Since most unfilled thermoplastics can be described as good insulators, an electrically conducting implant must be present at the joint line. A work coil, which is connected to a high frequency power supply, is then placed in close proximity to the joint. As electric current at high frequency passes through the work coil, a dynamic magnetic field is generated whose flux links the implant. Electric currents are induced in the implant and when these have sufficiently heated the conducting material, the surrounding thermoplastic parts melt and soften. If some pressure is applied to the joint, this aids wetting of the molten thermoplastics and a weld forms as the joint cools.