Shielded skin-effect current heated pipeline

In a pipeline to be heated by skin-effect current wherein a skin-effect-current heat-generating pipe as a heating source is laid along a fluid-transporting pipe of the pipeline so as to trace the same and consists of a ferromagnetic pipe and an insulated electric wire passed therethrough, and has a circuit so devised that, when a first alternating current is passed through said electric wire, a second alternating current flows through said ferromagnetic pipe in the direction opposite to that of the first alternating current in response thereto while being concentrated in the vicinity of the inner surface of said ferromagnetic pipe,

• the improvement comprising that said pipeline additionally includes an outer covering pipe of a ferromagnetic material provided so as to enclose the outer periphery of both said fluid-transporting pipe and said heat-generating pipe, said covering pipe being electrically connected to at least both end portions of said ferromagnetic pipe;
• and the thickness of said outer covering pipe being selected so that any alternating current that leaks out of said skin-effect-current pipe substantially does not appear on the outside of said outer covering pipe, but instead flows only through the inner skin portion of said outer covering pipe.