US2489378A - Modulated-oscillator type phonograph reproducing system - Google Patents

Description

Nov. 29 1949 H. P. KALMUS 2,489,378 MODULATED-OSCILLATOR TYPE PHONOGRAPH REPRODUCING SYSTEM Filed. March 31, 1945 2 Sheets-Sheet Fig. 5 44 54' 57 4 42 45 i p 4139 42 43 Fig.4

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MODULATED OSCSILLATOR TYPE PHONOGRAPH REPRODUCING SYSTEM Filed March 51, 1945 2 Sheets-Sheet 2 Fig. 8

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INVENTOR. HENRY P. KALMU 5 M 5.

H/s A TTORNEY Patented Nov. 29, 1949 MODULATED-OSCILLATOR TYPE PHONO- GRAPH REPRODUCING SYSTEM Henry I. Kalmus, Chicago, Ill., assignor to Zenith Radio Corporation, a corporation of Illinois Application March 31, 1945, Serial No. 585,826

5 Claims. (Cl. 179-1004) This invention relates to electro-acoustical devices of the type in which vibration of a member at sound frequencies produces corresponding variations in electrical current in an associated electrical circuit. The invention relates more especially to microphones and phonograph pickup devices and systems in which vibration of a member produces amplitude modulation of a high frequency wave. It is a primary object of the present invention to provide an improved signal translating apparatus of this type.

An object of the present invention is to provide an improved phonograph pickup system especially useful with phonographs, radio phonograph systems or the like.

Another object of the present invention is to provide an improved arrangement of an oscillatmg circuit with a phonograph pickup for pro-- ducing, demodulating and amplifying high irequency signals modulated in amplitude in accordance with vibrations imparted to the pickup.

Still another object of the present invention is to provide an improved phonograph pickup system which has none oi the disadvantages residing in the use of crystals because oi temperature, humidity, etc.

Yet another object of the present invention is to provide improved phonograph pickup structures particularly suitable. for use with a record of the so. called "constant velocity out in which the amplitude of the cut decreases 6 per octave.

Still a further object of the present invention is to provide an improved phonograph pickup Still a further object of the present invention 7 is to provide an improved vibratory element associated with a frequency determining element of a high frequency electrical oscillating system whereinmovementof the vibratory element pro- 2 duces no change in the frequency of the oscillating system but does produce substantial changes in the amplitude of oscillations in the system.

The features of the present invention which are believed to be novel are set forth with par-- ticularity in the appended claims. The present invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with accompanying drawings in which:

Fig. 1 illustrates a circuit arrangement embodylng certain features of the present invention.

Fig. 2 illustrates a vibratory member particularly useful in the arrangement of Fig. 1.

Figs. 3-5 illustrate details of a phonograph pickup construction particularly useful in the arrangement of Fig. 1, Fig. 3 being a section taken substantially on line 3-3 of Fig. 4.

Fig. 6 illustrates certain frequency response characteristics obtained with the use of apparatus described herein.

Fig. 7 illustrates a modified pickup structure for use in the arrangement of Fig. 1, Fig. '7 being a sectional view taken on a linesimilar to line 3--3 of Fig. 4

Figs. 8 and 9 illustrate modified circuit arrangements embodying the present invention.

The apertures described herein, illustrative of other apparatus which may embody the present invention, is arranged in Fig. 1 to transform intelligence recorded on a movable record medium I into suitable current variations of audible frequency in a utilization or amplifying device 2.

The intelligence on record disc I is in the form of laterally cut impressions of undulating charach ter in which a suitable stylus 3 travels as the record' disc I is rotated by conventional means whereby a vibratory memberl attached to the stylus is vibrated to produce corresponding changes in the impedance of an-inductanceelement 5 magrmetically coupled to the vibratory member.

Inductance element 5 whose effective resistance or inductance value may be cyclically altered by corresponding movement of stylus 3 constitutes one element of a single frequency determining circuit 5, 6, 1 in an oscillatory circuit including an electron discharge device 8--shown in Fig. 1 as a twin triode. As will be evident later from the description which follows, the first triode section ofdischarge device 8 with its I associated circuits functions not only as an oscil- 3 iatory device but also as a combined demodulating and amplifying device.

The series circuit formed by the serially connected inductances 5 and 6 is connected in parallel circuit relationship to condenser I to form the parallel tuned circuit 5, 6, 1, one terminal of that tuned circuit being connected through condenser 9 to the output electrode or anode In of the first triode section of device 8, the other terminal of the tuned circuit being connected through condenser H to the control grid or input electrode I2 of the first section of device 3 and the junction point of inductance elements 5 and 6 being connected to the common grounded cathode l3. Grid leak resistance I4 connected between grid I2 and cathode l3 serves a plural purpose as will become evident from the description which follows.

Space current for anode I is supplied from voltage source l which has its positive terminal connected through a load resistance IE to anode l0 and its negative terminal connected to the grounded cathode l3.

The operation of the electrical circuit thus far described in connection with Fig. 1 is as follows. Device 8, including anode I0, control electrode I2 and cathode l3 sustains an oscillatory current of constant amplitude in the tuned circuit 5, 6 and l in well known fashion when the vibratory member 4 does not move. Movement of vibratory member 4, for example, by stylus 3 traveling in the sound track of a conventional phonograph record groove, produces cyclical changes in the impedance of inductance element 5 to change the amplitude of oscillatory current flowing in the tuned circuit 5, 6, and 1 whereby amplitude modulation of such oscillatory current is produced. One important feature of the present invention is that device 8 including anode l8, electrode l2 and cathode [3 serves to produce simultaneously modulation components of the amplitude modulated wave across grid leak l4 and to amplify such components whereby they appear in amplified form on the anode I0.

Such amplitude modulation components are then applied to the second section including control grid or electrode l'l, anode l8 and common cathode is of the twin triode device 8 wherein the detected modulation components are further amplified.

I Anode I8 is supplied with space current from the positive terminal of source through resistance l9 and the control grid IT is maintained at a suitable negative continuous potential with respect to cathode l3 by connecting grid leak resistance between grid l1 and ground.

Amplified modulation components derived from the generated oscillation appear at anode I0 and are transferred to the control grid-cathode circuit of device IS, IT, l3 through the series circuit comprising, in turn, filter resistance 2|, isolating and coupling condenser 22 and filter condenser 23, grid I! being connected to the junction point of condensers 22 and 23, the other terminal of condenser 23 being connected to grounded cathode Filter condenser 23 has relatively small reactance at the frequency of oscillatory current generated in the stage l0, [2, I3 so that substantialiy all of the amplified voltage appearing at anode H! is of a frequency corresponding to the frequency of vibration of the vibratory member 4-. The amplified voltage is applied to device 2.

The vibratory member 4 of Fig. l magnetically coupled to coil 5 is preferably of high resistance conductive material and in one form of the present invention is .004" thick and is stainless steel known in the trade as American Iron and Steel Institute type No. 302. Stainless steel type 302 contains about 18% chromium, about 8 or 9% nickel, about 0.08 to 0.2% carbon, traces of manganese, silicon, phosphorus, sulphur and copper and the remainder iron. Stainless steel type 304 is preferred because it is substantially non-magnetic and contains less carbon. This type is preferred for it has been found that, by using such material, a minimum amount of cycle hum is introduced into the pickup from nearby motor driven turntables, transformers, etc. It has been found that stray 60 cycle fields shift the working point on the magnetization curve of the vibratory member with the result that the inductance and effective resistance of the coil 5 coupled thereto change. Such change produces undesirable am plitude modulation of the oscillating circuit. For these reasons where stray alternating magnetic fields are present the material preferably possesses high resistance and low permeability. In such case desirable modulation is produced substantially entirely by eddy current loss in the high resistance material, and the modulation is substantially unaffected by variations of permeability of member 4 in the presence of such stray fields.

Vibration of a member of non-magnetic, high resistance conductive material in the vicinity of inductance element 5 causes amplitude modulation in the oscillatory current because (1) the quality factor or Q of the inductance element is changed cyclically and (2) the amount of feedback voltage from anode 10 to grid i2 is changed cyclically due to cyclical change in the magnitude of inductance in the element 5, so that when the impedance of coil 5 is altered the feedback factor is altered in such a way that amplitude modulation produced by inductance change is in phase with amplitude modulation produced by the change in Q quality factor.

Viewed in another way, the inductances 5 and 6 and condenser 1 form a pi network through which oscillation voltage is transferred from anode ID to control electrode I2. In usual forms of electron discharge oscillation generators, and in the type illustrated in Fig. 1, there need be only a part of the voltage on the anode fed back to the control electrode, and, to maintain the generator in operation, that voltage so fed back must be maintained within certain limits. That is, if the amount of voltage fed back to the control electrode is too little, the generator ceases the production of oscillations. With the illustrated oscillater, the feedback is maximum, and oscillation intensity correspondingly maximum, when the impedances of coils 5 and 6 are equal. Between these limits where continuous operation is main tained, the amount of voltage fed back from the anode to the control electrode has a substantial influence on the intensity of the oscillations.

The pi network 5, 6, I is effective to alter in substantial degree the amount of voltage fed back from anode ID to control electrode 12 as the impedance of coil 5 changes. That is, variations of the impedance of coil 5 produce relatively large changes in the ratio of voltage across coil 5 to the voltage across coil 5 when the impedance of coil 5 is lower than the impedance of coil 6. For this reason, it is preferred that the inductance of coil 5 be less than that of coil 6.

In the fundamental form of the present inven- 5. tion, the material of member disv conductive and of high specific resistance. To make it magnetic as well as conductive is useful when the member 4need; not be used in the presence of stray magnetic field and when it is desired to reduce or eliminate inductance change of coil 5 and corresponding change of oscillation frequency. Also, to make member 4 magnetic introduces hysteresis loss which aids the eddy current efiect in member 4 to change the effective resistance of coil 5 and consequently to change the oscillation intensity. Cyclical change in inductance in element 5 causes a small amount of frequency modulation of the generated wave in tuned circuit 5, 6, 7 although only those components which produce amplitude modulation are detected on the grid leak resistance and amplified in device [-0, [2-, I3.

When the movable element 4 is constructed of magnetic and conductive material, movement of such element closer to coil 5 has a twofold effect on the inductance of that coil. Since element 4 has a greater permeability than that of air, the inductance tends to increase; since element 4 is conductive, the inductance tends to decrease. The net change in the inductance of coil 5 is equal to the difference between the opposing influences. At the same time, as element 4 approaches the coil, both the increased hysteresis loss and the increased eddy current loss operate to increase the damping resistance reflected in coil 5. In one arrangement incorporating an embodiment of the present invention, the permeability and specific resistance of element 4A (Figure 2) are so proportioned that substantially no net inductance change results from movement of the element with respect to the coil. Thus amplitude modulation of the oscillations generated in the tuned circuit 5, 6, i is effected with substantially no accompanying frequency modulation.

Figs. 3-5 show a preferred construction forembedded in a hard plastic base 24 which in turn is. clamped between two halves 25, 26 of a plastic casing 2'1, the, two halves being held together by cement or the like therebetween, or by bonding as by heat and pressure. Inductance coil 5 is mounted on coil form 28 which is fastened, for example by cement or the like, to the inside face of casing half 26 in magnetic relationship to member 4 with circular coil form 28 and the circularly shaped intermediate portion of member 4. substantially coaxial.

The free end of vibratory member 4 has a resilient L-shaped record contact stylus or needle 29 affixed thereon, for example by spot welding or the like, to give a desirable vertical compliance for the purpose of reducing needle chatter and record wear and for producing a desirable response curve which is substantially flat between 2000 and 4000 cycles per second with a sharp cutoff after 4000 cycles per second.

The phonograph pickup arrangement of Figs. 3-5 incorporated in a circuit such as shown in Fig. 1 has preferably a frequency response characteristic of the type shown in Fig. 6 wherein output voltage in the utilization device 2 is plotted as ordinate and frequency of the corresponding voltage, is plotted as abscissa. The sharp cutoff" after 4000 cycles is due mainly to the fact that,

remain substantially stationary and the middle.

portion adjacent the inductance element 5 vibrates with relatively large amplitude, so that the response at the frequency corresponding to this mode of vibration is large, and so that the response at increasingly higher frequencies is rapidly reduced. In order that the amplitude of vibration at the resonant frequency be not too large a damping element 30 such as a block of 'plasticized cellulose nitrate is fixed, as by heat. and pressure, on the vibratory member at thev upper end thereof in abutting relationship with the bottom surface of the base member 24. This material is preferred over ordinary rubber since it possesses more internal friction and less compliance per unit volume. Without the presence of a damping element 30 the characteristic curve iii in full line in Fig. 6 is peaked in the manner indicated by the dotted line portion 3-2 thereof but with a suitably proportioned vibratory member 4 and damping element 30 the characteristic curve Si is substantially flat in the range of 2000 to 4000 cycles per second. Without resonant effects in the mechanical vibratory member 5 the characteristic curve issubstantially as indicated by the dotted line 33.

In another form of the invention shown in Fig. '7 the stylus or recordv engaging portion. 3 at the end of vibratory member 4 may be straight instead of L-shaped as shown in the preferred embodiment in 4. Furthermore, instead of the upper end of the vibratory member 4 being embedded in a hard plastic base 24 as shown in the preferred embodiment in Fig. 3 the upper end. of member 4 may be embedded in an elastic base 36 such as rubber. In such case the elastic body 34 is held between the two halves 25, 25 of casing 21 in the identical manner as shown in Fig. 3.

Inductance coil or element 5 has its opposite leads 35, 3S permanently connected respectively to metallic contact elements 31, 38 which are embedded in and extend from the inside to the outside of casing 21, the outer ends of contact elements 3'5, 38 being adapted. to make good electrical and mechanical contact with corresponding contact elements 39, 40 embedded in and extending through the casing holder 4|. Leads 42, 43 permanently fastened respectively to the upper ends of contact elements 39, 40 extend through at least part of the tone arm 45 to the appropriate terminals 45, 4B in the oscillator circuit of Fig. 1.

The replaceable casing 21 which may be sold as an article of commerce contains in a unitary structure the vibratory member 4 and coil 5 magnetically coupled thereto in predetermined spaced relationship and is held as a unit snugly in the casing holder 4! by projections 41, 58 engaging correspondingly shaped depressions (not shown) in the walls of casing holder AI and by contact projections 69, engaging correspondingly shaped depressions in contact elements 39, cc.

The casing holder 4! is pivotally mounted about the axis of fixed pin 5! in a pair of arms 52 disposed on opposite sides of the casing holder 4!, each arm having one end thereof fastened to the tone arm, for example by screw 53, and, the other end thereof circularly formed to provide a.

bearing member with pin Arms 52 may be flexible but are preferably non-flexible.

The casing holder 4| i normally spring biased downwardly for rotation about the axis of pivot pin 5| by coil spring 54 having its ends fastened to tone arm 44 for example by screw 55 and an intermediate portion 56 engaging holder shoulder 51 to press the holder 4| downwardly in the direction of counterclockwise rotation about pin 5|.

The coil spring 54 is made sufficiently flexible such that the pickup unit is protected against injury in case the tone arm is dropped inadvertently, carelessly, etc. In such case the pickup unit including the casing holder 4| is deflected upwardly against the action of weak spring 54 so that the heel of casing 21 engages the record, arm 59 being fastened to the tone arm for eX- ample, by screw 53 to serve for a stop for excessive movement of casing holder 4| the free end of arm 59 is provided with a piece 58 of felt or other cushioning material to prevent undesirable rattling of casing holder 4| against arm 59. Protection is also afforded to the Lshaped stylus by extending the casing halves 25, 26 sufficiently far downwardly such that when more than a predetermined force is applied to the stylus, it moves upwardly within the space confined by the casing halves 25, 25. Since the end of arms 52 and 59 are both held by the same screw 53 it is obvious that if desired the arm 52 may form an integral extension of arm 59. This retractability feature is specifically claimed in United States Patent No. 2,444,218, issued June 29, 1948, to C. W. Carnahan, which patent is assigned to the present assignee.

The oscillator-detector arrangement shown in Fig. 1 is preferred but other arrangements may be utilized; for example, the arrangements of Figs. 8 and 9 may be utilized to carry out certain aspects of the present invention. The frequency of oscillation is in the order of 6 megacycles.

In Fig. 8 the rectification action between grid l2 and cathode is is not used as in the preferred form but a separate diode demodulator anode 58 is used with cathode I3. Amplitude modulated radio frequency signals are applied from the anode it to the diode anode 60, through condenser 6|, the cathode l3 being grounded with the result that only voltages of frequencies corresponding to the amplitude modulation components are present in large magnitude between anode 60 and ground.

The detected voltage of audible frequency between anode 60 and ground appears across resistance 62, connected between anode 55 and cathode l3, and is amplified through an electron discharge amplifier device including cathode l3, a control electrode and an anode 64, a load 2 being connected between cathode l3 and anode 64 for further amplification or utilization.

In Fig. 8 certain elements are like those shown in Fig. l and are given like reference numerals. That is, anode 55 is connected through resistance 2| and condenser 22, in series, to control electrode 63 and resistance and condenser 23 are connected in shunt between control electrode 63 and cathode i3. Anode 54 is connected through resistance is to the positive terminal of source 15. These elements function in the same manner as those shown in Fig. 1.

It is desirable that, where a separate diode detector is used, the two amplifying discharge de vices and one diode detector be in one envelope and have a single cathode, since cathode heating current and connections are thereby minimized.

It is of substantial advantage to use such a sep arate diode detector arrangement because of greatly increased detection efliciency.

When detection is accomplished in the oscillator, as in Fig. 1, a part of the detection is accomplished in grid l2 by rectification between that grid 12 and cathode l3, and a part is accomplished by what is called plate bend detection. The demodulated voltages appearing on anode II] from these two sources are, however, in opposite phase, with the result that the detection is not highly efficient. Also the amplification of the audio signal from grid [2 to anode I0 is not highly efiicient since limitations are imposed on the discharge device by its operation as an oscillator. Nevertheless, the arrangement of Fig. l is desirable where it be desired to provide a very minimum of parts at the sacrifice of some gain, since amplification from grid l2 to anode ID of the rectified signal is more than enough to offset the relatively inefficient detection attained when both grid rectification and plate bend detection are present.

In Fig. 9 the grid rectification action between grid 12 and cathode I3 is not used as in the preferred form but instead there is used a separate triode demodulator device 65 operating on the non linear portion of its plate current-grid voltage characteristic near a point thereon corresponding to plate current cutoff. Amplitude modulated waves are transferred from the junction point of elements 8, l, 9 to the grid 66 of device 65, the cathode El thereof being grounded through the parallel circuit comprising resistance 68 and condenser 59, and the anode I!) being con nected to the positive terminal of the anode supply source l5 through resistance H. The amplitude modulation components of audible frequency appearing at anode 10 are transferred through isolating condenser 12 to an amplifying and/or utilization device. This arrangement has the advantage of providin efficient detection and voltage gain, and the two discharge devices may be in the same envelope,

An important feature of the present invention embodied in the arrangements of Figs. 1, 8 and 9 is that but one single tuned circuit is utilized to determine the frequency of oscillation of the generated superaudible frequencies and no other tuned circuit is utilized, either in the oscillation generator, the detector, or any amplifier between the oscillation generator and detector. This feature is important since there is no likelihood of the detection of any frequency modulation components due to mistuning, drift, etc., occasioned in the use of a plurality of tuned circuits. In those arrangements herein where frequency modulation components may be developed they have no influence on the audio system since (1) they are not detectable and (2) they are filtered out along with the voltages of superaudible frequency. In brief, the use of a single frequency determining circuit avoids any necessity for alignment which would be necessary were a plurality of tuned circuits used and it is not necessary to take the special precautions which would then be necessary to avoid detuning between several circuits which would .be produced by eccentricity of record I forcing large inductance changes in coil 5 or by frequency instability of the oscillator.

While paiticular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects.

-9 and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the truespirit and-"scope of this invention.

1. Signal translating apparatus including: an electron discharge device 'havinga cathode, an input electrode, an'd'an output electrode; a single frequency-determining circuit coupled between said input electrode and'sai'd output electrode to constitute with said devicean'oscillationgenera- .torlfor producing oscillations of superaudible frequency, said circuit including the'parallel combination of a condenser and a pair of serially connected inductance coils; a connection from the junction of said coils to said cathode; a transducer including a conductive element magnetically coupled to one of said coils to reflect therein a damping resistance having a magnitude determined by the space relation of said element and said one coil for amplitude modulating said oscillations in response to relative movement between said element and said one coil; a condenser and a grid leak resistor included in said oscillation generator and coupled to said cathode and to said input electrode for detecting said modulated oscillations; and an audio frequency load impedance coupled to said cathode and to said output electrode for deriving an amplified audio frequency output signal in response to the detected amplitude modulation of said oscillations.

2. Signal translating apparatus including: an electron discharge device having a cathode, an input electrode, and an output electrode; a single frequency-determining circuit coupled between said input electrode and said output electrode to constitute with said device an oscillation generator for producing oscillations of superaudible frequency, said circuit including the parallel combination of a condenser and a pair of serially connected inductance coils; a connection from the junction of said coils to said cathode; a transducer including a magnetic and conductive element magnetically coupled to one of said coils to reflect therein a damping resistance having a magnitude determined by the space relation of said element and said one coil for amplitude modulating said oscillations in response to relative movement between said element and said one coil, the permeability and specific resistance of said element being so related that said relative movement produces substantially no net change in the inductance of said one coil; a condenser and a grid-leak resistor included in said oscillation generator and coupled to said cathode and Y to said input electrode for detecting said modulated oscillations; and an audio frequency load impedance coupled to said cathode and to said output electrode for deriving an amplified audio frequency output signal in response to the detected amplitude modulation of said oscillations.

3. A record player including: an electron discharge device having a cathode, an input electrode, and an output electrode; a single frequencydetermining circuit coupled between said input electrode and said output electrode to constitute with said device an oscillation generator for producing oscillations of superaudible frequency, said circuit including the parallel combination of a condenser and a pair of serially connected inductance coils; a connection from the junction of said coils to said cathode; a replaceable transducer unit comprising one of said coils and a vibratory element having a conductive portion of high specific resistance maresistor included in said oscillation generator and coupled to said cathode and to said input electrode for detecting said modulated oscillations;

and an audio frequency load impedance coupled to said cathode and to said output electrode for deriving an amplified audio frequency output signal in response to the detected amplitude modulation of said oscillations.

4. A record player including: a triode having a cathode, a control grid, and an anode; a single frequency-determining circuit coupled between said grid and said anode to constitute with said triode an oscillation generator for producing oscillations of superaudible frequency, said circuit including the parallel combination of a condenser and a pair of serially connected inductance coils; a connection from the junction of said coils to said cathode; a replaceable transducer unit comprising one of said coils and a vibratory element having a conductive portion of high specific resistance material magnetically coupled to said one coil and carrying a stylus adapted to track the undulations of a rotatable record disc to refiect in said one coil a damping resistance having a magnitude determined by the space relation of said conductive portion and said one coil for amplitude modulating said oscillations in response to vibration of said stylus; a condenser and a grid-leak resistor included in said oscillation generator and coupled to said cathode and to said grid for detecting said modulated oscillations; and an audio frequency load impedance coupled to said cathode and to said anode for deriving an amplified audio frequency output signal in response to the detected amplitude modulation of said oscillations.

5. A record player including: a triode having a cathode, a control grid, and an anode, a single frequency-determining circuit coupled between said grid and said anode to constitute with said triode an oscillation generator for producing oscillations of superaudible frequency, said circuit including the parallel combination of a condenser and a pair of serially connected magnetically independent inductance coils; a connection from the junction of said coils to said cathode; a replaceable transducer unit comprising one of said coils and a vibratory element having a conductive portion of high specific resistance material magnetically coupled to said one coil and carrying a stylus adapted to track the undulations of a rotatable record disc to reflect in said one coil a damping resistance having a magnitude determined by the space relation of said conductive portion and said one coil for amplitude modulating said oscillations in response to vibration of said stylus; a condenser and a grid-leak resistor included in said oscillation generator and coupled to said cathode and to said grid for detecting said modulated oscillations; and an audio frequency load impedance coupled to said cathode and to said anode for deriving an amplified audio frequency output signal in response to the detected amplitude modulation of said oscillations.

HENRY P. KALMUS.

(References on following page) REFERENCES CITED Number The following references are of record in the 136L315 file of this patent: UNITED STATES PATENTS 322%: Number Name Date 2 441 4 4 1,754,293 Weinberger Apr. 15, 1930 1,822,758 T011101! Sept. 22, 1936 2,055,107 Vieth Sept. 22, 1930 m Number 2,148,796 Barbieri Feb. 28, 1939 25 43 2,208,091 Zakarias July 16, 1940 314,560 2,228,163 Cohen Jan. 7, 1941 558 0 Name Date Thompson Nov. 4, 1941 Devine Feb. 20, 1945 Bobb July 3, 1945 Boys May 28, 1946 Gould May 11, 1948 Albright May 11, 1948 FOREIGN PATENTS Country Date Great Britain Sept. 26, 1926 Great Britain June 24, 1929 Great Britain Jan. 17, 1944 Certificate of Correction Patent No. 2,489,378 November 29, 1949 HENRY P. KALMUS It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 2, line 32, for the word apertures read apparatus;

and that the said Letters Patent should be read with this correction therein that the r same may conform to the record of the case in the Patent Office.

Signed and sealed this 4th day of April, A. D. 1950.

THOMAS F. MURPHY,

Assistant Oommz'ssz'oner of Patents.

Certificate of Correction Patent No. 2,489,378 November 29, 1949 HENRY P. KALMUS It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 2, line 32, for the word apertures read apparatus;

and that the said Letters Patent should be read with this correction therein that the \L same may conform to the record of the case in the Patent Ofi'ice.

Signed and settled this 4th day of April, A. D. 1950.

THOMAS F, MURPHY,

Assistant C'ommz'ssz'oner of Patents.

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