US2986963A - Electropiano - Google Patents

Description

E. M. JONES ELECTROPIANO June 6, 1961 Filed Aug. 12

2 Sheets-Sheet 2 ill INVENTOR. [Zn 4f JONES. BY

ATTORNEYS,

United States Patent 2,986,963 ELECTROPIANO Edward M. Jones, Cincinnati, Ohio, assignor to The Baldwin Piano Company, Cincinnati, Ohio, a corporation of Ohio Filed Aug. '12, 1955, Ser. No. 528,059 18 Claims. (Cl. 841.04)

This invention relates to percussive musical instruments and, in particular, to pianos of a type wherein strings are stretched on playing keys, and are set into vibration upon depression of the keys, as in the playing of a musical selection, the vibrations of the strings being translated electrically into sound. In a preferred embodiment of the invention, the strings are set into vibration by shock excitation.

Strings are felt to be the most practical vibrators, giving a long series of partials which are harmonically related with a fair degree of accuracy.

It is a primary purpose of the invention to provide a practical embodiment of such an instrument.

Other objects of the invention have to do with the solution of specific problems in connection with such instruments, all as will be set forth herein.

These and other objects of the invention, which will be clear to the skilled worker in the art as the description proceeds, are accomplished by that certain construction and arrangement of parts of which specific embodiments will now be described.

Reference is made to the accompanying drawings wherein:

FIG. 1 is a partial plan View of the exemplary instrument, with parts broken away to disclose interior construction.

FIG. 2 is a partial sectional view taken along the line 2-2 of FIG. 1.

FIG. 3 is an enlarged partial perspective view of the rear end of a key structure, showing the key mounting, the strings, tuning pins, and an electrical pickup.

FIG. 4 is an enlarged partial perspective view of the front end of a key structure showing the strings keymounted damper device.

FIG. 5 is an enlarged partial perspective view showing a construction for the shock excitation abutment members.

FIG. 6 is an enlarged partial perspective view showing a construction for an electrical contact mechanism.

FIG. 7 is an enlarged elevational view showing an entire key in the treble register, together with its appurtenances, the view being taken along the line 7-7 of FIG. 1.

FIG. 8 is an enlarged perspective view of a bridge member which may be used in the treble register.

FIG. 9 is an enlarged perspective view of a means for engaging the strings beyond the bridge.

FIG. 10 is an enlarged elevational view of a type of tuning pin which may be employed.

FIG. 11 is an enlarged partial elevational view showing the front end of a key with another form of damper device.

FIG. 12 is an enlarged perspective View of the damper and its pivotal mounting means.

FIG. 13 is an enlarged elevational view of a key element pivoted intermediate its length.

FIG. 14 is an enlarged transverse sectional view of a forward portion of a key element taken along the line 14-4-4 of FIG. 13.

FIG. 15 is an enlarged sectional view of a key element employing a frame of formed sheet metal.

In the copending application of Octavio Jose Alvarez, Serial No. 320,035, filed November 12, 1954, now Patent No. 2,799,196 and entitled Musical Instrument, a type of Patented June 6, 1961 musical instrument is shown in which strings are either stretched on keys as supports, or on supports which are actuated by playing keys. When the playing keys are depressed, the support is brought against a stop or abutment, causing the strings to be set into vibration by shock. Electrical pickup means are employed to convert the physical vibrations of the strings into electrical vibrations which are amplified and converted to acoustical vibrations by a loud speaker or similar device. There are, of course, various ways in which strings so mounted can be set into vibration, as by striking them with a hammer, by plucking, by bowing, etc. Excepting as set forth in the appended claims, salient features of this invention are not limited to any particular mode or mechanism for producing vibration. However, shock excitation as aforesaid produces a suflicient amplitude of vibration for the reproduction of sound by electrical means, and makes for economy because relatively expensive action elements are eliminated. It has already been indicated that strings are preferable to other vibrators such as reeds, tuning forks, and the like, not only for reasons of economy, but because of the nature and the amplitude of the harmonics produced. The stretching of the strings on individual supports further simplifies the construction, rendering unnecssary the provision of a plate or frame on which a plurality of strings must be stretched, and facilitates the individual excitation of the strings. The strings are limited to lengths of roughly 10 inches or so, so that the body of the key can be of a reasonable size. In the upper register the strings are relatively short, being of the same general order of length as in a standard piano, but are made of substantially thinner wire. In the lower register, the strings have a constant length limited by the lengths of the keys; but as the frequency goes down, the string tensions may be decreased and the strings may be wound in helices of varying pitches and diameters to increase their effective masses. This is preferable because the weight is distributed evenly along the eifective lengths of the strings, although actual weights can be attached .to the strings if desired. It will also be usual in an instrument of this type to provide a plurality of strings (generally two) for the keys in the lower register, although single strings will serve for the keys in the upper register.

The instrument does not require a sounding board. It can be made inexpensively, in a form which is light in weight and small in size. The over-all volume of the instrument is controlled electrically in ways known to those familiar with amplifiers and electro-acoustical transducers; and the instrument may be provided with an over-all volume control means. It is also possible for the operator to cause the sound to be reproduced only in headphones which he wears. Thus, a plurality of instruments of this class may be played or practiced upon in a single room without interference. Yet the instrument has the dynamic and percussive effects of the standard piano as obtained by the touch or the force with which the keys are struck.

Important features of the present invention include a non-vibrating support for the string or strings and the pickup device, a minimum noise, maximum-shock abutment or stop means for the keys, a system for determining the up-position of the keys, which system will be shock-free, an inexpensive, rigid pickup device, a means for making contact with the pickup device, eliminating stray electrical effects and permitting removal of the keys, a key mounted damping device for each note, and the like.

Preferably, the body of the key is made of metal so that a dimensionally stable frame is available on which to stretch the string. The exposed part of the key has a body of plastic, or wood faced with plastic or ivory. To avoid spurious vibrations of the body of the key,

which would cause relative motion of the pickup with respect to the string and-produce frequencies in the tone besides the desired ones, the body of the key is made very rigid. It has been found that making the body of the key from aluminum or aluminum alloy with a crosssection of approximately A in. x 1 /2 in. provides excellent rigidity in the vertical direction and sufficient rigidity to minimize transverse or torsional vibrations without making the key unduly heavy.

In FIGS. 2 to 4, 7, l1 and 14, the key body is indicated by the numeral 1. -It may be formed from aluminum or light metal alloy by casting or stamping. The front end of the key is shown as bearing a wooden element 3, hollowed out to accept the forward end of the key body and fastened to the key body in any suitable way, as by cementing. The wooden body may be faced with plastic or ivory, as at 4 and 5, to improve the appearance of the exposed surface. As will be clear from FIG. 1, the keyboard will be made up of white keys 6 and black keys .8 in the usual arrangement.

The instrument is provided, as in FIG. 1, with a key having side frames 9 and 1t which are spaced by a key bed structure 11 at the front and at the rear by other elements not shown but readily supplied by the skilled worker in the art. In a preferred embodiment, the keys are pivoted at the rear by means of leaf springs. To this end a rail 12 is carried across the keys of the instrument behind the key assembly. The rail may be attached to end plates 13 and 15 which are screwed to the inside surfaces of. the side frame members. The rail '12 is shown as having a forwardly extending flange 16 to which one end of a leaf spring 17 is attached, as l'l'y screws or bolts 18. The other end of the leaf spring is attached by screws or bolts 19 to the rear portion of the key body 1. There is a similar leaf spring mounting for each key body in the instrument.

Each key body has a downwardly extending nose or boss.20 which will serve as a striker for shock excitation. As most clearly shown in FIG. 4, this nose is drilled with one or more small holes 21 through which one or more strings 22 may be carried to a hitch post or screw 23. The rear end of the key body, as shown most clearly in FIG. 3, is provided with a downwardly extending tooth '24 which is grooved to accept the string or strings 22. The extreme end portion of the key body is shaped as indicated and is drilled for one or more tuning pins 25, 26. These tuning pins may take various forms. They may be threaded into the body of the key, or held in frictional bushings in said body; but it has been found sufficient to form the pins, as shown at 25 in FIG. 10, with a slightly tapered body, a hole 27 for the acceptance of the string end, and an upper squared portion 28 for engagement with a tuning wrench. Pins like those shown in FIG. may simply have a press fit in holes in the key bodies.

By reason of the construction outlined, the string or strings 22 will be in the lower register be stretched freely between the nose Ziland the tooth 24, lying well beneath the key body. The nose and tooth will in this instance determine the speaking length of the string, and the frequency of its vibration will be determined by the effective mass of its speaking lengthand the tension under which it is stretched by means'of the tuning pin. The invention in its broader aspect is by no means limited to the stretching of the strings beneath the key body, since the strings may lie above the key bodies by a simple reversal of parts, or may even lie alongside the key bodies in channels or recesses. However, locating the strings beneath the key bodies is an advantageous construction simplifying the parts required, and providing the maximum speaking length for the strings with respect to any given key body length. a 7

When the playing key is depressed by the operator, it is brought against a stop or abutment which causes a sudden cessation in its downward motion, shocking the string or strings borne by the key into vibration. To prevent noise from being transmitted to the frame of the instrument, the abutment means struck by the nose should be insulated from the frame. At the same time the abutment means must have an inertia effect large enough to provide suflicent shock excitation, yet must not be so large as to give rise to appreciable acoustic radiation. Also, any noise tending to be produced by the actual striking of the nose against the abutment should be damped.

These requirements can be met by a construction shown best in FIGS. 1, 2 and 5. The key bed 11 is provided witha channel 29 located beneath the noses of the several key bodies. This channel has a lining 30 of felt, rubber, or other shock absorbing material. In the channel is located a series of metallic blocks 31 of steel or other relatively heavy metal. The steel blocks may be insulated one from the other by layers of shock absorbing material at their ends, as at 32 in FIG. 1.

A single steel block can serve as an abutment for a plurality of key bodies. If it serves two keys, equal shock will be received by each key for the same playing velocity; but if it serves three or more keys, the intermediate keys of the group are likely to receive more shock for the same velocity than the end keys, thus causing unequal loudness of the tones. Consequently, it is preferred to fill the channel 29 with a series of'steel blocks of such length that each block serves a maximum of two, or at the outside, three keys. The tops of the blocks 31 are covered with thin pads of relatively hard felt or shock absorbing material 33 so that the shock effect will not be minimized, but any direct clicking of the noses against the blocks will be suppressed.

By the construction set forth, an adequate shock excitation is obtained. The mass of the key body is substantial and is greater than the normal mass of key bodies in conventional pianos. The inertia approaches the combined inertia of the key and the hammer and action elements in the conventional instrument, at least for the lighter hammers in the upper register. If, however, the mass of the key body is made too great, a difii culty may be encountered in that the upward travel of the key upon its release by the operator will be too sluggish with the maximum spring force permissible for comfortable playing of the instrument. With a metallic key body of the approximate cross-section shape noted hereinabove and a length of about 12 inches, in conjunction with a leaf spring capable of producing rapid key return, a touch is obtained which is not markedly dilferent from that of conventional pianos. Care must be taken that the strings are not shocked into vibration during the upstroke of the key. If stops are used to determine the non-actuated position, they should be of extremely soft material, but the material should have enough damping property to prevent troublesome rebound. An upper stop could be omitted entirely, and the quiescent position of the key determined by the equilibrium position of the spring providing adequate damping means, such as a dash pot, or a controlled friction device can be applied to each key to prevent rebound. In view of these considerations, where the keys are pivoted by leaf springs as shown, and where the leaf springs act to prevent undue sidewise motion of the keys, it has been found sufficient to combine a stop with a simple friction mechanism. Thus, the front board 34 of the instrument may'carry a soft felt stop 35 in a position to be contacted by the keys, While each key carries a curved spring finger 36 which engages a portion of a felt strip 37 on the rear surface of the front board. In this way both rebound and shock may be effectively minimized. V

Dampers must be provided for the string or strings on each key. It is of great advantage to mount these dampers on the keys, and to design them in such a way that a vibration-preventing element will be applied to the string or strings under suitableresilient pressure, and will be removed from the strings only when the keys are depressed or is actuated by a sostenuto device.

Such a damper is easily made by providinng a sheet metal element in the form or" an elongated resilient strip 38 (FIG. 4) which has an angularly related flange 39 at one end fastened to the key body, as by screws 40, and near its other end a lateral projection 41 overlying the strings 22. This lateral projection carries a pad of felt 42 or other damping substance. The forwardly projecting end of the body 38 is bent downwardly, as at 43, and then substantially horizontally, as at 44, to provide a foot. If the length of this foot is correctly correlated to the position of a stop mechanism, a depression of the key will raise the pad 42 out of contact with the strings, leaving them free for vibration. A fixed abutment could be provided in the key bed; but a sostenuto device is readily made by mounting a movable abutment rail 45 on a lever 46 at each end of the instrument, the levers being pivoted as at 47 on brackets 48 on the bed and connected by linkages 49 with a foot pedal (not shown). Since the abutment rail 45 is common to all of the keys, actuation of the foot pedal will raise the dampers from the strings even on the undepressed keys, providing the well-known piano sostenuto effect. In order to allow sufficient movement of the damper mechanism, the key body may be recessed as at 50.

The rail 45 may be covered with felt or similar damping material to prevent rebound of the damping device.

Various damper constructions are possible. In FIGS. 11 and 12 there is shown a type of damper comprising a wire element 51 having a horizontally bent end 52 which is pivoted to the key body in a hole 53. The body 51 has a plate 54 attached to it bearing a suitable block of damping material 55. The other end of the body is bent downwardly and across, as at 56 and 57, to form a foot which contacts the sostenuto rail 45. The damper mounting is urged toward the strings by a leaf spring 58 mounted on the key body.

Electrical pickup means should be mounted on the key body in proximity to the strings. Such electrical pickup means may take various forms, such as magnetic means comprising a permanent magnet and a winding thereon, electrostatic pickup means, and the like. Electrostatic means are provided for various reasons including simplification of structure. The pickups should be mounted on the key instead of on the stationary frame of the instrument to control more accurately the distance between the string and the pickup, and also because a release of the keys while the strings are vibrating and while the sostenuto pedal is actuated, would produce a sudden change in the intensity of the tones as the strings were carried away from the pickup means.

As indicated, the simplest type of pickup means is a capacitative pickup, since this involves mounting on the key only a single capacitative element in proximity to the strings. It is essential that the pickup be rigidly mounted on the metal frame of the key, not only to maintain a small precise distance between the string or strings and the pickup, but also to prevent vibration of the pickup with respect to the key. In the preferred construction a plate 59 of glass, ceramic, or other insulative material is atfixed to the body of the key, as by cementing. This plate of insulative material may bear a metallic coating or a plate of metal on its exposed surface, which may be connected to the amplifying system of the instrument. In practice, the string or strings are spaced a distance of about .020 to .080 in. from the metallic coating of the pickup, depending on the amplitude of vibration of the string and the desired loudness of the tone to be produced thereby in its relation to the tones of other keys of the instrument.

The metallic surfaces of the various pickups may be connected through flexible leads to a common bus bar. Where flexible leads are employed, care must be taken to prevent vibrations of the leads themselves or of the keys from producing extraneous variations in capacity which would introduce noises into the tones. It is possible completely to shield each lead, but in this event the dielectric should be firmly bonded both to the inner conductor and the shield to prevent relative movement. The use of flexible leads, moreover, makes it impossible to remove individual keys from the instrument for replacement, tuning, or repair, without disconnecting the leads, and unduly increases the total electrical capacity of the pickups.

A way has been found around this difficulty and is illustrated in such figures as 1 and 3. Here the insulative block 59 has fastened to it a metallic element having a flat surface 60 opposed to the strings 22 and a depending leg 61. A channel-shaped element 62 is fastened to the bed or frame of the instrument as a support. A second channel-shaped metallic element 63 is located within the first mentioned channel and its walls are separated therefrom by ceramic or other insulative blocks or elements 64, 65 and 66.

The inner channel is so located that the legs 61 of a group of pickup elements on various keys will extend down into it. At the positions of the depending legs 61 of the pickup devices, the inner channel is provided with spring conductors 67 adapted to embrace these legs closely enough to make dependable electrical contact therewith. The lower ends of the springs 67 are fastened to the base of the channel 63 in any suitable way. One convenient way involves locating the springs by means of a pronged template and then flowing some low melting point metal 68 into the base of the inner channel so as to fasten the spring ends thereto in electrical contact.

Thus, the depending legs 61 of the pickup devices are maintained at all times in electrical contact with the inner channel element 63 which serves as the bus bar aforesaid, and yet the depending legs 61 are free to move with respect to the inner channel as the keys are depressed and released. In addition, any key is readily removed from the instrument by disconnecting the pivoting leaf spring 17. The depending leg of the pickup can simply be pulled out of engagement with its spring and re-inserted therein when required. The inner channel will have an electrical connection 69 with an electroacoustic sound production system indicated broadly in FIG. 2 at 70. Electrical return is, of course, provided by the spring 17 and rail 12 to which electrical connection may also be made.

The particular form of contact device comprising inner and outer channels as described is advantageous not only because it provides a low loss insulating mounting for the inner channel, but also because the outer channel may serve as a shield, to which end it may be grounded. Where other forms of pickup are employed as, for example, a magnetic type, a similar connection arrangement may be employed; but it is not beyond the scope of the invention, where desired, to provide a plurality of contact legs on the key body and a plurality of contact elements on the key bed or base of the instrument. As will be understood by those skilled in the art, the placement of pickup means with respect to the length of the strings is of importance; but it may be varied to produce different eflects. Normally, where a high harmonic content is desired, the pickup means will be located relatively near one end of the string or strings. In FIG. 2, the pickup means is shown located near the rear ends of the strings 22, which is advantageous because it permits the location of the damping means near the front ends of the strings. However, since in the upper registers of the instrument the strings become shorter, the positions of the pickups on the key bodies will vary in the direction of the length of the key assembly, although it is usually possible to arrange for a similar positioning of pickups on groups of adjacent keys. The contact means hereinabove described comprising the inner and outer channel elements and the springs thus cannot be continuous throughout the length of the instrument, but will be divided into sections particularly in the upper register, as indicated at 71, 72, 73, 74 and 75in FIG. 1. The inner channels of the several sections may, of course, be electrically connected together.

FIG. 7 is illustrative of the structure employed in the upper register where the strings become shorter. A bridge element 76, triangular in cross-section, is used to define the speaking length of the strings as that portion of their length extending from the nose 20 to the bridge. The bridge may be grooved on its base, as at 77 (FIG. 8) to accept the lower edge of the key body. Its apex may be grooved, as at 7 8 to accept the string or strings. The bridge element 76 is adapted to be fastened to the key body at various distances from the nose 20 to define the speaking length of the string or strings as aforesaid. This may be done by cementing, soldering, brazing, or by a mechanical connecting means.

Beyond the bridge 76, the string or strings are engaged by a deflecting and damping device which may conveniently take the form of a channel-shaped member 80 lined with felt or other sound deadening material 81. The channel-shaped member is perforated to accept a screw 82 which may be threaded into the body of the key. Note in FIG. 7 the position of the pickup means 60 applied to the speaking length of the string. The string length beyond the bridge is, of course, maintained inactive by the element 80 and its lining; but the tuning pins are still located at the rear end of the key body beyond the tooth 24.

It is not necessary herein to describe amplifying and loud speaker systems suitable for use with an instrument of this class, since such systems are well-known in the art and may be selected by the skilled worker in accordance with the type of pickup employed. Where capacitative pickups are used, the vibrations of the strings may be converted into sound in any amplifier and speaker system designed for use with a condenser microphone, :for example. Also, it is possible and in some instances preferable to cause the small variations in capacity produced by the vibrations of the strings to modulate a radio frequency signal either in amplitude or in frequency. Amplification systems designed to produce sound from such modulated signals are well-known. United States Letters Patent 2,140,025 and 2,273,957 illustrate respectively systems for amplitude modulated signals and systems for frequency modulated signals.

Modifications may be made in the invention without departing from the spirit of it. For example, FIG. 13 illustrates a key having a body '83 which is pivoted intermediate its length as at 84. The return of the key is accomplished by a spring 85 engaging between a rail 86 and the rear end of the key body, where it is guided by a pin 87. Instead of being made of light metal, as in FIG. 14, the key body may comprise a formed, channelshaped sheet metal structure, as shown at 87 in FIG. 15. The weight requirements may be met by making the channel-shaped element of relatively heavy metal, such as steel; and the element may be filled with a wooden filler 88 throughout its length or at intervals, or at its ends metallic blocks may be inserted to hold such devices as the tuning pins 25, 26 and the hitch pin 23.

The invention having been described in certain exemplary embodiments, what is claimed as new and desired to be secured by Letter Patent is:

1. In an electrical musical instrument, an elongated, pivoted playing key, a string stretched between spaced supports on said key, electrical pickup means on said key for converting the vibrations of said string into electrical vibrations, means effective upon depression of said key to tact with said string when said key is depressed.

2. The structure claimed in claim 1, wherein said damper means is resiliently urged toward said string,

3. The structure claimed in claim 2, wherein said damper means has a projection extending below said key and adapted to strike an abutment when said key is depressed, whereby said damper means is brought away from said string.

4. The structure claimed in claim 3, wherein said abutment is arranged to be movable by a sostenuto pedal.

5. The structure claimed in claim 3, wherein said string is stretched below said key and wherein said damper means is mounted on a leaf spring cantilevered to said key body and having a forwardly and downwardly projecting portion acting aaginst said abutment.

6. In an electrical musical instrument, a key having an elongated, pivoted, metallic body providing spaced supports, a string stretched between said supports to provide a speaking length, electrical pickup means mounted on said body in proximity to said string, means acting upon depression of said key to set said string into vibration, damper means mounted on said key and resiliently urged toward said string, and means for moving said damper means away from said string upon depression of said key.

7. The structure claimed in claim 6, including releasable means for making electrical contact with said pickup means.

8. The structure claimed in claim 7, wherein the vertical cross-sectional dimension of said key is several times its transverse dimension to provide against extraneous vibration.

9. The structure claimed in claim 8, wherein said pickup means comprises a contact arm extending from said key, an electrically conductive element spaced from said contact arm, a spring attached to said electrically conductive element and embracing said contact arm whereby to maintain electrical contact between said arm and element irrespective of movements of said arm.

10. The structure claimed in claim 8, wherein said key is pivoted to a rail by means of a leaf spring causing return of said key after depressing, and wherein a frictional means is provided to prevent bouncing-ofthe key in the up position.

11. The structure claimed in claim 9, wherein said elec trical pickup means is capacitative and includes a metallic element approached to said string and insulatively mounted on said key, said contact arm being attached to said metallic element.

12. In an electrical musical instrument, a key having an elongated, pivoted, metallic body, supports adjacent the front and rear ends of said key, a string stretched between said supports, means for defining a speaking length for said string comprising a bridge element attached .to said key intermediate said supports and defining said speaking length in coaction with one of said supports, and damping and deflecting means between said bridge element and the other of said supports for engaging the remaining length of said string and rendering it inactive, electrical pickup means mounted on said key and effective in the speaking length of said string, and resilient, movable, damping means also mounted on said key and effective on the speaking length of said string.

13. In an electrical musical instrument, a key having an elongated, pivoted, metallic body, a string stretched between spaced supports on said body, electrical pickup means also mounted on said body for converting vibra- V sion of said key, said means comprising an abutment surface on said key and an external abutment mounted on said instrument in a position to be struck thereby upon depression of said key.

M. The structure claimed in claim 13, wherein said instrument has a key bed and wherein said external abutment comprises a metallic block supported on said key bed by sound deadening material.

15. The structure claimed in claim 13, wherein said instrument has a key bed and wherein said external abutment comprises a metallic block supported on said key bed by sound deadening material, and wherein the surface of said block, which is struck by said key, is covered with a layer of relatively hard sound deadening material.

16. The structure claimed in claim 13, wherein said instrument has a key bed and wherein said external abutment comprises a metallic block supported on said key bed by sound deadening material, and wherein the surface of said block, which is struck by said key, is covered with a layer of relatively hard sound deadening material, in which said block is common to a small number of keys therein, and in which other blocks are provided for other keys, said blocks being insulated from each other by sound deadening material.

17. The structure claimed in claim 13, wherein said key is pivoted to a conductive rail by a leaf spring serving as an electrical return, wherein said electrical pickup means is capacitative and comprises a metallic element approached to said spring and insulatively mounted on said key, a contact arm connected to said metallic element and extending away from said key, and a resilient electrical connection between said contact arm and an external circuit element.

18. in an electrical musical instrument having keys, and strings mounted on said keys, electrical pickups mounted on said keys, each having an arm connected to an external contact element by means of a spring, said external contact element comprising an inner metallic channel to which said spring is attached, an outer metallic channel supported on the instrument and grounded, and insulative means separating said channels,

References Cited in the file of this patent UNITED STATES PATENTS 272,314 Parsons et al Feb. 13, 1883 855,056 Hirschfeld May 28, 1907 2,293,372 Vasilach Aug. 18, 1942 2,767,608 Miessner Oct. 23, 1956 2,799,196 Alvarez July 16, 1957 FOREIGN PATENTS 164,349 Germany Oct. 27, 1905 509,259 Belgium Aug. 16, 1952

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