US2450434A

US2450434A - Piezoelectric crystal apparatus

Info

Publication number: US2450434A
Application number: US659469A
Authority: US
Inventors: Warren P Mason
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1946-04-04
Filing date: 1946-04-04
Publication date: 1948-10-05
Anticipated expiration: 1965-10-05
Also published as: BE471380A; GB632515A; FR940483A

Description

Oct. 5, 1948. w, P, MAsoN 2,450,434

PIEZOELECTRIC CRYSTAL APPARATUS Filed April 4, 1946 E THVLE NE DIAM/NE TAR TRA TE CRYSTALS Ly +x,+y QUAD/MNT 70er TENS/0N) LONG! TUD/NAL MODE L l' W ymvuLl/QJ?y A r TOR/v5 ff ene diamine tartrate (CeHuNzOe).

.vide an eillcient low Patented Oct.A 5, 1948 AUNITED STATE PIEZDELECTRIC CRYSTAL APPARATUS Warren P.

to Bell Telephone L New York, N. Y., a corpo Application April 4, 1946, Serial No. 659,469

This invention relates to crystal apparatus and particularly -to thickness-longitudinal mode type piezoelectric crystal elements comprising ethyl- Such. crystal elements may be used asirequency controlling circuit elements in electric wave illter systems, oscillation generator systems and ampller systems. Also, 4they may be utilized as modulators, or as harmonic producers, or as electromechanical 4transducers in sonic or supersonic projectors, microphones, `pick-up devices and detectors.

One of the objects of this invention is to pro- -loss cut or orientation capable of producing a longitudinal thickness mode' of motlonin crystal elements made from synthetic crystalline ethylene diamine tartrate.

Another object of this invention is to take advantage of the high piezoelectric coupling, the low ratio of capacities, the low cost and other advantages of crystalline ethylene diamine tartrate.

Other objects of this invention are to provide -a crystal element comprising ethylene diamine tartrate that may possess useful characteristics, such Ias effective piezoelectric lconstants, and small coupling of the desired mode of motion to undesired modes of motion therein.

A `particular object of this invention is to provide a synthetic ethylene diamine tartrate crys tal element having maximum piezoelectric coupling for the thickness-longitudinal mode oi motion.

Ethylene diamine tartrate is a salt of tartaric claims. (ci. 11i- 321i acid having a molecule which lacks symmetry elements. In its crystalline form, it lacks a center of symmetry and belongs to a crys-tal class which is piezoelectric and which is the monoclinic sphenoidal crystal class. By virtue of its structure.

ethylene diamine tartrate will form crystals oftering high piezoelectric constants. In addition. the crystalline material affords certain cuts with VlowI dielectric loss and mechanical dissipation.

Also crystalline ethylene diamine tartrate has no water of crystallization and hence will not dehydrate when used in -air or in vacuum.

Crystal elements of suitable orientation cut from crystalline ethylene diamine tartrate may be excited in different modes of mot-ion such as the longitudinal length or the longitudinal width modes of motion, or the thickness shear or the thickness longitudinal modes of motion controlled' mainly by the thickness dimension. 'I'hese var- 'o ious modes of motion are similar in -the general form of their motion 4to t'hose of similar or corresponding names that are already known in connection with other crystalline substances such Ias Mason," West Orange, N. J., assignor aboratories,

Incorporated, ratlonof New York quartz, Rochelle salt and ammonium dihydrogen phosphate crystal elements. It is useful to have a synthetic type of piezoelectric crystal element having a very high piezo; electric or electromechanical couplingl with low loss, for efficiently producing a thickness longitudinal mode of motion therein. In accordance with this invention, such a synthetic type crystal cut may be provided in the form of tartrate crystals and the tartrate crystals may be suitable cuts taken from crystalline'ethylene diamine tartrate adapted to operate in a thickness longitudinal mode of motion.

In the case or ethylene diamine *tartrate (CsHiiNzOs), which has no Water of crystallization. there lare among other useful cuts. thickness longitudinal mode crystal elements which may bey used, for example, in a delay circuit system for setting up longitudinal waves in-a mercury or water column for example, or asa circuit element inV 'an electric wave filter system, or asa circuit element for the Such crystal ele-ments may be used as acceptable substitutes `for quartz and other crystal elements in oscillator, filter and other crystal systems.

In accordance with this invention, the crystal elements cut from crystalline ethylene diamine Itartrate may be rotated Zcut type crystal elements-having the thickness dimension or the normal Z' to their major faces disposed at acute angles with respect to all three of the mutually per pendicular X, Y and Z axes, and operating in the f thickness longitudinal mode of motion .along the shortest or thickness dimension thereof. For maximum piezoelectric coupling, the plate normal or thickness dimension may be inclined at an angle 0 of about 50 degrees with respect tothe -l-Z axis and may lle in the -i-Y quadrant inthe plane of the ZZ' axes, the ZZ plane being inclined in the region of respect to the -i-X axis or about 20 degrees from the +Y axis as measured in the XY plane.

The synthetic tartrate'crystal elements Drovided in accordance with this invention have'high electromechanical coupling of the order of 20 to 25 per cent or more, and a small dielectric loss.

These advantageous properties together with the frequency control of oscillatorey c: about degrees with.

this invention and the additional advantages, features and objects thereof, reference is made to the. following description taken in connection with the accompanying drawings, in which like reference characters represent like or similar parts and in which:

Fig. 1y is a perspective view illustrating the form and growth habit in which a monoclinic crystal of ethylene diamine tartrate may crystallize, and also illustrating the relation of the surfaces of the mother crystal with respect to the mutually perpendicular X, Y and Z axes, and the crystallographic a, b and c axes;

Fig. 2 is an edge view illustrating the rectangular X, Y and Z and the crystallographic a, b and o systems of axes for monoclinic crystals, and alsoillustrating the lplane of the optic axes of ethylene diamine tartrate crystals; and

Fig. 3 is a perspective view illustrating thickness'longitudinal mode type ethylene diamine tartrate crystal elements rotated in eilect to a position corresponding to an angle oi' 0 in the region of about 50 degrees with respect to the -l-Z axis and an angle of llain the region oi' +70 degrees with respect to the |X axis.

This specification follows the conventionalo terminology, as applied to piezoelectric crystalline substances, which employs a` system of three mutually perpendicular Xl Y and Z axes as reference axes for defining the` angular orientation of a crystal element. As used in this specification and as shown in the drawing, the Z axis corresponds to the c axis, the Y axis corresponds to the b axis, and the X axis is inclined at an angie with respect to the a axis which, in the case of ethylene diamine tartrate, is an angle ,of about degrees. The crystallographic a, b and c axes represent conventional terminology as used by crystallographers.

Referring to the drawing, Fig. 1 is a perspective view illustrating the general form and growth for the X, Y and Z dimensions or of any sumcient size to suit the desired size for the piezoelectric circuit elements 2 that are to be cut therefrom. It will be understood that the mother crystal I may be grown to size by any suitable crystallizer apparatus such as, for example, by a. rocking tank type crystallizer or by a reciprocating rotary gyrator type crystallizer.

Crystals I comprising ethylene diamine tartrate have no water of crystallization and hence no vapor pressure, and may be put in an evacuated container without change, and may be held perature of about 130 C.,

habit in which ethylene diamine tartrate may crystallize, the natural faces of the ethylene diamine tartrate mother crystal I being designated in Fig. 1 in terms of conventional terminology as used by crystallographers. For example, the top surface of the crystal body I is designated as a 00 1 plane, and the bottom surface thereof as a 001 plane, and the other surfaces and facets thereof are as shown in Fig. l.

The mother crystal I, as illustratedln Fig. 1, may be grown from any suitablenutrient `solution by any suitable crystallizer apparatus or method, the nutrient solution used for growing the crystal I being prepared from any suitable chemical substances and the crystal I being grown from such nutrient solution in any suitable manner to obtain a mother crystal I of a size and shape that is suitable for cutting therefrom piezoelectric crystal elements in accordance with this invention. The mother crystal I from which the crystal elements 2 are to be cut is relatively easly to grow in shapes and sizes that are suitable for cutting useful crystal plates or elements 2 therefrom. Such mother crystals I may'be conveniently grown to sizes around 2 inches or more in temperatures as high as 100 C. At a temsome surface decomposition may start. A crystal I comprising crystalline ethylene diamine tartrate has only one cleavage plane which lies along the 0,0,1 crystallographic plane. While cleavage planes may make the crystals I somewhat more dimcult to cut and process, nevertheless satisfactory processing may be done by any suitable means such as, for example, by using a sanding belt cooled by oil or by a solution of water and ethylene glycol, for example.

Monoclinic crystals I comprising ethyelne diamine tartrate are characterized by having two crystallographic axes b and c, which are disposed 4a1; right angles with respect to each other, and

a third crystallographic axis a which makes an angle dierent than degrees from the other two crystallographic axes b and c. The c-axis lies along the longest direction of the unit cell of the crystalline material. The b axis is an axis of two-fold or binary symmetry. In dealing with the axes and the properties of such a monoclinic crystal I, it is convenient and simpler to use a right-angled or mutually perpendicular system of X, Y and Z coordinates. Accordingly, as illustrated in Fig. 1, the method chosen for relating the conventional right-angled X, Y and Z system oi axes to the a, b and c system of crystallographic axes of the crystallographer is to make the Z axis coincide with the c v'axis and the Y axis coincide with the b axis, and to have the X axis lie in the plane of the a and c crystallographic axes at an angle with respect to the a axisl the X axis angle being about 15 degrees 30 minutes above the a axis for ethylene diamine tartrate, as shown in Figs. 1 and 2.

The X, Y and Z .axes form a mutually perpendicular system of axes, the Y axis being a polar axis which is positive (4+) by a tension at one of its ends, as shown in Fig. 1. In order to specify which end of the Y axis is the positive end, the plane of the optic axes of the crystal I may be located. A monoclinic crystal I is an optically biaxial crystal and for crystalline ethylene diamine tartrate, the plane that contains these optic axes is found to be parallel to the b or Y crystallographic axis and inclined at an angle of about 24l/2 degrees with respect to the -l-Z axis, as illustrated in Fig. 2.

Fig. 2 is a diagram illustrating the plane of the optic axes for crystals I comprising ethylene diamine tartrate. As shown in Fig. 2l the plane of the optic axes of an ethylene diamine tartratc crystal I is parallel to the Y or b axis, which in Fig. 2 is perpendicular to the surface of the drawing; and is inclined in a clockwise direction at an angle of about 241A) degrees from the -l-Z or -l-c crystallographic axis. Since the -l-X axis lies at a counter-clockwise angle of 90 degrees from the -lc or -l-Z axis, and the +h=+Y axis makes a right angle system of coordinates with the X and Z axes, the system illustrated in Fig.

modes of motion in the crystal element 2.

Fig. 3 is a perspective view illustrating a crystal element 2 comprising ethylene diamine tartrate that has been cut from a suitable mother crystal I as shown in Fig. 1. The crystal element 2 as shown in Fig. 3' may be made into the form of a plate of substantially rectangular parailelepiped shape having a length dimension L, a breadth or width dimension W, and a thickness or thin dimension T, the directions ofthe dimensions L. W and T being mutually perpen- I dicular, and the thin or thickness dimension T being measured between the opposite major or electrode faces of the crystal element 2. The dimension L and the dimension W of the crystal element 2 may be made of values to avoid spurious The thickness or thin dimension T may be made of a value to suit the impedance or frequency of the system in which the crystal element 2 may be utilized as a circuit element. The dimensions L and W may be made of suitable values to avoid nearby vspurious modes of motion which, by proper dimensioning of the larger dimensions L and W relative to the smaller thickness dimension T, may be placed in a location that is relatively remote from the desired thickness longitudinal mode of motion 'along the thickness dimension T.

Suitable conductive electrodes l and 5 maybe provided adjacent the two opposite major or electrode faces of the crystal element 2 in order to apply electric eld excitation thereto. The electrodes 4 and l5 when formed integral with the faces of the crystal element 2 may consist of gold, platinum, silver, aluminum or other suitable conductive material deposited upon surfaces of the crystal element 2 by evaporation in vacuum or by other suitable process. The electrodes I and 5 may be electrodes wholly or partially covering the major faces of the crystal element 2, and may be provided in divided or nondivided form as lalready known in connection with other thickness mode crystals. Accordingly, it will be understood that the crystal elements -2 disclosed in this specification may be provided with conductive electrodes or coatings 4 and l5 on their faces of any suitable composition, shape and arrangement, such as those already known in connection with Rochelle salt or quartz crystals, for example; and that they may be mounted and electrically connected by any suitable means, such as, for example, by pressure type clamping means or by conductive supporting spring wires cemented by conductive cement or glued to the crystal element and to the metallic coatings 4 and 5 ldeposited on the crystal element 2, as already known in connection with quartz, Rochelle salt or other crystals having similar or corresponding thickness longitudinal modes of motion.

As illustrated in Fig. 3, the crystal element 2 has its major faces so disposed that the perpendicular or normal Z thereto and the thickness dimension T thereof is disposed in the -l-X, -l-Y quadrant and is inclined at an angle 0=about 50 degrees with respect to the +Z axis, and' lies in the ZZ plane which is inclined at an angle in theregion of about 20 degrees from the +Y axis and at a l angle of about 'l0 degrees with respect to the +X axis, as measured in the XY plane, theXY plane in the case of ethylene diamine tartrate being spaced about 151/2 degrees from the plane of the a and b axes. At the angles of 0=about 50 degrees with respect to the -l-Z axis and i =about 70 degrees with respect to the -l-X axis as particularly illustrated in Fig. 3. the crystal element 2 has a maximum value of piezoelectric coupling for its longitudinal mode of motion along the thickness dimension T.

As particularly illustrated in Fig. 3, while the dimension L edges of the major faces of the crystal element 2 lie substantially in the plane of the Z and Z axes, they may be disposed or inclined at other positions with the thickness dimension T extending along the Z axis. The electrodes l and 5 disposed adjacent the major faces of the crystal element 2 provide an electric ileld in the direction of the thickness dimension T of the crystal element 2 thereby Iproducing a useful longitudinal mode of motion along the thickness dimension T of the crystal element 2 with very high electromechanical coupling. l

The dimensional ratio of the width dimension W with respect to the length dimension L of the crystal element Z may be made of any suitable value such as in the region of 1.0, for example, and as particularly described herein is made about 1.0 for longitudinal thickness mode crystal elements 2. Other values of dimensional ratios of the width W with respect to the length L, as of the order of 1.0 more or less, may be used to space spurious modes of motion at a frequency which is remote from the fundamental longitudinal mode of motion along the thickness dimension T. Moreover, while rectangular shaped crystal elements 2 have been particularly described, the major faces thereof instead of being square or rectangular may be circular.

When the crystal element 2 is operated in the fundamental longitudinal mode of 'motion along .the thickness dimension T thereof, the crystal element 2 may be mounted and electrically connected by any suitable means such as by wires cemented to the crystal element 2 and the metallic coatings 4 and 5 in the edge regions of the crystal `element 2.

Fig. 3 also may be taken `to illustrate ethylene diamine tartrate crystal elements 2 cut from a mother crystal I such as that illustrated in Fig. 1, and having an orientation similar to that of the 9=50degree and l =70degree crystal element 2 particularly illustrated in Fig. 3, except for the position of the thickness dimension T thereof, which may be inclined at other acute angles with respect to all three of the mutually perpendicular X, Y and Z axes. The electrodes 4 and 5 disposed adjacent the major faces of the crystal element 2 provide an electric field in the general direction of the thickness dimension T of the crystal element 2, thereby producing a useful longitudinal mode of motion along the thickness dimension T of the crystal element 2 ywith a high electromechanical coupling.

The main mode of motion, which is the fundamental longitudinal mode of vibration along the Z' axis thickness dimension T has a frequency constant of about 1,520 kilocycles per second per millimeter of the thickness dimension T. Thus, as an example, a crystal element 2 having a thickness dimension T of one millimeter and a didimensionlyinginazz'planewhichlsdisposed substantiallylodegreeiromthe+xaxis measuredinthexYplaneinthe-f-x-l-Yquadrant. said thickness dimension of platebeingdispcaedatanangleofnxbstantially thickness longitudinal mode of motion is comparatively negligible.`v

The particular angle o! cut for the ethylene diamine tartrate crystal element 2 illustrated in Fig. 3 is suitable for setting up longitudinal waves in a mercury or water column of a delay, circuit for use in a radio system. In the delay Vline for a radio circuit, a considerable loss may be caused by a low coupling in the crystal and the Y high impedance of the mercury in the associated mercury column. A lower loss in such a delay system may be obtained by the use of an ethylene Adiamine tartrate crystal element 2, the constants of which are roughly 39x10* for the thickness longitudinal mode piezoelectric constant 1.5X1011, for the thickness mode stiffness or elastic constant Cn, 6.5 for the dielectric constant K, and 1,633 for the density p.

It will be noted that among the advantageous cuts illustrated in Fig. 3 is an orientation for which the piezo electric coupling coefllcient may be a maximum value. The high electromechanical coupling, the high reactance-resistance ratio Q, the ease of Iprocurement, the low cost of production and the freedom from water of crystallization in such ethylene diamine tartrate crystal elements are advantages of interest for use as circuit elements in electrical systems generally.

Although this invention has been described and illustrated in relation to specific' arrangements, it is to be understood that it is capable of application in other organizations and is, therefore, notl to be limited to the particular embodiments disclosed.

What is claimed is:

1. Crystal apparatus comprising an ethylene diamine tartrate crystal element of high electromechanical coupling having maior faces, the normal to said major faces lying in a plane which is disposed substantially 70 degrees from the -i-X axis and degrees from the -l-Y axis as measurecl in the XY piane, said normal beinginclined at an angle of substantially 50 degrees with respect to the -l-Z axis.

2. A thickness longitudinal mode ethylene diamine tartrate crystal element ofl high electromechanical coupling having substantially rectangular major faces; the normal or thickness dimansion between said major faces-lying a plane which is disposed substantially 20 degrees from the +Y axis and 70 degrees from the -l-X axis as measured in the +X, -l-Y quadrant, said thickness dimension being disposed at an angie of substantially 50 degrees with respect to the -l-Z axis.

3. A thickness longitudinal mode ethylene diamine tartrate crystal element of high electromechanical coupling having substantially square major faces, the normal or thickness dimension thereof lying in a ZZ plane which is disposed substantially 70 degrees from the -i-X axis and 20 degrees from the -i-Y-axis as measured in the XY plane, said thickness dimension being disposed at an angle of substantially 50 degrees with respect to the +Z axis.

thereoflyinginazz'planewhichisdispedin the +X,+Y quadrant substantially +701- greesfrcmthe-l-Xaxlsasmeasuredintben planasaidthicknessdimensionofsaidcrystai elementboingdisposedatanangieoisubstantiallydgreeswithrespecttothe+zaxis,and saidmaioriaccshavinganedgelyinginsaidlz' plane.K A l e i6. Piesoelectric crystal apparatus comprising an ethylene diamine tartrate crystal element having substantiaily'rectangular maior faces, the normaltosaidmaiorfaceslyinginazz'plane which is disposed in the +I. -l-Y quadrant substantially +70 degrees frcmthe +X axis as measured in the XY plane, said normal to said maior faces being disposed at angangie of substantially degrees with respect to the +Z axis, and means comprising electrodes disposed adiacent said major faces for operating said crystal element in a longitudinal mode of motion along said normal or thickns dimension otsaid crystal element.

= '1. Piezoelectric crystal apparatus comprising E an ethylene diamine tartrate crystal element having substantially square major faces, the normal to said maJor faces lying in a ZZ' plane which is disposed substantially degrees from the +X as measured in the XY plane, said normal to said maior faces being disposed at an angle of lsubstantially 50 degrees with respect to the +Z axis, and means comprising electrodes disposed adjacent said maior faces for operating said crystal element in a longitudinal mode of motion ong said normal or thickness dimension of said crystal element.

E 8. Piezoelectric crystal apparatus comprising an ethylene diamine tartrate crystal element having substantially parallel maior faces, the normal t said major faces lying in a ZZ' plane which is disposed substantially-H0 degrees from the -l-X axis as measured in the XY plane, said normal Y to said major faces being disposed'at an anale of substantially 50 degrees with respect to the +Z axis, and means comprising electrodes disposed adjacent said major faces for operating said crystal element in a. longitudinal model lof motion along said normal or thickness dimension of said crystal element. y

9. Piezoelectric crystal. apparatus comprising an ethylene diamine tartrate crystal element of high electromechanical coupling for its thickness longitudinal mode of motion, said crystal element having substantially square maior faces, the normal to said maior faces beingdisposed at acute .angles with respect to all three of the mutually perpendicular X, Y and Z axes, said angles being values corresponding to said high and substantially maximum value of said electromechanical coupling, and one of said angles being an angleA of substantially 50 degrees with respect Vto said Z axis, and means comprising electrodes disposed adjacent said maior faces for operating said crystal element in said longitudinal mode of motion along said normal or thickness dimension of said crystal element.

10. Piezoelectric crystal apparatus comprising an ethylene diamine tartrate crystal element of high electromechanical coupling for its thickness longitudinal mode of motion, said crystal element having substantially rectangular major faces, the normal to said major faces being disposed at acute angles with respect to all three of the mutually .perpendicular X, Y and Z axes, said angles being values corresponding to said high and substantially maximum value of said electromechanical coupling, and one of said angles being an angle of substantially 50 degrees with respect to said Z axis, and means comprising electrodes disposed adjacent said major faces for operating said crystal element in said longitudinal mode of motion along said normal or thickness dimension of said crystal element.

11. Piezoelectric crystal apparatus comprising an ethylene diamine tartrate crystal element of high electromechanical coupling for its thickness longitudinal mode of motion, said crystal element having major faces, the normal to said major faces being disposed at acute angles with respect to all three of the mutually perpendicular X, Y and Z axes, said angles being values corresponding to said high and substantially maximum value of said electromechanical coupling, and one of said angles being an angle of substantially 50 degrees with respect to said Z axis, and means com prising electrodes disposed adjacent said major faces for operating said crystal element in said longitudinal mode of motion along said normal or thickness dimension of said crystal element.

12. Crystal apparatus comprising an ethylene diamine tartrate crystal element, the thickness dimension between the major faces of said crystal element lying in a plane which is disposed substantially degrees from the -l-Y axis and 'l0 degrees from the -l-X axis as measured in the XY plane, said thickness dimension of said major faces being disposed at an angle of substantially 50 degrees with respect to the -l-Z axis, and means comprising electrodes disposed adjacent said major faces for operating said crystal element in a longitudinal mode of motion along said thickness dimension.

13. Piezoelectrlc crystal apparatus comprising an ethylene diamine tartrate crystal element of high electromechanical coupling adapted for 1ongitudlnal motion along the thickness dimension between its substantially square maior faces, said thickness dimension lying in a plane which is disposed substantially 20 degrees from the +Y axis and 'l0 degrees from the +X axis as measured' in the XYplane of the three mutually perpendicular X, Y and Z axes, and said thickness dimension being disposed at an angle of substantially 50 degrees with respect to said +Z axis,

said thickness dimension being a value corresponding to the frequency for said longitudinal mode of motion, said thickness dimension expressed in millimeters substantially from 1,400 to 1,600 divided by the value of said frequency expressed in kilocycles per second, and means comprising electrodes disposed adjacent said major faces for operating said crystal element in said longitudinal mode of motion. 1

14. Piezoelectric crystal apparatus comprising an ethylene diamine tartrate crystal element of high electromechanical coupling adapted for longitudinal motion along the thickness dimension between its substantially rectangular major faces, said thickness dimension lying in a plane substantially degrees from the +X axis as measured in the XY plane of the three mutually perpendicular X, Yand Z axes, and said thickness dimension being disposed at an angle of substantially 50 degrees with respect to said +Z axis, said thickness dimension being a value corresponding to the frequency for said longitudinal mode of motion, said thickness dimension expressed in millimeters being one of the o values substantially from 1,400 to 1,600 divided by the value of said frequency expressed in kilocycles per second, and means comprising electrodes disposed adjacent said major faces for operating said crystal element in said longitudinal mode of motion.

15. Piezoelectric crystal apparatus comprising an ethylene diamine tartrate crystal element of high electromechanical coupling adapted for longitudinal motion along the thickness dimension between its substantially parallel major faces. said thickness dimension lying in a plane which is in the +X, -l-Y quadrant and which is disposed substantially '10 degrees from the l-X axis as measured in the XY plane of the three mutually perpendicular X, Y and Z axes, and said thickness dimension being disposed at an angle of substantially 50 degrees with respect to said -l-Z axis, said thickness dimension being a value corresponding to the frequency for said longitudinal mode of motion, said thickness dimension expressed in millimeters being one of the values substantially from 1,400 to 1,600 divided by the value of saidI frequency expressed in kilocycles per second, and means comprising electrodes disposed adjacent said major faces for operating said crystal element inv said longitudinal mode of motion.

WARREN P. MASlONV REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Name Date Cady .f"- Aug. 22, 1939 Number being one of the valuesA