Abstract: A device for welding foils, webs or bags having differing thickness along a line to be welded at right angles to a conveying direction includes a pair of elongated welding dies movable relative to one another and one of which has an elongated, periodically heatable electric resistance heating element having serially-connected sections of differing electrical resistance along its length defined by zonal variations in cross sectional area of the heating element, e.g., areas of differing heating element width. Each section of differing resistance corresponds in length and position to the differing thickness along the line to be welded and is so dimensioned that, prior to initial contact with that line to be welded, the heating element has a higher temperature in the zones of lesser cross-sectional area corresponding to the thicker areas of the line to be welded than at a the at the zones of greater cross sectional area corresponding to the thinner areas thereof.