US2166763A

US2166763A - Piezoelectric apparatus and circuits

Info

Publication number: US2166763A
Application number: US131160A
Authority: US
Inventors: Warren P Mason
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1937-03-16
Filing date: 1937-03-16
Publication date: 1939-07-18
Anticipated expiration: 1956-07-18
Also published as: BE426805A; GB501691A; NL51407C; CH208062A; FR835337A

Description

July 18, 1939.,

w. P. MASON PIEZOELECTRIC APPARATUS AND CIRCUITS Filed March 16,1937 2 Sheets-Sheet 1 FIG. 6

//vv/v TOR W. R MA SON 8V ATTORNEY July 18, 1939. w. P. MASON PIEZOELECTRIC APPARATUS AND CIRCUITS Filed March 16, 1937 2 Sheets-Sheet 2 FIG. /3

lA/I/ENTOR WRMASON B) i V g ATTOIJNEV v Patented July is, 1939 4 UNITED STATES rmzonmc'rmc APPARATUS AND oiacurrs Warren P. Mason, West Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 16, 1937, Serial No. 131,160 19 Claims. (01.115-320) This invention relates to electrically operated relays and more particularly to piezoelectric apparatus and circuits.

Substantially all relays now in use having movi able contacts are of the electromagnetic type. An electromagnetic relay operates by virtue of a magnetic field which arises when an electric current flows through the relay winding and which persists substantially for the duration of that current. It follows that the relay actuating force which the magnetic field produces is a function of the intensity of current in the relay winding. In the case of relays operated over long lines or in circuits of high resistance the operl5 ating force may be so small as to make electromagnetic relays impractical without amplifiers or other devices for increasing the actuating force. Moreover, the flow of current in the circuit of therelay winding is attended with an engg ergy loss which in the case of long continued periods of operation, or large numbers of relays may be very costly. v

As is well known, a piezoelectric body responds to a properly imposed electric fleld to alter its position or conformation in consequence of the mechanical stresses induced in it by the field. If such a body, as for example, aplate of Rochelle salt crystal be mounted by fixing one surface or margin on a support and be provided with a pair 39 of electrodes coated or otherwise attached on its surfaces or position adjacent thereto, an electromotive force applied between the electrodes will set up such a stress as to cause a corresponding displacement of the free portions of the body.

Accordingly, an egctric contact element carried by such a displaced portion may be brought into engagement with a fixed position electric contact element upon application of an electromotive force to the electrodes of the piezoelectric body. so If a local circuit including a source of current and a translating device as, for example, a lamp be connected to the twocontact members, the piezoelectric body will operate as a relay to close the local circuit so that the lamp may be lighted 45. by energy from the source of current. Piezoelectric relays have several inherent advantages. Because they respond to potential upon their electrodes they may be operated by the very small quantity of electricity necessary to so charge their electrodes to the required operating potential. Inasmuch as the charging currents are extremely small, the operating circuit losses are correspondingly small. Moreover, the operating circuit may sufiice to transmit suflicient 55- energy to operate the relay even if its resistances are very high. It is quite possible to operate a piezoelectric relay over an operating circuit having a resistance of, for example, 1 megohm. Having once charged the electrodes and actuated the relay no further operating current is rer, quired except the infinitesimal amount necessary to compensate for leakage and maintain the charge and the relay remains operated until the charge on its electrodes is removed. The energy required is substantially only that necessary to lo initially actuate the relay and the usual holding current loss is almost entirely avoided. Moreover, these advantagesare attained by a structure which is relatively simple and inexpensive to construct and which is rapid in operation. 15

An object of the invention is to provide a relay of the piezoelectric type which shall be quickacting.

Another object of the invention is to prevent chattering at the contacts of a piezoelectric re- 50 lay.

An additional object of the invention is to control the time of operation after application of the operating electromotive force and of release after withdrawal of the operating electromotive force 26 to secure any desired time delay or duration of operation-of a piezoelectric relay.

A further object of the invention is to produce a piezoelectric element that may be quick-acting and at thesame time may impart a suificient 0 motion to its movable contact to operate circuits requiring'considerable contact separation.

A still further object of the invention is to provide a piezoelectric relay element which may be rugged enough to withstand the stresses encoun- 135 tered in rapid operation for long periods.

Other objects of the invention are to improve the sensitivity of piezoelectric relays and to render the sensitivity more uniform throughout a wide range of operating temperatures. 40

A still further object of the invention is to render the piezoelectric element of a piezoelectric relay relatively immune to deteriorating atmospheric effects.

It is also an object of the invention toimprove the forceiactor of a piezoelectric relay.

In accordance with the invention the operating element of a piezoelectric relay which carries the movable contact c'onsists of thin blades of plezo'a electric material clamped at one end at which they are made considerably thicker in the interest of ruggedness. A rigid beam structure of material of low density may be cemented to the piezoelectric blades near. their free ends to-carry the movable electric contact at the center 01 66 f percussion of the composite structure and to multiply the displacement available. Chattering of the contacts may be reduced by mechanical and detailed speciflcation. in connection with the drawings in which:

Fig. 1 illustrates in perspective the mounting of the piezoelectric blades which form the operating part of a piezoelectric relay;

- Fig. 2 a side view of the blades;

Fig. 3 an elevation of the structure shown in Fig, 2; v

Fig. 4 a diagram to explain the calculations of the displacement available;

Fig. 5 a modification of the structure of Fig. 1;

Fig. 6 a side view of the piezoelectric blades of Fig. 5;

Fig. '7 an elevation of the structure shown in Fig. 6.

Fig. 8 is a circuit diagram of a system for determining the time of operation of a piezoelectric relay;

'Fig. 9 shows the electrical equivalent circuit of a piezoelectric relay; v

Fig. 10 shows a System for damping a piezoelectric relay;

Fig. 11 illustrates an expedient for rendering the sensitivity of a piezoelectric relay more nearly uniform under varying temperature conditions;

Fig. .12 shows a modification of the device of Fi 11 in which the piezoelectric relay is incor-.

porated as one element of a Wheatstone bridge; and

Fig. 13 shows the electrical diagram of a cir cuit for rendering a piezoelectric relay quick-acting and more sensitiveto a small applied voltage.

Referring to Fig. 1 there is shown a supporting frame I of insulating material having a base member to mount the relay members in vertical position. attached thereto by screws 4 which may support the ,terminals for the respective leads 5 and 5'. An angle portion 6 of the bracket is provided with a split screw-threaded opening through which works a contact carrying screw 1 having a contact member 8 and an adjustingv head 8. Also,

mounted on frame i by means of screws I8 is a axes and perpendicular to the A axis of the virgin bracket II with which a plate l2 connected to bracket II by screws i8 cooperates to clamp the piezoelectric members I 4 and i5.

Piezoelectric elements I and ii are preferably formed of Rochelle salt and their principal faces, indicated in Fig. 2, are parallel to the B and C Rochelle salt crystal. The rectangular plates ii and ii are so cut that their central longitudinal axis 0-0 indicated in Fig. 2 is in a direction inclined 45 degrees to the B or C axes of the virgin crystal. The lower end portions of the elements I and i5 indicated at l8 and H are made thicker than the principal portions so that they are less fragile and hence less likely to be fractured by the clamping action of plate i2 and bracket Ii.

The inner flat faces of elements I and II which- On the frame I are two brackets 8 But from elastic theory 14, is provided with a contact member 28 insulated from the coating on element I4 and having a very flexible leacl'or conductor 2| connected thereto. Elements l4 and i5 are so designed that upon application of an electromotive force between input terminals I8 and I8 one element lengthens and the other shortens. If the electromotive force is of the proper polarity it will cause the assemblage of adhering elements H 26d l5 to bend in the manner of a bimetallic ermostat to carry contact element 28 into engagement with contact element 8 thus completing an electrical path from lead 5 to lead 2|. If an electromotive force of opposite polarity is applied elements l4 and I5 will bend in the opposite div rection to cause the similar contacts on the opposite side to engage.

The displacement of the piezoelectric relay contact 20 may be calculated as follows by reference to Fig. 4. Assuming an effective length 1 from the bending fulcrum of the piezoelectric elements adjacent clamp ll up to the contact 2|, 9. thickness t of the two elements i4 and I5 and a radius r of bending with the final arc subtending an angle 0 But from piezoelectric theory i l d,,V (3) J 300t Where dis is the piezoelectric constant of extension in the direction'of axis OO for a Rochelle salt plate cut as described in connection with ele-- ments l4 and i5 and subjected to an electric field in the direction of the A axis, V is the applied electromotive force and t the thickness of one of the crystals. For Rochelle salt d1: has a magni tude of about 2x10- at 245 C. and decreases on each side of that temperature. Solving for the radius of curvature Where 1: isthe displacement of the center line of the crystal from its equilibrium position and a: the distance from one end of the crystal, hence V escapes For the relay considered here with the crystal plate clamped at one end y -0 when x0 Accordingly d V 2 (8) y: 3

and the displacement at the end of the crystal of length 1 1S lated result checks quite closely with the measured displacement of such a crystal plate at 24.5" 0.

Figs. 5, 6 and '7 illustrate a modification of the piezoelectric element of-Flgs. -1, 2 and 3 in which the displacement of the moving contact element is increased by positioning it on a rigid beam extension of the piezoelectric element. As indicated, the moving contact element 20 is mounted on one side of an I beam 23 preferably consisting of two channel shaped members placed back to back to constitute a rigid or substantially unbending beam. A similar contact 20' is mounted on the opposite side. To minimize the mass of "the beam it is constructed of some low density material as, for example, aluminum. The piezoelectric elements l4 and 15 are slotted at their ends as indicated at 25 to accommodate the I beam which is attached to the piezoelectric elements by some adhesive material such as shellac. The conductive coatings forming the driving electrodes are interrupted shortof the margins adjacent the beam so as not to be electrically connected thereby. Assuming a length of piezoelectric plate Z1 and an additional length 12 of aluminum bar measured up to a contact point carried thereby we may calculate the displacement of the contact point for small displacements from Equation 7 since the aluminum bar will not bend but will have approximately the slope of the end of the piezoelectric plate to which it is cemented. Assuming the piezoelectric element to its Junction with the end of the rigid bar to have a length 11 and the rigid bar a length In it may be readily shown since the slope of the rigid bar is from Equation '7 E 150:: that the total displacement of the movable con tact is The force which the relay will exert on its contact points isalsooi interest. This can be calculated closely by using the formula for the deflection of a beam clampedat one end. This is where i is the len th to is the width, 2 the thickness of one piezoelectric element, that is, half the thickness of the relay tongue, dis the deflection,

E is Young's modulus ofthe piezoelectric material and F the applied force in dynes. -E for Rochelle salt is about 1.5 10 dynes per square cm.'for a plated crystal. Hence solving for the force required to displace thecrystal by an amount d we find 3 X lol glwt'id To find the force applied by the relay 'to its contact Equation 11 can be used if d represents the displacement beyond the fixed relay contact surface that the contacting surface of the movable relay contact would experience if the fixed relay contact oifered no obstruction. For example, with the relay element of Fig. 1, suppose the contact separation is normally 5 mils. Then'the value d for 45 volts will be 16.5-5=11.5 mils. We

assume that the width of the relay is 1" or 2.54 cm.

u x (15) 3 10 X2.54(;f5(;l636) X.0292

13.6X l0 dynes=l4 grams approx.

Measured values appear to be of" this order of" magnitude.

The speed of operation of the relay or the time at which it closes its contacts after application of an electromotive force to its circuit is' another important element. Since the piezoelectric element possesses an electrostatic capacity it takes a certain time to build up therein an electromative force suiilcient to actuate the piezoelectric elements. Moreover, once the capacity device is charged to the proper operating potential there is an additional time required for the operation of displacing the movable contact to take place.

Assuming the requisite charge or potential on its electrodes, a piezoelectric element will respond in an interval which is a function of its resonance frequency. The longitudinal or extensional mode of vibration of a Rochelle salt element has a natural frequency expressed by where as before t is the thickness of the Roch lie salt blade and 1 its effective length. Since e fixed contact stop arrests the motion of the mov'Q time interval T1 occurring during the motion will be approximately To the time T1 required for the relay to operate after it has been subjected to the operating potential there must be added the time T2 required for bringing the potential difference between the relay electrodes up to the operating magnitude. Consider, for example, the circuit of Fig. 8 in which a source 25 of actuating electromotive force is connected in a circuit with a the piezoelectric elementand a resistance element M is shunted across the circuit between the key and the series resistance 21. The time required for the

capacity elements

29 and 30 to Jointly charge up to the operating potential will be approximately T==RC in seconds where R is the resistance of the series elementl'l and C is the joint capacitance of

capacity elements

29 and 30. If, for example, R is a 10 megohm resi'stance and the capacitance of 30 and 2! together is made as large as 1 m1. it requires about 10 seconds for the relay to close. It is accord,- ingly readily possible to increase or decrease the preoperating period by varying the capacity and resistance. The capacity of the electrodes of such a piezoelectric element as was disclosed in Fig. 1 is of theorder of .01 mf. It is, therefore,- apparent that with the capacity 30 decreased to such an extent as practically to eliminate it and with the series resistance reduced from 1 megohm "to 1000 ohms the preoperating time or delay period may be reduced to approximately .00001- second.

After the key 26 is closed and the capacitance 2! becomes charged up to the operating potential as has been described the relay tongue will bend to cause its contacts to engage. Thereafter substantially no energy is taken by the piezoelectric relay which remains actuated as long as its electrodes are held at the requisite potential. Consequently, ifthe key 20- be permitted to open immediately after the contacts of the relay 28 close, the contacts will remain closed until the charge on capacitances 29 and" leaks 01! to such a de- .1 tree that the-actuating force of the residual potential difference between the electrodes will no longer sustain'a displacement sufllcient to hold the contacts closed. The time T: required for the charge to leak of! is a function of the capacitance of the element to be discharged and of the

series resistances

21 and 3| through which it must discharge. Consequently,v the holding time ofthe relay may be varied as desired by variation of the magnitudes of resistances 21. and II and of shunt capacity 30. :This. characteristic of a determinable holding time for an electromechanical relay without supply of substantial additional energy to hold the relay operated after once it is actuated is believed to render this device unique. Chattering of relays at their contacts is an important factor particularly in high speed operation. The equivalent circuit of the relay for its fundamental mode of motion is illustrated in Fig. 9 where Re, Lo and Ce are respectively the mechanical resistance, inductance, and compliance of the crystal for its first mode of motion. See Fig. 3 of the article entitled An electromechanical representation of a piezoelectric crystal" by W. P. Mason, published in Proceedings of the Institute of Radio Engineers, vol. 23, pp. 1252-1263,

.October 1935. If the mechanical resistance Re is very small the crystal will oscillate around its final displacement for a considerable time. If, however, there be introduced a mechanical resistance of such magnitude as to critically damp the crystal the crystal will come to its equilibrium position without oscillation. Such a resistance may comprise an oil-damping element 32 as illustrated in Fig. 1 in which motion of the relay drives a piston which pushes out the oil between the piston and a flat plate behind it. This produces a mechanical resistance which can be adjusted'to any desired value by adjustment. of the flat plate by means of an external knob 33.

It is also feasible to use the piezoelectric property of the vibrating body to advantage in intro- .front piezoelectric blade and the back arcoma ducing damping. Since in its motion-the body generates piezoelectriceiectromotive forces the vibration of chattering produces an alternating eiectromotive force. If an electrical resistance element be connected between points at which such an alternating eiectromotive force is generated the resulting energy dissipation may be employed to effectively damp the vibration of the piezoelectric body. A structure of this kind is illustrated in Fig. 10 in which the front face of the face of the back blade are each'provided with three separate coatings, I and 36. Each of the three front coatings is directly connected electrically with the corresponding back coating. When key 31 is 1 closed an actuating eiectromotive force is applied directly between the coating 35 and the single central coating 38 between the two piezoelectric blades. The relay thereupon responds. Any chattering or vibration sets up an alternating piezoelectric eiectromotive force between front and back coatings 34 and front and back coatings It, all acting as a coating of one potential and the central coating SI of the opposite potential.

Consequently, resistance R1 connected between 25 these coatings furnishes an energy dissipation path which provides the desired damping effect. This damping effect can also be obtained by inserting an electrical resistance R in series with the relay of -Flg. 1. This follows from the 30 fact that when the relay tongue strikes the contact it is bent and this produces an additional charge on the relay which is dissipated through the resistance R in a similar way to that employed in the relay of Fig. 10. It' has been shown theoretically that the best value for this resistance R is a value such that it equals the impedance of the relay capacitance at the resonant frequency of the relay; for example, if the relay has a static capacitance of .01 mi. and a resonant frequency of 150 cycles, the best value of R is 40 about 100,000 ohms. This will not appreciably slow up the action of the relay.

Rochelle salt has a maximum capacitance and piezoelectric response at about 24.5" C. In order to give a more nearly" uniform response with varying temperatures a condenser C1 of considerably less capacity than the piezoelectric capacitance 29 at 245 .C. may be connected in series with the piezoelectric element as is indicated in Fig. 11.- Hence at 24.5 C. most of the driving electron otive force will be expended across the seriescondenser and the sensitivity of the piezoelectric device will be reduced. As the temperature changes from 245 C. the capacity 29 falls away rapidly and soon becomes less than that of condenser C1 so that the relative portion of the driving voltage eflective on the piezoelectric element is increased in such manner as to tend to compensate for its, reduced'piezoelectric sensitivity. The applied charge may leak of! through the inherent leakage resistance of' the crystal structure as indicated in' dotted lines or a suitablephysical shunt resistance may be provided.

A circuit capable of more exact compensation balanced and the piezoelectric element will be 7 same time its capacity and that of elements Ca increase to a magnitude approximating the capacity of elements 02. The bridge thus becomes more nearly. balanced so reducing the electromotive force applied to the piezoelectric elenient and compensating for the increased sensitivity of the piezoelectric element.

Since Rochelle salt crystal relays are, in general, most sensitive at temperatures of about 24.5

C., that is 76 F., it is advantageous in many instances to operate them in air conditioned rooms or enclosures in which the temperature is at about that point. vantage in rooms where humidity is regulated that thecrystal is protected against dehydration. It is, therefore, to -be understood that piezoeiectricrelays in accordance with the present invention may be placed singly or with numbers of other relays in enclosures provided with temperature regulation or with both temperature and-humidity regulation. The broken line recan enclosure for the piezoelectric relay element If it is desirable to make the piezoelectric relay very fast it is necessary to increase the resonance frequency of the piezoelectric element either by making it shorter or thicker. In either case, the sensitivity of the relay will be considerably decreased so that. the displacements obtainable with the driving electromotive forces ordinarily available for signaling will be considerably reduced. To overcome this a circuit arrangement in accordance with that of Fig. 13 may be used in which the piezoelectric element is operated by a relatively large actuating electromotive force controlled by an electron discharge device to the input circuit of which a relatively small electromotive force is applied. Although a high voltage source is used to actuate the piezoelectric relay very little power is consumed and, accordingly, such a system is of relatively high emciency.

As shown in Fig. 13, the signal input is represented diagrammatically by. the source E1 and the signaling key 39. in series with resistance Re which may represent the resistance of a long line or other circuit over which the signals are to be transmitted. An electron discharge device of the usual type has its input electrodes connected to the signal path just described. The anode circuit of the device 40 includes in series an inductance coil L. a resistance R3 and a twopart space current source E2, Ea the negative terminal of which is connected to the cathode of the discharge device. A piezoelectric relay 4| has its actuating terminals connected respectively to the anode and tothe common junction point of source'Ez, E3. The circuit is so adjusted that the normal space currentpassing through the tube and. through the inductance L and the resistance Rs in series causes a drop of potential in the inductance L and the resistance R3 which is just equal to the electromotive force of source E2. Under those circumstances there is a sub- There is the additional addusted thatthe normal potential drop across L and Re Just equals the electromotive force of E2 the normal potential diiference across the actuating terminals of relay M is zero. It will be obvious that under these circumstances, the source E2 in effect supplies the potential drop across Ra and the source Ea supplies that within the internal space current path, or, in other words, that the electromotive forces of the sources E2 and E3 will bear the same ratio as R3 and Rp. In other words, Ez with the magnitudes of Ra and R as assumed; will be approximately 666 volts and E3 of 333 volts electromotive force.

.The normal space current in the circuit of Fig. 13 in the absence of impressed signal electromotive forces is I =fi%= io q=6 6 milliam o v-ia 50,000+ 100, peres The change in current which occurs in consequence of introduction of a signaling electromotive force V of 33.3 volts into the grid circuit is Now, if the electromotive force V is so directed that the change in current is a reduction the final space current Io-Iv will be substantially zero. Under these conditions the full electromotive force of E2 will be applied across the piezoelectric element M. It is, therefore, apparent that a signal voltage of 33.3 volts has changed the actuating electromotive force applied to element 4| from an initial zero value to 666 volts. This very great efl'ective amplification will enable high speed operation to be attained with short or thick piezoelectricelements of relatively low sensitivity.

The inductance L of the circuit of Fig. 13 is' preferably given such magnitude and so incorporated into the circuit that together with the .inherent capacitance 29 of the relay 4| it constitutes a' one-half section low-pass filter of characteristic impedance R3. Since this one-half section is terminated in its characteristic impedance the combination will, up to. the cut-off frequency,

have a pure resistance impedance equal to that of R3. The magnitude of the inductance of L is a representative value for a fast relay and R3 sive to an impressed electromotive force to cause a displacement of a portion of the device, a movable contact mechanically fixed with respect to the displaced portion so that its position is shifted by displacement of said portion, a second contact mounted adjacent said movable contact and adapted to be engaged by the movable contact, and means predesigned to control the rate of application of the eiiective' electromotive force to the electrostatic device to predetermine the interval which elapses after impression of the electromotive force upon the input terminals before the engagement of the contacts.

plates having juxtaposed flat surfaces, means holding the plates together as a unit with their lengths so arranged that one plate tends to shorten when the other plate lengthens, supto and between which an electromotive force may be applied to cause displacement of a portion of the plate, a first contact carried by the displaceable portion, a second contact engageable by the first contact, the piezoelectric plate comprising a thin long member having a much thicker portion adjacent a margin remote from the first contact, and supporting means clamping the thickened portion of the plate whereby the tendency of the clamping to shatter the plate is materially reduced.

4. An electrically operated device comprising two relatively thin long blades of piezoelectric material each having two principal surfaces coated with conductive material, means causing one principal face of one plate to adhere to a principal face of the other, an input terminal connected to the coatings on the exposed faces of both blades a second input terminal connected to the coatings on the adhering faces, each bla'de having an integral thickened portion at one end,

and supporting means clamping the blades at their thickened portion.

5. A relay comprising an elongated member of piezo-electric material, input electrodes associated with two opposite faces thereof, an individual input contact connected toeach electrode,

means for supporting the piezoelectric member at one end, a rigid bar of material having low specific gravity carried by the piezoelectric memher at a point remote from the supported endand extending in the same direction as the elongated member, and a contact carried by the rigid bar whereby upon application of an electromotive force between the input electrodes the contact member moves a greater distance than does the movable portion of the piezoelectric member and the motion ismore rapid than it would be for a piezoelectric member having a dimension equal to the over-all length of the piezoelectric member and the bar. a

6. A relay comprising a plate of piezoelectric material, the piezoelectric constant of which varies with temperature, electrodes associated with the plate, means for mounting the plate to hold one portion thereof relatively fixed, a stationary electrical contact member on said mounting means, a second electrical contact member on the plate and in position to engage the stationary contact member upon application of an electromotive force to the electrodes, and means for maintaining the temperature of the piezoelectric plate within a limited range of temperatures including that at which the piezoelectric constant of the piezoelectric plate is a maximum.

7. An electrostatic relay comprising a member having two electrodes in position to subject the 2,166,763 2. A relay comprising a pair of piezoelectric member to an electric field so as to cause a dis-; placement of the member upon application of an electromotive force to the electrodes, and means to discharge any residual charge upon the electrodes after withdrawal of the applied electromotive force.

8. The combination according to claim '7, char-. acterized in this, that the means to discharge the residual charge comprises a resistance, the magnitude of which is so related to the capacity of the electrodes as to predetermine the time during which the relay remains actuated after the actuating electromotive force has been withdrawn.v

9. A relay comprising a piezoelectric element, electrodes mounted adjacent thereto, terminals connected to the electrodes to permit an electrical electromotive force to be impressed thereon to cause fiex'ure of the piezoelectric element, and a yieldable mechanical damping device connected to the element to tend to damp oscillations resulting from the displacement occurring when the condition of charge of the electrodes is suddenly varied.

10. In combination an electron discharge ampli-- fier having a cathode, an anode and an impedance contrcfi element, input terminals connected to the cathode and impedance control element; an output circuit connected to the anode and cathode, said output circuit including in series two sources of space current and a resistive impedance, the source of current adjacent the resisti'veiimpedance having an electromotive force just sufllcient to compensate for the drop of potential in the resistive impedance, and a piezoelectric relay connected in shunt to the resistive impedance and its adjacent source whereby a normal zero potential difference exists across the piezoelectric relay and a large potential difference is impressed on the relay when an electromotive force is applied between the input terminals.

ance having one terminal connected to the in-.

ductance and the other to the piezoelectric element to terminate the filter section in. its

characteristic impedance whereby the filter sec- I tion will present a pure resistance impedance in the output circuit of the electron discharge device.

12. In a delay action piezoelectric relay, in combination, a pair of input terminals, a resistance connected therebetween, a second resistance and a piezoelectric relay element in series therewith connected in shunt to the first resistance,

and a capacity element connected in shunt to the piezoelectric relay element.

13. A relay comprising a long element of R0- chelle salt having its two principal flat faces in planes parallel to each other and parallel to. the B axis and perpendicular to the A axis of the mother crystal, the longitudinal axis of the piece lying at an angle of 45 degrees with the B axis, a second similar shaped element of Rochelle salt similarly cut, electrodes for each of the pieces of Rochelle salt associated therewith, means con-' necting the electrodes of one piece respectively to the electrodes of the other, means holding the two pieces in fixed back to back relation such 7-5- afrearcs that an electromotive force appliedbetween the electrodes tends to cause one piece to shorten and the other piece to simultaneously lengthen, a

movable contactor carried by the Rochelle salt trodes to the operating voltage of the relay.

14. An electric relay comprising a pair of piezoelectric plates clamped together, electrodes associated with the platesto subject the plates to an electric field when the electrodes are electrically charged, a pair of input terminals connected to the electrodes in such manner that an electromotive force applied to the input terminals tends to cause one plate to increase in dimension along one direction and to simultaneously cause the other plate to decrease in dimemion along the same direction whereby the assemblage of plates warps from its normal position, a stationary contact'element mounted adjacent the plates, a movable contact element carried by the assemblage of plates formotion into engagement with the stationary contact element, means in series with the input terminals to determine the initiation of theresponse of the relay after an electromotive force is applied to the terminals, means for suppresslng chattering of the contacts upon engagement to insure the precision of the contact interval, and means in shunt to the terminals and having such resistance relative to the magnitude of'the capacity of the electrodes as to predetermine the time of release of the relay after the actuating electromotive force is withdrawn from the input terminals.

15. A relay comprising a pair of piezoelectric plates having juxtaposed fiat surfaces, electrostatic capacity elemerits associated with the plates in fixed relation whereby in response to an electromotive force impressed upon the elements the plates are each subjected to an electric field,v

means holding the plates together as a unit with their dimensions so arranged that in response to an application of a given electromotive force to the elements one plate increasesin dimension in a given direction and the othersimultaneously shortens, a contact carried by the plates at a point remote from the holding means so as to be moved upon change in dimensions of the plates, a stationary contact adapted to be engaged by the movable contact, a resistor in series with the electrosatic capacity elements and .having such magnitude of resistance with respect'to the electrosatic capacity of the elements and the electric field required to initiate operation of the piezoelectric plates as todetermine the initiation of the operating period of the relay, and a resistance' path in shunt to the capacity elements whereby upon eifective disconnection of an actuating source of electromotive force the charge accumulated upon the elements and the electric hearing such relation to the capacitance of the elements as to determine the time of dissipation of the charge and release of the relay after efiective withdrawal of the source of actuating electromotive forces whereby the placing of the period of operation of the relay as well as its duration may be predetermined,

16. The combination presented in claim 15 together with meanstending to eliminate chattering at the contacts upon initiation of the operation of the relay whereby the effective operation period is not disturbed by chattering.

17. A relay system comprising anelectrostatic relay which includes two input tefminals; two output terminals, a pair of electrostatic capacity elements respectively. connected to the input ter- .minals and the electrostatic capacity of which is of the order of .01 microfarad, a pair of contactor elements respectively connected to the output terminals, means physically connected to one of the contactor elements and responsive to relative motion of the capacity elements upon the application of a charging electromotive force to the input terminals to cause engagement oi the contactor elements, and an electrical network connected to the input terminals of the relay which includes a conductive resistive path shunted across the capacity elements to determine the period required for the capacity elements to discharge to a release potential after withdrawal of the applied clectromotive force from the input terminals.

18. A relay system comprising an electrostatic relay which includes two input terminals, two

output terminals, a pair 01' electrostatic capacity elements respectively connected to the input tergminals and the electrostatic capacity of which is of the order of .01 microiarad, a pair of contactor elements respectively connected to the output terminals, means physically connected to one of the contactor elements and responsive to relative motion-of the capacity elements upon the application of a charging electromotive force to the input terminals to cause engagement oi. the contactor elements, and an electrical network. connected to the input terminals and including a 1 resistance in series between one of the input ter ,minals and the respectively, connected capacity element to determine the time required for the electrostatic capacity to charge up to a desired operating potential after application of the,

charging electromotive force to the input terminals.

19. A relay comprising a plate of piezoelectric material, the piezoelectric constant of which is a function of the condition of the surrounding atmospheric medium, electrodes associated with the plate,. means for mounting the plate to hold one portion thereof relatively fixed, a stationary electrical contact member on said mounting means, a second electrical contact member on the plate and in position to engage the stationary contact member upon application of an electromotive' force to the electrodes, and means for preventing change of the magnitude of the piezoelectric constant in consequence of change in the condition of the atmospheric medium surrounding the relay.

' WRW P. MASON.