Nondimensional variables are described that take account of changes in flow geometry, gas velocity and pressure and arc current on the behaviour of axial-flow forced-convection arcs. Calculated steady-state characteristics are shown that are in good agreement with measurements made in nitrogen gas. An elementary theory is presented for the dynamic arc in nozzle flow that agrees qualitatively with the known behaviour of such arcs. The nondimensional scaling principle is extended to include the interaction between the arc and an electric circuit. It is shown that, for a simplified arc model, the zero-current energy balance of a given type of circuit can be described by a unique set of clearance/failure boundaries. Such a set of curves represents all combinations of gas-flow variables and circuit values.