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The exposure standard for low energy X-rays has been established at the Electrotechnical Laboratory. The details of construction of the standard and the procedure of absolute measurement are described. In obtaining the exposure from the ionization by X-rays in the standard ionization chamber, the most important of corrections necessary for undesirable factors are for the following three, the electric field distortion, air attenuation of X-rays and lack of saturation of ionization due to recombination of ions, which are related to one another and difficult to determine separately by usual ways. A new method to measure the shape of the electric field directly has been developed and applied to determination of optimum conditions of geometry of the chamber to minimize the influence of the field distortion. By this method the field distortion is obtained separately from the other correction factors. An investigation is carried out into the correction for the lack of saturation based on the modified Scott and Greening's theory in principle. The space charging effect is analyzed using the relation between the exposure rate and the slope of the corresponding saturation curve in the near saturation region, and the applicability of the above-mentioned theory is examined. The error is estimated to be about 1.3%