a pencil point like structure for generating short current path across the ends 26 and 28 of the electrodes 12 and 14 respectively fig5 a illustrates tip 30 sharpened to a pencil point like structure wherein the inner electrode 12 is a solid electrode fig5 b illustrates a hollow inner electrode 13 which may also be sharpened as shown in phantom at 15 alternatively applicants have found that it may be possible to form the bi polar needle in a less expensive manner as shown in fig4 a b and c in this case applicants use a pre made insulated wire the wire includes an inner core such as 12 and a dielectric jacket heat recoverable material comprising the means 16 the wire is expanded preferably the outer material only to an axially stretched radially reduced condition as seen in fig4 b an outer electrode such as 14 may then be slipped over the stretched wire and recovered in the fashion described above and illustrated in fig4 c thus for manufacturing ease applicants have found that it is desirable to select a particular diameter wire already having the insulating locking means preferably the jacket comprising is pre expanded before being extruded.