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A superconducting cable having improved transmission efficiency facilitates the laying operation and reduces heat intrusion. The superconducting cable comprises a first pipe 105 within which are accommodated a refrigerant passage section 101, a superconducting member 102 and an electrical insulatin

A superconducting cable having improved transmission efficiency facilitates the laying operation and reduces heat intrusion. The superconducting cable comprises a first pipe 105 within which are accommodated a refrigerant passage section 101, a superconducting member 102 and an electrical insulating section 103, and a second pipe 106 arranged on the outer side of the first pipe. A vacuum heat insulating section 104 is provided between the first and second pipes. The second pipe is formed of a ferromagnetic material.

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What is claimed is: 1. A superconducting power transmission cable at least comprising a first pipe having a superconducting member accommodated therein, and a second pipe arranged on an outer side of said first pipe; wherein a vacuum heat insulating section is provided between said first and second

What is claimed is: 1. A superconducting power transmission cable at least comprising a first pipe having a superconducting member accommodated therein, and a second pipe arranged on an outer side of said first pipe; wherein a vacuum heat insulating section is provided between said first and second pipes; and said second pipe is formed of a ferromagnetic material. 2. The superconducting power transmission cable as defined in claim 1 wherein said first pipe has accommodated therein a refrigerant passage section, said superconducting member and an electrically insulating section, in this order, when looking from an inner side. 3. The superconducting power transmission cable as defined in claim 1 wherein said second pipe comprises an iron pipe. 4. The superconducting power transmission cable as defined in claim 1 wherein said second pipe includes a magnetic gap along the longitudinal direction thereof. 5. The superconducting power transmission cable as defined in claim 4 wherein said second pipe is provided with a non-magnetic material within said magnetic gap. 6. The superconducting power transmission cable as defined in claim 1 wherein an insulating member composed of a plurality of sheets is provided in said vacuum heat insulating section provided between said first and second pipes, said sheets each having aluminum deposited thereon. 7. The superconducting power transmission cable as defined in claim 1 wherein said superconducting member is composed of a former and a superconducting strand wound thereon. 8. The superconducting power transmission cable as defined in claim 1 wherein said second pipe performs the role of vacuum heat insulation and retention of magnetic energy. 9. The superconducting power transmission cable as defined in claim 1 wherein said second pipe at least includes one pipe accommodated therein via a vacuum heat insulating section; and another pipe having said one pipe accommodated therein. 10. The superconducting power transmission cable as defined in claim 9 wherein said one pipe is used for keeping vacuum tightness, and said other pipe is used for keeping magnetic energy. 11. The superconducting power transmission cable as defined in claim 9 wherein a cushioning member is provided in a gap between said one pipe and said other pipe. 12. The superconducting power transmission cable as defined in claim 1 wherein a magnetic shielding member is provided at a site of installation of the superconducting power transmission cable. 13. The superconducting power transmission cable as defined in claim 1 wherein facing ends of straight pipe sections of the superconducting power transmission cables are connected to each other by a bellows tube; and the straight pipe sections of the superconducting power transmission cable are secured on at least a plurality of sites separated from one another. 14. The superconducting power transmission cable as defined in claim 1 further comprising: a first superconducting cable having one end connected to a positive terminal of a first power supply; a second superconducting cable having one end connected to a negative terminal of a second power supply; a copper cable having one end connected to a connecting point between a negative electrode of said first power supply and a positive terminal of said second power supply; a first inverter connected across the other end of said first superconducting cable and the other end of said copper cable; and a second inverter connected across the other end of said second superconducting cable and the other end of said copper cable; wherein the connecting points between said first and second inverters and the other end of said copper cable are grounded; and an a.c. output is delivered from said first and second inverters. 15. The superconducting power transmission cable as defined in claim 13 further comprising: a first switch connected in parallel with said first power supply and adapted to be controlled to be on/off; a second switch connected in parallel with said second power supply and adapted to be controlled to be on/off; a first changeover switch supplied with an output of said first inverter and adapted for connecting said output to an a.c. output or a dummy output in a switching fashion; and a second changeover switch supplied with an output of said second inverter and adapted for connecting said output to an a.c. output or a dummy output in a switching fashion. 16. The superconducting power transmission cable as defined in claim 1 wherein said first pipe has accommodated therein a first refrigerant passage section, a first superconducting member, a first electrically insulating section, a ferromagnetic section, a second superconducting member through which the current flows in a reverse direction to that in said first superconducting member, a second electrically insulating section and a second refrigerant passage section, in this order, when looking from an inner side. 17. The superconducting power transmission cable as defined in claim 1 wherein said ferromagnetic section is composed of a plurality of rings of a ferromagnetic material arranged along a longitudinal axis of said first pipe. 18. The superconducting power transmission cable as defined in claim 16 wherein neighboring ones of said rings of the ferromagnetic material are interconnected by a connection wire. 19. The superconducting power transmission cable as defined in claim 1 wherein said first pipe includes a bellows pipe. 20. A superconducting power transmission cable wherein at an ambient temperature end of the superconducting power transmission cable as defined in claim 1, a superconducting strand, forming said superconducting member, is connected via a lead to a first feed-through performing the role of vacuum sealing and electrical insulation, and is further connected from said first feed-through via a lead to a second feed-through disposed between a vacuum and an atmospheric side; and wherein a thermoelectric transducer is connected to said second feed-through to reduce heat intrusion; said thermoelectric transducer being connected from said second feed-through over a cable to an external power supply. 21. The superconducting power transmission cable as defined in claim 20 wherein said second pipe is formed of a stainless steel pipe on the side of a flange mounting portion mounting said second feed-through, a part of said second pipe surrounding said first pipe is formed by an iron pipe and wherein said stainless steel pipe and the iron pipe are interconnected at a connecting portion. 22. The superconducting power transmission cable as defined in claim 20 wherein a pipe for influx of a refrigerant flow and a pipe for efflux thereof are provided to said stainless steel pipe at the end of said second pipe forming an ambient temperature end portion and are connected to said first pipe via an electrically insulating pipe. 23. The superconducting power transmission cable as defined in claim 20 wherein said second feed-through comprises a first electrode provided on a vacuum side and adapted for carrying said thermoelectric transducer thereon; an electrically insulating first insulating plate, adapted for carrying said first electrode; a second electrode sandwiching said first insulating plate with said first electrode and connected over a cable to a power supply on an ambient temperature atmospheric side; and an electrically insulating second insulating plate, adapted for carrying said second electrode. 24. The superconducting power transmission cable as defined in claim 20 wherein said second feed-through comprises: a first electrode provided on the vacuum side and connected to one side of said thermoelectric transducer; a third electrode provided on the vacuum side and connected to the other side of said thermoelectric transducer and to a current lead; electrically insulating first and third insulating plates adapted for carrying said first and third electrodes, respectively; a second electrode sandwiching said first insulating plate with said first electrode and connected over a cable to a power supply on the ambient temperature atmospheric side; and an electrically insulating second insulating plate, adapted for carrying said second electrode. 25. The superconducting power transmission cable as defined in claim 23 wherein said second electrode is provided with a fin. 26. The superconducting power transmission cable as defined in claim 23 wherein said first insulating plate comprises a ceramic plate. 27. The superconducting power transmission cable as defined in claim 23 wherein said second insulating plate comprises an FRP plate. 28. The superconducting power transmission cable as defined in claim 24 wherein said third insulating plate comprises an FRP plate. 29. The superconducting power transmission cable as defined in claim 1 wherein said first pipe has accommodated therein a refrigerant passage section, said superconducting member and an electrically insulating section, in this order, when looking from an inner side; said refrigerant passage section comprising an ongoing refrigerant passage section and a return passage section disposed on inner and outer sides, respectively; said ongoing refrigerant passage section and said return passage section being separated from each other. 30. The superconducting power transmission cable as defined in claim 29 wherein a pipe for separating said ongoing refrigerant passage section and said return passage section from each other is provided between said ongoing refrigerant passage section and said return passage section. 31. The superconducting power transmission cable as defined in claim 1 wherein said first pipe has accommodated therein an ongoing refrigerant passage section, said superconducting member, a return refrigerant passage section and an electrically insulating section, in this order, when looking from the inner side. 32. A superconducting power transmission cable comprising a cooling station for supplying the refrigerant to the superconducting power transmission cable as defined in claim 1, wherein a connecting portion for connecting one of the superconducting power transmission cables laid for extending from said cooling station to another of the superconducting power transmission cables is mounted at a lower altitude than the site of installation of said cooling station. 33. A superconducting power transmission cable comprising a cooling station for supplying the refrigerant to the superconducting power transmission cable as defined in claim 29, wherein a connecting portion for connecting one of the superconducting power transmission cables laid for extending from said cooling station to another of the superconducting power transmission cables is mounted at a lower altitude than the site where said cooling station has been installed; and wherein said cooling station supplies the refrigerant to an ongoing refrigerant passage section of one of the superconducting power transmission cables; said refrigerant having its path reversed at said connecting portion and being returned via a return refrigerant passage section of said one superconducting power transmission cable to said cooling station. 34. A power transmission system comprising the superconducting power transmission cable as defined in claim 1.