IN 1925, Fricke and Glasser (1) constructed small thimble chambers of many different materials and shapes in an attempt to test a theoretical formula by means of which the ionization current within the thimble chamber could be calculated with sufficient accuracy to be used as a primary standard for roentgen-ray quantity. In this attempt they were not entirely successful. Although much valuable information regarding the behavior of various thimble chambers was obtained, the so-called “air wall” chamber of carbon and aluminum, calculated according to the Fricke-Glasser formula to have the same effective atomic number as atmospheric air (7.69), failed to agree with the standard air ionization chamber. However, this disagreement stimulated extensive studies on the problem of “air wall” chambers. H. T. Meyer (2) was the first to call attention to the fact that thimble chambers which were made to agree with the readings of a standard air chamber had an effective atomic number which was lower than that required by the previously mentioned formula. Investigations which we have carried out over a number of years have supported this contention. Curve A in Figure 1 represents a previously published (3) curve of the ratio of ionization currents in the Fricke-Glasser “air wall” chamber to those of the standard air chamber for various effective wave lengths. Curve B shows the same ratio for a thimble chamber built to produce ionization currents which run parallel to those in the standard chamber and which, for a given volume of 2.48 c.c., had a bakelite wall with an effective atomic number of 6.32. Other investigators, notably Albrecht (4), Friedrich, Zimmer, and Schulze (5), Glocker and Kaupp (6), Kiistner (7), Mayneord (8), Meyer (2), Miehlnickel (9), Murdoch and Stahel (10), and Sievert (11), have investigated further the factors which contribute to the ionization within the enclosing walls of a thimble chamber and although their results as a whole are similar to those of Fricke and Glasser, their studies have brought out some new points of importance. It is the purpose of this paper and of others which are to follow to correlate those observations with our own. As a rule, better calculated agreement is obtained between the thimble chambers of different materials, provided the materials are absolutely pure, than between chambers of different volumes and shapes. The effects of different wall areas and thicknesses and, in addition, the effects of the total volume of the chamber or the diameter upon the ionization produced within are complicated and therefore difficult to predict. A consideration of the mechanism of absorption and an actual study of the various effects which result in the production of ions will illustrate this point. In the process of absorption, a photoelectron is ejected by an impacting photon, the photo-electron absorbing the full energy of the photon.