US2290930A - Magnetic material detector - Google Patents

Description

July 28, 1942.

H. E. WURZBACH v MAGNETIC MATERIAL DETECTOR Filed Jan. 13,11941 4 Sheets-Sheet 2 phi.

HUGHE': Tif /Ema;

July 28, 1942. H. E. WURZBACH MAGNETIC MATERIAL DETECTOR Filed Janfl3, 1941 V 4 Sheets-Shed :5

HUGHE IVURzBAcH,

July 28, 1942. H. E. WURZBACH MAGNETIC MATERIAL DETECTOR Filed Jan. 13, 1941 4 Sheets-Sheet 4 Patente d July 28, 1942 UNITED- STATES PATENT OFFICE Hugh E. Wurzbach, Magma, Utah Application January 13. 1941, Serial No. 374,183

9Claims.

This invention relates to devices for detection of ferromagnetic objects, and pertains particularly to a device for detecting tramp iron in a moving body of ore or like material. This invention is directed to improvements in devices of the general character set forth in my issued United States Patents Nos. 2,228,293 and 2,228,-

It is well known that in milling operations in which crushing machinery or the like is used to.

pulverize ores in the process of handling the material for further concentration in the recovery of the mineral values, it is not unusual for mine drills or parts thereof, or'parts of mining machinery, or other ferromagnetic objects, all of which come under the designation of "tramp iron, to be present in the ore. If the tramp iron is of sufllcient size to cause damage when allowed to enter the crushing machinery, it is essential that the presence thereof be noted and that it be removed before entering the crushing machinery. In large operations, where considerable tonnage of ore is involved, it is common to cause this ore to be conveyed from one position to another in the mill through the agency of rather wide, heavy capacity conveyor belts, which owing to their width present a rather wide zone which must be subjected to scrutiny in order to detect and remove the offending tramp ironl It will be appreciated that if a comparatively small piece of tramp iron is present in the ore being handled, such as a tobacco tin ornail or the like, there is no necessity for removing it from the ore stream inasmuch as the possibility oi' such a small object causing any damage is quite remote and elimination of the expense and delay attendant the shut-down for removal of such a non-objectionable object is desirable. It

as it passes through the detection zone on .the'

conveying belt, and which-will at the same time be insensitive to all pieces of tramp iron which may bein the moving body of ore which are smaller than this allowable'size.

It is, therefore, one of the particularobjects' of this invention to provide a device for the detection of tramp iron in a movingbody of ore or like material, which will serve to indicate the presence of any piece of tramp iron which is above the established allowable size, independwithin the ore body carried by the conveyor belt which is under scrutiny.

A further object of the invention is to provide a device which serves to subject a given body of ferromagnetic material such as tramp iron in a moving body of non-magnetic material (diamagnetic or feebly paramagnetic material) such as ore to substantially the same degree of scrutiny independent of the shape of such ferromagnetic body, wherefore a given mass of tramp iron will be subjected to substantially identical scrutiny whether it happens to pass through the detection zone in close proximity to the conveying surface .or whether it is high in the body of are on .the belt, even though it is an irregularly shaped body with its major axis extending horizontally, vertically, or at some other angle. These considerationsapply to all hithertoesuggested forms of detecting means but have not been all'realized in one compact apparatus.

A further object of theirfivention is to provide a ferromagnetic material detector adapted to subject a rather large body of moving material (such as may be carried by a conveyor belt or the like) to uniform scrutiny, and which will be substantially uniformly responsive to ferromagnetic objects located at all positions across the width of the belt whether closely adjacent the belt or upwardly removed from the belt surface, and which is at the same time substantially insensible to external magnetic disturbances and to internal magnetic disturbances resulting from variations in the energizing power supply, and the like.

' The invention comprises, essentially, the provision of means defining two cooperating magnetic fields or groups of lines of flux, one group being so arranged as to extend from below the belt upwardly through the material on such belt 40 in-a generally vertical relation, and the other group being arranged to extend transversely through the material and the space immeciately above the material in a generally horizontal relation, together with a detector coil assembly positioned to have an appreciable voltage induced therein in response to the disturbance produced in either of the groups of lines of force by the presence of a ferromagnetic object above a given minimum size. V

As a preferred broad example of my invention as above defined andas applied to the detection of the presence of tramp iron in the non-magnetic material carried in a longitudinal path on a moving conveyor belt, my apparatus may comeat of the position of that piece of tramp iron prise frame means defining a pair. of longitudinally spaced low-reluctance magnetic circuits extending around the belt. Carried on the frame means below the belt is magnet means defining a plurality of respectively vertically and respectively horizontally extending magnetic fields psitioned to extend through the material on the belt, whereby substantially all of the material is caused to pass through at least one horizontally extending and one vertically extending field in its movement past the frame means along the path of the belt. Also carried on the frame means is detector coil means located above the belt'in spaced relation to the material carried thereon and having a magnetic core portion included in both the low reluctance magnetic circuits.

The movement of a ferromagnetic body above a given size through one or both of the abovementioned vertically or horizontally extending tage and at the same time minimize the effects of stray magnetic fields produced by electrical equipment operated inthe vicinity of the apparatus. In this last connection, it is preferable that the detector coil means include a plurality of spaced detector coils connected to have opposed voltages induced therein by the action of foreign stray fields or by a uniform change in the fields strength so that substantially no current will flow in the detector circuit from these infiuences.

Other objects and features of the invention will be'brought out in the ensuing description of certain preferred embodiments thereof, or will be apparent from such description. The accompanying drawings illustrate such embodiments, and referring thereto:

Fig. 1 is an end elevation of one form of the device, the view being taken transversely through the conveyor belt and illustrating an advantageous relative disposition of the flux generating means and the detector coil means with relation to the conveying belt and the material carried thereby;

Fig. 2 is a plan view of the form of construction shown in Fig. 1, with the conveyor belt and its carried material being omitted;

Fig. 3 is a transverse section through a portion of the device, as taken alongline 3-3 in Fig. 2;

Fig. 4 is a view corresponding to Fig. 3, taken on line 4-4 in Fig. 2;

Fig. 5 is a schematic representation of a signal circuit which may be associated with the detector coil means in the form of construction shown herein;

Figs. 6 and 7 illustrate an alternative layout of electromagnets to establish a desired arrangement of flux, taken in plan and side. elevation, respectively;

Fig. 8 is a wiring diagram illustrating one way of connecting the detector coils illustrated herestruction shown in Fig. 9, the view being taken transversely through the conveyor belt.

Referring to .Figs. 1-4 of the drawings and more particularly to Figs. 1 and 2, the apparatus is shown as comprising a plurality of ferromagnetic pole members I, 2, 3, 4, and 5 disposed beneath a conveyor belt 6. The outer pole members I and 5 are preferably located with their outer edges substantially below the lateral edges of the belt 6 and, as may be seen by inspection of Fig; 2, each pole member is spaced from its adjacent pole member by substantially the same distance. I

The respective pole members are provided with coils la, 2a, 3a, 4a, and 5a, which are adapted upon connection to a suitable source of direct current (not shown) to polarize the upper ends of the poles I, 2, 4, and 5 similarly, for example to provide north poles, and to polarize the upper end of the pole 3 oppositely, for example to provide a south pole. The pole members and their energizing coils provide means defining two cooperating magnetic fields or groups of lines of flux. The adjacent like poles I and 2, [and 4, and 4 and i provide groups of magnetic fields or lines of flux which extend generallyvertically upward through the belt 4 and the material 1 carried thereon. The adjacent unlike poles I and 3, 2 andl, I and 4, and I and I provide magnetic fields or groups of lines of flux which extend between the poles of these respective pairs of poles and pass through the belt and the materlal carried thereon in a generally horizontal direction. Obviously, for these last-mentioned lines of fiux to pass through the material on the belt above the poles requires that they also extend in a vertical direction. However, at positions in the space between these unlike pole pairs the direction of the lines of flux is generally horizontal.

Stillreferring to Figs. 1-4, but with particular reference to Figs. 3 and 4, the groups of lines of flux as produced by pairs of unlike and like poles are diagrammatically indicated at l and 8 in Figs. 3 and 4 respectively. It will be understood, of course, that the-illustration of the fiux lines is not theoretically perfect, no attempt being made to show the actual density of the field or the distortion thereof produced by ferromagnetic core members in the field as by the subsequently described ferromagnetic detector core I members. The main purpose of the figures is to show the general direction of the lines. The position of the conveyor belt 0 and the outline of the material' can'ied thereon are indicated in dot-dash lines at I and I in Figs. 3 and 4. It will be noted from Fig. 3 that except atposiitons closely adjacent the faces or-upper ends of the pole members I and l the flux lines 8 for the most part extend in a generally horizontal direction through the material I. From Fig. 4 it may be seen that the flux lines from the poles 2 and 4 for the most part extend in a generally vertical direction through the material I.

The device is further provided with detector coil means comprising a plurality of detector coils ll, I2, l3, and I4, which are located above the belt C adjacent the material I. The coils may be mounted in pairs on ferromagnetic core members with their axes in substantial parallel alinement, with the coils II and I! on a core member II and the coils I I and I4 on a parallel core member II for example; Although the detector coils may be connected numerous ways, as in series or parallel, or

parallel, as shown in Fig. 8. The coils so 0011- nected are shown connected to a pair of terminals H9 and I291. The coils on either one of the cores 55 or it are said to be connected in opposition when a uniform change in flux throughout the length of either one of the cores generates equal and opposite voltages in th coils carried by that core.

The detector coils are each'preferably so psitioned as to be linked by lines of fiux from .I I I I 3 protection over the weak area. Another serious fault of the simple use of the known art is that the shape of the .body passing through the protected zone materially governsthe degree of detection. For example, if an elongated body. whose lineal dimension is several times its diameter, such as an iron rod one inch in diameter by eight inches long, passes through the weak area of a simple magnetic field in which a detector is placed. the amount of current induced in the detector circuit will greatly depend on I whether the rod passes through the magnetic the above-mentioned dissimilar groups of lines of flux; and each of the coils preferably has its turns so positioned that a distortion or movement of the flux linking the coil (as produced by movement of a magnetic object through one or more of the groups of.lines of flux) will induce I appreciable voltage in the coil.

It is well known that an elongated ferromagnetic object in passing through'a magnetic field will change and/or redistribute the flux lines in the field to a greater or less extent dependent upon the orientation of its longitudinal axis with respect to th direction of the flux lines. It will i substantially horizontally extending field pro-' duced by cooperation of the unlike pole members l and 3.

If an elongated piece of tramp iron, such as shown at It! in Fig. 1, has its axis so alined as tomaterially change the reluctance of the magnetic circuit and/or redistribute the flux lines in the horizontally extending field it may have little effect on the vertically extending field and yice versa. In other words, with a field arranger ment having all lines of fiux directed in substantially one direction it is possible for a piece of tramp iron above the allowable minimum size to pass through the field without operating the detector, depending upon the orientation of the piece of iron with respect to the field lines. with my arrangement a piece of tramp iron located at any position in the material on the belt is subject to scrutiny by both types of fields, i. e., a vertically directed anda horizontally directed field. Obviously then, irrespective of the orientation of the iron, its presence is indicated by its effect on either one or both of the fields.

It is to be pointed out that in previously prof posed detectors utilizing only the field between oppositely polarized poles, the eifective field was too weak in the central area between the poles and this weakness had to be compensated for byhighly amplifying the pickup or detector 'circuit so as to give it strength enough to operate a commercial relay when the iron passed through the weak area. This makeshift, however, has the disadvantage of givingan extremely high degree of detection over the strong part of the protected area while it affords only normal or minimum field with its axis parallel to the lines of force or at right angles to them, th ratio of detection being approximately in the ratio of six to one. By using the field arrangement disclosed, herein, the degree of detection is sufiiciently uniform and high throughout the area under scrutiny as to make it unnecessary to use amplification.

It should be borne in mind that detection is required for a substantial vertical distance as well as for a substantial horizontal distance. This problem is not the same as obtaining either uniform detection or ultimate detection through a small vertical or horizontal distance.

The pole members I, 3, and 5 are preferably carried on a ferromagnetic core member H which extendstransversely of the conveyor belt, and the pole members 2 and 4 are preferably carried on another ferromagnetic core member extending generally parallel to the first core member. These core members are preferably ied together by a rectangular ferromagnetic core member 2| which is provided at itscorners with uprights 22 of angle iron or the like which extend upwardly alongthe sides of the conveyor belt to a position above the level of the material I. The framework is further provided with an upper rectangular frame 23 of angle iron or the like which joins the side frame members 22 and supports the cores i5 and It. The framework provides a lower reluctance return path for .the flux from the pole members.

As may be seen by inspection. the upper frame 23 and lower frame 2| in combination with the side frame members or posts 22, define a plurality of low reluctance magnetic circuits extending around the belt. For example, laterally extending frame members 23a and 23b of the upper frame 23 constitute parts of two circuits around the belt. These circuits help produce a desirable distribution of the flux lines through the material and help in decreasing the effect of stray magnetic fields. The cores l5 and I6 extend between the laterally extending members 23a and 23b and are also included in both of the above-mentioned low reluctance magnetic circuits. v

Although the detector coils may be positioned in a number .of different ways, I prefer, in order to secure the maximum strength and uniformity of detection, to position them substantially as shown in Figs. 9 and 10 when employing the energizing arrangement illustrated in Figures 1, 2, 9 and 10, although the arrangement shown in Figs. 1 and 2 may be employed with satisfactory results. With such an arrangement as shown in Figs. 1 and 2 the longitudinal spacing of the coils is not extremely critical, but it is preferable to place the coils Ii and I3 directly above the transversely alined poles I, 3, and 5, and the coils l2 and I4 directly above the transversely alined poles 2 and 4'. It is also preferable to locate the detector coils at a heightabove the upper end of the pole 3, which is about equal to the spacing between adjacent poles, substantially as shown. The lateral spacing of the detector coils is somewhat more critical than the lons tudinal spacing thereof, and it is preferable to place them with their axes located substantially in line with the inner edges of the poles .2 and l. as illustrated in Figs. 1 and 2.

Another modification of my invention is illustrated in Figs. 6 and! and is shown as comprising a central core member ll provided with three upstanding spaced pole members 32, 38, and 34 each provided with a suitable energizing coil ll, 36, and 31. The apparatus further comprises a pair of core members a and 30 respectively provided at their ends with upstanding pole members Mi, ll, 42, and If, the last-mentioned pole members being provided with energizing coils ll, 45, 46, and 41, respectively. All of the upstanding pole members are-shown located at equal distances from the pole member 33, and each pole member is spaced the same distance from its adjacent pole member.

All of the energizing coils are adapted for connection to a suitable source of direct current and are preferably connected thereto in such manner as to polarize the central pole 33 in one manner and the remaining poles in an opposite manner. For example, the central pole may be a south pole and all the remaining poles may be north poles. With such an arrangement positioned beneath a conveyor belt (not shown), it is obvious that any'piece of material carried on the belt will be subjected to a field produced by two unlike poles and another field produced by two like poles in the same manner as in the arrangement illustrated in Fig. l.

The detector coils which may be used with the apparatus illustrated in Figs. 6 and 'l are not shown, but they may be located in accordance with the teachings of Figs. 1 and 2, as at the positions designated by the ":r marks along the center lines CC and C'-C'. Obviously the cores ll, 38, and 38 may be interconnected by suitable paramagnetic members to form a rigid framework, and such framework may be continued about the conveyor belt to support the detector coils and provide the low-reluctance magnetic circuits around the belt in the same manner as illustrated in Fig. 1.

A form of work circuit which may be associated with the apparatus above described. as by connection to the terminals Ill and ill of the detector coil assembly of Fig. 8, is illustrated in Fig. 5. Referring to said figure, a motor lli which may be used to operate the conveyor belt I in Fig. 1, is shown connected through a starting switch I12 to a three-phase power supp y. One lead I82 of the power supply is shown connected through actuating coil ll! of a starting switch I12, thence through back contact ll! of a relay 7 Ill and connections lll'and III" to another lead I84 of the power supply. through a normally open push-button ill. Upon pressing push-initton ill the circuit of the coil I'll is cloud, operating the starting switch Ill and maintalning coil ill in energized relation by establishing contact through normally closed push-button Ill, contact ill of the starting switch I12 to line connection Ill, rendering the push-button I'll inoperative.

A normally open relay is shown at I'll with its terminals Ill and ill adapted for connection to a pair of the above-mentioned detector coil terminals. This relay should be of sufllcient sensimum voltage impulse from the associated detector coils in response to the presence of a body of magnetic material of given minimum size. Upon receiving an impulse this relay will momentarily close, energizing the coil. of relay Ill for a sumcient time to establish connection at its front contact I". This relay I'll may be of a type will known to the art which will close its contact in response to a short energizing impulse and will maintain its contact closed for a given time interval after'the cessation of the impulse, returning to its position in contact with its back contact "3 at the end of the time interval. Such a relay may be dashpot or motor operated after a manner well known to the art and need not be described in detail herein. The break of the circult of relay Ill with its back contact I83 will break the circuit to coil Ill, opening switch I12 to stop the motor Ill. Closing of relay I'll with its front contact I86 will energize a warning horn I19 and operate the horn for the above-menby a plurality of pole members comprising a pair of pole members [I and 52 located beneath a conveyor belt I! and held betweena pair of iron angle bars II and I which are suitably secured to iron plate members I! and It. The space between the pole members ii and 52 is preferably bridged by an iron core havingsubstantially the same section as the poles and which extends between the bases thereof and is located between the angles II and i4.

A second set of pole members are indicated at 81, i8 and II and are held between iron angles 6| and ii which are secured at their ends to the plates II and II. The magnetic circuit between these pole members is preferably bridged by core members of substantially the same section as the pole members and which extend between the angles II and ii.

The line joining the center of the poles BI and B2 is preferably parallel to the line Joining the centers of the poles I1, 58, and I9, and these lines preferably extend to right angles to the path of movement of the conveyor belt ll and the material thereon indicated at I. The spacing between the poles II and I! is preferably substantially equal tothe spacing between poles I! and El and between the poles II and 59. Furthermore, it is preferable that the distance between the above-mentioned parallel lines joining the poles of the respective sets is substantially equal to the distance between poles II and 82. It is also preferable to position the magnet pole substantially symmetrically with respect to the line of travel of the belt. Thus the pole I8 is located substantially beneath the center of the belt-and the distance from the center of the belt to the pole II is substantially equal to the distance from the center of the belt to pole II.

Each of the poles is provided with a coil or winding I! which is adapted for connection to a suitable source of direct current; The coils 68 tivity to close its contact upon receiving a miniare so connected as to polarize the upper ends of the poles 5|,52, 51, and-59 similarly, for example to make them north poles, and to polarize the upper end of the pole 58 oppositely, for example to make it a south pole.

The type of field produced by this energizing arrangement is quite comparable to that produced by the arrangement illustrated in Figs. 1 to 4. The adjacent unlike poles produce groups of lines of flux which extend in a generally vertical direction through the material 62 and the adjacent unlike poles produce groups of lines of flux which extend in a generally horizontal direction through the material 62. From an inspection of Figs. 9 and it may be seen that substantially every particle of material carried on the belt is subjected to the action of both of thesegroups of lines of flux in passing through the detector.

With this embodiment of my invention the detector coils indicated at 5%, 65, 66, and 6? are carried at spaced positions on a single ferromagnetic core member 68 which is preferably located directly above the central pole 58 and extends parallel to and in the direction of movement of the conveyor belt 53. The detector coils are obviously located sufliciently above the belt to clear the material 62 and the axis of the coil is preferably located at a distance above the poleSB which is slightly greater than the distance between the poles "5| and 52. .As a preferred limit the lowest portion of the detector coil is located at a distance above the pole 58 which is substantially equal'to the distance between the poles 5! and 52. The coils 64, 65, 66 and. 61 may be spaced substantially equally from one another.

The core 68 is shown supported on a frame comprising cross frame members 69 and I0 which are tied together at their ends by longitudinally extending frame members H and 12. These members are in turn supported by vertically extending posts 13, 14, 15, and 16 which are attached at their lower ends to the plates 55 and 56. This frame is preferably fabricated from ferromagnetic material such as angle iron and forms part of the magnetic circuit between the detector coils and the energizing magnets. The frames and posts cooperate to form a pair of lowreluctance magnetic circuits which extend around the belt. The two circuits are defined by the cross frame . members 69 and 16 of the upper frame, and the ferromagnetic core portion 68 bridges these circuits and is included in both of them.

The faces of the poles are preferably chamfered to conform to the contour of the conveyor belt and each of the faces thereof are preferably spaced the same distance from the bottom of the belt. This practice may be employed with the arrangement illustrated in Fig. 1. In order to prevent the material carried on the belt and other extraneous material from falling into the magnet windings I have provided guard plates TI and 1 of non-magnetic material such as copper or brass which fit over the pole members and are supported above the magnet coils 68 by means of suitable support members such as pins 19 which are attached to the base framework 50.

As brought out above, any magnetic body above a given minimum size and irrespective of its orientation, upon being subjected to lines of flux from both groups, will change the reluctance and/or redistribute the flux lines in such manner as to induce an appreciable voltage in the detector coils. The detector coils may be suitably connected to an electrical circuit such as illuscircuit be substantialy insensitive to a uniform change in magnetic field such as caused by outside influence or by variation in the energizing current supplied to the magnet coils. This may be accomplished by connecting the coils 64 and 65 so that the voltages induced therein by such a uniform change are additive and are opposed to the voltages induced in the coils 66 and 61.

It is to be understood that my invention is subject to considerable modification without departing from the spirit thereof. Hence I do not choose to be limited to the examples described herein, but rather to the scope of the appended claims. 20

I claim: 1. In a device for detecting tramp iron in the material carried generally along a plane by a belt conveyor or the like, magnet means disposed on one side of such plane and defining a zone of magnetic detection extending the full width of such belt, and comprising: a plurality of magnetic pole members of one poiarityarranged to establish a plurality of groups of magnetic lines of force directed through such material in a generally vertical direction and transversely with respect to the direction oftravel of material along such plane; an additional magnetic pole member of opposite polarity magnetically coupled to the respective first-mentioned pole members and cooperating therewith to establish a plurality of additional groups of magnetic lines of force directed generally horizontally through such material and transverse to the direction of travel of such material; and detector coil means located adjacent to such plane and disposed on the other side of such plane in position to be linked by flux from all of said groups of lines of force and responsive to changes in flux linkage therein produced by passage of a magnetic body carried by said belt through one or more of said groups of magnetic lines of force.

. 2. A deviceas set forth in claim-l, and combers, said pole members extending toward said belt from said one of said other portions.

3. In a device for detecting tramp iron in the material carried along a plane by a belt conveyor, means defining a zone of magnetic detection, which comprises: flux producing means including spaced magnetic pole members of one polarity arranged to establish a group of magnetic lines of force directed transversely through such material with respect to the direction of travel thereof along such plane, and an additiona] magnetic pole member of opposite polarity magnetically coupled to said first-mentioned pole said plane and to said flux producing means in position to be linked. by both of said groups of lines of force and responsive to changes in flux linkage therein produced by passage of a magnetic body through said groups of lines of force.

4. In an apparatus for detecting the presence,

of tramp iron in non-magnetic material carried on a conveyor belt in a longitudinal path, the combination which comprises: frame means defining a pair of longitudinally spaced low-reluctance magnetic circuits extending around said belt; magnet means carried on said frame means i below the belt and defining a plurality of respectively horizontally and respectively vertically extending magnetic fields positioned 'to extend through the material on said belt whereby subsuch material and transversely of said first-men- .tioned plurality of groups of lines of force; and

stantially all of such material is caused to pass through at least one horizontally extending and one vertically extending field in its movement past said frame means along said path; and detector 'coil means carried on said frame means above the belt in spaced relation to the material carried thereby and having .a ferromagnetic core portion included in both of said low-reluctance magnetic circuits.

5. In an apparatus for detecting the presence of tramp iron in non-magnetic material carried on a conveyor belt in a substantially horizontal path, the combination which comprises: a pair of frames of ferromagnetic material located respectively above and below the belt and extending substantially parallel to the path of the belt, said frames each comprising members extending longitudinally of the belt and members extending transversely of the belt; a plurality of polar projections mounted on one of said frames with their faces disposed toward the belt; coils carried bysaid projections for polarizing the face of 'at least one of said projections with one magnetic polarity and polarizing the faces of a plurality of the remaining projections with another magnetic polarity, said projections being so polarized and located that the path of material carried on the belt passes through a field produced by adjacent similarly polarized poles as well as through a field produced by adjacent oppositely polarized poles; ferromagnetic-members located on laterally opposite sides of the belt and a ferromagnetic core l connecting the frames; member extending longitudinally of' the belt in spaced relation thereto and supported between laterally extending members on the other one of said frame members; and a plurality of spaced detectorlcoils on said core member..

6. ma device for detecting tramp iron in the material carried generally along a plane by a belt conveyor or the like, magnet means disposed below such plane and defining an upwardlyextending zone of magnetic detection extending the full width of such belt, and comprising: a

plurality of magnetic pole members of one polarity arranged to establish a plurality of groups of magnetic lines of force directed upwardly detector coil means located adjacent such plane but upwardly removed therefrom in position to be linked by all of saidgroups of lines of force and responsive to changes in flux linkage therein produced by passage of a magnetic body along said beltin said plane and passing through one or more of said groups of magnetic lines of force. 7. The improvement set forth in claim 6, and comprising in addition an external ferromagnetic circuit means surrounding the zone of magnetic detection defined by said pole members and said detector coil means, said external ferromagneticcircuit comprising .a ferromagnetic framework extending below said pole members and magnetically connected thereto and having portions extending upwardly on opposite lateral sides of said zone and across the top of said zone to provide a common magnetic return for the magnetic flux of said plurality of groups of lines of magnetic flux.

8. The improvement-set forth in claim 6, and comprising in addition an external ferromagnetic circuit means surrounding the zone of magnetic detection defined by said pole members and said detector coil means, said external ferromagnetic circuit comprising a ferromagnetic framework extending below said pole members and magnetically connected thereto and having-portions extending upwardly and opposite lateral sides of said zone and across the top of said zone to provide a common magnetic return for the magnetic flux of said plurality of groups of lines of magnetic flux, and ferromagnetic core members extending through said detector coil means and magnetically connected to said external ferrobelow such plane and defining an upwardly extending zone of magnetic detection extending the full width of such belt, and comprising: a

plurality of magnetic pole members of one polarity arranged to establish a plurality of groups of magnetic lines of force directed upwardly through such material and transversely with respect to the direction of travel thereof along such plane, and an additional magnetic pole member of opposite polarity magnetically coupled to the respective first-mentioned pole members to establish a plurality of additional groups of magnetic lines of force directed upwardly through such material and transversely of said first-mentioned plurality of groups of lines of force; and detector coil means located adjacent such plane but upwardly removed therefrom in position to be linked by flux from all of said groups of lines of force and responsive to changes in flux linkage-therein produced by passage of a magnetic body along said belt in said plane and passing through one or more of said groups of magnetic lines offorce, said additional magnetic pole member of opposite polarity being sy'mmetrically'disposed with respect to said plurality of pole members of one polarity, and substantially uniformly spaced from each of said pole members of one polarity.

HUGH E. WURZBACH.

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