Quoting from INDUCTION COILS HOW TO MAKE AND USE THEM by Marshall and Stoye (Spon & Chamberlin, 1906), pages 65-66: "For ordinary sparking experiments the negative electrode should terminate in a fairly large brass disc, the positive being a needle point adjustable as to distance, but moving opposite the centre of the disc. By this means the longest sparks can be obtained. It will be noticed that the sparks constantly strike fresh places, and they almost invariably traverse very crooked paths through the air. In this respect they are like lightning flashes -- 'small editions' of which they really are, as a matter of fact. These crooked paths represent 'lines of least resistance,' the electric current, no doubt, finding irregularly distributed particles of conducting matter floating in the atmosphere." Is that really why electric arcs follow an irregular path? Is the mechanism for this phenomenon known? Is it the presence of dust, or something? Would an arc struck in dust-free air follow a smooth path? Or could there be an invisible grain structure in air? Such as local pockets with different chemical composition, as compared to the matrix? Would an arc struck in pure nitrogen behave any differently than an arc struck in air?