reference will now be made in detail to various embodiments of the present invention ( s ), examples of which are illustrated in the accompanying drawings and described below . while the invention ( s ) will be described in conjunction with exemplary embodiments , it will be understood that present description is not intended to limit the invention ( s ) to those exemplary embodiments . on the contrary , the invention ( s ) is / are intended to cover not only the exemplary embodiments , but also various alternatives , modifications , equivalents and other embodiments , which may be included within the spirit and scope of the invention as defined by the appended claims . an exemplary embodiment of the present invention will hereinafter be described in detail with reference to the accompanying drawings . fig1 is a cross - sectional view of a water pump of clutch type to which a coil according to an exemplary embodiment of the present invention is applied . as shown in fig1 , a water pump of clutch type 1 includes a pump housing 40 , a pump cover ( not shown ), a pulley 10 , a hub 80 , a shaft 20 , and a coil 110 . the pump housing 40 has a disk shape and is provided with a penetration hole formed at a middle portion thereof . coupling means for coupling the pump housing 40 with the pump cover are disposed at an external circumferential portion of the pump housing 40 . generally , a bolt ( not shown ) is used as the coupling means . the shaft 20 is disposed in the penetration hole , and an impeller 30 is fixed to one end of the shaft 20 . a bearing for smoothly rotating the shaft 20 is disposed between the shaft 20 and the penetration hole . the pump cover is coupled with the pump housing 40 so as to form a chamber ( not shown ) therebetween in which coolant is pressurized . the impeller 30 is disposed in the chamber . in addition , the chamber is connected to an inlet ( not shown ) so as to receive the coolant circulating a cooling circuit ( not shown ), and is connected to an outlet ( not shown ) such that the pressurized coolant is supplied to the cooling circuit . the impeller 30 rotates together with the shaft 20 and pressurizes the coolant flowing in the chamber . the pump housing 40 , the pump cover , and the impeller 30 according to an exemplary embodiment of the present invention are similar to those according to conventional arts and are well known to a person of an ordinary skill in the art . therefore , a detailed description thereof will be omitted . the pulley 10 has an annular shape having an exterior circumference and an interior circumference . a belt ( not shown ) is mounted at the exterior circumference of the pulley 10 . the pulley 10 is connected to a crankshaft ( not shown ) through the belt . therefore , the pulley 10 rotates according to a rotation of the crankshaft . therefore , the pulley 10 always rotates when an engine operates . the pulley 10 is provided with a friction pad 12 mounted at one surface thereof ( a surface facing the pump housing 40 ), and a clutch disk 60 rubbing with the friction pad 12 is provided . the clutch disk 60 is always rotated together with the pulley 10 by frictional force of the friction pad 12 . a first hole 62 is formed at the clutch disk 60 . the hub 80 has a disk shape , and a shaft mounting portion is formed at a middle portion thereof . the shaft 20 is mounted in the shaft mounting portion , and accordingly , the hub 80 rotates with the shaft 20 . in addition , a bearing 50 is disposed between the hub 80 and the interior circumference of the pulley 10 so as to permit relative rotation of the pulley 10 to the hub 80 . that is , the pulley 10 always rotates when the engine operates . however , the hub 80 is selectively connected to the pulley 10 and selectively rotates together with the pulley 10 . the bearing 50 includes an inner ring 52 fixed to the hub 80 , an outer ring 54 fixed to the interior circumference of the pulley 10 , and rolling elements 56 mounted between the inner ring 52 and the outer ring 54 . a second hole 70 corresponding to the first hole 62 is formed at an external circumferential portion of the hub 80 . a pin 82 for selectively connecting the hub 80 with the pulley 10 is inserted in the second hole 70 . the pin 82 inserted in the second hole 70 is selectively inserted in or came out from the first hole 62 according to magnetic force generated by the coil 110 and elastic force of an elastic member 90 fighting against the magnetic force . if the pin 82 inserted in the second hole 70 is inserted in the first hole 62 , the hub 80 is connected to and rotates with the pulley 10 . if the pin 82 inserted in the second hole 70 is not inserted in the first hole 62 on the contrary , connection between the hub 80 and the pulley 10 is cut off and the hub 80 does not rotate . the elastic member 90 is disposed between the hub 80 and the clutch disk 60 and applies the elastic force fighting against the magnetic force generated by the coil 110 to the hub 80 . a coil spring is mainly used as the elastic member 90 . the coil 110 is wound on a coil case 100 . the coil case 100 has an annular shape , one surface thereof ( a surface facing the pump housing 40 ) is open , so as to receive the coil 110 , the opened surface is blocked by a coil cover 120 after the coil 110 is wound on the coil case 100 . therefore , the coil 110 wound on the coil case 100 is not uncoiled by the coil cover 120 . if the current is applied to the coil 110 , the magnetic force is generated at the coil 110 . the magnetic force pulls the pin 82 such that the pin 82 comes out from the first hole 62 . therefore , if the current is applied to the coil 110 , the connection of the hub 80 with the pulley 10 is cut off . hereinafter , referring to fig2 and fig3 , the coil 110 according to the exemplary embodiment of the present invention will further be described . fig2 is a cross - sectional view of a coil according to an exemplary embodiment of the present invention , and fig3 is an enlarged view of fig2 . as shown in fig2 and fig3 , the coil 110 includes a first coil 112 and a second coil 114 . cross - sectional area of the first coil 112 is different from that of the second coil 114 . herein , it is exemplarily described that the cross - sectional area a 2 of the second coil 114 is smaller than that a 1 of the first coil 112 . the first coil 112 is wound on the coil case 100 or the second coil 114 , and the second coil 114 is wound on the first coil 112 . cross - sectional centers of the first and second coils 112 and 114 form a row . in addition , the first and second coils 112 and 114 are wound by a predetermined numbers so as to form a plurality of rows . the first coil 112 and the second coil 114 are disposed to form a zigzag shape . in addition , the first coil 112 is wound as a cubic packing . that is , the cross - sectional centers of the first coil 112 are disposed to form a plurality of columns vertically disposed to the rows . therefore , ( m , n ), ( m , n + 1 ), ( m + 1 and n + 1 ), and ( m + 1 , n ) cross - sectional centers are disposed to form a square shape . that is , an angle θ between a connecting line of the ( m , n ) cross - sectional center with the ( m , n + 1 ) cross - sectional center and a connecting line of the ( m , n ) cross - sectional center with the ( m + 1 , n ) cross - sectional center is 90 °, and length of one connecting line is the same as that of another connecting line . herein , m and n are any natural numbers , and a ( m , n ) cross - sectional center means a cross - sectional center of the coil disposed at an intersecting point of a m - th row and a n - th column . generally , if the first coil 112 is wound as the cubic packing , porosity increases compared with a porosity if the first coil 112 is wound as the rhombohedral packing . however , the second coil 114 , the cross - sectional area a 2 of which is smaller than that a 1 of the first coil 112 , is disposed in the pore formed among the ( m , n ), ( m , n + 1 ), ( m + 1 and n + 1 ), and ( m + 1 , n ) cross - sections of the first coil 112 according to the exemplary embodiment of the present invention . therefore , porosity may be reduced . it is preferable that the pore among the first coil 112 is completely filled with the second coil 114 . accordingly , the second coil 114 contacts with the first coils 112 surrounding it at four points p 1 , p 2 , p 3 , and p 4 . if diameter of the first coil 112 is 1 , the porosity is about 0 . 546 . therefore , diameter of the second coil 114 completely filling the pore ( generally , density of the coil 110 is increased by pressing the coil 110 .) is about 0 . 417 . therefore , a ratio of the cross - sectional area a 1 of the first coil 112 to that a 2 of the second coil 114 is about 1 , 0 . 174 . therefore , it is preferable that the cross - sectional area a 2 the second coil 114 is 17 % of or smaller than that a 1 of the first coil 112 . reduction in the porosity means that the numbers of the coils 110 wound on the coil case 100 increases . the magnetic force generated at the coil 110 is proportional to the numbers of the coils 110 wound on the coil case 100 and the numbers of the poles . since the number of the coils 110 wound on the coil case 100 increases , the magnetic force generated at the coil 110 is stronger according to the exemplary embodiment of the present invention . in addition , the current of the first coil 112 flows in an opposite direction that of the second coil 114 . therefore , the number of the pores increases . further , since porosity of the coil 110 is reduced , the coils 110 are more pressed to each other . therefore , slip of the coil 110 may be prevented , and accordingly , spread of adhesive for preventing slip of the coil 110 is unnecessary . according to an exemplary embodiment of the present invention , a stronger magnetic force is generated at coils without increasing size of an object as a consequence of winding more coils on the object having a limited space . therefore , capacity of devices ( for example , a motor , a generator , a pump , and so on ) generating the magnetic force by flowing current through the coil may increase without increasing size thereof . for convenience in explanation and accurate definition in the appended claims , the terms “ upper ”, “ lower ”, “ interior ”, “ exterior ”, “ inner ”, and “ outer ” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures . the foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description . they are not intended to be exhaustive or to limit the invention to the precise forms disclosed , and obviously many modifications and variations are possible in light of the above teachings . the exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application , to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention , as well as various alternatives and modifications thereof . it is intended that the scope of the invention be defined by the claims appended hereto and their equivalents .