A method and apparatus for resistance welding of objects (39) by a spot welding gun which has a C-shaped arm (9), a front electrode (11), a back electrode (13) and a pneumatic cylinder (21). The back electrode is axially moveable a limited distance by the cylinder. The gun is arranged to clamp the objects between the electrode tips (15, 17) by the cylinder (21) and to provide a welding current to flow between the tips. The back electrode is arranged to be released from, moved axially in relation to, and fastened to the cylinder to enable or simplify positioning the gun so that the electrodes are on each side of the objects, removal of the gun from the welded objects and adjustment of the electrode distance (a) based on the thickness (t) of the objects.