Space charge and current in alternating corona

Joseph Slepian: One thing that Mr. Willis brings out and which is worth emphasizing is that the beginning of corona is not the point where ionization by collision begins. This is important because very frequently in the past it has been assumed that the beginning of ionization by collision is also the beginning of breakdown or of corona under various conditions. Mr. Willis has shown by his experiments here that the corona comes at a higher gradient than that at which appreciable ionization by collision occurs and that corona sets in when a sufficiently great cumulative reaction takes place between the ionization produced by collisions of the negative ions and that produced by collisions of the positive ions. Hence the gradient at which the corona begins is not a constant of the gas but also depends upon the geometrical configuration of the electrodes; that is, it depends also upon the radius of the wire and the distance to the other electrode.