This work was performed to screen potential cathode materials for use in arc heaters with cylindrical electrodes. It focused on hot isostatic pressing consolidated copper composites; 10% composites of copper with tungsten and niobium as well as 1.5% niobium were used. The composite cathodes were tested in pure Ar, He and N₂ as well as in mixtures of Ar with CO, N₂ and H₂, at atmospheric pressure and currents up to 150 A. The arc was driven around the circumference of the cathode with an external magnetic field. The results are compared with those using pure copper cathodes. The Cu-W cathode showed a better arc stability and lower erosion rates than copper. The Cu-Nb cathode showed much lower erosion rates and much better arc stability than copper for almost all the different operating conditions and may be considered a promising new electrode material. The experimental results corroborated previous interpretations of the relationships between arc velocity, erosion rate, surface drag and electrode surface composition.