we will now describe the variable reluctance sensor of our invention in detail in terms of the revolution sensor illustrated in fig1 and 2 and therein generally designated 10 . in fig1 we have shown the revolution sensor 10 together with part of a wheel 12 of magnetic material having a series of teeth 14 formed thereon with a prescribed pitch p . this magnetic wheel 12 represents an example of movable object whose revolution is to be detected from variations in reluctance and translated into corresponding voltage variations by the revolution sensor 10 . the revolution sensor 10 includes a tubular enclosure 16 of nonmagnetic material having a closed front end 18 and an open rear end 20 . the closed front end 18 of the enclosure 16 is disposed opposite the toothed periphery of the magnetic wheel 12 with a spacing therebetween . mounted within the enclosure 16 is a permanent magnet assembly 22 which is held against the closed front end 18 of the enclosure . the permanent magnet assembly 22 of this particular embodiment comprises three permanent magnets m 1 , m 2 and m 3 arranged in a row in the diametric direction of the tubular enclosure 16 and in the plane of rotation of the magnetic wheel 12 . it is essential according to our invention that the first and third magnets m 1 and m 3 have their poles of the same character oriented toward the magnetic wheel 12 , and the second magnet m 2 have its pole of the opposite character oriented toward the magnetic wheel . thus , in this particular embodiment , the s poles s 1 and s 3 of the first and third magnets m 1 and m 3 , and the n pole n 2 of the second magnet m 2 , are shown held against the closed front end 18 of the enclosure 16 . the n poles n 1 and n 3 of the first and third magnets m 1 and m 3 and the s pole s 2 of the second magnet m 2 are oriented away from the magnetic wheel 12 . it is also important that the center to center distances between the three magnets m 1 - m 3 be predetermined as follows in relation to the pitch p of the teeth 14 of the magnetic wheel 12 . our invention requires that the first and third magnets m 1 and m 3 be each opposed to a different tooth 14 of the magnetic wheel 12 when the second magnet m 2 is opposed to the spacing between any two adjoining teeth of the wheel , and that the first and third magnets be each opposed to the spacing between two adjoining teeth when the second magnet is opposed to one tooth . toward this end the center to center distance d between the first and second magnets m 1 and m 2 is made equal to that between the second and third magnets m 2 and m 3 . further , in this particular embodiment , each distance d is equal to one half of the pitch p of the teeth 14 , so that the center to center distance d &# 39 ; between the first and third magnets m 1 and m 3 is equal to the pitch p . therefore , in this embodiment , speaking more broadly , however , the distance d &# 39 ; can be an integral multiple of the pitch p for the fulfillment of the above requirement of our invention . thus , according to the broader aspect of our invention , where n is a positive integer or zero , and m is a positive integer . also mounted within the enclosure 16 and disposed behind the permanent magnet assembly 22 is a member 24 of magnetic material for carrying variable magnetic flux that is produced as the teeth 14 of the magnetic wheel 12 successively travel past the permanent magnet assembly . we have shown the magnetic member 24 as a core around which a coil of wire 26 is wound via a bobbin 28 . substantially cylindrical in shape , the magnetic core 24 has a midportion 30 having the coil 26 wound thereon , a front end portion 32 of greater diameter held against the permanent magnet assembly 22 , and a rear end portion 34 of still greater diameter . the front end portion 32 of the magnetic core 24 is in contact with the n poles n 1 and n 3 of the first and third magnets m 1 and m 3 and with the s pole s 2 of the second magnet m 2 . the permanent magnet assembly 22 , the magnetic core 24 and the coil 26 are retained in positions within the enclosure 16 by a filler 36 of nonmagnetic , electrically insulating material , preferably a plastic . the filler 36 serves also to make the revolution sensor 10 impervious to fluids . the revolution sensor 10 of the foregoing construction senses the rotation of the toothed magnetic wheel 12 from variations in magnetic flux in the magnetic core 10 of the coil 26 . we have illustrated in fig3 and 4 how the magnetic flux varies in the magnetic core 10 with the rotation of the toothed wheel 12 . fig3 shows the magnetic wheel 12 in such an angular position that its two neighboring teeth 14 are opposed to the first and third magnets m 1 and m 3 , and the spacing between these teeth is opposed to the second magnet m 2 . it will be seen that the magnetic wheel 12 , the permanent magnet assembly 22 and the magnetic core 24 constitute in combination a magnetic circuit of variable reluctance . opposed to the first and third magnets m 1 and m 3 as in fig3 the two teeth 14 of the magnetic wheel 12 shown here serve to provide paths 38 and 40 of less reluctance for the magnetic fluxes due to the first and third magnets than the path for the magnetic flux due to the second magnet m 2 , the latter being now opposed to the spacing between the two illustrated teeth 14 . let φ 1 , φ 2 and φ 3 be the individual magnetic fluxes produced by the three magnets m 1 , m 2 and m 3 at the midportion 30 of the magnetic core 24 . then the total magnetic flux φ at the midportion 30 of the magnetic core 24 can be expressed as assume that the three magnets m 1 , m 2 and m 3 are of the same strength . since the path for the flux φ 2 due to the second magnet m 2 is of relatively high reluctance when the magnetic wheel 12 is in the angular position of fig3 φ 2 is less than each of φ 1 and φ 3 . consequently , the total magnetic flux φ at the midportion 30 of the magnetic core 24 is greater than if all the magnets m 1 , m 2 and m 3 were opposed to the different teeth , or the same tooth , of the magnetic wheel 12 . in fig4 is shown the magnetic wheel 12 in such an angular position that one of its teeth 14 is opposed to the second magnet m 2 , and the spaces on its opposite sides are opposed to the first and third magnets m 1 and m 3 . in this case , as will be understood from the foregoing explanation of fig3 the fluxes φ 1 and φ 3 due to the first and third magnets m 1 and m 3 are each less than the flux φ 2 due to the second magnet m 2 . the total magnetic flux φ at the midportion 30 of the core 24 is therefore much less than that when the magnetic wheel 12 is in the angular position of fig3 . the rotation of the magnetic wheel 12 with respect to the revolution sensor 10 is , essentially , the repetition of its two angular positions depicted in fig3 and 4 . accordingly , the above discussed difference in φ when the magnetic wheel 12 is in these angular positions is tantamount to the difference between the maximum and minimum values of the voltage cyclically induced in the coil 26 during the rotation of the magnetic wheel 12 . we have previously defined this difference between the maximum and minimum values of the sensor output voltage as the sensitivity of the variable reluctance sensor . it will therefore be understood that the increased difference in φ when the magnetic wheel 12 is in the positions of fig3 and 4 leads to the higher sensitivity of the revolution sensor 10 according to our invention . thus the revolution sensor 10 makes it possible to more accurately detect the rate of revolution of the magnetic wheel 12 , or to use less expensive equipment for shaping , amplifying , and otherwise processing the sensor output signal , than heretofore . the alternate revolution sensor 10a represents the simplest embodiment of our invention in having but two permanent magnets m 1 and m 2 as a permanent magnet assembly 22a . the first magnet m 1 is shown to have its s pole s 1 disposed opposite the magnetic wheel 12 and its n pole n 1 against the core 24 . the second magnet m 2 is shown to have its n pole n 2 disposed opposite the magnetic wheel 12 and its s pole s 2 against the core 24 . the center to center distance between the two magnets m 1 and m 2 should preferably be equal to half the pitch of the teeth 14 on the magnetic wheel 12 . the other details of construction of this revolution sensor 10a can be identical with those of the fig1 and 2 sensor 10 . in operation , the first magnet m 1 is opposed to one of the teeth 14 of the magnetic wheel 12 when the second magnet m 2 is opteeth 14 of the magnetic wheel 12 when the second magnet m 2 is opposed to one of the spaces on both sides of that tooth , and vice versa . it will therefore be understood from the foregoing description of fig3 and 4 that this revolution sensor 10a can also detect the rate of rotation of the magnetic wheel 12 much more sensitively than if only one magnet were provided in place of the two magnets m 1 and m 2 or if the two magnets were disposed in the same polarity . fig6 shows another modified permanent magnet assembly 22b which takes the form of a single block of magnetizable material permanently magnetized to possess three pairs m 1 , m 2 and m 3 of poles of opposite nature in a row . the second pole pair m 2 is in opposite polarity to the first and third pole pairs m 1 and m 3 . used in place of the permanent magnet assembly 22 of the fig1 and 2 sensor 10 , the modified magnet assembly 22b will make it easier of assemblage . still another modified revolution sensor 10b shown schematically in fig7 represents a different arrangement of the core 24 with respect to the magnetic wheel 12 . it will be seen that the core 24 , with the coil 26 wound thereon , generally extends parallel to the axis of rotation of the magnetic wheel 12 . the permanent magnet assembly 22 ( or 22a or 22b ) is attached to one side of the front end portion 32 of the core 24 and so is disposed opposite the toothed periphery of the magnetic wheel 12 . we recommend this arrangement in applications where a sufficient space for the installation of the complete revolution sensor is not available in the radial direction of the magnetic wheel 12 . the variable reluctance sensor of our invention is applicable for the detection of not only the rate of revolution , as in the foregoing embodiments , but also the rate of linear movement . thus , in fig8 we have shown a linearly movable member 12a of magnetic material having a series of teeth 14a formed thereon with a predetermined pitch . although the rate of movement of the linearly movable member 12a may be sensed by any of the sensors 10 , 10a and 10b disclosed in the foregoing , we have shown in fig8 another modified sensor 10c together with the linearly movable member 12a . the modified sensor 10c features a permanent magnet assembly 22c having five permanent magnets ( or five magnetized pole pairs of a single piece of magnetizable material ) m 1 - m 5 . the five illustrated magnets m 1 - m 5 are all attached to the magnetic core 24 and arranged in a row in the direction of movement of the linearly movable member 12a . the first , third and fifth magnets m 1 , m 3 and m 5 are shown to have their s poles oriented toward the movable member 12a whereas the second and fourth magnets m 2 and m 4 are shown to have their n poles oriented toward the movable member . in short , according to our invention , the permanent magnet assembly can be constituted of any number of magnets , or discretely magnetized pairs of poles of opposite nature , not less than two . despite the foregoing detailed disclosure we do not wish our invention to be limited by the exact details of the illustrated embodiments . the following , then , is a brief list of possible modifications or alterations of the foregoing embodiments which we believe all fall within the scope of our invention : 1 . magnets of different strengths could be employed as the permanent magnet assembly . 2 . the individual magnets of the permanent magnet assembly could be magnetically isolated from one another , either with intervening gaps or with intervening members of suitable material , or by the staggered arrangement of the magnets . 3 . the magnets of the permanent magnet assembly could be of other than the rectangular shape . 4 . a member or members of magnetic material could be disposed on the front end of the permanent magnet assembly . 5 . magnetoelectric converter means such as a hall effect element could be employed in place of the coil for providing an electric signal representative of the variations in magnetic flux caused by the movement of the toothed wheel or other movable member . 6 . not all but only the teeth of the movable member could be made of magnetic material . 7 . the permanent magnet assembly could be disposed out of contact with the magnetic core or with an equivalent magnetic member for carrying magnetic flux .