US2845829A - Key action for musical instrument - Google Patents

Description

Aug. 5, 1958 B. F. MIESSNER KEY ACTION FOR MUSICAL INSTRUMENT Original Filed Oct. 3, 1950 7 Sheets-Sheet 1 N WW KN B. F. MIESSNER KEY ACTION FOR MUSICAL INSTRUMENT Original Filed Oct. 3, 1950 Aug. 5, 1958 7 Sheets-Sheet 2 BENJAMIN F. MESS/v55 INVENTOR- T 70/? N Y5 Aug. 5, 1958 B. F. MIESSNER 2,845,829

KEY ACTION FOR MUSICAL INSTRUMENT Original Filed 001:. s, 1950 7 Sheets-Sheet 3 2/ lmu vli q HEM/A M/N- F. M/ESSNER IN V EN TOR.

' TTORN s Aug. 5, 1958 B. F. MXESSNER 2,845,829

KEY ACTION FOR MUSICAL INSTRUMENT Original Filed Oct. 3, 1950 7 Sheets-Sheet 4 I26 /22 I I /23 25 j /2/ /fig [25 f 7% 1 5" BE/VJA MIN /-7 M/ESS/VER I N V EN TOR.

BY 2 %4Q%my M A TOR/V 5 Aug. 5, 1958 B. F. MIESSNER 2,845,829

.KEY ACTION FOR MUSICAL INSTRUMENT Original Filed Oct. 3, 1950 7 Sheets-Sheet 5 AMPL TUDE M tom 5 .1.- l5 BE/VJA Ml/V M/ESS/VER I N V EN TOR.

Aug. 5, 1958 B. F. MIESSNER 2,845,829

'KEY ACTION FOR MUSICAL INSTRUMENT Original Filed Oct. 3. 1950 7 Sheets-Sheet 6 /5o I7 /52 L /62 A we HEM/A m/v F. M/ESS/VER INVENTOR.

Aug. 5, 1958 B. F. MIESSNER 2,845,829

KEY ACTION FOR MUSICAL INSTRUMENT Original Filed 001:. 3, 1950 7 Sheets-Sheet 7 BENJAM/IV F: M/E55/VER 7E -7 INVENTOR.

United States Patent KEY ACTION FOR MUSICAL INSTRUMENT Benjamin F. Miessner, Harding Township, Morris County, N. J., assignor to Miessner Inventions, Inc., Harding Township, Morris County, N. .I., a corporation of New Jersey Continuation of application Serial No. 188,106, October $619550. This application October 28, 1954, Serial No.

6 Claims. (Cl. 84-258) This invention relates to musical instruments and more particularly to novel arrangement for exciting tuned vibrators through the action of suitable mechanisms controlled by playing keys of a keyboard.

In musical instruments such as the piano and harpsichord the vibrators are excited, or set into vibration, by a hammer or a plucker and the vibrations are terminated by separate damper mechanisms upon release of the playing key. Additionally, other controls are provided for holding certain or all of the dampers away from the tonedamping positions while still providing control of the hammer or plucker mechanism through the keyboard.

In the conventional piano, a hammer strikes a string thereby deflecting the string at the point of contact. The string deflection is, of course, a maximum at the point of contact with the exciter (referred to as the striking point) and the amount of string deflection decreases at points more distant from such striking point. The sudden blow sets up a longitudinal wave motion which travels in both directions along the string, reaches the terminating ends, and is referred back to the opposite ends. Standing waves develop in the string producing nodes and antinodes of vibration.

Another entirely unrelated system of partial vibrations is also set up in the string by the hammer blow. This vibration is of purely longitudinal type as contrasted with the desired lateral type. It travels molecularly along and through the string to both terminations, is there re-' fiected, and so continues back and forth in both directions until it finally is damped out. Its frequency is determined chiefly by the material of which the string is made, the string length and, to some extent, by the string tension. While its partial vibration frequencies are, unlike those of the lateral vibrations, exactly in tune with one another, this whole system of partials is totally unrelated harmonically to the lateral system of partials. Its fundamental pitch is generally much higher than that of the lateral system of partials, being, typically, about octaves or so higher and it manifests itself to the ear as a clang," wolf tone, iron, guts, or etc., terminology well known in the art. It is wholly undesirable and has never been eliminated in the best of pianos.

A third source of sound in such percussively-excited instruments is a broad, continuous band of frequencies, which are heard as noise, due to the physical impact of the hammer against the string. Where, as in the high treble, the hammer strikes the string very close to the bridge, the hammer blow is, in eflect, largely coupled directly to the bridge so that its noise is like that produced by striking the bridge, or the sound board, or the string frame, directly. Obviously, this sound is wholly unrelated to the desired, lateral vibration tone spectrum of frequencies.

If the hammer, as conventionally employed, is covered thickly with felt, or other resilient material, it will remain in contact with the string for some period of time and thus acts as a damper until it is thrown back by the or 2,845,829 1C6 Patented Aug. 5, 195

reaction of the string. This contact time period will vary with the mass, kinetic energy, and resiliency of the covering material of the hammer, and with the mass and compliance of the string. In general, the hammer mechanisms are designed to provide variable kinetic energies in the ranges suitable for the particular strings with which they are used. The hammer mass is made of a low order while the hammer velocity is made fairly high so that the hammer remains in contact with the string for only a short time interval. The covering material is made variable in resiliency, to some extent, by ironing, needling, or partial impregnation with thin 'cellulosic cement, as a control of tone brightness or dullness and for regulating all tones into a smooth progression of quality.

If the hammer remains in contact with the string for only /2 cycle of its fundamental frequency, that componentl will suffer only small damping-out of its hammerimparted energy of vibration. However, for the second vibration partial II the hammer contact time is equal to one full cycle and, consequently, this partial will sufiier a greater degree of damping. The damping increases for vibration partials of higher order so that the tenth partial X is damped through five cycles of vibration.

Since piano hammers remain in contact with the associated string for much longer periods than /2 cycle of their fundamental frequency, especially in the high treble region, this hammer damping is a very pronounced factor which governs the harmonic richness of the output tone and also, to a large degree, the tone power or volume. For example, if the hammer for string C 88, having a fundamental vibration frequency of 4186 cycles per second, remains in contact with the string for a time period of only 0.1 seconds, its damping influence will act on the string through 418.6 cycles of its fundamental frequency (partial I), for 837 cycles of its vibration for partial II, 1256 cycles of its vibration for partial III, and so on. Since this is a typical case, the tones of a tensionedstring piano, in the high treble regions, are relatively deficient in overtones. These tones are also relatively weak. This is due to the fact that the hammer imparts energy to the string during the first cycle after which the string returns considerable of its energy to the hammer in accelerating the hammer in the reverse direction. Typical curves of hammer motion, for high velocity, indicate that the velocity of the hammer as it leaves the string is about 0.8 to 0.9 of its striking velocity. Since kinetic energy is equal to MV and the mass M is constant, the energy returned to the hammer by the string is from 35 to of the energy which the hammer delivers to the string. At low hammer velocities the return percentage is still higher. So it is seen that of the total energy delivered to the, keys by the fingers only a relatively small percentage remains in the string available for its vibration. Pianos, therefore, require a very considerable amount of work by the performer in supplying energy, most of which is wasted. Consequently, piano tones, especially in the treble region, are relatively weak and blanketed by the relatively large, wide-band noise frequencies heard as hammer thump, crack, percussive noises, etc., according to various accepted descriptive terms.

It may here also be pointed out that if the hammer remains in contact with the string during more than one cycle of its vibration rate at any particular, partial frequency, the frequency of that partial will be higher during the hammer-contact cycles than after the hammer leaves the string, due to the fact that the hammer acts as a new bridge point for the string. Under this condition the active length of the string is divided into two sections, one of which extends from the hammer to the far bridge and the other of which extends from the hammer to the near bridge-terminating end.

The tone-terminating dampers, as distinguished from the temporary damping influence of the exciter-hammers, above described, are separate devices with very soft, relatively long (axially with respect to the string) pads. These pads are moved by key depression, from normal string-contact positions to positions clear ofthe vibrating string before the hammer strikes the strings. Conversely, when the keys are released these pads move against the string and quicklydamp-out the string vibrations.

In the harpsichord a plucking type of vibration exciter is employed. A plectrum is normally out of contact with the string and when a playing key is depressed the plectrum rises to touch the string, continues onward to deflect the string to some fixed maximum deflection when the plectrum suddenly releases the string so that its own built-up potential energy causes it to vibrate. While the overall dynamic effects may be very similar to that of the percussively-excited piano string, however, in the harpsichord arrangement there is no damping of the string vibrations whatever-by the plectrum exciter which is freed of string contact the instant the string oscillation starts. Furthermore, there is no longitudinal system of vibrations (characteristic of hammer excitation) set up by the plectrum, nor is there produced a broad-band of noise when the plectrum releases the string. A very low order of lateral string vibrations, at a higher than normal frequency, is setup when the plectrum firsttouches the string, but this is inconsequential and is immediately followed by the very much louder lateral vibrations when the string is released.

Contrasted with the piano tone, the tone of the harpsichord is extremely rich in partial frequencies due chiefly to the absence of the hammer damping influence. While the dynamic power of a piano tone is controllable by velocity of. key depression and the resulting variation of velocity and kinetic energy delivered to the string by the hammer, such characteristic is lacking in the harpsichord. The harpsichord tones are, therefore, uniformly loud irrespective of the strength or the velocity of the key blows, similar to those of organ tones. As in the piano, the harpsichord strings are provided with separate dampers for silencing the strings when the playing keys are released.

While the piano, due to its'very important touch-responsive, dynamic control of tone, is much preferred over the harpsichord it, nevertheless, possesses many disadvantages such as:

(1) The undesirable damping of the string vibrations by the hammers;

(2) Its unavoidable production of the longitudinal, inharmonic vibrations; and

(3) Its large blanket of impact noises particularly noticed in the upper register.

The present invention is directed to the provision of a novel and exceedingly simple excitation method and apparatus combining the advantages of a plectrum and the desirable characteristic of hammer impact.

An object of this invention is the provision of an excitation arrangement for musical instruments in which the vibrators are excited by normally deflecting them and then releasing them, whereby the vibrators vibrate due to their own potential energy derived from such deflection.

An object of this invention is the provision of a vibrator exciter of the key operated type, wherein the vibrator is deflected and then released at varying velocities in response to key-depression velocities, thereby regulating the amplitude of the vibrator vibration.

An object of this invention is the provision of a vibrator-exciting device that deflects the vibrator in response to key depression and which also serves as a vibration damper for terminating the vibrations of the vibrator.

An object of this invention is the provision of a keyresponsive mechanism for exciting vibrators which 4 mechanism combines the advantages of a plectrum exciter and a hammer impact exciter.

An object of this invention is the provision of an exciter-damper device for vibrators comprising a resilient member normally deflecting the vibrator from its vibrational axis, and means actuated by a playing key for removing the resilient member from the vibrator, whereby the stored potential energy of the vibrator will set the latter into vibration.

An object of this invention is the provision of a vibrator exciter and damper comprising a member encircling the vibrator, a resilient pad normally deflecting the vibrator, means for removing the resilient pad from contact with the vibrator, and means eflective upon such removal of the resilient pad to impart a slight plectrum action to the vibrator.

An object of this invention is the provision of an exciter-damper for vibrators comprising a closed frame surrounding the vibrator, a relatively-hard pad of resilient material secured within the frame, a vibrator-accommodating opening in the said relatively hard pad, said opening having a constricted entrance slightly smaller than the width of the vibrator, and a pad of relatively soft material secured adjacent to the first-mentioned pad.

An object of this invention is the provision of a keyresponsive' instrument having tone-producing vibrators, said instrument comprising key-actuated exciters for imparting a plectrum action to the vibrators, and for selective damping of the vibrator oscillations, and selectively-operated means for rendering ineffective the damping of the vibrator oscillations. An object of this invention is the provision of a musical instrument comprising a vibrator for tone-generation, an exciter-damper having a normal position wherein said exciter-damper deflects the vibrator from its vibrational axis, a playing key, means for moving the exciter-damper upon key depression, means forming part of the exciterdamper for exciting the vibrator upon such key-actuated movement of the exciter-damper, and means efiective upon removal of the key depression for returning the exciter-damper to its normal position. An object of this invention is the provision of a piano wherein excitation of the tone-generators is accomplished by a novel exciter-damper device and including novel means for providing the loud pedal and sostenuto effects of'a conventional piano,

'An object of this invention is the provision of a novel key-actuated exciter-damper device for setting a vibrator into vibration in combination with novel means for imparting to said exciter-damper device a velocity exceeding that of the key depression.

These and other objects and advantages will be apparent from the following description when taken with the accompanying drawings illustrating several embodiments of the invention. The drawings are for purposes of illustration and are not to be construed as defining the scope or limits of the invention, reference being had for the latter purpose to the appended claims.

In the drawings wherein like reference characters denote like parts in the several views:

Figure l is a fragmentary, side-view of a tensionedstring piano, with certain parts in cross-section, and illustrating the general arrangement of the parts in an instrument made in accordance with this invention;

Figure 2 is a cross-sectional view taken along the line A-A of Figure 1 and drawn to an enlarged scale to show the construction of one form of my exciter-damper device;

Figure 3 is similar to Figure 2 and showing another construction of the exciter-damper for imparting a slight amount of plectrum action to the tensioned string;

Figure 4 is a cross-sectional view taken along the line B-B of Figure 3 and illustrates the normal, deflected position of the tensioned string;

Figure 5 is a fragmentary view, similar to Figure 3,

showing the vibration of the tensioned-string upon excitation by the vibration damper;

Figure 6 is a fragmentary view, similar to Figure 3, but showing an exciter-damper designed for use with a vibratory reed;

Figure 7 is similar to Figure 6 illustrating the vibration of the reed upon excitation by the exciter-damper;

Figures 8 and 9 are fragmentary, side views, with certain parts in cross-section, showing an arrangement for obtaining the sostenuto eifect in an instrument provided with my novel exciter-damper;

Figures 10 to 12 illustrate an arrangement for continuing the vibrations of excited vibrators even though the playing key has been released, this effect corresponding to the loud pedal effect of a piano;

Figure 13 illustrates another arrangement for obtaining the effect provided by the construction shown in Figures 10 to 12;

Figure 14 are curves showing relative string vibration and the motional velocity of the exciter-damper to produce string vibration of maximum amplitude;

Figure 15 illustrates a lever-arm and hammer arrangement for increasing the motional velocity of the exciterdamper in response to a relatively lower key-depression velocity;

Figure 16 is somewhat similar to Figure 15 but in this case the exciter-damper responds directly to movement of the playing key, whereas in the Figure 15 arrangement movement of the exciter-damper is the result of an impact force;

Figure 17 is also somewhat similar to Figure 15 but showing a different arrangement of the lever arm;

Figure 18 is a side view, with certain parts in crosssection, of a simplified exciter-darnper device;

Figure 19 is a cross-sectional view taken along the line A-A of Figure 17.

Figure 20 is a fragmentary front view showing a novel arrangement for retaining the struck playing keys in the depressed position to thereby hold the eXciter-damper device clear of the vibratory string; and

Figures 21, 22 and 23 are sectional views taken along the lines AA, BB and CC, respectively, of Figure 20.

While I shall describe my novel exciter-damper device specifically with respect to a string type vibrator, it will be apparent that it is equally applicable to and useful with other vibrators of readily yieldable type such as reeds, thin rods, etc. The novel exciter, while fully capable of exciting maximum vibrations in a string or other vibrator, acts in a reverse manner to that of hammers and somewhat similar to a plectrum. Unlike the hammer, however, which introduces its energy into the normally-undeflected string very rapidly by percussion, my exciter introduces energy relatively slowly into the string and stores it there as potential energy for the normal condition ready at all times for instant release on key depression. It is, therefore, quite the opposite of the percussion type of excitation. In fact, it is more like the plectrum action in that it deflects the string and releases it. The deflection is relatively slow and instead of rapidly deflecting and releasing the normally undeflected string in one rapid sequence of operations, the exciter slowly deflects the string for the normal condition and releases it more or less rapidly when the playing key of the keyboard is depressed so that there is a notable and important distinction in method and apparatus. Additionally, I may utilize the string deflector as a tone-terminating damper. By these means I provide an extremely simple and inexpensive exciter and damper for string, or other flexible, vibrators.

For electronic instruments utilizing strings, reeds, etc., this normally-deflected vibrator principle olfers an important advantage since the spaced capacitive or other pick-up device may be so placed that the spacing between such pick-up and the vibrator is relatively large. By this 6 arrangement the normal capacity between a group of vibrators and pick-ups is much reduced. This normal capacity determines the amount of modulation (of the total capacity) producible by a single vibrator and pickup electrode, and this, therefore, determines the translation etliciency. For example, if 88 vibrator reeds and their closely-spaced, end wise pick-ups have an aggregate capacity of 500 mmf., vibration of one of the reeds at any maximum amplitude (including removal of the reed entirely) could change only 1/88 of the total capacity. If, however, all reeds are normally deflected from their associated, axially-positioned pick-ups by an amount equal to the maximum desired vibration amplitude, the normal capacity between all reeds and pick-ups may be reduced to the order of 1% or less of the assumed aggregate capacity of 500 mmf. or to a total capacity of only 5 mmf. Since the maximum capacity of each reed and pick-up is 500/88:5.68 mmf., and since the residual capacity of all the parallel-connected reeds and pick-ups is 5 mmf., it is seen that the capacity modulation of the total capacity produced by a single reed, at the assumed maximum vibration amplitude, would vary this capacity between a minimum of 5 mmf. and a maximum of 10.68 mmf., thus yielding a capacity modulation of over 50%. In the first mentioned, conventional arrangement, the modulation is only 1 part in 88 or about 1.4%. The translating efliciency of the normally-deflected vibrator excitation system, as will be described in detail hereinbelow, is, therefore, 50/ 14 or 35 times as great as that for percussively or plectrum excited vibrators with similarly located pick-ups.

Referring now to Figure 1, there is shown a side view, with certain parts omitted and others drawn in crosssection, illustrating the general arrangement of the parts in a piano made in accordance with this invention. The instrument may be housed in a cabinet comprising a base 20, a lid 21, a slidably-removable front board 22, and a back (not shown), said cabinet being supported by suitable legs 23 provided with castors 24, as is common in piano construction. The keyboard of the instrument is of the conventional type and includes a series of keys such as the key 25. Each key is retained in position by a pivot pin 26 extending upwardly from the base 20 and passing through a tapered hole in the key, substantially as shown. Smooth, rocking motion of the key, in response to linger pressure applied to the outer end thereof, is provided by a curved pivot rail 27 spaced from the key by a resilient washer 28. Excessive lateral motion of the key is prevented by the key guide pin 29 that is secured to the base 20 and extends into a bore in the key. The pad 3t) serves to limit the downward motion of the key and to deaden the contact noise when the key is depressed vigorously. A wood strip 31, secured to the cabinet base by a screw 32, prevents the entrance of foreign objects between the key and the base and also enhances the general appearance of the instrument. The inner end of the key has a resilient pad 33 secured thereto, as by cement, which pad cooperates with a similar pad 34 secured to the base to prevent rebounding of the key as it returns to its normal position. Alternatively, only the pad 34 comprising a long, continuous strip for all the playing keys, need be used, as in a conventional piano.

The tensioned-string 39 is stretched across a rather massive iron string frame 40, one end of the string being fastened to a string hitch pin 41 and the other end entwined around the tuning pin 42, said pin being threaded into the iron frame. A fixed bridge 43 secured to the frame 40 and a bridge 44 carrying the bridge pins 45, establish the effective length of the string. The bridge 44 is supported by and in direct glued contact with the sound board 46, whereby vibrations of the string are imparted to the sound board to produce an audible tone, as is well known. As in conventional practice, .the sound board is arched between heavy board planks 47, 48 the latter being rigidly secured to the iron frame 40 and .through offset, end-sections of. each. Cemented to the inner surface of the upper member 50 is a pad 54 of resilient material, such as felt, sponge rubber or, etc., said pad being provided with a triangular opening, substantially as shown, for accommodation of the string 39. The threaded rod 55, having a head 56 on one end, passes through a hole in the guide rail 57 and the sound board 46, and is threaded into the lower, curved member 51. It may here be pointed out the guide rail 57 extends substantially the entire width of the piano and is secured firmly in a fixed position below the sound board 46. The helical spring 58, disposed between the guide rail and the rod head 56 serves to bias the entire exciterdamper downwardly so that the nut 60 is in contact with the resilient pad 61 carried by the guide rail 57. Consequently, when the playing key is in the normal position, as shown in Figure 1, the string 39 is engaged by the pad 54 and the string, therefore, is deflected from its otherwise normal, straight position. The maximum downward normal deflection of the string may be regulated by the nut 60 threaded on the rod 55, said nut seating on a resilient pad 61 secured to the upper surface of the guide rail 57. When the playing key is struck the entire exciter-damper moves upward, thereby removing the string-deflecting pressure and allowing the string to vibrate. After key depression and initiation of string vibration, release of the playing key results in a downward motion of the exciter-damper, under the action of the coiled spring 58, whereby the V-shaped pad 54 is brought into contact with the string 39 thus terminating the string vibration and again deflecting the string in preparation for a subsequent depression of the playing key.

The above described arrangement is suitable only for strings and vibrators of relatively low frequency and for low to moderate vibration amplitude since the maximum velocity of removal of the exciter-damper device is limited to the maximum velocity of key depression.

An adaptation of the same general principle for more general application can be accomplished by providing a small amount of plectrum action when the string is excited into vibration. Such plectrum action, in combination with the normal deflection pressure on the vibrator, provides a certain minimum of vibrator vibration irrespective of the velocity of key depression. Performance somewhat of this character is achieved in the plectrum action of the harpsichord, but at the expense of an undesired, uniformly-loud output tone at all velocities of key depression. In my novel arrangement the amount of plectrum excitation is very small and just suflicient to produce a very low minimum amplitude of vibration for each complete key depression. Such performance is never attained in piano actions at key depression velocities below a set, appreciable minimum so that in the rendition of pianissimo passages even the best artists frequently have blank tones due to too low depression velocity.

An exciter-damper arrangement incorporating a small amount of plectrum action is illustrated in Figures 3 to 5. Figure 3 is somewhat similar to Figure 2. In this case, however, the looped exciter-damper is formed of two strips 70, 71, of metal or plastic. The curved portions of the metal strips are secured together by a screw 72 and nut 73 while the lower, flat portions may be secured together as by welding at the points a, b and c. The ends of the strips are offset at right angles to provide the head 74 that bears against the resilient pad 59 carried by the playing key (Figure 1). This head also serves to support a flat felt washer 75 and the coiled spring 76 is retained between this washer and a similar washer 77 that abuts against the lower surface of the guide rail 57. A portion of the edges of the straight section of the strips 70, 71 is provided with a thread for accommodation of the regulating nut 78 that seats against the resilient pad 79 and serves the purpose explained above with reference to the nut 60 of Figures 1 and 2.

Cemented to the inner surface of the loop formed by the strips 70, 71 is a member 80 of hard felt or soft rubber and having a circular opening 81 therein said opening lying on the center line of the loop. This opening 81 is slightly larger in diameter than the vibratory string 39, but its dimension across the opening in the arcuate face of the member 80 is slightly less than the string diameter. Hence a definite small amount of pressure is required to force the string 39 into and out of the opening 81. Two arcuate members 82, 83, of relatively resileiut material, such as soft felt, are cemented within the loop and to the member 80. These members are spaced to clear the opening to the circular opening 81 in the relatively hard member 80. As shown in Figure 4, which is a central cross-sectional view taken along the line B-B of Figure 3, the relatively soft and wide members 82 and 83 have a dimension along the string axis which is fairly long so that these members damp the string along an appreciable portion of its length when i the playing key is released, as in conventional piano dampers. This prevents a sharp line of division of the string into two segments. The relatively hard member 80, however, is relatively short along the string axis to provide definite such string segments.

In the Figure 3 illustration the exciter-damper is shown in its normal at-rest position and the string 39 lies within the opening 81. In this position of the exciter-damper device the string is deflected downward due to the action of the coiled spring 76. When the playing key is depressed the exciter-damper is raised relative to the string. Until such time as the string returns to its striaght line position nothing happens. However, as the exciter-damper continues to move upward beyond this point the string tends to leave the opening 81. However, since the entrance of this opening is slightly less than the string diameter, a small upward deflection is imparted to the string before it slips through this opening into the relatively large clearance area 85 within the loop. Although the material of the member 80 is yieldable it is sufiiciently rigid to impart a slight plectrum action to the string as the latter snaps through the narrow passageway between the opening 81 and the clearance area. In other words, the member 80 has sufficient radial yield to allow the string to pass into and out of the hole 81 and sufficient stiffness to hold the string there against the small deflecting force acting upon the string as the device is moved upward.

Once the string 39 snaps out of the opening 81 it is set into vibration. It will be apparent that at low key depression velocity the plectrum action imparted to the string as it leaves the opening in the member 80 is very small and, consequently, the string vibration amplitude is small. The clearance area 85 is designed so that the string will not come into contact with any part of the exciter-damper at maximum vibration amplitudes in either the vertical, horizontal or orbital modes. This is shown in the fragmentary view of Figure 5 wherein the maximum vibration of the string 39, in all planes, falls within the dotted line 86.

When the playing key is now released the exciterdamper moves downward under the action of the coiled spring 76. The vibrating string vibrates into contact 9 with the resilient, relatively- soft pads 82, 83 which damps the string vibration. At the termination of the string vibration the string is forced through the passageway between the member 82, 83 and into the circular opening 81 where it comes to rest as the exciter-damper descends further. It will be understood that the lower, soft- material pads 82, 83 are relied upon for damping the string vibration to substantially zero before the string enters the slot aperture and that no biasing deflection of the string is produced thereby. If such biasing deflection occurred while the string is still vibrating the frequency of the string vibration would be increased as this damper would act momentarily as a new bridge point thereby effectively shortening the vibration length of the string.

When the exciter-damper is made of strip material as described with reference to Figures 35, the hole 90 in the guide rail 57 may have a rectangular form corresponding to the cross-sectional form of the shank portion of the exciter-damper. In such arrangement there can be no substantial turning of the exciter-damper and, therefore, a proper positioning of the device with respect to the string is maintained at all times. To promote smooth, noise-free action, the rectangular hole in the guide rail may be lined with felt 91.

The exciter-damper device is adapted for use with other than tensioned-string vibrators. Figures 6 and 7 which are fragmentary views showing only the upper portion of the device, show an exciter-damper designed for use with vibratory reeds. Here the loop may be formed by joining together strips 100, and 101 in a manner similar to that described with reference to Figure 3. The upper, relatively hard pad 102 has a constricted opening 103 therein, the length of said opening being slightly less than the width of the flat reed 104. Soft pads 105, 106 of vibration damping material are cemented to the pad 102 and to the inner surface of the strips 100, 101, substantially as shown. As the exciter-damper is raised, in response to depression of the playing key, the reed snaps through the constricted passageway into the clearance area 107, the latter being of a size to permit unrestricted, maximum vibration of the reed, as shown by the solid and dotted reed positions in Figure 7.

As has been stated above, the normally-deflected vibrator principle is very advantageous in electronic musical instruments, wherein the vibrator vibrations are translated into musical tones by means of capacitive pick-up and associated translating, amplifying and electro-acoustic apparatus. Inasmuch as the vibrator normally is deflected from its vibrational axis, the aggregate capacity of all vibrators and associated pick-ups is materially reduced since such pick-ups are best located in line with such vibrational axis. Also, when the exciter-damper device releases the vibrator (thereby exciting the vibrator into vibrations about its longitudinal axis), the capacity between the vibrator and its pick-up increases considerably.

Consequently, the modulation of the total capacity, by vibration of a single vibrator, is substantially greater than s possible in the conventional arrangement wherein the at-rest position of the vibrator normally corresponds to the axis about which the vibrator vibrates upon excitation. Therefore, it is apparent the excitation of the vibrators by the novel means herein described results'in a greatly increased translation efficiency of the electronic apparatus as a whole.

My novel exciter-damper arrangement also lends itself to mechanical combinations affording playing techniques corresponding to those of the conventional piano. A simple arrangement may be provided to hold the exciter-damper in the upper position, after tone excitation, without continued pressure on the playing key. Such operation, set into effect by a foot-operated pedal, corresponds to the sostenuto control used in grand pianos.

Figure 8 is a fragmentary view, similar to Figure 1, illustrating a simple arrangement for obtaining the sostenuto effect. The exciter-damper device is constructed as shown in Figures 3 and 4, but only the lower portion thereof is shown in Figure 8. Such lower portion comprises the straight sections of the strips 70, 71 together with the coiled spring 76 having an end abutting against the fiat washer 75 that is retained by the offset end sections of the members 70, 71. Whereas, on the Figure 3 embodiment these offset end sections are of equal length to provide the leg or base 74 (Figure 3), in the Figure 8 construction the offset section 106 of the strip 71 is made somewhat longer to extend beyond the end of the playing key 25. Disposed above the section 106 is a slide plate 107 that is slidably secured to a fixed rail 108 by the screw 109 and the flat washer 110. It may here by pointed out that the fixed rail 108 extends across the full complement of playing keys as does also the slide plate 107. This slide plate is normally biased away from the exciter-damper devices by a series of coiled springs. One such spring 111, having its ends secured to an eye bolt 112 carried by the plate 107 and an eye bolt 113 carried by the fixed member 108, is shown in the Figure 8 illustration. Horizontal movement of the slide plate is accomplished by depression of the pivoted foot-pedal 114, the latter being connected to the eye bolt of the slide plate by a cord 115 operating in the pulley 116. As shown in Figure 8, the slide plate normally is biased by the springs 111 so that the edge of said plate lies clear of the edge of the leg 106 of the exciterdamper and, consequently, the latter is free to move vertically in response to depression of the playing key in the normal manner. When the sostenuto effect is desired the playing keys corresponding to the desired musical chord are depressed and held momentarily until the footpedal 114 is pressed downward. This condition is shown in Figure 9 wherein operation of the foot-pedal has moved the slide plate 107 so that its forward edge lies below the leg 106 of the exciter-damper. When the playing keys are released, while maintaining the foot-pedal in the depressed position, the legs 106 of the actuated exciterdampers will rest upon the slide plate 107 and in this position of the exciter-damper the string is free for unrestricted vibration, as shown and described with reference to Figure 5 above. Such string, or strings, continue vibrating so long as the foot-pedal is held down until, of course, the vibrations terminate in a natural manner. Upon release of the foot-pedal the slide plate moves to the right under the action of the coiled springs, thereby allowing the exciter-dampers to return to the normal position whereby the string vibrations are damped and the string is placed in the normally-deflected position as described hereinabove.

An arrangement for producing, in this apparatus, the loud pedal elfect of a conventional piano is illustrated in Figures 10 to 12. Here a fixed rail 120, that may be secured to the bottom 20 of the cabinet, extends the full length of the entire complement of playing keys. A slide plate 121 having a length corresponding to the rail '120 is slidable on the top surface of the said rail being secured to the latter by a series of screws 122 and washers 123. The slots through which the screws pass through the slide rail are elongated thereby providing stops to limit the horizontal, sliding movement of the plate 121. In the normal, ineffective position, the slide plate 121 occupies the position shown in Figure 10 being biased to the right by a series of leaf springs 124 secured to the fixed rail by the screws 125. In this position the edge of the slide plate lies spaced from the vertical line of travel of a flat spring 126 that is attached to the leg extension 106 of the exciter-damper. The leaf spring 120 which may be attached to the leg extension 106 as by welding at the point d, forms a flexible arm which is free to move downward relative to the extension 106, but cannot move upward because its length is materially reduced by the overlapping of the extension 106, as will become more apparent hereinbelow.

When the slide plate 121 occupies its normal position,

11 Figure. 10, the entire exciter-damper is free to move vertically, that is, upward in response to depression of the playing key and downward under the action of the coiled spring 76 (see Figure 1) when the playing key is released. The slide plate 121 may be moved to the left (toward the exciter-damper) by a pedal action similar to that described with reference to Figures 8 and 9, such mechanism being omitted from Figures lO-12 for purposes of clarity. When the slide plate 121 is so moved to the left its forward edge crosses the vertical line formed by edge of the spring 126 as it moves upward and downward with the exciter-damper. Thus, when the exciter-damper is raised, in response to depression of the playing key, the leaf spring 126 contacts the lower surface of the slide plate 121 and is bent downwardly, as shown in Figure 11. In this direction of flexure the spring 126 hasa relatively long, effective length and, therefore, it deflects downwardly (away from the extension 106) quite readily. Such deflection of this spring continues as the exciter-damper moves further upward until the said spring snaps past and over the slide plate 121. Now, when the playing key is released, as shown in Figure 12, the exciter-darnper is held up by the spring 126 resting on the upper surface of the slide plate 121. In this position of the exciter-damper the vibratory string is free to vibrate as shown in Figure 5. When the footpedal is released, the springs 124 move the slide plate 121 tinuance of all tones which have been started by key depression but will not permit a re-excitation of these particular strings until the loud pedal is released. Thus, a series of tones, say a long arpeggio of harmonically-related tones, may be sounded and held, while other tones are played, without undamping all the strings of the instrument, as is the case in a conventional piano. When the sustaining or loud pedal of a conventional piano is depressed every string in the instrument is free to vibrate as such action removes all the tone dampers. Therefore, any struck strings may communicate their vibrations to other strings by resonance action through the mechanical coupling of the bridge and sound board. Furthermore, since a very appreciable part of the hammer impact is coupled directly into the bridge as an impulse, this sets all strings into vibration, especially those close to the struck strings since here the degree of coupling is the greatest- Thus, when a number of strings (throughout the compass of the instrument) are struck, every string in the instrument is effectively, impulsively excited by the shocks communicated to the bridge at various points. The resulting complex sound takes on a strong characteristic of noise, which is merely a broad, continuous band of frequencies. Additionally, of course, the struck strings, which receive considerably more energy from their hammers than do the large number of other, unstruck strings, vibrate at a greater amplitude so that the tones of these strings stand out above the general noise level of all the unstruck strings, at least so long as their amplitude of vibration is high. This excitation of the undamped, unstruck strings serves no real musical purpose. It produces only a strong background of roaring noise which blankets the desired tones.

The arrangement which I have just described, therefore, produces much better musical results than does the conventional sustaining pedal of the piano. Its only limitation, not found in the conventional piano, is that strings once struck cannot be restruck until the sustaining pedal has been released (slide-plate 121 removed to its right hand position as shown in Figure 10). This is not considered of serious importance in view of the other advantages.

Figure 13 is similar to Figure 11 but shows a different arrangement of the flexible leaf spring for obtaining the sustaining or loud pedal effect. In this case the leaf spring 126' is secured to the upper surface of the slide plate 121 instead of the leg extension 106 of the exciterdamper and the general action of the apparatus is similar to that described with reference to Figures 10-12.

As previously mentioned, the complete apparatusdescribed to this point is useful in connection with vibrators that vibrate at relatively low velocity, at least at that part of the vibrator Where the exciter-damper is applied. Since strings, or clamped-free reeds, have a zero mean velocity at their fixed ends, it is desirable that these exciter-damper devices be applied at points relatively close to such vibrator ends. However, the degree of pressure required at such points for the production of the desired maximum amplitude of vibration is greater than at points further along the vibrator, that is, at the free end of clamped-free vibrators and at the center of tensioned strings. Since the pressure of the spring, which maintains the exciter-damper and the vibratory string in a normally-deflected position, must be overcome by the playing key, and since it may be desirable (particularly for high-velocity vibrations of the higher pitched vibrators) to provide higher velocities of release of the stringdeforming exciter-damper, it may be desirable to utilize a velocity step-up lever arrangement to accomplish these ends.

In order to realize an amplitude of string vibration equal to the amplitude of deflection at the point of application of such deflecting force, the deflecting device must be removed at a velocity at least equal to or slightly in excess of the velocity of the string through one-half /z) of its vibration cycle, such cycle beginning at a point of maximum amplitude. This will be clear by reference to Figure 14. If the string be normally deflected downward by the excited-damper to an amplitude of minus ten (10) units and if it be desired to vibrate the string initially at this amplitude at the point of stringdeflection, the string-deflecting exciter must move away from the string at a velocity at least as great (and preferably slightly greater) than the unrestrained string velocity. The unrestricted string velocity varies along the String Vibration Curve from zero velocity at the point C to a maximum velocity at the point D, the velocity reducing thereafter, during the first half cycle of vibration, to zero at the point E. Consequently, the exciterdamper should have a motional characteristic correponding substantially to the curve identified as Exciter- Damper Curve on Figure 14. Point E, for damped oscillations, will always have a lower amplitude than the starting amplitude at point C, the actual difference being determined by the various damping influences acting on the string, such as air damping, physical hysteresis losses of the string, losses at the string end supports, losses in the bridge and sound board, sound radiation losses, etc.

If the frequency of vibration of the string he, say, 500 cycles per second and the peak amplitude of the maximum vibration be 0.1 inch, the string will move from point C, through point D, to point B, a distance of approximately 0.2 inch in of a second with an average velocity of 200 inches per second. This, by actual measurement, is the terminal velocity (as the hammer begins string contact) of a relatively large bass type piano hammer, at maximum velocity, as produced by a very strong key blow through a step-up lever motion between the key and the hammer head of approximately 5 to 1. High treble-range hammers of lighter weight may have terminal velocities of 40050O inches per second. The maximum terminal velocity of the playing key is of the order of /s of these velocities or only 40 to inches per second which is too low for use directly in the exciter-dam-per arrangement herein described when used at some appreciable distance from the fixed end of a vibrator, and when the vibrator must be vibrated 13 at large amplitudes of vibration. rangement, therefore, is provided.

The step-up lever arrangement may have a mass considerably in excess of that of the exciter-damper assembly in order that the exciter-damper may be accelerated very rapidly without appreciably lowering the velocity of the lever arrangement. The exciter-damper device is, therefore, preferably made as light in weight as practicable while retaining sufficient vertical stiffness to absorb a hammers accelerating forces without appreciable deformation.

Figure 15 illustrates an arrangement of a step-up lever arrangement for attaining high-velocity release of the string-deflecting, exciter-damper. The arrangements for the sostenuto and sustaining pedal effects are omitted for the sake of simplicity. This particular arrangement includes a weight at the end of the lever arm for rapidly accelerating the eXciter-damper although such weight is not essential to the operation of the arrangement, as will become apparent hereinbelow. The vibratory string 39, its attaching members and the associated components, are the same as shown in Figure 1 and the exciterdamper device is identified by the numeral 130. The lever action comprises an arm 131 pivotally attached to the flange 132 that is secured to a fixed hammer rail 133 by a screw 134. A weight 135, having a soft pad 136 thereon, is attached to the free end of the arm 131, whereby the arm normally rests upon a soft pad 137 affixed to a block 138 supported by the cabinet base 20. In this arrangement the inner end of the playing key 25 carries a capstan screw 139 adjusted for normal contact with a pad 140 aifixed to the pivoted portion of the arm 131, as shown. When the playing key is depressed the arm 131 is rotated in a counter-clockwise direction and the weight 135 strikes the head 56 of the exciter-damper device causing the latter to move upward and thereby excite the string 39 into vibration, as has already been explained. It will be apparent the velocity-multiplying action of the mechanism is determined by the ratio of the distances of the weight 135 and the capstan screw 139 from the pivot point 143 of the arm 131. Consequently, the weight 135 strikes the ex-citer-damper device at a velocity substantially greater than that of the key depression.

Figure 16 illustrates a similar lever action. In this case the weight carried by the striker arm is dispensed with and the end of the lever arm 131 normally is in contact with the head 56 of the exciter-damper, through the medium of a soft pad 141. This is accomplished by making the supporting rail 142 of proper height. Figure 16 also illustrates a modified construction wherein the sound board 46 terminates in a heavy supporting member 145 which also serves as the guide rail for the support of the exciter-damper devices, substantially as shown.

Figure 17 illustrates a lever arrangement that is more economical of space since the lever arm 131, its supporting members, and approximately one-half of the playing key 25 are telescoped under the string 39. In such arrangement the end of the lever arm rests upon a pad 146 afiixed to the upper surface of the playing key at a point near the key pivot-pin 26. At the moment of key depression the lever arm is set into motion by forces applied by the capstan screw 139 and the key pad 146.

This arrangement is effective to overcome, in the initial instance, the inertia of the lever arm and the exciterdamper, after which an accelerating velocity is imparted to exciter-damper by the multiplying action of the mechanism through the capstan screw. The height of the supporting post 148, carried by the key frame 149 is such that the entire assembly of keys, lever arms, etc., may slidably be removed from the front of the piano after the front panel 22 has been removed and the exciterdamper assembly has been unscrewed, as a whole, from the supporting member 150. For this purpose, the guide A step-up lever arrail 151, supporting the series of exciter-damper devices, is secured to the sound board support by screws 152 that are located between the eXciter-dampers.

From the description to this point it will be apparent my normally-deflected vibrator arrangement operates on a principle which has not been applied, within my knowledge, to instruments of the type employing vibrators for tone generation, and which offers many unique advantages. While the exciter-damper arrangement employing a slight amount of plectrum action is preferred, the simple vibrator deflector and release arrangement, as described with reference to Figure 2, is satisfactory for use in instruments in which there is imparted a certain minimum velocity to the playing keys. It is especially suitable for the excitation of vibrators having small physical dimensions, such as those unsuited for direct, acoustic tone production, but which are highly suitable for use with electronic devices for translating their m1 nute vibrations into musical tones of ample power. In such instruments but small deflecting forces are required and small normal key depression forces are ample for counteracting the vibrator-deflecting forces.

Figures 18 and 19 illustrate a somewhat simplified arrangement for applying a vibrator-deflecting force that is removable upon key depression whereby the vibrator is set into vibration by its stored potential energy. As in the Figure 2 device, the velocity of removal of this normal deflecting force determines the amplitude of vibra tion so that this device is fully touch responsive in key operation, being quite sensitive to low velocities of key depression and producing gradually increasing vibration amplitudes up to the amplitude of reed deflection produced by the exciter-damper deflecting force. As shown in Figures 18, 19, the exciter-damper comprises an L- shaped rod having a threaded end screwed into the playing key 161. The other end of the rod carries a tube 162, made of rubber, felt or leather, that may be secured thereto by cement. The inner end of the playing key has a weight .163 secured thereto by a screw 164, said weight serving to retain the playing key in the normal position as shown in Figure 18. The vertical length of the rod 160 is such that when the playing key is in the normal position the tube 162 deflects the vibratory reed 165 from its normal, straight position, said reed being secured to a reed block 166 as by the plate 167 and the screw 1158. Upon depression of the outer end of the playing key the reed deflecting force is removed and the stored potential energy of the reed sets the reed into vibration, it being apparent the reed vibration amplitude is determined by the velocity of key depression, that is, the velocity of removal of the exciter-damper from contact with the reed. Upward movement of the inner end of the playing key is limited by a key stop comprising a bracket 170 secured to the key frame 171 by the screw 172, said bracket having a resilient pad 173 secured thereto for contact by the similar pad 174 affixed to the key 161, as shown. Upon removal of key depression the exciter-darnper returns to its normal reeddeflecting position whereupon the tube 162 contacts the upper surface of the reed, terminating the reed vibrations and again placing the reed in the deflected, stored-energy position ready for the subsequent key depression.

Figures 20 to 23 illustrate a novel arrangement for retaining a struck playing key in the depressed position to permit unrestricted vibration of the associated vibrator. In Figure 20, which is a fragmentary, front view of a piano, the playing keys 1% and 132. are shown in the normal position, While the playing key 181 is shown in the depressed position. Disposed in front of each key is a flat leaf spring, such as the springs 183, 184 and 185, which are secured at the lower ends to a slide rail 186 by the rivets 187, or other suitable means.

As shown in Figure 21, which is a sectional view taken along the line A-A of Figure 20, each leaf spring, in this case the spring 183, has a head 189 and the slide 15 rail 186 is slidable in a slot 190 that extends the full width of the base 191. The slide rail 186 normally is biased to the left by a compression'spring 192 whereby the leaf springs normally are spaced so. the heads thereof do not interfere with the normal operation of the playing keys, as shown.

When the slide rail 186 is moved to the right, as shown in Figure 22 which is a sectional view taken along the line BB of Figure 20, the leaf springs (spring 184 in this case) are brought up against the front face of the base 191 in which position the heads (189 in this case) overly the front edge of the associated playing keys (181 in this case). Consequently, when the playing key is now depressed the associated leaf spring will latch the playing key in the depressed position and such key will be so held until the slide rail 186 is again moved to the left to remove the leaf spring head (and all others) from contact with the playing keys.

The position of the slide rail 186 may be controlled selectively by the artist by means of a foot pedal; As shown in Figure 23, which is a sectional view taken along the line CC of Figure 20, the foot pedal 195 is pivoted at the inner end 196. A cord 197, or chain, is secured between the foot pedal and the slide plate 186, said cord passing over a pulley 198 that is mounted in an opening 199 provided for this purpose in the base 191. When the pedal 195 is in the normal position, as shoswn in Figure 23, the slide rail 136 is biased to the left by the compression springs 192 of which there are several disposed behind the slide rail at spaced points along the range of the keyboard. When the pedal is pressed downward the slide rail 186 is moved to its active position, as shown in Figure 22, wherein the individual leaf springs will catch such playing keys as are depressed. From what has been explained hereinabove with respect to the operation of the exciter-damper device, it will be apparent that when one or more of the playing keys are retained in the depressed position the corresponding exciter-dampers will remain in a position wherein the associated strings are free to vibrate in the natural, undamped manner. Release of the foot pedal removes the latching leaf springs, whereupon the playing keys return to the normal position permitting the exciter-damper to terminate the string vibration, as has been explained.

A unique advantage of the key-latching arrangement just described is the fact that there is provided a visual indication by the depressed key, of the strings, or vibrators, that cannot be re-excited until the latching mechanism is released.

From the above described invention those skilled in this art will recognize the following advantages of my exciter-damper arrangement for exciting a vibrator into vibration and for terminating such vibrations:

l) I eliminate the damping by the exciter device after imitiation of the vibration,

(2) I eliminate the longitudinal vibrations produced by percussive exciters,

(3) I eliminate the broad band spectrum of noise frequencies produced by hammer excitation,

(4) I retain the full spectrum of string frequencies as with the plectrum exciter,

(5) I retain a full control of tonal dynamics through control of the velocity of key depression,

(6) I utilize the exciter device as a tone-terminating damper.

These and other advantages are secured with an extremely simple, reliable and inexpensive mechanism which requires no intricate devices for the prevention of multiple string excitation for single key blows, and will operate at extremely low or high speeds. The dynamic resistance to key depression, caused by the necessarily very rapid acceleration of relatively large masses in conventional percussively-excited systems is herein greatly reduced, so that much less effort is required of the fingers in playing the instrument,

Having now described my invention in detail in accordance with the patent statutes, various changes and modifications will suggest themselves to those skilled in this art, and it is intended that such changes and modifications shall fall within the spirit and scope of the invention as recited in the following claims. I

Thisapplication is a continuation of my co-pending application Serial No. 188,106, filed October 3, 1950, now abandoned.

I claim:

1. In a musical instrument, the combination of a vibrator, means normally deflecting the vibrator from its vibrational-mean position, a playing key, and means effective upon actuation of the playing key for moving said deflecting means away from its vibrator-deflecting position, said deflecting means comprising a member formed into a loop circumscribing the vibrator, a resilient pad within the loop and an opening in said pad, said opening being sufficiently large to accommodate the vibrator but the entrance thereto being smaller than the dimension of the vibrator transverse to the direction of movement of the deflecting means.

2. In a musical instrument, the combination of a vibrator, a damper normally contacting and damping the vibrator, a support carrying the damper and arranged for movement to remove the damper from the vibrator, and means, also carried by said support, effective in said damper-removing movement of the support to engage and deflect the vibrator and thereafter to release the sodeflected vibrator.

3. In a musical instrument, the combination of a vibrator, a damper normally contacting and damping the vibrator and deflecting it from its vibrational-mean position, a support carrying the damper and arranged for movement to remove the damper from the vibrator, and means, also carried by said support, effective in said damper-removing movement of the support to engage and deflect the vibrator and thereafter to release the sodeflected vibrator.

4. In a musical instrument, the combination of a vibrator, a damper normally contacting and damping the vibrator, a support carrying the damper and arranged for movement to remove the damper from the vibrator, and a pair of yieldable jaws also carried by said support and moved by the support, in said movement thereof, past the vibrator, said jaws being separated by less than the dimension of the vibrator transverse to the direction of said movement whereby to pluck the vibrator during said movement.

5. In a musical instrument, the combination of a vibrator, a damper normally contacting and damping the vibrator and deflecting it from its vibrational-mean position, a support carrying the damper and arranged for movement to remove the damper from the vibrator, and a pair of yieldable jaws also carried-by said support and moved by the support, in said movement thereof, past the vibrator, said jaws being separated by less than the dimension of the vibrator transverse to the direction of said movement whereby to pluck the vibrator during said movement.

6. An exciter-damper device for use with a vibrator, said device comprising a loop of rigid material surrounding the vibrator in a plane substantially normal to the vibrational axis of the vibrator, a resilient pad carried by the loop, a vibrator-accommodating opening in said pad, means biasing the loop to a normal position wherein the vibrator is disposed within the opening in the pad, and means for moving the loop a predetermined distance sufficient to remove the vibrator from the opening in the pad, said opening being provided with a constricted aperture through which the vibrator may pass but which is normally slightly smaller than the dimension of the vibrator transverse to the direction of loop movement.

(References on following page) 17 References Cited in the file of this patent 2,588,295 UNITED STATES PATENTS 2701414 254,910 Blake Mar. 14, 1892 1,169,832 Johnson Feb. 1, 1916 5 922,777 1,824,706 Casciotta Sept. 22, 1931 18 Rowe Mar. 4, 1952 Marshall May 3, 1955 FOREIGN PATENTS France June 18, 194-7

Images