re coated respectively with O-pipes 628a and 628b secured by such means as soldering or bonding with each other at the rear ends The insulating tube 625a has such length that its rear end surface may be substantially in the same plane as of the rear end surface of the O-pipe 628a but the other insulating tube 625b projects rearward from the rear end of the O-pipe 628a, has the wire 623 and I-pipe