Gap welding is different from the procedure described above: Before welding starts, the stud is positioned in a defined and adjustable distance above the work piece (gap). After triggering the welding process, the stud is accelerated by a spring to the plate surface. If there is contact between the ignition tip on the work piece, the process continues as described above. Welding time is approx. 1 msec; thus e.g. welding of aluminum becomes feasible without using a shielding gas atmosphere. Recommended plate thickness should be 1/10 d, but not less than 0.5 mm.