A new method of determining the external dynamic characteristics of an electric arc, without reference to physical events occurring within the arc, is described, and a testing procedure is developed. The principles are introduced in the first place by reference to a simple example, and a summary of more general rules is given in an Appendix. Although the method can be fully described without reference to any others, a brief account of the arc equations of Cassie and Mayr is given to illustrate essential differences.There follows a description of the first attempt to apply the theory, in which tests were carried out on a 1-cm carbon arc carrying currents of the order of 20 amp. Part of a first-order differential equation is obtained and comparison is made with the Cassie and Mayr equations. A consistent form of discontinuity was observed during the testing and a criterion for this is given.This method of approaching the problem raises some questions about electrical conductors or circuits in general which may eventually prove to be important. The discussion of these is brief, as it is necessarily speculative.