Since Davy’s discovery of the electric arc, a century ago, down to the present time, the nature of the physical processes going on in it, and the mechanism by which it conducts electricity, have been the subject of almost uninterrupted discussion and experiment. In order to explain the fact that the equation connecting P. D. current and length appears to contain a large practically constant term, experimenters have assumed that the arc possesses resistance and E. M. F., though which of the two is the more important in obstructing the flow of the current, or whether both must be considered, has been, and is still, a matter of controversy, the settlement of which, it is hoped, will be furthered by the experimental results described in this communication. A priori it is highly probable that the resistance and E. M. F.’s of the electric arc, if they exist, will be functions of the current; it is therefore necessary to first consider the definitions of these quantities, as it will largely depend on the definitions adopted whether the arc can be said to possess a resistance, an E. M. F., or neither. The ordinary text-book definitions of resistance and E. M. F. generally start with the assumption that they are constant quantities independent of the current flowing, and their possible variation is generally developed as a secondary effect due to the current altering the state or nature of the body or apparatus considered, these alterations in the state being the primary cause in the change in resistance and E. M. F. observed.