B is an enlarged view of the series-connected portion in FIG. 19A, andFIG. 19C is a sectional view of the portion shown in FIG. 19B;FIG. 20 is a schematic view showing a method of supplying a non-contact medium;FIG. 21 is a graph showing the relationship between the mean square surface roughness of the non-contact medium and its welding reliability; andFIG. 22 is a graph showing the relationship b