The University of Wisconsin‐Madison Accredited Dosimetry Calibration Laboratory has designed and constructed an Attix style variable‐length free‐air ionization chamber for application in mammographic energy x‐ray beams. The chamber is designed to enable absolute in‐air exposure measurements. A unique feature of this design is that the variation in plate separation is accomplished by use of two stepping motor controlled translation stages. This paper details the characterization of this instrument and presents some preliminary exposure measurement results. ©1995 American Institute of Physics.