skilled in the art to supply them as required in fig1 two lengths of pipe heat tubes 10 and 12 and delimit two tracing segments insulated conductor 14 is connected to heat tube 10 at a first end runs within heat tube 10 and passes without heat tube 10 at a second end similarly an insulated conductor 16 is connected to heat tube 12 at a first end runs within heat tube 12 and passes without heat tube 12 at a second end a window type current transformer 18 has a first conductor 180 a second conductor 182 electromagnetically coupled to conductor 180 and a winding 184 electromagnetically coupled to both conductor 180 and to conductor 182 a first terminal of a first alternating current source 20 is connected to a first terminal of a second alternating current source 22 at a node 21 a second terminal of alternating current source 20 is connected to a first end of conductor 180 a second end of which is connected to insulated conductor 14 a second terminal of alternating current source 22 is connected to a first end of conductor 182 a second end of which is connected to insulated conductor 16 the second ends of heat tubes 10 and 12 are connected to node 21 a first terminal of a current detecting apparatus 19 is connected to a first end of winding 184 a second terminal of current detecting apparatus 19 is connected to a second.