Humbucking pickup and method of providing permanent magnet extending through opposing coils parallel to string orientation

A humbucking pickup for a musical instrument has a bobbin assembly with first and second bobbins. The musical instrument can be an electric guitar. A first coil is wound around the first bobbin, and a second coil is wound around the second bobbin. A permanent magnet or a plurality of permanent magnets extend through the first coil and second coil of the bobbin assembly. A housing is disposed over the bobbin assembly. Blades are disposed over side surfaces of the housing to redirect the magnetic flux toward the strings. A plurality of strings of the musical instrument is routed over a top surface of the housing parallel to opposing poles of the permanent magnet. Movement of the strings of the musical instrument disturbs a magnetic flux of the permanent magnet proximate to a first pole of the permanent magnet and a second pole of the permanent magnet.

FIELD OF THE INVENTION

The present invention relates in general to musical instruments and, more particularly, to a humbucking pickup with a permanent magnet extending through opposing coils in parallel to string orientation to convert movement of the strings into corresponding electrical signals representative of the sounds depicted by the strings, while providing cancellation of external interference.

BACKGROUND OF THE INVENTION

Many musical instruments include strings which the player imparts movement to generate sound. In the case of an electric guitar, the audio sound is produced indirectly from the motion of the string, typically steel strings, over a magnetic pickup. The magnetic pickup includes a bobbin wrapped with a coil of fine enameled copper wire and a permanent magnet with a core material, such as alnico or ferrite. The pickup is most often mounted on the body of the guitar. The permanent magnet creates a magnetic field, and the strings are routed through the magnetic field. When the electric guitar is played, the motion of the vibrating steel strings disturbs the magnetic field and changes the magnetic flux to induce a voltage in the coil. The changes in voltage from the vibrating strings produce a current in the coil representative of the string motion and, correspondingly, the sound intended by the player. Accordingly, the pickup operates as a transducer or variable reluctance sensor that converts mechanical string vibrations to an electrical signal, which is routed to an amplifier and loudspeaker to reproduce an audible sound.

In a traditional single coil magnetic pickup, the turns of wire in proximity to each other have an equivalent self-capacitance that, when added to cable capacitance, resonates with the inductance of the winding. The resonance accentuates certain frequencies, giving the pickup a characteristic tonal quality. More turns of wire in the winding produces a higher output voltage and lower resonance frequency. The inductive source impedance inherent in a single coil pickup makes it less linear than other forms of pickups, such as piezo-electric or optical transducers. The tonal quality produced by such nonlinearity is, however, subject to taste, and some artists consider the sound aesthetically superior to a more linear transducer.

The single coil pickup is susceptibility to external electromagnetic interference (mains hum), e.g., from electrical power cables, power transformers, and fluorescent light ballasts in the area, as well as the magnetic interference with the natural vibrations of the strings. Mains hum typically includes a fundamental frequency at 50-60 Hz and some harmonic content. The changing magnetic flux caused by the mains current links with the windings and induces an undesired voltage variation in the pickup.

FIG. 1shows a typical humbucking pickup10for overcoming the mains hum effect by using two coils12and14, with each coil wound or connected in opposition to the other around bobbin16. A plurality of cylindrical permanent magnets20are disposed through bobbin16in an interior area of coil12in humbucking pickup10, and a plurality of cylindrical permanent magnets22is disposed through bobbin16in an interior area of coil14. Magnets20are aligned with magnets22in the respective windings. Since ambient hum from power-supply transformers, radio frequencies, or electrical devices reaches the coils as common-mode noise, the external interference induces an equal and opposite voltage in each coil, which tends to cancel each other out due to the opposition of coils12and14.

In particular, humbucking pickup10is mounted to electric guitar body30with cylindrical magnets20and22oriented perpendicular to guitar strings32, as shown inFIG. 1. Magnet flux lines emanating from the north pole N of permanent magnet20are perpendicular or normal to the longitudinal axis of strings32. Strings32vibrate through the magnetic field across the north pole N of cylindrical magnets20, which changes the magnetic flux and induces a voltage in coils12and14. The changes in voltage from the vibrating string32produces a current in coils12and14representative of the string motion and, correspondingly, the sound intended by the player. The electrical signal is routed to an amplifier and loudspeaker to reproduce an audible sound.

Coils12and14are wound or connected in opposition so that external interference, e.g., ambient hum from power-supply transformers, radio frequencies, or electrical devices, induces an equal and opposite voltage in each coil which tends to cancel out external interference. Humbucking pickup10with permanent magnets20and22oriented perpendicular to guitar strings32achieves a level interference cancellation, albeit with the associated higher cost and larger form factor.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 2aillustrates a bobbin assembly100with bobbin102and bobbin104of uni-body construction with 6/6 nylon, 30% glass filled nylon, fiberglass, or other electrically insulating material. Bobbin assembly100includes an opening110extending through outer flange112, bobbin102, middle flange114, bobbin104, and outer flange116. Outer flange116includes a printed circuit board or contact pads for making electrical connection to route electrical signals from the pickup to electronic components of the guitar. Opening110is elongated with a rectangular shape or rounded ends and extends completely through bobbin assembly100. In one embodiment, bobbin assembly100has outside dimensions of 6.7 centimeters (cm) across middle flange114and 1.5 cm between outer flange112and outer flange116. Opening110is 4.8 cm by 0.32 cm.

FIG. 2cshows a permanent magnet120as a solid body of core material, such as alnico or ferrite, with a north pole N designated at surface122and south pole S designated at surface124. The form factor of magnet120matches opening110through bobbin assembly100. In one embodiment, magnet120has dimensions of 4.8 cm by 0.32 cm by 1.5 cm. Bobbin102is wound with numerous turns, e.g., thousands of turns, of coated copper wire to form coil126, and bobbin104is likewise wound with numerous turns of copper wire to form coil128. Coil126and coil128are wound or connected in opposition around bobbins102and104, respectively, to cancel external interference.

FIG. 2dshows permanent magnet120press fit or friction fit into opening110of bobbin assembly100with surface122flush or coplanar with outer flange112and surface124flush or coplanar with outer flange116. In another embodiment, surface122extends outward beyond a surface of outer flange112, and surface124extends outward beyond a surface of outer flange116. Bobbin assembly100with coil126and coil128, and permanent magnet120inserted into opening110constitute humbucking pickup130.

InFIG. 2e, housing cover132includes top surface134and mounting flange136with openings138for securing attachment to the guitar body. Opposing sides of housing cover132include openings140and142. Humbucking pickup130is positioned over housing cover132with opening140vertically aligned with outer flange112and surface122of permanent magnet120, and opening142vertically aligned with outer flange116and surface124of the permanent magnet.

FIG. 2fshows humbucking pickup130enclosed within housing cover132. In particular, humbucking pickup130is disposed within housing cover132so that north pole N surface122of permanent magnet120is oriented toward opening140and south pole S surface124is oriented toward opening142of the permanent magnet. Accordingly, the body and opposing poles of permanent magnet120are oriented in parallel to top surface134and ultimately will be oriented in parallel to the guitar strings and guitar body. Blade150is mounted to an exterior surface of housing cover132in a form-fitted slot of opening140over surface122of permanent magnet120with an adhesive, screws, magnetically, or other securing mechanism. Likewise, blade152is mounted to an exterior surface of housing cover132in a form-fitted slot of opening142over surface124of permanent magnet120, as the final centerless centerpoint humbucking pickup assembly154. Blades150and152can be mounted to an interior side surface of housing cover132. Blades150and152are made with steel and redirect the lines of magnetic flux of permanent magnet120toward the guitar strings. Humbucking pickup assembly154exhibits a centerless centerpoint operation by nature of permanent magnet120extending completely through coils126and128in bobbin assembly100in parallel to the guitar strings, and blades150and152picking up guitar string motion at two points, i.e. proximate to the north pole N surface122of permanent magnet120and proximate to the south pole S surface124of the permanent magnet, neither of which exhibits a central pickup point as found in conventional pickups.

FIG. 2gshows humbucking pickup assembly154mounted to guitar body156with screws158fastened through openings138in mounting flange136. Screws158can be used to adjust the height of humbucking pickup assembly154relative to strings160. The body and opposing poles of permanent magnet120are oriented in parallel with a surface of guitar body156. Six guitar strings160are routed across top surface134of housing cover132. Given the orientation of permanent magnet120in housing cover132, the longitudinal axis of strings160run parallel with the north pole N surface122and south pole S surface124of the permanent magnet, as shown in the cutaway view ofFIG. 2h.

FIG. 2ishows strings160in magnetic field162of permanent magnet120. Blades150and152redirect the lines of magnetic flux of permanent magnet120toward guitar strings160. Humbucking pickup assembly154responds to movement of strings160at point166and point168, i.e., proximate to blade150and the north pole N surface122of permanent magnet120, and proximate to blade152and the south pole S surface124.

In another embodiment, blades150and152may extend above top surface134of housing cover132, seeFIG. 2i. A plurality of notches can be cut into blades150and152to set the response of humbucking pickup154to different gauges of guitar strings, similar toFIG. 4f.

FIG. 3illustrates guitar170including body172, neck174, and strings176. Guitar170can be an electric guitar, electric bass guitar, or any other string musical instrument incorporating magnetic pickups. A bridge178is affixed to body172using adhesive, screws, clips, or other suitable attachment mechanism. Bridge178anchors and supports one end of strings176. Neck174of electric guitar170includes headstock180and fretboard182. Machine heads184are attached to headstock180and anchor an opposite end of strings176. The tension of strings176is adjusted and guitar170is tuned by turning machine heads184. A pickguard or scratch plate186is attached to body172.

Humbucking pickup assembly154is mounted to body172using adhesive, screws, clips, or other suitable attachment mechanism. Humbucking pickup assembly154is disposed under strings176to convert string movement to electrical signals representative of the intended sounds from the vibrating strings. An audio output jack188is provided on body172. The electrical signals generated by humbucking pickup assembly154are output from guitar170through audio output jack188. The audio signals and control signals are routed from audio output jack188to external devices, e.g., an amplifier and speaker, for signal conditioning and sound reproduction.

In particular, permanent magnet120of humbucking pickup assembly154is oriented in parallel to guitar strings176. The longitudinal axis of strings176run parallel with an axis between the north pole N surface122and south pole S surface124of permanent magnet120. Permanent magnet120creates a magnetic field and strings176are routed through the magnetic field. When electric guitar170is played, the motion of the vibrating steel strings176disturbs the magnetic field, and the changes the magnetic flux to induce a voltage in coils126and128, as shown inFIG. 2h. The changes in voltage from the vibrating string produces a current in coil representative of the string motion and, correspondingly, the sound intended by the player. The pickup operates as a transducer or variable reluctance sensor that converts mechanical string vibrations to an electrical signal that is routed to an amplifier and loudspeaker to reproduce an audible sound.

Coil126and coil128in humbucking pickup assembly154are wound or connected in opposition. Any external interference, e.g., ambient hum from power-supply transformers, radio frequencies, or electrical devices reaches coils126and128as common-mode noise, induces an equal and opposite voltage in each coil which tends to cancel each other out. Accordingly, the parallel orientation of permanent magnet120in humbucking pickup assembly154with respect to guitar strings176produces a voltage in coils representative of the string motion and, correspondingly, the sound intended by the player. The opposing windings or connection of coil126and coil128induces an equal and opposite voltage in each coil to cancel out external interference. Humbucking pickup assembly154with permanent magnet120oriented in parallel to guitar strings176achieves the desired interference cancellation with a single magnet in a smaller form factor, i.e., similar to the form factor of a single coil pickup.

FIGS. 4a-4hillustrate another embodiment with individual separate permanent magnets extending through the humbucking pickup.FIG. 4ashows bobbin assembly200with bobbin202and bobbin204of uni-body construction with 6/6 nylon, 30% glass filled nylon, fiberglass, or other electrically insulating material. Bobbin assembly200includes a plurality of tubular openings210extending through outer flange212, bobbin202, middle flange214, bobbin204, and outer flange216. Outer flange216includes a printed circuit board or contact pads for making electrical connection to route electrical signals from the pickup to electronic components of the guitar. Tubular openings210extend completely through bobbin assembly200. In one embodiment, bobbin assembly200has outside dimensions of 6.7 cm across middle flange114and 1.5 cm between outer flange212and outer flange216. Tubular openings210are 0.32 cm in diameter.

FIG. 4bshows a plurality of permanent magnets220, each with a solid body of core material, such as alnico or ferrite, with a north pole N designated at surface222and south pole S designated at surface224. In one embodiment, magnets220have dimensions of 0.32 cm in diameter and 1.5 cm in length. Bobbin202is wound with numerous turns, e.g., thousands of turns, of coated copper wire to form coil226, and bobbin204is likewise wound with numerous turns of copper wire to form coil228. Coil226and coil228are wound or connected in opposition around bobbins202and204, respectively, to cancel external interference.

FIG. 4cshows permanent magnets220press fit or friction fit into openings210of bobbin assembly200with surfaces222flush or coplanar with outer flange212and surfaces224flush or coplanar with outer flange216. In another embodiment, surfaces222extend outward beyond a surface of outer flange222, and surfaces124extend outward beyond a surface of outer flange216. Bobbin assembly200with coil226and coil228, and permanent magnets220inserted into openings210constitute humbucking pickup230.

InFIG. 4d, housing cover232includes top surface234and mounting flange236with openings238for securing attachment to the guitar body. Opposing sides of housing cover232include openings240and242. Humbucking pickup230is positioned over housing cover232with opening240vertically aligned with outer flange212and surfaces222of permanent magnets220, and opening242vertically aligned with outer flange216and surfaces224of the permanent magnets. Humbucking pickup230is then enclosed within housing cover232. In particular, humbucking pickup230is disposed within housing cover232so that north pole N surfaces222of permanent magnets220are oriented toward opening240and south pole S surfaces224are oriented toward opening242of the permanent magnet. Accordingly, the body and opposing poles of permanent magnets220are oriented in parallel to top surface234and ultimately will be oriented in parallel to the guitar strings and guitar body. Blade250is positioned over opening240and blade252is positioned over opening242.

FIG. 4eshows blade250mounted to an exterior surface of housing cover232in a form-fitted slot of opening240over surfaces222of permanent magnets220with an adhesive, screws, magnetically, or other securing mechanism. Likewise, blade252is mounted to an exterior surface of housing cover232in a form-fitted slot of opening242over surfaces224of permanent magnets220, as the final centerless centerpoint humbucking pickup assembly254. Blades250and252are made with steel and redirect the lines of magnetic flux of permanent magnets220toward the guitar strings. Humbucking pickup assembly254exhibits a centerless centerpoint operation by nature of permanent magnets220extending completely through coils226and228in bobbin assembly200in parallel to the guitar strings, and blades250and252picking up guitar string motion at two points, i.e. proximate to the north pole N surfaces222of permanent magnets220and proximate to the south pole S surfaces224of the permanent magnets, neither of which exhibits a central pickup point as found in conventional pickups.

In another embodiment, blades250and252can be mounted to an interior side surface of housing cover232. Blades250and252may extend above top surface234of housing cover232, as shown inFIG. 4f. A plurality of notches255can be cut into blades250and252to set the response of humbucking pickup254to different gauges of guitar strings.

FIG. 4gshows humbucking pickup assembly254mounted to guitar body256with screws258fastened through openings238in mounting flange236. Screws258can be used to adjust the height of humbucking pickup assembly254, relative to strings260. The body and opposing poles of permanent magnets220are oriented in parallel with a surface of guitar body256. Six guitar strings260are routed across top surface234of housing cover232. Given the orientation of permanent magnets220in housing cover232, the longitudinal axis of strings260run parallel with the north pole N surfaces222and south pole S surfaces224of the permanent magnet, as shown in the cutaway view ofFIG. 4h. As described forFIG. 2h, humbucking pickup assembly254responds to movement of strings260proximate to blade250and the north pole N surfaces222of permanent magnets220and proximate to blade252and the south pole S surfaces224.

FIGS. 5a-5cillustrate humbucking pickup280, similar to humbucking pickup130inFIGS. 2a-2c, with permanent magnet282as a solid body of core material, such as alnico or ferrite, with a north pole N designated at surface284and south pole S designated at surface286.FIG. 5ashows the form factor of magnet282matching the dimensions of opening288through the bobbin assembly from outer flange290through middle flange291to outer flange292. Coil294and coil296are wound or connected in opposition around the bobbin assembly to cancel external interference. Permanent magnet282is press fit or friction fit into opening288with surface284flush or coplanar with outer flange290and surface286flush or coplanar with outer flange292. Alternatively, surface284extends outward beyond a surface of outer flange290, and surface286extends outward beyond a surface of outer flange292. The bobbin assembly with coil294and coil296, and permanent magnet282inserted into opening288constitute humbucking pickup280.

In another embodiment, humbucking pickup280may use a plurality of permanent magnets, similar to magnets220in humbucking pickup230, disposed through coils294and296in the bobbin assembly.

Pole shoe300and pole shoe302are made of steel and magnetically coupled to the north pole N surface284of permanent magnet282and south pole S surface286, respectively. Pole screws304are inserted into threaded openings306of pole shoe300. Likewise, pole screws308are inserted into threaded openings310of pole shoe302. Pole shoe300and pole shoe302operate as respective extensions of the north pole N surface284and the south pole S surface286, vertically adjustable at six locations on each pole surface of permanent magnet282by pole screws304and pole screws308. That is, pole screws304and pole screws308can be individually threaded and unthreaded in openings306and openings310to make small adjustments of the relative locations of pole shoe300and pole shoe302with respect to the north pole N surface284and the south pole S surface286. The small adjustments of the relative locations of pole shoe300and pole shoe302by individual threading and unthreading pole screws304and pole screws308have the effect of altering the magnetic field relative to the guitar strings. Accordingly, the individual adjustments to pole screws304and pole screws308tailor the response of humbucking pickup280to different gauges of guitar strings.

In another embodiment, there are separate pole shoes, each with a threaded opening corresponding to each pole screw. Again, the pole screws can be individually threaded and unthreaded to make small adjustments of the relative locations of the individual pole shoes with respect to the north pole N surface and the south pole S surface. The separate pole shoes are particularly applicable to the embodiment of humbucking pickup230.

Housing cover320includes top surface322and mounting flange326with openings328for securing attachment to the guitar body. Openings334are provided in top surface322to accommodate pole screws304and pole screws308. Housing cover320is positioned over humbucking pickup280with side surface330vertically aligned with outer flange290and surface284of permanent magnet282, and side surface332vertically aligned with outer flange292and surface286of the permanent magnet.

InFIG. 5b, humbucking pickup280is enclosed within housing cover320, as the final centerless centerpoint humbucking pickup assembly344. In particular, humbucking pickup280is disposed within housing cover320so that north pole N surface286of permanent magnet282and pole shoe300are oriented toward side surface330and the south pole S surface286and pole shoe302are oriented toward side surface332. Pole screws304and pole screws308are accessible through openings334on top surface322.

Humbucking pickup assembly344is mounted to guitar body346with screws348fastened through openings328in mounting flange326. The body and opposing poles of permanent magnet282are oriented in parallel with a surface of guitar body346. Six guitar strings350are routed across top surface322of housing cover320. Given the orientation of permanent magnet282in housing cover320, the longitudinal axis of strings350run parallel with the north pole N surface284and south pole S surface286of the permanent magnet. Humbucking pickup assembly344responds to movement of strings350proximate to pole screws304and pole screws308. Pole screws304and pole screws308can be individually threaded and unthreaded in openings306and openings310to make small adjustments of the relative locations of pole shoe300and pole shoe302with respect to the north pole N surface284and the south pole S surface286. The small adjustments of the relative locations of pole shoe300and pole shoe302by individually threading and unthreading pole screws304and pole screws308have the effect of altering the magnetic field relative to the guitar strings to tailor the response of humbucking pickup280to different gauges of guitar strings.

FIG. 5cshows a cross-sectional view of humbucking pickup assembly344with pole shoe300disposed between sidewall330and the north pole N surface284, and pole shoe302disposed between sidewall332and the south pole S surface286. Pole screw304and pole screw308redirect the lines of magnetic flux352of permanent magnet282toward guitar strings350. Humbucking pickup assembly344exhibits a centerless centerpoint operation by nature of permanent magnet282extending completely through coils294and296in the bobbin assembly in parallel to guitar strings350, and pole screw304and pole screw308picking up guitar string motion at point354and point365proximate to pole shoe300and pole shoe302.

FIG. 6aillustrates another embodiment of a single coil pickup360with bobbin362of uni-body construction with 6/6 nylon, 30% glass filled nylon, fiberglass, or other electrically insulating material. An opening extends through outer flange364, bobbin362, and outer flange366. Permanent magnet370has a solid body of core material, such as alnico or ferrite, with a north pole N surface and south pole S surface. The form factor of magnet370matches the opening through bobbin362. Permanent magnet370is press fit or friction fit into the opening of bobbin362with the north pole N surface flush or coplanar with outer flange364and the south pole S surface flush or coplanar with outer flange366. In another embodiment, the north pole N surface extends outward beyond a surface of outer flange364, and the south pole S surface extends outward beyond a surface of outer flange366. Bobbin362is wound with numerous turns, e.g., thousands of turns, of coated copper wire to form coil376.

InFIG. 6b, housing cover380includes top surface384and mounting flange386with openings388for securing attachment to the guitar body. Opposing sides of housing cover380include openings390and392. Single coil pickup360is positioned over housing cover380with opening390vertically aligned with outer flange364and the north pole N surface of permanent magnet370, and opening392vertically aligned with outer flange366and the south pole S surface of the permanent magnet.

Single coil pickup360is disposed within housing cover380so that north pole N surface of permanent magnet370is oriented toward opening390and south pole S surface is oriented toward opening392of the permanent magnet. Accordingly, the body and opposing poles of permanent magnet370are oriented in parallel to top surface384and ultimately will be oriented in parallel to the guitar strings and guitar body. Blade400is mounted to an exterior surface of housing cover380in a form-fitted slot of opening390over the north pole N surface of permanent magnet370with an adhesive, screws, magnetically, or other securing mechanism. Likewise, blade402is mounted to an exterior surface of housing cover360in a form-fitted slot of opening392over the south pole S surface of permanent magnet370, as the final centerless centerpoint single coil pickup assembly404. Blades400and402can be mounted to an interior side surface of housing cover360. Blades400and402are made with steel and redirect the lines of magnetic flux of permanent magnet370toward the guitar strings. Single coil pickup assembly404exhibits a centerless centerpoint operation by nature of permanent magnet370extending completely through coils376in bobbin362in parallel to the guitar strings, and blades400and402picking up guitar string motion at two points, i.e. proximate to the north pole N surface of permanent magnet370and proximate to the south pole S surface of the permanent magnet, neither of which exhibits a central pickup point as found in conventional pickups.

FIG. 6cshows single coil pickup assembly404mounted to guitar body406with screws408fastened through openings388in mounting flange386. Screws408can be used to adjust the height of single coil pickup assembly404relative to strings410. The body and opposing poles of permanent magnet370are oriented in parallel with a surface of guitar body406. Six guitar strings410are routed across top surface384of housing cover380. Given the orientation of permanent magnet370in housing cover380, the longitudinal axis of strings410run parallel with the north pole N surface and south pole S surface of the permanent magnet.

Strings410vibrate in the magnetic field of permanent magnet370, similar toFIG. 2i. Blades400and402redirect the lines of magnetic flux of permanent magnet370toward guitar strings410. Single coil pickup assembly404responds to movement of strings410at points proximate to blade400and the north pole N surface of permanent magnet370and proximate to blade402and the south pole S surface of the permanent magnet.

In another embodiment, blades400and402may extend above top surface384of housing cover380. A plurality of notches can be cut into blades400and402to set the response of single coil pickup assembly404to different gauges of guitar strings, similar toFIG. 4f.

FIG. 7illustrate humbucking pickup420, having features of humbucking pickup assembly154fromFIGS. 2a-2iand features of humbucking pickup280fromFIGS. 5a-5c. Blade422is mounted to an exterior surface in a form-fitted slot of opening426in housing cover428over the north pole N surface of the permanent magnet, similar toFIG. 2f. Pole screws430are disposed through openings434in top surface436of housing cover428, and further through a pole shoe magnetically coupled to the south pole S surface of the permanent magnet, similar toFIGS. 5a-5c. Accordingly, humbucking pickup420has blade422on one side of top surface436of housing cover428and pole screws430on the other side of the housing cover.

Humbucking pickup assembly420is mounted to guitar body440with screws442fastened through openings in mounting flange444. Six guitar strings450are routed across top surface436of housing cover428. Given the orientation of the permanent magnet in housing cover428, the longitudinal axis of strings450run parallel with the north pole N surface and south pole S surface of the permanent magnet. Humbucking pickup assembly420responds to movement of strings450over blade422proximate to the north pole N surface of the permanent magnet, and further to the movement of strings450over pole screws430proximate to the south pole S surface of the permanent magnet.