An improved arc gap structure for a lightning arrester comprises first and second annular electrodes concentrically arranged relative to one another. The first electrode has a generally cylindrical inner surface which tapers to a smaller diameter toward one end. The second electrode has a generally cylindrical outer surface which is sized to be spaced apart from the inner surface of the first electrode when received therein. The electrodes are axially displaced relative to one another such that the spacing between the upper corner of the second electrode and the tapered portion of the first electrode define the ignition arc gap for the lightning arrester. The second electrode is preferably axially adjustable relative to the first electrode to select a desired arc over voltage for the lightning arrester. A magnet inserted within the second electrode is positioned to generate a field within the gap between the electrodes to move an arc axially away from the ignition arc gap and to circulate the arc around the cylindrical surfaces of the electrodes until it is extinguished.