The introduction covers briefly the principal characteristics of single- and multiple-arc loads, a tentatively suggested method of electrode classification and the systems for supplying direct current to the arcs.A comparison of generators which supply single and multiple arcs shows that “copper” designs are required for single-arc generators, and “iron” designs for multiple-arc generators. Load variations in single-operator machines involve flux changes causing sluggish response and inducing e.m.f.'s in the coils under commutation. In both types of generator the m.m.f.'s of the coils under the brushes and other field forcing methods may be used to enhance response to load changes.Present design practices and standard ratings are thought to give too small single- and too large multi-operator sets. It is suggested that larger and more efficient single-operator generators and more highly rated multi-operator units of the fully compensated type will lead in both cases to overall economy.A review of the existing systems for arc supply points out some serious effects of phase unbalance and suggests that future developments lie along the lines of rotary a.c. multi-operator and d.c. single-operator generators.