An improved form of standard ionization chamber

A modification of the open-air parallel plate ionization chamber, rradilv nortable and adapted to field use, is described. In this chamber, narrow guard , at s are supplemented by a system of 10 guard wires across the ends of the cha n ', The wires are so spaced that X-ray scattering from them is negligible Bv , < means a uniform electrostatic field between electrodes is obtained without tK use of wide guard plates and the large plate-to-shield spacing necessary in the primary standard. This design has permitted a decrease in the volume and weight of the secondary standard by a factor of 15.