At various light intensities, resistance (D. C. and 30c/-5Mc/s) and dielectric constant (30c/s-5Mc/s) measurements of ZnO-Resin dispersion system are reported. It was observed that the dielectric constant was increased with decrease of the resistance contrary to them at the dark. The observed impedance changes can be explained as the behavior of photoconductor with space charge.

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Electrofax papers were made by making use of various kinds of Zinc Oxide six kinds of Zinc Oxide were examined.
Their components, size distributions and origins are known.
These Zinc Oxides were manufactured by the socalled French mathod or American method.
Both kinds of Zinc by electrolysis or distillation were used in French method.
After charging our fax papers by 6000 valt coronal discharge, the dark and light decay curves were obtained and analysed.

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Effects of some ambient molecules on the surface conductivity of zinc oxide at room temperature were studied. The dark conductivity was found to increase by adsorbing electron donating molecules such as NH₃ and CH₃NH₂, decreased considerably by adsorbing electron acceptors (O₂NO, NO₂ etc.) and unchanged in neutral molecules (N₂ and CO₂). The photoconductive sensitivity was of considerable amount only when the measurement was done in the electroaffeinitive gases. It was suggested that there exists a parallel relation between the chemical activity of the ambient molecules and the rate of current decay process after cease of the irradiation.

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The mechanism of the photoconductivity in zinc oxide used in electrophotography is considered to be complex and the phenomena are known to be affected by the impurities contained in the materials and the absorption at the surfaces. For these study it has been believed beneficial to use the single crystals.
In this preliminary work, the single crystals was manufactured by the special method, and the fundamental data of the electrical and the optical properties of zinc oxide single crystals were developed. The dimensions of the crystals obtained. were 0.2-1 mm in diameters and several centimeters in lengths for the bar-typs, and several square milimeters for the plate-like ones.

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Recent experiment on electrophotographic development is described, with emphasis on high resolution development and the relationship of initial paper potentials to powder image density. The magnetic brush consists of powdered iron (300-1000 mesh granular particles) mixed with very fine toner Particles.
The, toner was produced by spray drying the solution of raw materials and collecting in a electrostatic precipitator. The resulting image has a resolution of about 120lines/millimeter on the electrophotographic papers.
The powder image density was measured on three kind of papers printed in contact to a half tone positive or enlarged from a transparent gray scale.

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In this report we announce the characteristics of the vibrating reed type electrometer with negative feedback circuit, and show the reason why this electrometer is suitable for measuring of surface potential of electrophotographic plates. In this electrometer a new rectifying circuit is used.

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The lenses used for copying and enlarging in electrophotography should be different from those used in ordinary photography, with regard to the principle of chromatic correction, the position of the selected object plane, the transmittance of component optical glasses for actinic rays, etc. For instance, ordinary enlarging lenses never give satisfactorily sharp pictures of a microfilmed negative on white (non color-sensitized) electrofax papers, as the main response of these papers, combined with a tungsten light source, lies in near ultraviolet region where ordinary lenses are not corrected.
It was discussed how should be the characteristics of lenses used for copying and enlarging in electrofax process as well as in xerography, and some lenses were actually manufactured to exemplify our discussions, with very satisfactory results.

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The present author expressed his opinion on the sensitivity of silver halide emulsion in 1950²⁾ maintaining that its change by light is rather more physical contrary to Mott's theory 1938. He defines¹⁾ also the photography of silver halide as a sort of Electrophotography in the sense that the light change of photosensitive material is physical.
This report is written in order to make clear the above mentioned maintenance.
The result is as follows.
Silver halide crystals are made as p type semiconductor at the first ripening. The crystals are desired to be of moderate size and complete. Those with defects are not good. The second ripening process is to adhere to the surfaces of those p type crystals the active gelatine or the active addition agents, which have semiconductive and photoconductive property, and also are desirable to be n type.
Thus, we obtain the emulsion crystals which have p n contact as semiconductor. This p n contact in emulsion crystals is the true form and essence of the socalled sensitivity speck.
But, this p n contact is electronically not in thermal equilibrium before exposure, namely the Fermi levels on both sides of p n junction are not equivalent. By exposure, charge carriers are transferred from the one side to the other in p n contact, and space charges emerge like Fig. 1(b) or (c) at the p n contact in proportion to the numbers of absorbed light quantums by moderate light intensity, so that Fermi levels on both sides move toward the equilibrium. The emergency of space charge at the p n contact by light or other effects is the latent image or the cause of fog. This charged part of p n contact also becomes the development centre.

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