US 3142035 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
`luly 21, 1964 w. T. HARRIS 3,142,035
RING-SHAPED TRANSDUCER Filed Feb. 4, 1960 4 She'ets-Sheet 1 imi July 21, 1964 w. T. HARRIS 3,142,035
RING-SHAPED TRANSDUCER Filed Feb. 4, 1960 4 sheets-sheet 2 INVENTOR July 21, 1964 w. 1'. HARRIS RING-SHAPED TRANSDUCER 4 Sheets-Sheet 3 Filed Feb. 4, 1960 FIG. 5
INVENTOR. Maig/,e z' wwf/S )www July 21, 1964 w. T. HARRIS 3,142,035
RING-SHAPED TRANSDUCER Filed Feb. 4, 1960 4 Sheets-Sheet 4 pas 3,142,035' RING-SHAPED TRANSBUCER Wilbur T. Harris, Woodbury, Conn., assigner to The Harris Transducer Corporation, Woodbury, Conn., a corporation of Connecticut Fiied Feb. 2-, 1969, Ser. No. 6,722 le Claims. (Cl. Seil-l) This invention relates to the construction of a transducer particularly adapted for the reception and transmission of vibrations in the sonic and ultrasonic ranges.
Transducers capable of operating in these frequency ranges, and capable of handling vibrations at appreciable power levels, are often cylindrical in configuration, and formed of a plurality of transducing elements, usually in the form of elongated strips, which are assembled t0- gether to produce a substantially ring-shaped structure. These devices have suffered from a number of substantial drawbacks which have been made for difiiculties and expense in assembly and operation.
One of these drawbacks resides in the fact that the resonant frequency of such transducers is ordinarily controlled almost exclusively by their size. For a resonant frequency of 5 kc., a typical such transducer is one foot in diameter, while for a resonant frequency of 500 cycles the diameter would have to be increased to something on the order of l0 feet. The construction of the instant invention permits attainment, with a given size transducer, of lower resonant frequencies than have heretofore been possible.
A second drawback of the prior art structures revolves about the difliculty of assembling the various strips which make up the complete transducing unit and reliably securing them in assembled condition. Even with special assembling fixtures, it has proved to be practically impossible on a production basis to form the ring-like structure by securing strips to one another in proper positional relationship. In the course of assembly the various individual strips must be tightly pressed against one another with their films of bonding agent therebetween, the bonding agent being subsequently hardened. The very act of pressing the strips against one another has with prior art constructions tended to cause one or more of the strips to shift from its proper position. A ring-like structure of uncertain or variable periphery and with discontinuities in circumferential connection tends to result. The structure of the present invention permits the ready assembly of the strips into a ring-like structure to accurately predetermined configuration, the various strips being so shaped as to cooperate with one another and hold one another in place when they are pressed together.
In prior art constructions, the strips of transducing material are of appreciable peripheral length. This poses an electrical and electro-mechanical problem. The strips must change dimensions in a circumferential direction. lf they are provided with electrodes on their radial inner and outer surfaces, thereby presenting a comparatively low electrical impedance in the circuit in which they are connected, a low electro-mechanical coupling coefi'icient results. lf they are provided with electrodes at their peripheral ends, so that they are polarized circumferentially, and hence in the same direction as their mechanical expansion and contraction, higher andrtherefore more desirable electro-mechanical coupling coetiicients are Obtained, but the electrical impedance is correspondingly increased, with a consequent loss in electrical effectiveness. The structure of the present invention permits the attainment of relatively high eiectro-mechanical coupling coefficients, thus making for good transducing efiiciency, and at the saine time, provides for relatively low electrical impedance, thus making for high overall efiiciency.
Structures of the type under discussion, particularly 3,142,035 Patented July 2l., 1964 when designed for underwater operation, are adapted to be covered by a protective sheath of rubber or the like. If there are any gaps, even of the most minute nature, between this sheath and the transducer ring itself, the mechanical transfer of vibratory energy between the transducer ring and the medium in which it is placed will suffer, thus reducing the overall efficiency of the devices. In an attempt to solve this problem, the prior art has often permanently bonded the protective sheath to the ring. This, however, greatly increases the time and expense involved in maintenance and replacement, and is not always effective in eliminating gaps between sheath and ring. In accordance with the present invention, the transducer ring defined by the strips of transducing material plus the other strips in the makeup of that ring form a structure the periphery of which is not truly cylindrical, but which may readily be covered by a thin layer of plastic material which is bonded thereto. The outer surface of that layer may be made truly cylindrical and exceptionally smooth. The protective rubber sheath may easily be slid over this outer surface, thus permitting ready disassembly of the transducer proper from the sheath, and because of the extreme smoothness of the outer surface of the thin plastic layer, the sheath and the ring will be in intimate physical, and therefore acoustical, contact, thus insuring maximum transfer of energy.
In accordance with the above, the transducer ring is formed of a plurality of strips of appropriate transducing material, which may be substantially rectangular in crosssection and hence easily and efciently formed. These transducing strips are provided at their sides facing circumferentially along the ring with appropriate electrodes. interposed between selected pairs of these transducing strips are spacing strips preferably formed of conductive material. The side surfaces of these spacing strips facing circumferentially along the ring are so shaped and oriented as, when engaged by the electrode-covered side surfaces of the adjacent transducing strips, to cause the strips collectively to assume a substantially ring shape, and to cause the adjacent transducing strips to be urged radially in a given direction relative to the ring when the ring is compressed. The spacing strips and the adjacent transducing strips are provided with cooperating parts which, when the ring is compressed, prevent relative movement between those strips in the opposite radial direction. Consequently all of the strips will retain their proper radial and circumferential position.
In the form here specifically disclosed, the spacing strips have a cross-sectional shape comparable to that of a keystone, with ledges formed thereon at the narrow end of the keystone. The inclination of the sides of the keystone prevent the transducing strips from moving radially outward and engagement between the transducing strips and the ledges on the spacing strips prevent the transducing strips from moving radially inwardly. The relative positions of the strips are therefore fixed.
The combined cross-sectional areas of the spacing strips form an appreciable proportion of the total crosssectional area of the ring. Consequently, the physical characteristicsV of the material of which the spacing strips are formed will appreciably contribute to a determination of the resonant frequency of the ring. Hence, that resonant frequency can be varied within limits for a given size ring through judicious selection of the material of which the spacing strips are formed, the use of material of less stiffness giving rise to a lower resonant frequency. The stiffness of spacing strips of a given material may also be lowered, thereby to lower the resonant frequency of the ring, by weakening the spacing strips as by providing passages therethrough.
The spacing strips, when conductive in nature, define electrical `terminals adapted to be electrically connected to an external source of voltage, and an electric circuit is defined from one conductive spacing strip to the next conductive spacing strip in the ring via the transducing strips located therebetween. Where minimization of electrical impedance is a factor, and whether the spacing strips are conductive or not, it is preferred to interpose a plurality of individual transducing strips between each pair of spacing strips, and to connect those transducing strips in parallel with one another. This is readily accomplished by interposing a thin conductive element such as a mesh sheet between each adjacent pair of transducing strips, that element being pressed into intimate physical and electrical connection with the electrodes on the sides of the transducing strips and extending out radially beyond the ring, the radially extending portions of the mesh sheets being electrically connected to electrical terminals of appropriate polarity.
To the accomplishment of the above, and to such other objects as may hereinafter appear, the present invention relates to the structure of an electro-mechanical transducer, as dened in the appended claims and as described in this specification, taken together with the accompanying drawings, in which:
FIGURE 1 is an elevational view of one embodiment of the present invention formed of a plurality of transducer rings disposed in longitudinal coaxial array;
FIGURE 2 is a fragmentary cross-sectional view, on an enlarged scale, taken through one end of the device of FIGURE 1 as indicated by the line 2 2 of FIGURE 3;
FIGURE 3 is a cross-sectional view taken along the line 3 3 of FIGURE 2, with certain parts omitted for purposes of clarity;
FIGURE 4 is a schematic view of a portion of the transducer ring of FIGURE 3 illustrating the electrical connections;
FIGURE 5 is a schematic View similar to FIGURE 3, but showing an alternate embodiment;
FIGURE 6 is a front elevational view of a conductive strip;
FIGURE 7 is an end elevational view thereof;
FIGURE 8 is a cross-sectional View, on an enlarged scale, taken along the line 8 8 of FIG. 3, semischematically indicating the manner in which electrical connection is made to the electrodes of adjacent transducing strips;
FIGURE 9 is a front elevation view of a transducing strip; and
FIGURE 10 is a view as taken along the line 10 10 of FIGURE 8.
The transducer assembly illustrated in FIGURE 1 comprises a plurality of coaxially arranged individual transdusing rings, each generally designated by the reference numeral 2 (see FIG. 2). The assembly may consist of two sections designated 4 and 6, each connected at one end to an intermediate mounting bracket 8 and each having at its other end a unit 10 which seals off the end of the assembly and provides for electrical connection to external circuitry. Each of the sections 4 and 6 may consist of a plurality of individual transducer rings 2.
In the form specifically disclosed in FIGURES 3 and 4, the ring 2 comprises a plurality of elongated transducing strips 12 and a plurality of individual conductive spacing strips 14, all of equal length, there being three times as many transducing strips 12 as there are conductive strips 14. The transducing strips 12 are shown as substantially rectangular in cross-section, and their side surfaces 16 are substantially parallel and are provided with conductive electrodes 18 (see FIG. 8) over substantially their entire area. These transducing strips 12 are formed of any appropriate transducing material, such as barium titanate, and they are electrically polarized between their electrodes 18 as schematically indicated in FIGURES 3 and 4. As the potential applied between these electrodes 18 variesin polarity, the width of the strips 12 (as measured between their electrode-covered side surfaces 16) will vary, or if the width of the strip is physically varied, the potential across the electrodes 18 will vary. Electro-mechanical transducing therefore occurs.
The spacing strips 14 (see FIGS. 6 and 7) are formed of an appropriate conductive material, such as steel or brass. The conductive strips 14 are provided with outer surfaces 20 from which side surfaces 22 extend in converging relation, the inner ends of those side surfaces 22 terminating in outwardly disposed ledges 24 defined by protrusions 26. The inner surface of the conductive strips 14 may be grooved, as at 28, and a terminal lug 30, which may take the form of a brass tube, is secured to the strip 14 within the groove 28, as by being received within aperture 32, the lug 30 being located longitudinally olf-center, adjacent one end of the strip 14, and extending out beyond the groove 28 formed in the inner surface thereof.
The shape of the conductive strips 14, and particularly the side surfaces 22andledges 24 thereon, is such as topermit one or more of the transducing strips 12 to be interposed between a pair of conductive strips 14 in such a manner as to insure that appropriate radial and circumferential relationship between the strips will be maintained when the ring is formed. Thus, with reference specifically to FIGURE 3, the transducing strip 12 immediately adjacent a given side of a conductive strip 14 engages both the side surface 22 and ledge 24 of the latter, and in so doing is oriented so as to conform to the desired ring-shape of the completed assembly of strips. Because of the inclination of the side surface 22 of the conductive strip 14, circumferential compression of the ring will tend to cause the transducing strip 16 to move radially inwardly and will prevent radial outward movement of that strip 12. Radially inward movement of the strip 12 is positively prevented by the ledge 24. Thus, the shape of the conductive strips 14 control, determine and reliably fix the circumferential and radial positions of all of the strips 12 and 14. This is the case even when, as disclosed in FIGURE 5, only a single transducing strip 12 is interposed between each conductive strip 14.
The conductive strips 14 function as electrical terminals with respect to the transducing strips positioned therebetween. To that end, each alternate conductive strip 14 is electrically connected to have a dierent polarity, as indicated in FIGURES 3, 4 and 5. To facilitate the attainment of this polarity, each alternate conductive strip 14 is arranged with its terminal lug 30 differently positioned, the terminal lugs 30 on each set of conductive strips 14 of the same polarity being at approximately the same level along the axis of the ring. Since the terminal lugs 30 are located nearer one end of the strips 14 than the other, this is readily accomplished by assembling the ring with one strip 14 having its terminal lug 30 near the top of the ring, the next strip 14 having its terminal lug 30 near the bottom of the ring, and so on. Hence, one wire 32 may be connected between all of the terminal lugs 30 of one set, and another wire 34 may be connected between all of the terminal lugs 30 of the other set, the wires 32 and 34 being connected to external circuitry so as to have opposite polarity applied thereto at any given instant. (It will be understood, of course, that the signals applied to the wires 32 and 34 will usually be alternating in character so that the polarity of those wires, and of the conductive strips 14 to which they are connected, as indicated in FIGURES 3 and 4, represents an instantaneous condition. The polarities of the transducing strips 12 as indicated in FIGS. 3 and 4 represent the permanent piezoelectric polarization thereof.)
The basic mode of operation of the transducing ring 2 will in the main be apparent from the above description. The individual transducing strips 12 are polarized in the direction of the circumference of the ring 2, and as the polarity of the electrical potential applied to the electrodes 18 is caused to vary, the corresponding dimension of the transducing strips 12 will also vary, thus causing the ring to expand and contract. Since the dimensional change in the transducing strips 12 is in the same direction as their electrical polarization, they operate in the 3-3 mode, and hence relatively high electro-mechanical coupling coef'licients are obtained.
In the embodiment illustrated in FIGURE 5, the circumferential distance between adjacent conductive strips 14 is relatively small, and hence the effective capacitance between those conductive strips 14 is correspondingly large, leading to relatively low impedance. In the embodiment of FIGURES 3 and 4, however, the circumferential spacing between adjacent conductive strips 14 is appreciably greater, and hence if a single transducing strip 12 were interposed therebetween, as in the embodiment of FIGURE 5, an excessively high electrical impedance between the strips 14 might result. To avoid that undesirable eventuality, each pair of conductive strips 14 has a plurality of transducing strips 12 interposed therebetween, those transducing strips 12 being electrically oppositely polarized as indicated. The strips 12 are adapted to be electrically connected in parallel between the adjacent conductive strips 14 in accordance with their respective electrical polarizations. With this mode of connection, each of the transducing strips 12 has a capacitance between its electrodes 18 which is approximately one-third of the value which would obtain if a single transducing strip were employed, and this reduction in capacitance, coupled with the parallel electrical connection, results in an electrical impedance which is only approximately one-ninth of that which would be obtained if a single transducing strip 12 of comparable circumferential length were employed. Hence, optimum electro-mechanical efficiency and optimum electrical characteristics are achieved simultaneously.
To facilitate the parallel electrical connection of the plurality of transducing strips 12 interposed between each pair of conductive strips 14, a conductive sheet 36 formed of copper mesh is interposed between each pair of adjacent transducing strips 12 in intimate physical and electrical connection with the electrodes 18 over substantially the complete area of those electrodes, as may best be seen from FlGURE l0. A portion 3S of this copper mesh sheet 36 extends radially inwardly beyond the inner surfaces of the transducing strips 12. A buss wire 40 is soldered to the edge of the mesh portion 38 along substantially its entire length, and a lead 42 is electrically connected to and extends from the buss wire 46, the leads 42 extending to and being connected to the terminal lugs Sti on the appropriate adjacent conductive strips 14. The function of the buss wire 4u is to ensure that substantially uniform electrical polarity is applied over the entire area of the electrodes 1S of the transducing strips 12.
Copper mesh sheets similar to the sheets 36 except that they do not extend radially beyond the ring 2 may be compressed between the conductive strips 14 and the transducing strips 12 adjacent thereto, in the embodiments both of FIGURES 3 and 5, in order to facilitate electrical connection between adjacent strips.
Assembling the rings 2 (see FIG. 2), the facing surfaces of the various strips 12 and 14 are cleaned and then coated with a suitable adhesive, usually of a heat-setting type. The strips 12 and 14 are laid out in a generally circular pattern corresponding to that of the desired ring, and the mesh strips 36 are properly positioned between the electrodes 13 of adjacent transducing strips 12. Clamps are then piaced around the outside of the ring, preferably at spaced locations axially therealong, and tightened. This will cause all of the strips to be pressed into ciose circumferential engagement with one another. The clamped ring is then subjected to suitable heat treatment until the adhesive has set.
The inwardly projecting mesh portions 38 may be reinforced with plastic material, such as epoxy resin, to rigidify them and prevent breakage.
If desired each set of three transducing strips 12 may be consolidated into subassemblies by bonding the strips to one another before such subassemblies are assembled to the spacing strips 14.
The outer surface of the thus-formed ring is not truly cylindrical, as may be clearly seen from FIGURE 3. The ring 2 is therefore preferably positioned Within a suitable mold having an extremely smooth and accurately cylindrical inner surface, and the space between the ring 2 and the mold is filled with a suitable coating material, such as bubble-free epoxy resin. This resin defines a coating 44 (see FIG. 2) which is bonded to the outside of the ring 2 and which has an outer surface which is substantially truly cylindrical and glassy-smooth.
The assembly of FIGURE l is formed by locating a ring 2 on insulating base plate 46 (see FIGS. 2 and 3) which is in turn mounted on end plate 48, an insulating spacer 50 being positioned on top of the ring 2, the next ring 2 being located on top of the spacer Sti, and so on to desired axial length. Sealing material (not shown) may be located between the ring 2 and the elements 46 and 5t?. A steel cylinder 54 is also mounted on the end plate 4g, a space 56 being defined between it and the ring 2, within which space leads 58 and 60 are received which are connected to external circuitry in any appropriate manner, as by terminals 61, leads 63, and glands 65. These leads 53 and 60 are in turn electrically connected to the wires 32 and 34 for each of the rings 2.
A rubber sheath or boot 62 is received over the periphery of the axially aligned rings 2. This sheath 62 engages the outer surface of the coating 44 on the ring. Since that outer surface is smooth, the sheath 62 may readily be applied or removed, and when applied, it will engage the coating 44 tightly and uniformly and without any cavities, pits or gaps therebetween. Hence, the sheath 62 will be an intimate and efficient acoustical contact with the ring 2, as is necessary if the device is to function in an eflicient manner.
The resonant frequency of each ring 2 will be determined by the physical characteristics of the transducing strips 12 and the spacing strips 14. Since the transducing strips 12 must be made of materials having appropriate electro-mechanical characteristics, and since there are only a limited number of such materials available, there is not much that can be done to control the resonant frequency insofar as the transducing strips 12 are concerned except by making the ring 2 of appropriate diameter and thickness. However, the presence of the spacing strips 14, the total cross sectional area of which constitutes an appreciable proportion (which may range between 5% and 40%, and is preferably between 10% and 25%) of the total cross sectional area of the complete ring 2, permits control of the resonant frequency of the ring 2 without changing its dimensions, but rather by changing the overall stiffness of the spacing strips 14. This variation may be accomplished in several ways. Different materials may be used for the spacing strips 14.v Fewer or more spacing strips 12, or spacing strips of greater or lesser cross section, may be used. The use of brass instead of steel will reduce the resonant frequency. The use of a lead alloy will still further reduce the resonant frequency, because of its low stiffness and high density. The stiffness of the spacing strips 14 can be modified by forming passages therethrough, such as the axially extending apertures 64 shown in FIGURE 5, the presence of the passages 64 causing a further reduction in the resonant frequency of the ring.
The electrical conductivity of the lead alloy is satisfactory, but if desired for any reason, as if strips 14 were not satisfactorily conductive, means other than the strips 14 alone may be used to make electrical connection with the transducing strip electrodes 13 adjacent the strips 14. Hence, although the strips 14 have often here been termed H conductive strips, since they usually have that characteristic, it is not essential that they be conductive. They can be considered more generically as spacing strips interposed between predetermined numbers of transducing strips.
By way of example, a typical one foot diameter ring with a one inch wall thickness employing twelve 3-bar ceramic (barium titanate) strips and 12 steel conductive strips 14, as in FIGURE 3, would have a resonant frequency of about 5 kc. By changing the steel conductive strips 14 to brass strips, the resonant frequency would be reduced to approximately 4.5 kc. By forming the apertures 64 such as shown in FIGURE 5, the resonant frequency would be reduced still further to approximately 4 kc. This reduction in resonant frequency is accomplished Without change in the overall dimensions of the unit and without adversely changing the acoustic behavior of the device.
From the above it will be apparent that the cooperation between the spacing strips 14 and the transducing strips 12 greatly facilitates assembly of the ring 2. Once the strips have been initially assembled in a roughly ring Shape, the engaging side surfaces of the strips act in a cam-like manner to force the several strips into proper position as the loosely assembled ring is compressed into iinal shape. The greater the compressive force exerted on the ring, the greater is the force urging the strips into their proper relative positions. The fact that this will occur even when transducing strips 12 of substantially rectangular cross-section are employed is noteworthy. The spacing strips 14 which thus facilitate the assembly of the ring serve other important functions. They contribute appreciably to a determination of the resonant frequency of the ring 2, thus permitting the attainment, within limits, of different resonant frequencies for rings of the same size. The spacing strips 14 may be made of conductive material, in which case they can serve as terminals by means of which electrical connection is made to the transducing strips 12. By placing a plurality 0f transducing strips 12 between each adjacent pair of spacing strips 14, and by connecting those transducing strips 12 in parallel, the strips 12 have appropriate electrical polarization, optimum electrical and electro-mechanical eiciency is obtained. By covering the outer surface of the ring formed by the strips with a thin layer of plastic material bonded to the strips, the thus-formed ring can be given a cylindrical, glassy-smooth outer surface which facilitates assembly and disassembly with the covering sheath 62 and ensures that the sheath 62 will be in close acoustical engagement therewith.
While but a limited number of embodiments of the instant invention have been here specifically disclosed, it will be apparent that many variations may be made therein, all within the scope of the invention as defined in the following claims.
1. A transducer comprising a plurality of strips of transducing material and a plurality of spacing strips interposed between selected transducing strips, said strips collectively deiining a substantially ring-shaped structure, said spacing strips having non-parallel side surfaces terminating at one end in substantially laterally extending ledges, said transducing strips being in operative engagement with said side surfaces and being retained in position by said ledges.
2. The transducer of claim 1, in which said spacing strips are provided with passages, thereby to modify the resonant frequency of said transducer.
3. The transducer of claim 2, in which the collective cross-sectional area of said spacing strips denes a substantial proportion of the total cross-sectional area of said ring, whereby the physical characteristics of said spacing strips contribute appreciably to the determination of the resonant frequency of said transducer.
4. The transducer of claim l, in which the collective cross-sectional area of said spacing strips defines a substantial proportion of the total cross-sectional area of said ring, whereby the physical characteristics of said spacing strips contribute appreciably to the determination of the resonant frequency of said transducer.
5. The transducer of claim l, in which a plurality of said transducing strips are interposed between each pair of spacing strips and are electrically connected in parallel.
6. The transducer of claim 5, in which a thin conductive element is interposed between each pair of adjacent transducing strips, said element being in electrical connection With said transducing strips and extending radially beyond said transducing strips, said electrical connections to said transducing strips comprising said elements.
7. A transducer comprising a plurality of strips of transducing material and a plurality of spacing strips interposed between selected transducing strips, said strips collectively defining a substantially ring-shaped structure, said spacing strips being essentially of keystone-shape in cross-section and having laterally extending ledges at the narrow end of said keystone shape, said transducing strips being in operative engagement with the side surfaces of said spacing strips and being retained in position by said ledges.
8- The transducer of claim 7, in which spacing strips are provided with passages, thereby to modify the resonant frequency of said transducer.
9. The transducer of claim 8, in which the collective cross-sectional area of said spacing strips defines a substantial proportion of the total cross-sectional area of said ring, whereby the physical characteristics of said spacing strips contribute appreciably to the determination of the resonant frequency of said transducer.
10. The transducer of claim 7, in which the collective cross-sectional area of said spacing strips denes a substantial proportion of the total cross-sectional area of said ring, whereby the physical characteristics of said spacing strips contribute appreciably to the determination of the resonant frequency of said transducer.
ll. The transducer of claim 7, in which a plurality of said transducing strips are interposed between each pair of spacing strips and are electrically connected in parallel.
l2. The transducer of claim 11, in which a thin conductive element is interposed between each pair of adjacent transducing strips, said element being in electrical connection with said transducing strips and extending radially beyond said transducing strips, said electrical connections to said transducing strips comprising said elements.
13. A transducer comprising a plurality of strips of transducing material and a plurality of spacing strips of conductive material, said strips collectively defining a substantially ring-shaped structure, a plurality of said transducing strips being arranged circumferentially between each pair of spacing strips and electrically connected in parallel between said pairs of spacing strips.
14. A transducer comprising a plurality of strips of transducing material having essentially parallel side surfaces, and a plurality of spacing strips interposed between selected transducing strips, said strips collectively defining a substantially ring-shaped structure7 said side surfaces of said selected transducing strips being in operative engagement with the side surfaces of said spacing strips, said side surfaces of said spacing strips being inclined to prevent movement of said transducing strips radially of said ring in one direction, and cooperating parts on said spacing strips and said selected transducing strips for preventing movement of said transducing strips radially of said ring in the other direction.
(Other references on following page) 9 UNITED STATES PATENTS Harrison Feb. 25, Benioi Apr. 22, Lanphier July 11, Massa Sept. 4, Ely July 31, Harris Jan. 21,
OTHER REFERENCES Rand et al.: Proceedings of the National Electronics Conference, 1958, vol. 14, pages 180481.