usages application_number US 46445809 A description in the following description this invention will be explained with reference to embodiments thereof however these embodiments are not intended to limit this invention to any specific environment applications or particular implementations described in these embodiments therefore description of these embodiments is only for purposes of illustration rather than limitation it should be appreciated that in the following embodiments and the attached drawings elements unrelated to this invention are omitted from depiction and dimensional relationships among individual elements in the attached drawings are illustrated only for ease of understanding but not to limit the actual scale in reference to fig1 through 5 the coil assembly 100 of this embodiment comprises a wire assembly 1 a joint assembly 2 a sleeve 3 and a flexible core 4 the wire assembly 1 is formed by winding the first wire layer 11 and second wire layer 12 onto the outer surface of the flexible core 4 and is then wrapped around by the sleeve 3 e g a thermal shrinkable sleeve on the outer surface unlike the prior art the flexible core 4 of this embodiment is a linear and flexible hollow cylinder with two opposite ends the length.