Further studies have been made on the effect of the diaphragm system upon the calibration measurements made with the large free air ionization chamber. Air absorption coefficients were obtained, and it was found necessary to use these values to correct the ionization chamber readings. Calibrations of a thimble chamber, made with and without a restricting diaphragm close to the X-ray tube, differ by several per cent. Dependent upon the position of the thimble chamber with respect to the tube target, the calibration may vary over a range of 10 per cent. When the radiation is not restricted by a diaphragm close to the tube, there may be a difference in the radiation quality received by the thimble and air chamber, as large as 10 per cent.