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A free-air ionization chamber for the measurement of supervoltage X-rays is described. It is built on the guarded-field principle and of such size as to permit measurements for X-ray excitation voltages from 150 to 350 kv at atmospheric pressure and 200 kv upward at elevated pressures. The electrode system is contained in a pressure cylinder of such size as to give an equivalent plate spacing of 4 m at a pressure of 10 atm. The chamber characteristics have been studied in the range of 200 to 400 kv. The most important finding is the required increase in plate separation at the higher voltages. While reasonable, this is at variance with the findings of other workers. From the m easurement of the plate separation for full electronic equilibrium, electron ranges may be obtained.To carry out these measurements it was necessary to design and construct a high-voltage plant capable of extreme steadiness of operation. This was accomplished with a four-stage rectifier having valves operated on a single pumping system. Potentials were carefully distributed through the system with the result that the voltage could be measured and controlled within ± ~ percent, which figure also represents the variation in tube output.The X-ray tube is of the simple four-stage type, made up of Pyrex-glass cylinders. By means of an insulated target, only the target current is measured.

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