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The magnet is the central element of the spectrometer system, where it will be used to momentum-analyze secondary heavy-ion fragments at relativistic energies. High bending power and large acceptance apertures are necessary for simultaneous multiple fragment measurements. Free access over large angles to the magnetic volume is necessary to accommodate a variety of experiments and particle detection systems. The magnet has pole tips two meters in diameter, a one meter gap, and a maximum central field of three tesla. The coils are designed to be cryostable, with a helical winding pattern. A window-frame steel yoke limits the stray field and augments the central field. In terms of its magnetic energy of 55 MJ it will be one of the twelve or so largest magnets in the world. Unusual engineering features of the magnet are the large magnetic forces (one million kg) between the coils at liquid helium temperature and the yoke at room temperature, and the large diameter (7 m) hydraulic cylinder base to provide rotation for the magnet. The magnet will be installed at the BEVALAC heavy-ion facility at Lawrence Berkeley Laboratory.