embodiment 1 of the invention will be described hereinafter with reference to fig1 to 5 . fig1 is a vertically left side view of an example of an induction coupling apparatus in accordance with embodiment 1 of the invention with a partially sectional view in a direction of an arrow along a line i - i in fig2 , showing a state that a first core element part is most separated from a second core element part by means of a core parallel moving mechanism . fig2 is a front view of a whole structure of an induction coupling apparatus in accordance with embodiment 1 of the invention , the view from the left side of fig1 . fig3 is a vertically left side view corresponding to fig1 of an induction coupling apparatus in accordance with embodiment 1 of the invention , the induction coupling apparatus having a first core element part and a second core element part put close to each other by means of a core parallel moving mechanism so that the length of a gap would be a predetermined gap length . fig4 is a schematically plan view of a clamp mechanism in accordance with embodiment 1 of the invention . fig5 is a right side view of the periphery of a clamp mechanism in accordance with embodiment 1 of the invention . same signs and numerals denote same components in the drawings . as shown in fig1 to 5 , an induction coupling apparatus in accordance with embodiment 1 of the invention comprises an upper side first core element part ( simply referred to as “ an upper core ”, hereinafter ) 1 , a lower side second core element part ( simply referred to as “ a lower core ”, hereinafter ) 2 , a predetermined gap length regulating member 3 , a clamp mechanism 4 , an elastic member ( referred to as “ an upper core fixing spring ”, hereinafter ) 5 , a positioning pin 7 , a bolt ( referred to as “ a core driving bolt ”, hereinafter ) 8 , an induction coupling apparatus main body part 9 , a positioning member ( referred to as “ a power line fixing tool ”, hereinafter ) 10 , a core case 11 , a primary winding 12 , a holding plate 14 , a nut 18 and a bolt 19 . the induction coupling apparatus is mounted to a power line 13 used as a communication medium . as clearly shown in fig1 , the upper core 1 on the upper side is fixed to the core case 11 while the lower core 2 on the lower side is molded into one body with the induction coupling apparatus main body 9 in molding the induction coupling apparatus main body 9 . the primary winding 12 , which is a part of a signal line , is wound around the lower core 2 . the upper core fixing spring 5 is fixed at one end thereof to the core case 11 . the upper core fixing spring 5 is further fixed to one end of the holding plate 14 . the holding plate 14 is formed from a sheet of plate substantially formed into the shape of l as shown in fig1 . on the other end of the holding plate 14 , fixed is the nut 18 into which the core driving bolt 8 is screwed . moreover , the positioning pin 7 provided in the induction coupling apparatus main body 9 is inserted into a schematically long hole 141 drilled in the middle part of the holding plate 14 . the core case 11 , the upper core fixing spring 5 and the holding plate 14 form a core holding mechanism part 1145 for holding the upper core 1 . the holding plate 14 , which has the schematically long hole 141 , and the positioning pin 7 form a positioning mechanism 147 for preventing the core holding mechanism part 1145 from rotating about the bolt in accordance with rotation of the core driving bolt 8 to hold the core holding mechanism part 1145 at a predetermined position . the core holding mechanism part 1145 , the positioning mechanism 147 , the nut 18 and the core driving bolt 8 form a core parallel moving mechanism 1478 for moving the upper core 1 parallel to the lower core ( in a vertical direction in the case shown in the drawings ). the predetermined gap length regulating members 3 are provided between a pair of legs of the upper core 1 and a pair of legs of the lower core 2 , respectively . as clearly shown in fig2 , the clamp mechanisms 4 are provided on the both sides in a direction of extension of the power line 13 ( on the both of left and right sides in fig2 ) symmetrically with respect to the induction coupling apparatus main body part 9 so that the power line 13 would be arranged under clamp arms 41 of the clamp mechanism 4 . between the power line 13 and the induction coupling apparatus main body part 9 , provided is the power line fixing tool 10 , which is fixed to the induction coupling apparatus main body 9 by means of the bolt 19 . the upper core 1 and the lower core 2 are both formed from a magnetic material such as ferrite . top end surfaces of the legs of the above cores , which face to and contact with the predetermined gap length regulating member 3 , are finished with accuracy . the predetermined gap length regulating members 3 are arranged between the both of left and right legs of the upper core 1 and the lower core 2 so as to closely adhere to the upper core 1 and the lower core 2 . the predetermined gap length regulating members 3 , the upper core 1 and the lower core 2 are integrated into one body to form a magnetic core 123 formed from a gap core . a combination of the primary winding 12 for transmitting a signal , the magnetic core 123 and the power line 13 forms the induction coupling apparatus . the predetermined gap length regulating member 3 is formed from a non - conductive material so as not to generate magnetic shielding due to eddy current since a magnetic flux passes through the predetermined gap length regulating member 3 per se in a direction of the gap length ( in a vertical direction in fig1 to 3 ). the predetermined gap length regulating member 3 is fixed to the lower core 2 by adhesion to be mounted on the lower core 2 side as shown in the drawings in embodiment 1 . the core case 11 holding the upper core 1 is formed from a rustproof material such as stainless steel . the core case 11 is in the shape of a container whose opening is on the lower side . the core case 11 is placed with its opening up to put the upper core inside the opening . pouring and fixing a resin material and the like allows the upper core 1 to be fixed to the core case 11 . due to a fixed bonding material 111 such as resin , the upper core 1 is firmly fixed to the core case 11 even when the upper core 1 is provided under the core case 11 as shown in fig1 to 3 . the upper core fixing spring 5 is a leaf spring . the upper core fixing spring 5 has the function of pressing at one end thereof the upper core 1 downward in the drawings ( in other words , in a direction of the gap length of the predetermined gap length regulating member 3 ) through the core case 11 . an end of the holding plate 14 is welded to the other end of the upper core fixing spring 5 . on the other end of the holding plate 14 , fixed is the nut 18 , which is screwed at the top of the core driving bolt 8 . the core driving bolt 8 is rotatably mounted to a first arm 91 of the induction coupling apparatus main body part 9 so as not to move in the axial direction of the rotation . the holding plate 14 and the core driving bolt 8 hold the upper core 1 , the fixed bonding material 111 , the core case 11 and the upper core fixing spring 5 . the respective components have sufficient rigidity so as not to be deformed in the holding . a rotation operating part 81 of the core driving bolt 8 is provided under the first arm 91 . a rotating operation of the rotation operating part 81 causes the core driving bolt 8 to rotate . the holding plate 14 starts to rotate so as to follow the rotation of the core driving bolt 8 but the positioning pin 7 inserted into the schematically long hole 141 of the holding plate 14 prevents the following rotation . accordingly , when the rotating operation of the rotation operating part 81 causes rotation of the core driving bolt 8 , the nut 18 moves in the axial direction of the rotation of the core driving bolt 8 since the core driving bolt 8 does not move in the axial direction of the rotation . that is to say , rotation of the rotation operating part 81 in one direction causes the nut 18 to be raised up and rotation in the other direction causes the nut 18 to be raised down . the holding plate 14 , the upper core fixing spring 5 , the core case 11 and the upper core 1 are also raised up and down in accordance with the movement of rising up and down of the nut 18 . as described above , the holding plate 14 having the schematically long hole 141 and the positioning pin 7 form a positioning mechanism 147 for preventing the rotation about the bolt of the core holding mechanism part in accordance with the rotation of the core driving bolt 8 to hold the core holding mechanism part at a predetermined position . the axial direction of the rotation of the core driving bolt 8 ( in other words , a direction of extension of the core driving bolt 8 ) is parallel to a direction of extension of the positioning pin 7 and a gap length direction of the predetermined gap length regulating member 3 . accordingly , movement of rising up and down of the upper core 1 in accordance with the rotation of the core driving bolt 8 is parallel movement of the upper core 1 to the lower core 2 . the clamp mechanism 4 has the clamp arm 41 tapering in shape of a side surface as shown in fig1 . the clamp mechanism 4 is provided with a recess 42 in the shape corresponding to the external form of the power line 13 so as to be able to easily hold the power line 13 at the base of the projected thin part of the clamp arm 41 . the recess 42 prevents the power line 13 from coming off when the clamp arm 41 is depressed . the clamp arm 41 is fixed to the induction coupling apparatus main body part 9 by means of the clamp bolt 16 . rotation of the clamp bolt 16 is arranged to cause the clamp arm 41 to rise up and down . the power line 13 fixing tool 10 is formed from a metal material and holds the power line 13 between the power line fixing tool 10 and the clamp arm 41 . the clamp bolt 16 is rotatably mounted to each of second and third arms 92 and 93 , which are provided symmetrically on the both sides in a direction of extension of the power line 13 ( on the both of left and right sides in fig2 ) from the induction coupling apparatus main body part 9 with respect to the induction coupling apparatus main body part 9 , so as not to move in the axial direction of the rotation . the clamp arm 41 is screwed into each top screw part of each clamp bolt 16 . the rotation operating parts 161 of the respective clamp bolts 16 are provided under the corresponding second and third arms 92 and 93 . the respective rotation operating parts 161 and 161 are provided on the lower side , which is the same side as the rotation operating part 81 of the core driving volt 8 and the induction coupling apparatus main body 9 . the power line fixing tools 10 are screwed to the respective mounting surfaces of the second and third arms 92 and 93 on the power line 13 side by means of the bolt 19 so as to correspond to the respective clamp arms 41 and 41 . a rotating operation of the rotation operating part 161 causes the clamp bolt 16 to rotate . when the clamp bolt 16 rotates , the clamp arm 41 also starts to rotate so as to follow the rotation of the clamp bolt 16 . the power line fixing tool 10 , however , prevents the rotation for the following . accordingly , when the rotating operation of the rotation operating part 161 causes rotation of the clamp bolt 16 , the corresponding clamp arm 41 moves in the axial direction of the rotation since the clamp bolt 16 does not move in the axial direction of the rotation . that is to say , rotation of the rotation operating part 161 in one direction causes the corresponding clamp bolt 16 to rise up and rotation in the other direction causes the corresponding clamp bolt 16 to rise down . now , an operation of mounting the induction type signal coupling apparatus in accordance with embodiment 1 mentioned above to the power line 13 will be described one by one . first , the induction coupling apparatus is checked that the upper core 1 is in the upper position and the clamp arm 41 is also in the upper position as shown in fig1 in embodiment 1 . then , a holding pole for overhead wiring ( not shown ) is used on the ground to hold the core driving bolt 8 . the clamp arm 41 of the clamp mechanism 4 is then completely hung on the power line 13 provided at a height of several meters to move the whole apparatus so that the power line 13 would correspond to the recess 42 of the clamp arm 41 . after the power line 13 is settled in the recess 42 of the clamp arm 41 as shown in fig1 , the core driving bolt 8 is rotated to raise down the upper core 1 in order to closely stick the upper core 1 and the lower core 2 to each other through the predetermined gap length regulating member 3 , as shown in fig4 . in this case , the predetermined gap length regulating member 3 is not deformed due to excessive power added to the predetermined gap length regulating member 3 since the core case 11 is fixed to the holding plate 14 through the upper core fixing spring 5 even when the core driving bolt 8 is tightly fastened , and therefore , accuracy of a gap can be maintained . the height of the clamp 4 in the upper position , the clamp 4 holding the power line 13 , is designed to be lower than the height of the inner surface of the upper core 1 completely raised down . accordingly , an operation for closely sticking the upper core 1 and the lower core 2 is not hindered . in other words , the upper core 1 , the lower core 2 and the power line 13 are arranged not to be in contact with each other when the upper core 1 and the lower core 2 closely adhere to each other via the predetermined gap length regulating member 3 . the holding pole is then removed from the core driving bolt 8 to hold and rotate one clamp bolt 16 . this causes the clamp arm 41 to be depressed , and thereby , the power line 13 is fixed ( that is , held ) between the clamp arm 41 and the power line fixing tool 10 . following to the above , the other clamp bolt 16 in the corresponding position is also rotated similarly to the above to hold the power line 13 between the clamp arm 41 and the other power line fixing tool 10 . fig5 shows a condition that the clamp arm 41 is raised down to hold the power line 13 in the clamp mechanism 4 . an operation of mounting the induction coupling apparatus in embodiment 1 to the power line 13 is completed as described above . the induction coupling apparatus in accordance with embodiment 1 is arranged to hold the power line 13 by means of the clamp mechanism 4 to fix the induction coupling apparatus to the power line 13 , as described above . this allows the induction coupling apparatus not to be shifted in position with respect to the power line 13 even in the case of exposure to wind and rain , so that an effect that movement of the induction coupling apparatus due to wind and such does not cause a beak of a communication line occurs . further , the gap of the core is determined in accordance with the thickness of the predetermined gap length regulating member 3 . this allows the gap to be highly accurate . moreover , a problem is not caused such that the predetermined gap length regulating member 3 is deformed due to plasticity to cause deterioration in accuracy of the gap since the force equal to or more than the elastic force of the upper core fixing spring 5 does not add to the predetermined gap length regulating member 3 even when the core driving bolt 8 is tightly fastened in mounting the apparatus . an inductance l in the gap core is obtained by the following formula : wherein “ s ” denotes a cross section , “ g ” denotes a gap , “ μ0 ” denotes permeability of a vacuum , “ la ” denotes a length of a magnetic path of a gap core and “ μ ” denotes relative permeability of a core . as clearly seen from the formula , a highly accurate gap “ g ” in the invention allows a value of the inductance “ l ” to be highly accurate . the inductance of a core is an important circuit constant in an induction coupling circuit . accordingly , the highly accurate inductance value results in an effect that the strength of a signal , which can be transmitted in the power line , is stabilized , and thereby , the information transmission is stabilized . both of the core driving bolt 8 and the clamp bolt 16 are provided on the same side of the induction coupling apparatus main body part 9 at the same angle . this causes an effect that the simplicity in operation is improved such that the same operation can be carried out every time for the holding pole for overhead wiring at the same angle and that operating only one bolt enables the induction coupling apparatus to be raised to and hung on the overhead line , enables two cores to be closely stuck with each other and enables the core to be fixed . furthermore , using the core parallel moving mechanism 1478 for relative and parallel movement of the first core element part and the second core element part in a direction that the gap length varies allows stable and excellent induction coupling efficiency and high reliability to be secured and a low cost to be practically achieved since the core parallel moving mechanism 1478 can be formed from one holding plate 14 , one core driving bolt 8 and one positioning pin 7 , for example , as described above . moreover , the magnetic cores 1 and 2 are arranged not to be in contact with the power line 13 in a condition that the power line 13 is held in the clamp mechanism 4 on the both sides of the magnetic core in a direction of extension of the power line . this allows a collision of the power line 13 and the magnetic cores 1 and 2 due to a swing or an oscillation of an aerial power line to be avoided , so that the accuracy in coupling of the induction coupling apparatus can be improved , the accuracy in coupling can be stabilized and the reliability can be improved . a structure of an induction coupling apparatus in accordance with embodiment 2 is substantially same as that of embodiment 1 mentioned above . in the structure of the induction coupling apparatus in accordance with embodiment 2 , a clamp spacer 17 is provided under the power line fixing tool 10 as shown in fig6 . the clamp spacer 17 is made so as to have the height corresponding to a change in diameter of the power line 13 . the clamp spacer 17 is provided at the center thereof with a through hole 171 into which the fixing bolt 19 is inserted . the shape of the clamp spacer 17 can be simpler than that of the power line fixing tool 10 since only a function of adjusting the height is required . an operation in embodiment 2 is same as that of embodiment 1 described above except for a point that the clamp spacer 17 is provided in advance in order to correspond to the power line 13 having a different diameter . the above - mentioned structure of the induction coupling apparatus in accordance with embodiment 2 allows an induction coupling apparatus for a power line having a different diameter to be provided with a slight modification made , so that an effect that a cost can be reduced is achieved . an induction coupling apparatus in accordance with embodiment 3 has a structure in which several structures are added to the structure of the induction coupling apparatus in accordance with embodiment 1 described above and which is shown in fig7 and 8 . in fig7 and 8 , a position adjusting screw 6 is added . the upper core fixing spring 5 and the holding plate 14 are coupled by welding in embodiment 1 . in embodiment 3 , however , they are coupled by means of the position adjusting screw 6 . a positioning pin fixing screw 15 is also added in fig8 . the upper core fixing spring 5 is provided with a screw for the position adjusting screw 6 . the holding plate 14 is provided with a through hole 142 into which the position adjusting screw 6 is inserted . the through hole 142 of the holding plate 14 has a diameter a little bit larger than the diameter of the position adjusting screw for the purpose of giving a space for adjustment by means of the position adjusting screw . the holding plate 14 having the through hole 142 , the position adjusting screw 6 , the upper core fixing screw 5 form a position adjusting mechanism 146 having a later - mentioned position adjusting function . fig8 shows the shape of the positioning pin fixing screw 15 . the positioning pin fixing screw 15 is welded to the holding plate 14 in the vicinity of the through hole 142 for the positioning pin 7 drilled in the holding plate 14 . the positioning pin fixing screw 15 allows a part in contact with the positioning pin 7 to have a curvature to fix the positioning pin 7 in one direction . now , described will be an operation . a process of mounting the induction coupling apparatus in accordance with the invention to the power line 13 is same as that of embodiment 1 . in embodiment 3 , however , the following adjustment is carried out before shipping the apparatus as a product in order to improve the accuracy in providing the upper core 1 and the lower core 2 . first , the upper core 1 and the lower core 2 are closely stuck to each other by means of the core driving bolt 8 under a condition that the position adjusting screw 6 is loosened . at that time , a gap material 3 is provided between the both cores . then , fine adjustments are made to the position of the upper core 1 so that end surfaces of the upper core 1 and the lower core 2 would accurately correspond to each other . the position adjusting screw 6 is tightened after the adjustments are completed to fix the upper core fixing screw 5 and the holding body 14 . performing such an adjustment allows the upper core 1 to be accurately positioned to the end surface of the lower core 2 even when the core driving bolt 8 is rotated to raise up and down the upper core 1 . further , the positioning pin 7 is pressed to a side surface of the through hole of the holding body 14 by means of the positioning pin fixing screw 15 in rotating the core driving bolt 8 to raise up and down the upper core 1 . accordingly , an error in providing the upper core 1 and the lower core 2 due to tolerances of diameters of the positioning pin 7 and the through hole of the holding body 14 does not occur , so that the upper core 1 and the lower core 2 are accurately positioned to each other . as described above , it is possible to carry out the position adjustment in a relation of position between the upper core 1 and the lower core 2 after assembling the core driving bolt 8 , the positioning pin 7 , the holding body 14 , the upper core fixing screw 5 and the core case 11 . this allows the accuracy in positioning to be improved regardless of an individual variation of each component and dispersion in assembly , so that the function of an induction coupling apparatus is stabilized .