normal magnetic field strength of the double-layer sample was stronger than that of the single-layer sample.In order to discuss the in tensity of the normal magnetic field strength of the single tube and the double tube,the cross-section diagrams are shown in Fig. 11.The lines of the cross-section diagram are shown as C–C0 in Fig. 10. TheC–C0 line was located at the position 60 mm in height,which was along the weld area.The magnitude of DB was the