US2521353A - Transmitter - Google Patents

Description

Sept. 5, 1950 c. J. FITCH ETAL TRANSMITTER Filed NOV. 27, 1945 mv NM ii m wE 5 W R H Y mmm 2 Wm VJ N T c Wm Y B Patented Sept. 5, 1950 UNITED STATES FATENT OFFICE TRANSMITTER Glyde J. Fitch, Endi-cott, and Robert N. Eichorn,

Vestal, N. Y., assignors to- International Business Machines Corporation, New York, N. Y., a corporation of New York Application November 27, 1945, Serial No. 631,096

ble. of reliable high speed performance.

Another object is to provide a keyboard; operated telegraph transmitter permitting a partial overlap in operation of two successive keys.

Another object is to provide a device of the-kind described providing stable operation over a wide range of supply line. voltages.

The invention is in the nature of an improvement upon the transmitter shown in Patent No. 2,210,577, being distinguished from said transmitter by the fact that, the signal elements are stored in trigger tubes, ratherthan in relays.

' It is a particular object of the invention to provide a' start-stop transmitter of the type shown in Patent No. 2,210,577", which has a smaller size andweight than the earlier transmitter.

Other objects of the invention will be pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principle of the invention and the best mode, which has been contemplated, of applying that principal.

In the drawings:

Fig; I is a complete circuit diagram of a transmitter embodying the invention.

Fig. 2 is a diagram of the voltage waves of the sequence tubes.

Fig. 3 is a diagram of a typical tone signal transmitted.

4' is a diagram of the code signal corresponding to the tone signal shown in Fig. 3.

For the purpose of illustration, the invention is shown embodied in an electronic startstop transmitter of the type shown in Patent No. 2,210,577. Voltage for operating the transmitter is taken firom an AC. line through a plug i and rectified by a selenium rectifier 8. The D. C. voltage appears across a condenser 9 at the polarity indicated, and across a voltage divider consisting of resistances- I 2, 3, 4*, 5, anda potentiometer 5 A transformer l supplies alternating current to the tube filaments, partially represented at H, and to a selenium rectifier l2, which sets upa bias vo1-t ageacrossa condenser" F3.

The transmitterincludes a sweep circuit comprising seguence vacuum tubes Vfi-Vt and transhirm'ers Fly-T1 The cathode of the tube V0 is connected by a Wire 20- to a point l8 on the voltage divider l--6, while the cathodes of tubes 26' maintain its grid slightly positive.

Vl-V8 are connected through a common wire 21 to a point l9 on said voltage divider. The'plate of the Vacuum tube V0 is connected through the primary winding of transformer T0" and awireifl to the positive end of voltage divider I--E, while the plates of tubes Vi -VT are connected through the primar windings of transformers Tl--T'| and a wire 23 to a point i!- on said voltage divider. The plate of tube V8 is connected through a plate resistor 2 and wires 25 and 22 to-the' positive end of the voltage divider l--6Z The grid of thetui ie V0 is connected to the cathode of the same tube through a resistance 26. It is also connected through a resistance 28", and wire- 2 2 to the positive end of the voltage divider l -6'. The grids of the tubes VI-VB' are connected through the secondary windings of the respective trance formers T0-T1- and a wire 30 to the arm of poten-iometer 6.

Due to the connections described, the sequence tubes Vl-V8 are normally biased off, while the tubeVD is normally conductive, becausecurrent is flowing through the resistance 26' in a directiomto The sweep circuit comprising the tubes V0-V8 and transformers Til-Tl is-designed for sequentialoperationof the tubes, according to the principle described' in the copending application of O. J. Fitch, Serial No. 465,604, filed November 14, 1942 now Patent No. 2,421,606. The sweep is initiated by biasing off the tube V0, in a manner to be. de-

scribed later, thus causing the field in transformer T0 to collapse, inducing a voltage wavein the secondar winding of the transformer and driving th grid of tube VI positive with respect to its cathode. Current now passes through the primary winding of transformer Tl and tube i l. Also, grid current flows in tube Vi, retarding: the collapse of the held in transformer To; When the potential on the grid of tube Vi drops to cathode potential, grid current stops and the field in transformer T0 collapses quickly. grid potential of tube; Vldrops suddenly, cutting oiT the flow of current through the tube V! and the primary winding of. transformer Tl A voltage 7 wave is: induced in the secondary winding: of

transformer Tl which drives the grid of. tube V2 positive with respect to its cathode. The. sweep passes down through the sequence tubes in the same manner, each: tube remaining conductive for a definite interval, for example, 10 milliseconds (see Fig. 2"). The intervals can be adjusted by shifting the arm of potentiometer 5- and thereby changing the normal bias on the grids of the sequence tubes.

There are six gas trigger tubes Gi -G8, one for each code element of the signal. The cathodes of these trigger tubes are connected through a wire 32 to the negative end of the voltage divider |5. The plates of the tubes GiG8 are connected through plate ersistors 36 and a wire 3? to the cathode of a vacuum tube Vi l. The plate of this tube is connected by wires 25 and 22 to the positive end of voltage divider l-E. The grid of the tube V! l is connected through resistances 38 and 39 (the former shunted by a condenser 4l) to the plate of the tube V8. The cathode of tube Vi? is connected through a. resistance 40 to the negative end of voltage divider [-6.

The control grids of tubes Gl-G6 are connected through grid resistors 42 and wire 4! toan intermediate point 45' of a voltage divider 45 extending from Wire 32 to a wire 44 leading to the negative side of condenser I3. The shield grids of the tubes G!G6 are connected through grid resistors 43 and wire 44 to the negative side of condenser [3. The upper part of voltage divider 45 is shunted by a condenser 46. The screen grids of tubes Gl-GG can be connected through respective normally open contacts (II-C and a wire 41 to the cathodes of said tubes. The contacts C1-C6 may be the permutation contacts of a keyboard transmitter, such as shown in Patent No. 2,355,297, or they may be the contacts of a tape reading unit such as shown in Patent No. 2,320,788. The dotted line 48 represents the plug connection of the keyboard transmitter or tape reader to the transmitter circuit.

' There are six coupling tubes VIIV!6, one for each trigger tube GIG5. The cathodes of .the tubes Vl |Vi5 are connected, respectively, by wires 49 to the plates of the tubes Gl-GE. The plates of the tubes VI EV!6 are connected to a common signal wire 50. The grids of the tubes VI |-Vl6 are connected through grid re- ;sistors 5|, grid lead resistors 52, and. wire 32, to the negative end of voltage divider l--5. They are also coupled, respectively, by condensers 53, to the secondary windings of transformers TlTfi. When a sweep is initiated, the grids of the tubes V! l-V! 6 receive in succession, through v the condensers 53, positive Voltage waves, which condition them for conduction during successive time intervals corresponding to V2V'! in Fig. 2.

It will be seen, from the connections described, 2 that the vacuum tube V! I is normally conductive, its grid being connected to its plate, and that current flows in resistance 40. Due to the voltage drop in resistance 40, the vacuum tubes VI IVI6 are normally biased off, and continue to be so even when the related trigger tubes are conductive. Also, the control grids and shield grids of trigger tubes G|-G3 are normally biased negative and this bias is sufficient so that either grid of each tube will prevent the tube from being ignited. The potential on the control grids can 1 be raised to unlock the tubes Gl-G6, by causing current to flow through resistance 45, in a manner to be described presently, in such a way as to lift the potential of the intermediate point 45'.

The screen grids can be brought selectively to "cathode potential by closure of the related contacts Cl-CB. When the unlocking of a tube by lifting of the potential on its control grid coin- 1 cides with closure of the related contact Cl-C6 the tube will fire. This has the eiiect of providing a conductive path from the cathodes of the respective coupling tubes VI IVI6 to the negative end of voltage divider l-6. When the 1 potentials on the grids of tubes VIlVl6 are lifted in succession, by operation of the sweep circuit, any tube V! l--Vl 6 connected to a trigger tube GIG6 which is in a conductive state will establish a conductive path from signal wire 50 to the negative end of voltage divider l5 and will thereby control a signal output device in a manner to be described presently.

The signal output device comprises a vacuum tube V20, which is an oscillator continuously in operation. Its plate is connected through wire 22 to the positive end of voltage divider |-5. Its cathode and grid are connected to a tuned circuit comprising a condenser 55 and the primary winding of a variable transformer 56. A point 5? on the tuned circuit is connected by wire 32 to the negative end of the voltage divider I--B. A grid resistor 58 tends to maintain the frequency of oscillation stable while a cathode resistor 54 provides a bias. The transformer 56 couples the oscillator circuit to the grids of a pair of vacuum tubes V2l and V22 connectedin a push pull arrangement. The cathodes of these tubes are connected by wire 23 to the point I! on the voltage divider l5, and are connected to a center tap of the secondary winding of transformer 56 by a connection including a resistance 59. The left end of resistance 59 is connected by a wire 60, and resistance 6| to the signal wire 50. The plates of the tubes V2! and V22 are connected to the end of the primary winding of a transformer 62 connected at a center tap through wire 22 to the positive end of voltage divider |6. The transformer 62 couples the amplifier V2!- V22 to a pair of amplifier tubes V23 and V24, the grid of V23 being connected through a grid resistor 63 to one end of the secondary winding of transformer 62, while the grid of tube V24 is connected through a grid resistor 64 to a volume control potentiometer 65 shunting the secondary winding of transformer 62. The cathodes of tubes V23 and V24 are connected through cathode resistors 65 to one end of the secondary winding of transformer 62 and, by Way of wire 32, to the negative end of the voltage divider l8. The plates of tubes V23 and V24 are connected through tuned circuits each comprising a condenser 6'! and the primary winding of a transformer TM or TS, and through the wire 22, to the positive end of the voltage divider l-6. The output of transformer TM goes to a monitoring device (not shown), While the output of transformer TS is sent through any suitable communication channel.

Operation Normally, that is, during the standby period, while the vacuum tube V20 is oscillating, the oscillations are shut off from the transformers TM and TS by a voltage drop in the resistance 59, which biases ed the amplifier tubes V2l and V22, This voltage drop is caused by current flowing from the point I! of voltage divider l-6 through wire 23, resistance 59, wire 50, a resistance 10, the plate to cathode path through a standby trigger tube G9 (which is normally conductive for a reason to be described presently), wire H, branching through resistance 45 and wires 32 and 44, to the negative end of voltage divider I6 and the negative side of condenser [3, respectively. The potential of the point 45 is elevated above that of wire 32 and the trigger tubes are thereby unlocked.

In accordance with the signaling system chosen for illustration, each complete signal consists of a start signal, six code signal elements, and a 'stop'signal (see Fig. 4).

. a condenser 8 to the ati ve The start signal and spacing signal elements are represented by oscillation's', while the stop signal and marking signal elements are represented byno oscillations (see Fig. 3). i I

Associated with the contacts C-l-C6 is a start contact CS, which is closed when any one of the contacts ClC,8'is closed (see contactQfi of Patent No. 2,355,297) One side'of contact'CS is connected by Wire 4! to the negative end of voltage divider I6, while the other sideis connected to the'shield grid of a gaseous start tube G 8. This grid is normally biased to cut-ofi the tube byconnection through a resistor 90 and wire 44' to the negative bias terminal of rectifier [2. The cathodeand control grid of "the tube G8are connected 'bywire 32 to the negative end of the'voltage divider l-G. The plate of tube G8 is connected to the cathode of a vacuum tube Vii], whose plate is connected by a wire 15, resistance 26 .-andwi1'e' 29, to the point l8 of voltage divider 1- 6, The grid of the tube VH1 is connected through a gridiresistor Hi to wireZi! and thus the tube VIO. is normally conductive. v

Assume that the standby tube G9 is in a conductivefstate, so that signal output is shutoff and .the potential on the control grids of .tubes GIG6 is high. Now if, by operation of the, typewriter keyboard in Patent No. 2,355,297, or "the'tape reader in Patent No. 2,320,788, the contacts, Cl, C4, C5, and CS are closed, tubes 011 64., and G5 Will be ignited immediately.

When contact CS. is closed, the shieldgrid, of

tube G8 is brought up tocathode potentialand the 'tube. is fired. and current flows through a circuit extending from the .point [8 of the voltage divider l-S through Wire 20, resistance 26, wire 15, tubes V13 and G8, and wire 32, to the negative end of voltage divider l-. The'voltage drop through resistance 25: biase off the normally conductive tube Vii and starts the sweep. sequence. Italso transmits a negativevoltagewave through plate of tube Qlland ex- -tinguishes this tube. This cuts off the flow .of current through resistance 45, .caus'inga negative biasing potential to be.appliedto'thecontrol grids of the tubes (ii-G6, which prevents any of these "tubes not initially ignited from being ignited, so long as. the gas tube G9 remains non-conductive. .Also, whenthe tube G9 .is extinguished, current now through the resistance ts ceases and oscillations are passed to the TMand'TS ransmrmc s. This constitutes the start element of-th i.s.ignal ,(see Figs. 3 and 4).

During the start signal element the weep tube fVl is conductive. When its conductivity is terminated by the collapse of the. field in transformer T0, apositive pulse is transmittedthrough'trans-- former T! to the grid of tube V2 and, through condenser 53, to the grid of tube -VI I, rendering bothof these tubes conductive. This starts the readout of the set of trigger tubes GlG 6,.in

which the signal l, 4, 5 is stored. Since, inatcordance with the example; the tube Gr'l is in a conductive state when the coupling tube Vtljbe j comes conductive, a circuit i completed from the point i? of the voltage divider .l-*E through wire '23, resistance 59, wire 'fiiresistancefii, wire 59, the plate-cathode path of tube Vll, wire-49, the plate-cathode path of tube'G l wire 32, tothe negend of voltage divider -!'6.i The voltage drop across the resistance 59 biases ofi the amps fier tubes V2 l "and V22, cutting offthe oscillations from the transformers TM and TS. This' constitutes a marking element, in-conformance-with the first signal element-ofthe signal select d-fortilustrationlsee Figs.3and4). Whenthe field in transformer Tl collapses and the tube V2 becomes non conductive, a positive pulse' 'is transmitted 'to the grids-"of tubes V3 and W2, rendering these tubes conductive.

Since the tube G2 is in a non-conductive state, current does not flow threugh resistance .59 and oscillations are transmitted to the transformers TMv and TS during the second element of the signal. During the third element of the sig'nal,

the corresponding tube G3 interrupts the circuit 'throughthe resistance 5-9 and signal wire Wand oscillations continue to be transmitted :to the transformers TM and TS. Tubes G4 and ing ignited, during the 'fou'rth and fifth signal elements current will pass through resistance 59 and-oscillations will not reach the transformers TM and TS. During the sixth signal element,

the tube GB being non-conductive, oscillations will be transmitted to the transformers TM and TS. When vacuum tube V] is cut on *by the collapse of the field in transformer. T6, "a positive pulse is transmitted to the grid of normally'n'onconductive vacuum tube V8, rendering this tube conductive. A negative pulse is transmitted from the plate of tube V8 through condenser ll and grid resistor '39 to the grid of normally conducitive tube Vl'l, rendering this tube non-conductive. This cuts off the plates of the trigger-tubes (EL-G6 from the voltage'supply and extinguishes any tubes which were in conductive state. ';At

the same time, the negative voltage wave .frcin the plate of tube V8 is transmitted through a wire 80 and condenser 8| and part of grid'resister 16 to the grid of tube Vlfl, biasingoflthis tube .andextinguishin'g gas tubeflfl. Whentuble V1!!! is biased off, the potential o'fits plate rises and a positive voltage wave is transmitted through a condenserBZ to the control grid of'gas tube'GS, rendering this tubev conductive.

This restores the standby condition, butv the nextsignal can'- not be. startediminediatelhibecause the tubes Vlll and VI! remain non-conductive as long as the tube V8 continues to conduct. After a brief tiine, the field .in the transformer T1 coliapsesuand the tube V8 is' cutoff; This restores the :conductivity of .tube 'Vll, applying positive potential :to plates of the tubes GL-GG. -Also, it :restor'es the .tube V M to a condition .in which it can .con-

duct.

If after this a .new combination, .say 245, is setup on the contacts Cl-CB, the trigger tubes i .G2, G4 and Giwill be ignited. Also, fthecohtacts CS will be. closed and the start :tu'be G 8 will :be ignited. The transformers TM and-TS will accordingly be controlled "in a mannersimilar to that previously described-to transmit the signal 124.5.

The circuit parametersmay be determined so that each signal element .is equal to zit-{milliseconds, for example. Since there are ei t e 8- -ments in each signal, namely, start, :stop and.

six code elements, the total time. of nne complete cycle of operation would then be 80 milliseconds.

This is equivalent to 12%; cycles orchoracters per secondior .125 words per minute. "Bathe-keyboard permutation selector controlling the contacts CIWCB and .CSis so constructed and .conditioned as to keep the contacts :closed for about 45 milliseconds, it is possible for the-secondkey to'be depressed 40 milliseconds af terthe. first key, because the contacts closed by the second key .will remain closed 5 .millisecondsbeyond Lthe Memes termination of the first signal. For example, if a key is depressed which closes the contacts 2, ,;4 and 5, 40 milliseconds after the previous key was operated, this has no immediate effect upon the trigger tubes GI'G6 because at this time the control grids of these tubes have a negative bias, which makes the tubes indifferent to potentials applied to the shield grids. During the last milliseconds of the first signal, the tubes VI! and VIO are biased ofi, and during this time the standby tube G9 is conductive, the signal generator is biased off, and the both grids of the tubes G2, G4 and G5 are so conditioned that the tubes would be ignited if their plates were positive.

1; Likewise, although the shield grid of the tube (58 is atcathode potential because the contact ,CS is closed, this tube is non-conductive because the tube VH3 is cut off. When the tube V8 is biased'ofl at the end of the 80 milliseconds which :marks the termination of the first signal, the vacuum tube Vll is restored to conductivity and the tubes G2, G4 and G5 are ignited. Also, the vacuum tube VIu becomes conductive and the tube G8 is ignited; This starts the second signal; The contacts C2, C4 and C5 are opened 5 milliseconds after the start of the second Sig- .nal.

The effect of this overlap is that for two successive signals, keys can be operated 40 milliseconds ,apart or at the rate of 250 words per minute. The third key, of course, cannot be operated .until 80 milliseconds after the second key.) This Iallows a high speed typist to average over 100 words per minute, giving in effect a free keyboard.

During partial overlap, as described above,

tube Vl'l is unlocked at the same instant that .G9 is extinguished. Since the tubes Gl-G5 will not ignite until after VI! unlocks, and since they cannot ignite after G9 is extinguished, it is :necessary to provide some delay in the decay of the bias voltage on the control grids of the tubes GI -G9. This delay is provided by the condenser When power is first applied to the circuit. the standby tube G9 is non-conductive, and oscillations are continuously transmitted to the transformers TM and TS. By closing the contact CS :the standby condition is established, the tube G9 being ignited and the oscillations being cut off from the transformers TM and TS. While there have been shown and described and .pointed out the fundamental novel features of tubes for storing respective signal elements, means including a set of signal element contacts per- :taining respectively to said trigger tubes for selectively igniting said trigger tubes, signal output means, readout means including a plurality of sequentially operable devices for subjecting said signal output means to the control of each of said trigger tubes in succession, means including a normally non-conductive trigger tube for starting said readout means, means for igniting said start- Y ing trigger tube including a start contact adapted to be closed whenever any one of saidsignal element contacts is closed, means responsive to the last one of said sequentially operable devices for extinguishing said starting trigger tube, a standby device for controlling said signal output means during the standby period, means coupling said standby device to said starting trigger tube so that said standby device is turned oiT and on as said starting trigger tube is ignited and extine guished, respectively, and means controlled by said standby device for locking said signal element trigger tubes during the time said standby device is off and releasing the same for selective ignition while said standby device is on.

2. In a transmitter for operation by a keyboard having a plurality of combinationally operated signal element contacts and a start contact which is closed whenever any one or more of th signal element contacts is closed; a set of gaseous trigger tubes, one for each of said signal element contacts, for storing respective signal elements, each of said trigger tubes having an anode, a cathode, and two grids; a voltage source; means connecting the anodes, the cathodes, and the grids of said trigger tubes to said voltage source so that each grid is normally. biased to a potential sufficiently low to prevent ignition of the tube, each of said signal element contacts being connected to one grid of the related trigger tube and to said voltage source in such a way that when a contact is closed the bias on the related grid is reduced sufiiciently to release the tube to the control of its other grid; a gaseous standby tube connected to said voltage source and to all of the second grids of said trigger tubes in common, in such a way as to remove the bias potential from said second grids when said standby tube is ignited; signal output means; readout means including a plurality of sequentially operable devices for subjecting said signal output means to the control of each of said trigger tubes in succession; start means controlled by said start contact for starting the operation of said readout means; means controlled by said start means for extinguishing said standby tube and restoring the cut-ofi bias to said second grids of the trigger tubes; means controlled by the last of said sequentially operable devices for cutting ofi said voltage source from the anodes of said trigger tubes, to extinguish any ignited trigger tubes; and means also controlled by the last of said sequentially operable devices for igniting said standby tube, to remove the cut-off bias from the second grids of said trigger tubes.

3. In a transmitter, a voltage source, a signal output circuit including a control element, a set of storage trigger tubes for storing respective signal elements, a start trigger tube, each of said trigger tubes comprising an anode, a cathode and a grid, means for connecting the anodes of said storage trigger tubes to said voltage source comprising a single grid controlled vacuum tube for all anodes and a plurality of current limiting resistors for the individual anodes, means for conmeeting the anode of said start tube to said voltage source including a grid controlled vacuum tube, means for connecting the cathodes and grids of said trigger tubes to said voltage source so that said grids are normally biased to a cut-ofi potential, storage control means for selectively controlling the potential on the rids of said trigger tubes to ignite said tubes selectively in combinations, start control means operated in conjunction with said storage control means for controlling the potential on the grid of said start tube to cause said tube to ignite, means for additionally connecting the anodes of said storage trigger tubes to said voltage source through the control element of said signal output circuit, said last connecting means comprising parallel circuit means for the respective anodes, each including a sequence control device, sweep circuit means to condition said sequence control devices sequentially, to provide conductive paths from the voltage source through the control element of said signal output circuit to the anode of each of said trigger tubes in succession, means controlled by said start tube for initiating an operation of said sweep circuit means, and means operated by said sweep circuit means at the end of a sweep for applying a negative voltage impulse to the grids of said vacuum tubes, to extinguish any ignited storage tubes and the start tube.

4. A transmitter as described in claim 3, characterized by the fact that said storage trigger tubes have second grids connected to said voltage source so as to have a normal bias preventing the ignition of said storage tubes, a gaseous standby tube, means connecting said standby tube to said second grids and to the voltage source and adapted to reduce the bias on said second grids, when said standby tube is conductive, to release said storage tubes for control by their first mentioned grids, and means coupling said standby tube to the vacuum tube connected to said start trigger tube, so that said standby tube is ignited when said vacuum tube is made non-conductive by a negative voltage impulse at the end of a sweep and is extinguished when said vacuum tube becomes conductive again.

5. In a transmitter for operation by a keyboard having a plurality of combinationally operated signal element contacts and a start contact which are held closed for a definite period of time when operated, said start contact being closed whenever any one or more of said signal element contacts are closed; a set of gaseous trigger tubes, one for each of said signal element contacts, for storing respective signal elements, each of said trigger tubes having an anode, a cathode, and control electrode means, a voltage source, means connecting the anodes, the cathodes, and the control electrode means of said trigger tubes to said voltage source, locking means whereby the voltage applied to said control electrode means can be conditioned to prevent the ignition of said trigger tubes, voltage controlled means for conditioning said locking means to unlock or to lock said trigger tubes, voltage controlled means for interrupting the anode connection of said trigger tubes to said volotage source, means including said signal element contacts for selectively igniting said trigger tubes when said locking means is conditioned to permit them to be ignited, signal output means, readout means including a plurality or sequentially operable devices for subjecting said signal output means to the control of each of said trigger tubes in succession, means controlled by said start contact for starting the operation of said readout means, means controlled by the last of said sequentially operable devices for transmitting a voltage impulseto each of said voltage controlled means, whereby any ignited trigger tubes are extinguished and the tubes are released by said locking means for the duration of said voltage impulse, means to delay the return of said locking means to locking condition until a short time after the anode connection to said trigger tubes is restored to conductive condition at the end of said voltage impulse, and means whereby said locking means is additionally controlled by said starting means, to maintain said trigger tubes in an unlocked condition, after the transmission of one signal, until said start contacts are closed to initiate another signal.

6. A transmitter as described in claim 5, characterized by the fact that said last mentioned control means for said locking means comprises a trigger tube and a vacuum tube in series connection, said last mentioned trigger tube being controlled by said start contact and said vacuurn tube being controlled by the impulse from said voltage impulse transmitting means.

CLYDE J. FITCH. ROBERT N. EICHORN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,555,870 Nyquist Oct. 6, 1925 2,168,198 F'rink Aug. 1, 1939 2,210,574 Fitch Aug. 6, 1940 2,247,167 Edwards June 24, 1941 2,400,574 Rea et a1 May 21, 1946 2,415,944 Fagen Feb. 18, 1947

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