An electric arc reactor having a substantially straight feed material passage (8) extending through both the anode and the cathode (1, 2). Gas is fed into the gap (5) between the two electrodes (1, 2) so as to swirl about the axis (17) of the feed passage and thereby confine the lateral extent of the arc (15) and also confine the location at which the upstream arc root (18) attaches to the upstream electrode (1). An enlargement (12) of the passage in the gap end of the upstream electrode provides an internal sloping root attachment surface (31). A similar enlargement may be provided in the downstream electrode, but spaced from the gap (5), if the downstream electrode (2) forms the cathode. Gas flow (16) in a direction towards the upstream end of the feed passage is used to control the location at which an arc root (18) attaches to the sloping surface (31) of a passage enlargement (12). Magnetic means (14) is provided to induce rotation of the arc (15) such as to form a hollow arc column which is coaxial with the feed passage (8) and through which feed material can pass.