prior to description of a manufacturing method according to the present invention , the structure of a superconducting cable to be manufactured by the method will first be described by reference to fig1 . the cable has three cores 2 housed in a thermally insulated pipe 1 . the thermally insulated pipe 1 is constituted by providing thermal insulation material , such as multi - layer thermal insulation ( not shown ), between an outer pipe 3 and an inner pipe 4 , and evacuating a space defined between the pipes 3 , 4 . the cable cores 2 are stranded with sufficient slack to enable management of thermal contraction . the outer and inner pipes 3 , 4 are formed into corrugated pipes . each of the cable cores 2 comprises , in order from the center , a former 5 , a superconductor 6 , an electrical insulation layer 7 , and a shield layer 8 . a protective layer ( not shown ) may be provided outside of the shield layer 8 . in the present embodiment , the former 5 is hollow . the internal space of each former 5 and the space defined between the inner pipe 4 and the cable cores 2 serve as coolant channels . as the superconductor 6 , oxide superconductors , such as a y - based superconductor or a bi - based superconductor , are preferable . materials that can be used as the electrical insulation layer 7 include a paper tape soaked with coolant , or a composite paper composed of a paper tape and a plastic tape , and the electrical insulation layer 7 is formed by winding such paper tape . liquid nitrogen or liquid helium can be utilized for the coolant . in order to allow slack in the strands of three cores , the cable is manufactured according to a method provided below after manufacture of cable cores . spacers are interposed among the three cores at the time of stranding of the cores as shown in fig2 . rectangular slits 11 are formed in a batten plate 10 which is to act as a guide at the time of stranding of cores . a tape - like spacer 12 is caused to pass through each of the slits 11 . as shown in fig3 , the spacers 12 are sandwiched between the cores 2 , thereby producing three cores with the side surfaces of the spacers 12 exposed through grooves among the strands . the three cores with the spacers are housed in the inner pipe 4 . as shown in fig4 the inner pipe is usually produced by covering an outer circumferential surface of the thus - stranded cores 2 with a metal plate 20 ( e . g ., a stainless plate ) and sequentially welding joints in the metal plates 20 with a welder 21 . in this stage , the inner pipe is not a - corrugated pipe but a linear pipe . subsequently , the inner pipe 4 is introduced into a corrugator 22 , where the pipe 4 is corrugated . the thus - corrugated pipe 4 is taken up on a drum 23 . the spacers 12 are removed before the stranded cores is introduced into the welder 21 . the spacers 12 can be easily removed by being drawn from the grooves defined among the stranded cores . for instance , before the time of introduction of the stranded cores into the welder 21 , the ends of the stranded cores have been subjected to terminal treatment for preventing spreading of the cores , and the respective spacers 12 have been drawn from the cores . since the spacers 12 are sandwiched merely among the cores 2 , the spacers 12 can be easily pulled by hand . preferably , the spacers are drawn sideways of the production line and taken up . by means of such a production step , the three cores can be housed in the corrugated inner pipe 4 with slack corresponding to the thickness of the spacers 12 . the slack is maintained not only during the process of processing the corrugated inner pipe but by the end of assembly of the cable . trial calculation of how thick a spacer is to be for practical used is carried out . explanations are given of a case where stranded three cores are housed in a thermally insulated pipe ( i . e ., an inner pipe ) of a superconducting cable having an outer diameter of 135 to 136 mm so that the cable can be stored into a pipe line having an inner diameter of 150 mm . the present embodiment adopts a core outer - diameter d of 39 . 5 mm ; an inner diameter b of 93 mm for a corrugated inner pipe ; and a core stranding pitch of 1000 mm . a standard inner diameter of the corrugated inner pipe is 95 mm . however , in consideration of tolerances of + 0 mm and − 2 mm , a smallest inner diameter of 93 mm is adopted . as described previously , metal constituting the cable material is subjected to a contraction of about 0 . 3 %. here , a required amount of slack ao is taken as 0 . 3 %, and trial calculation this time takes into consideration full management of the required amount of slack a0 . in order to impart a slack of 0 . 3 % to the core after assembly of the superconducting cable , desirably as much slack as possible is imparted to the cores at the time of manufacture of the corrugated inner pipe . a conceivable measure is to maximize a spacer thickness . if the spacer thickness is made excessively large , the profile of strands of three cores becomes apt to deformation at the time of manufacture of a cable core . there must also be taken into consideration constraints on production ; that is , the outer diameter of a core being sufficient to house the core into the inner pipe ; and obviation of a risk of causing a failure to weld the inner pipe at the time of manufacture of the inner pipe or a risk of inflicting damage to - cores . on the basis of a result of review of these factors , trial calculation of a spacer thickness is carried out with a view toward housing the stranded cores into the inner pipe without inflicting damage to the cores and assuring a design slack of about 0 . 4 %. an amount of design slack al is expressed mathematically by eq . 1 , provided that the length of one pitch of cores with spacers is taken as l1 and the length of one pitch of cores without spacers is taken as l2 . the thickness “ t ” of a spacer is determined such that 0 . 3 %≦ a1 and the diameter of an enveloping circle circumscribing the three cores with spacers assumes a value of ≦ 93 mm . lengths l1 , l2 can be determined as follows . fig5 is a cross - sectional view of a three - core strand into which cable cores 2 , each having a diameter d , are stranded while tape - like spacers 12 having a thickness “ t ” are sandwiched between the cable cores . as is obvious from the drawing , a distance r1 from the center of the enveloping circle circumscribing the three cores having the spacers to the center of each core is determined by eq . 2 . r1 = t + d 2  cos   30  ° eq .  2 a distance r2 from the center of the enveloping circle circumscribing the three cores not having spacers ( unillustrated ) to the center of each core is determined by eq . 3 . r2 = d 2  cos   30  ° eq .  3 in general , the length l per pitch of the stranded three cores is expressed by eq . 4 , provided that a distance from the center of the enveloping circle circumscribing the three cores to the center of each core is taken as “ r .” corelength ={ square root }{ square root over (( 2 r π ) 2 + pitch 2 )} eq . 4 by means of substituting r1 or r2 into “ r ,” l1 and l2 are expressed by eq . 5 and eq . 6 , respectively . l1 = ( ( t + d ) cos   30  °  π ) 2 + pitch 2 eq .  5 l2 = ( d cos   30  °  π ) 2 + pitch 2 eq .  6 the diameter of the enveloping circle circumscribing the three cores having spacers is expressed by eq . 7 provided below . from equations 1 , 5 , 6 , and 7 , there is selected “ t ” which satisfies 0 . 3 %≦ a1 and such that the diameter of the enveloping circle circumscribing the three cores having spacers becomes ≦ 93 mm . t + d cos   30  ° + d eq .  7 trial calculation was carried out in accordance with the procedures set forth . a graph shown in fig6 denotes a relationship between the spacer thickness and the amount of design slack , and a relationship between the spacer thickness and the diameter of an enveloping circle circumscribing the three cores having spacers . as is found through review of a spacer thickness which assures maximum slack for strands of three cores and which inhibits damage to the cores at the time of manufacture of the corrugated inner pipe , a design slack of about 0 . 4 % is achieved by adoption of a spacer having a thickness of 7 mm . the example trial calculation has described the case of three cores . even when the number of cores is less than or greater than three , a spacer thickness is determined on the basis of the same concept and in the same manner . as has been described , under a method of manufacturing a superconducting cable according to the present invention , cores can be housed in a thermally insulated pipe with sufficient slack for managing thermal contraction , by temporarily interposing spacers between cores . hence , there can be manufactured a superconducting cable capable of managing thermal contraction without integrating another member with cable cores .