A lightweight (1150-gram) instrument for measuring the flux of relativistic heavy primary cosmic ray nuclei of Z ≥ 6 and separating these into three categories according to charge is described. The detector is an ionization chamber 11 cm. in diameter and 11 cm. long filled with 9 atmospheres of argon. The associated electronics which consists of a 400-volt supply for the ionization chamber, a high input impedance amplifier, and a three-channel pulse height analyzer is completely transistorized and consumes 54 mw.