before describing the preferred embodiments , an explanation will be given on the structure of a prior art electromagnetic driving device in reference to an elevational view in fig1 and a plan view in fig2 . fixed magnetic cores 11 and 12 are arranged in parallel at a distance therebetween . additional fixed magnetic cores 18 and 19 are arranged laterally in parallel to form a closed magnetic path . a movable member 31 is located between the fixed magnetic cores 11 and 12 , and is capable of moving freely in the longitudinal direction . a coil 21 is wound on the fixed magnetic core 11 . a unidirectional magnetic field is formed between the fixed magnetic cores 11 and 12 by the movable member 31 . the movable member 31 is moved in the longitudinal direction by the interaction between the unidirectional magnetic field and the current i 1 of the coil 21 . an electromagnetic driving device according to an embodiment of the present invention is illustrated in the elevational view of fig3 and the plan view of fig4 . fixed magnetic cores 13 and 14 are arranged in parallel at a distance therebetween . the ends of the fixed magnetic cores 13 and 14 are not closed . the movable member 31 is located between the fixed magnetic cores 13 and 14 and is capable of moving freely therealong . the coil 21 is wound uniformly on the fixed magnetic core 13 . the cross - sectional area of the central portion of each of the fixed magnetic cores 13 and 14 is greater than the cross - sectional area of the end portions thereof , as illustrated in fig4 . the principle of operation of the device of fig3 will be understood with reference to fig5 showing the distribution of the magnetic flux in the upper portion and the change of magnetic flux density in the fixed magnetic core in the lower portion . in the prior art device of fig1 removal of the additional fixed magnetic cores 18 and 19 will result in magnetic flux φ ( 21 ) caused by the coil 21 , leakage magnetic flux φ ( 21l ) from the coil 21 , and magnetic - flux φ ( 31 ) caused by the movable member 31 . in the upper portion of fig5 the arrows φ ( 21l ) represent the directions of magnetic flux by their directions and the values of magnetic flux by their lengths . the value of the leakage magnetic flux φ ( 21l ) is greater in the end portion than in the central portion , while , to the contrary , the magnetic flux density in the fixed magnetic core caused by the coil 21 is greater in the central portion than in the end portion . the abscissa of the graph in the lower portion of fig5 denotes the distance in mm of movement in the rightward direction of the movable member 31 , taking the left end of the fixed magnetic cores 11 and 12 as the origin . if the central portion of the fixed magnetic cores 11 and 12 is in the saturated state due to the magnetic flux φ ( 21 ) caused by the coil 21 , the magnetic flux φ ( 31 ) of the movable member 31 is prevented from entering into the fixed magnetic cores 11 and 12 , and , hence , the driving force for the movable member 31 is reduced . accordingly , in order to avoid such reduction of the driving force , it is necessary to ensure a sufficient area of the cross - section of the fixed magnetic cores 11 and 12 to avoid the saturation of the magnetic flux . however , since the magnetic flux density is not so large in the end portions of the fixed magnetic cores 11 and 12 because of leakage of magnetic flux , it is possible to make the cross - sectional area of the end portions of the fixed magnetic cores 11 and 12 smaller . hence , it is possible to keep down the weight of the fixed magnetic cores 11 and 12 while keeping the driving force of the movable member constant . the magnetic leakage flux from the movable member 31 is omitted from the explanation of fig5 for simplification of explanation . the reduction of the driving force due to the magnetic leakage flux is negligible . a speedometer of the horizontal movement type to which the electromagnetic driving device of fig3 is applied is illustrated in fig6 . a plan view of the fixed magnetic core 13 of fig6 is given in fig7 . a cross - sectional view of the device of fig6 is given in fig8 . in the device of fig6 the movable member 31 is fixed by means of adhesive material to a guide rail 41 having a slot . the movable member 31 can be moved freely horizontally along two slide bars 42 , 42 arranged in parallel at a predetermined distance , keeping contact at the slot portions of the slide bars 42 , 42 . the slide bars 42 , 42 are fixed to the yokes 101 and 102 bridging the fixed magnetic cores 13 and 14 . the front panel 45 on which the numerals are indicated is fixed to the yokes 101 and 102 . an indicator 44 is fixed to the guide rail 41 . a spring is provided between the movable member 31 and one of the fixed magnetic cores 101 and 102 . thus , it is possible to indicate the speed of the automobile by passing current corresponding to the automobile speed through the coil 21 . if a potentiometer is coupled with the movable member to detect its position , it is possible to carry out servo control . the present invention is not limited to the above described embodiment . various modifications and alterations are possible without departing from the scope and spirit of the present invention . for example , it is possible to have the fixed magnetic cores 13 and 14 wound by coils 21 and 22 to double the driving force , as illustrated in fig9 and 10 . also , it is possible to provide a fixed magnetic core 15 and a pair of movable members 31 and 32 having opposite polarities and arranged in parallel at a predetermined air distance with respect to the fixed magnetic core 15 , as illustrated in fig1 and 12 . in the device of fig3 and 4 the change of cross - sectional area of the fixed magnetic core 13 is attained by changing the width of the fixed magnetic core 13 , while the thickness of the magnetic core 13 is maintained constant . it is also possible , however , to attain the change of cross - sectional area of the fixed magnetic cores 16 and 17 by changing the thickness of the fixed magnetic core , while the width of the magnetic core is maintained constant , as illustrated in fig1 and 14 .