chloride, vinyl acetate copolymer) on the glass plate on which the chromium is evaporated. The surface charge was given by the corona discharge and measured by sector type electrometer as surface potential (Voltage).
The adsorbed quantities of carbon black powder were observed by using the optical densitometer. The experimental equipment is shown in Fig. 1. Fig. 2 shows the charge decay characteristics of insulator surface in both air and ligroine only. In the range of carbon black concentration of 0. 01∼0.1 Wt% under the surface voltage 200 V, the adsorption characterristics are shown in Fig. 3∼6.
The initial rate of adsorption in Fig. 3∼6 increases linearly with the carbon black concentration as shown in Fig. 7. The maximum adsorption in each concentration approaches rapidly the saturation state, shown in Fing. 8. The charge decay characteristics by carbon black adsorption are shown in Fig. 9 and 10, and the relation between both intial charge decay rate and maximum charge decay and the carbon black concentration are shown in Fig. 11 and 12 separately. It was observed that these results correspond to the characteristics of adsorption in Fig. 7 and 8. Fig. 13 shows the linear relation between the initial rate of adsorption and the initial decay rate of charge.
Adsorption characteristics under the constant carbon black concentration of 0.05 Wt%, varying the surface voltage 0~350 V, are shown in Fig. 14~17. It seemed that the charge characteristics of the initial rate of adsorption with positive surface voltage is not so simple as in the case of constant voltage, except negative surface voltage in which the linear relation can be found as shown in Fig. 18. The relation between the maximum adsorption of carbon black and the initial surface voltage was also not linear but more complicated as shown in Fig. 19.

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Process of liquid develoment was discussed on the basis of electrophoretic theory.
The behavior of the development was analyzed by neglecting space charges in liquid, and simple expressions for edge effect, time constant and efficiency of the development were derived.
Experimental results were compared with the theory, and general agreements were obtained.
Some questions about the reasonableness of the assumptions employed here for simplification were also discussed.

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Electrophotographic properties of carbazole, N-alkyl carbazoles, N-acyl carbazoles and polyvinyl
carbazole were studied.
Following results were obtained.
(1) N-A]kyl derivatives can be compared with carbazole in their sensitivity.
(2) Appreciable sensitivity is not observed in the case of N-acyl derivatives.
(3) The sensitivity of polyvinyl carbazole is about ten times higher than that of N-alkyl derivatives.
(4) Positively charged layers generally show higher sensitivity than negatively charged ones.

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The decay of surface potential and the layer current were simultaneously measured of the
white and rose bengal sensitized ZnO-silicone resin layers in the dark or illuminated states.
The corona discharge currents through said layers were also measured.
The following results were obtained,
(1) The Electrical resistivity of the ZnO-silicone resin layer changes gradually during the dark decay and light-decay periods.
(2) The tendency of the resistivity change is comparable to the rate of R C change calculated from the surface potential change.
(3) The saturation periods of value Σ_ti_l: and τ₀^* are found to be almost the same, Σ_ti_l: The accumulated current under illumination
τ₀^*: The time of the potential drop by the light from the initial potential to Ov.
(4) After the saturation of Σ_ti_l, the value increased more, under higher energy radiation. This shows the existence of the charge Qb which is undetectable neither by the electrometer nor by the powder development. Qb is considered as photorelease of internal polarization.
The corona discharge current was also measuared and the peculiar property of the ZnO layer was found, in the dark the corona current through the layer increased considerably as compared to the current through metal plate. This pecuriar phenomena extinguished, if the layer was illuminated. This characteristic can be explained as the corona acceleration by the layer in the dark.
This mechanism and the more detailed data are now being studied.

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Electrical Communication Laboratory of Nippon Telegraph and Telephone Public Corporation
The electrostatic character image is made on the electrostatic recording paper by applying the high voltage to character plate In which each character is insulated and which contact with recording paper. This electrostatic image is developed and fixed by the usual process.
30,000 character/sec is expected in this method theoritically.

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Two methods of reproduction of gray scales on film from the originally casted one, density scales of which are more accurate, were given, The first method is by contact printing and the other is by projection printing (enlarging and reduction). To avoid curving of under exposure part of characteristic curve of film, auxiliary scale which was prepared compensating and just for straightening the part above mentioned, was adopted.
The printing was made on films through gray scale and auxiliary scale where the latter scale was placed so that the denser part was contacted with the densest part of the casted scale.
By accurate exposure and development, various sizes of reproduced gray scales were able to obtain and also possible to reproduce gray scales the gradient of which were steeper or softer through change of the developing time.

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A photographic copying lens of F/2.0 in relative aperture and of 135 mm in focal length was designed and manufactured, which makes an image, magnified 2.3 times, of letters appea ring on the fluorescent screen of 7″ Charactron tube, onto an electrophotographic sensitive plate, such as running electrofax paper or rotating Se drum in Xerography. The whole system serves as a “high speed printer” for the output of electronic computers etc.
The aberrations of the lens was corrected in the wavelength region of 400∼600 mμ, as near ultraviolet rays are almost fully cut off by the lens itself, and the lens is equally suitable for dye-sensitized electrofax papers as well as Se plate used in Xerography. At full aperture of F/2. 0, it showed a resolving power (at the object plane) of at least 20 lines/mm at any point in the object plane of 120 mm in diameter, and an even illumination over the entire image field, and makes sharp images of letters on electrofax papers sensitized with Rose bengal. The “printing speed” of the system was limited within ca. 2000 letters/sec in the case of electrofax papers, owing to the low sensitivity of these papers. This speed is expected to reach 10,000∼20, 000 letters/sec by using Xerographic recording, as the sensitivity of Se plate is 5∼10 times higher than that of electrofax papers.

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The object of this work is the formation of halftone images on Electrofax paper by contact printing of halftone positives.
The best results are obtained by using a strobo flash as the light source. The light intensity is controlled by irises attached to the front of the flash and the charging voltages of the main condenser. As the beam is approximately parallel by the attachment of the iris, images of good quality are built up.
On exposing, contact is made in a vacuum frame. To prevent discharge effect, a sheet of
transparent highly-insulating film is inserted between the charged layer and the positive. In our work, Myler No. 25 and No. 50 made by Du Pont are used with good results.
The halftone positive scales used in the work are 150 line/inch and 300 line/inch.
As the result of examining the images built up, because the particle of the toner is large, the edge of every halftone dot is not sharp and in that, there is microscopic unevenness of density. When these images are compared with those of the sensitive materials now being used, for example, Kodalith film, the images of Electrofax prove to be inferior to them. But Electrofax shows better reproduction than the halftone images reproduced by printing at present.

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In regard to images developed by Electrophotography, the fog involved in high-light is likely to have a bad effect on the image quality. In order to remove this fog, we studied methods of reduction and removal of the fog on Electrophotography, until at length we devised an new method. This method brushes on the developed images with the carrier soonafter developing, in the dark. We applied this method not only to continuous tone images, but also to halftone images, with good results.
This reduction method develops a tendency to cutting reduction and to subtractive reduction when the brushing times are increased.
The finer the meshes of the carrier are, the capability of the reduction is increased. Of course, brushing times are increased and so, the reduction effect is advanced. And the capability and the effect of reduction are changed when a different carrier is used.
For example, on applying this method to the continuous tone images, the reduction of the density is 0.1 at the shadow area and the reduction effect of 0.3 by density is showed at the fog area.
These methods are applied practically to halftone images, so the quality of the images is improved remarkably.

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