By the use of a parallel plate ionization chamber with laboratory air and a
60Co γ-ray source that gives the ion density of 0.02_??_1.2 e.s.u./cc, sec, volume recombination characteristics are in-vestigated. With perspex, aluminum, graphite and copper as the wall material, 0.5_??_5000 volt/cm as the field intensity and 2_??_15mm as the plate spacing, the
m-value is observed to vary from 39.0 to 27.5 as the collection factor ƒ increases from about 0.2 to nearly 1.0. The
m-value seems dependent not on the wall material, plate spacing or ion density, but on the field intensity alone. Since the
m-value remains steady when ƒ<0.5, intentional use of the intensity range in which the ionization current is not saturated seems of advantage to obtain the saturation current with high accuracy. Treating of the
m-value as a constant as has customarily been done is proved meaningless. In this paper, some of stem effects are also investigated.
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