US2843782A - Electric discharge tubes - Google Patents

Description

July 15, 1958 Filed Aug. 25, 1954 A. H. w. BECK ET AL 2,8 3,782

ELECTRIC DISCHARGE TUBES 5 Sheets-Sheet l Inuenions A.H W4 BECK M. JACKSON J. LY OL L IS Attorney July 15, 1958 A. H. w. BECK ET AL 2,843,782

ELECTRIC DISCHARGE TUBES Filed Aug. 25, 1954 5 Sheets-Sheet 2 Inventors W. BEC K- JAC KSON- Y OLLIS Attorney July 15, 1958 'A. H. w. BECK ET AL 2,843,782

ELECTRIC DISCHARGE TUBES Filed Aug. 25, 1954 5 Sheets-Sheet 3 3 Inventors A. H. W. B [C K- M. JACKSON J. LY OLLIS y Attorney July 15, 1958 A. H. w. BECK ET AL 2,843,782

ELECTRIC DISCHARGE TUBES Filed Aug. 25, 1954 5 Sheets-Sheet 4 INVENTORS A. H. W. BECK, T.M. JACKSON 8| J.LYTOLL|S ATTORNEY July 15, 1958 A. H. w. BECK ET AL 2,843,782

ELECTRIC DISCHARGE TUBES Filed Aug. 25, 1954 5 Sheets-Sheet 5 INVENTORS A. H. W. BECK, T. H. JACKSON a J. LYTOLLIS BY was ATTORNEY United States Patent ELECTRIC DISCHARGE TUBES Arnold Hugh William Beck,

and John Lytollis, London, ternational Standard Electric Application August 25, 1954, Serial No. 452,034

Claims priority, application Great Britain September 2, 1953 5 Claims. Cl. 313-189) Thomas Meirion Jackson, England, assignors to In- Corporation, New York,

Discharge from the cathode is shared between the two anodes and speech voltages are applied between them. According to the said patent there is provided a cold cathode gas-filled electric discharge tube comprising a cathode and a plurality of anodes cooperatingtherewith so that a glow discharge may pass from the cathode to each anode, the said anodes being so disposed relative to one another and to the cathode that during discharge between the cathode and the said anodes an electric current transmission path is provided across the gap between the or a pair of the said anodes;

A low anode-to-anode impedance can be obtained if the two anodes are disposed with respect to the cathodes so that they are both at the edge of the cathode dark space. This means, however, that for each gap the anode cathode striking'and maintaining voltages are then similar, which, in general, is not desired. In the above mentioned patent, therefore, there is described an electric discharge device comprising, within an envelope containing an ionisable gas, a cold cathode and two anodes disposed with respect to the said cathode so that the maintaining voltage of the gap between the said cathode and either anode is less than the striking voltage and so disposed with respect to each other that, during discharge between the cathode and the said anodes, a path for signals is provided of low impedance at least for signals in the voiee-frequency range.

From the point of view of obtaining a maximum difference .between the striking and maintaining voltages for the anode-cathode gaps, the anodes should be as far distant as possible from the cathode. If the anodecathode gap is too great, however, not only does the anode-anode impedance, rise, as will be explained later, but noise is introduced so soon as the anode-cathode gap is longer than that appropriate to a limited region of the Faraday dark space. The above mentioned patent describes a cold cathode gas filled electric discharge tube comprising a cathode and two mutually adjacent anodes each so positioned that during abnormal glow discharge between the cathode and both anodes, the said anode is situated between the edge of the cathode dark space and the end of that region of the Faraday dark space in which there is substantially no electron space charge sheath surrounding the anode.

The requirements above specified with regard to the Faraday dark space can be translated into terms of the pressure of the gas filling and the gap lengths from the cathode to either anode, and the measured characteris tics of the tube. The said patent, therefore, defines the cold cathode gas filled electric glow discharge tube, comprising a cathode and a pair of mutually adjacent anodes, as being characterized in this that the product of the pressure of the gas filling and the gap length from the cathode to either said anode is such that, for a given glow discharge current from the said cathode, the said product lies within the range of values for which, at that said current, the resistance of the gap between the said anodes is near its minimum value and varies slowly with change of the said product when compared to its rate of variation outside the said range.

As applied to automatic telephone practice, said patent provides means, inter alia, for substituting an electromechanical selector switch as used in a step by step system by equivalent cold cathode circuitry, and it is, therefore, expedient to provide in a single envelope the equivalent of several individual tubes of the kind disclosed in the parent specification.

According to the present invention there is provided a cold cathode gas filled electric glow discharge tube according to said patent comprising a plurality of pairs of anodes each pair of anodes co-operating with a cathode common to the pair to define a pair of discharge gaps having like electrical characteristics with the cathode current divided between the two anodes in such manner that a transmission path, at least for audio frequency signals, is established between the said two anodes, the said tube further comprising means isolating from other like gaps the discharges at separate of said pairs of gaps to prevent undesirable cross-fire or cross-talk between the several transmission paths.

Such a tube has conveniently, a cathode common to each said pair of discharge gaps, the constructional .a'rrangement being characterised by a hollow cylindrical cathode and circumferentially disposed pairs of anodes together with suitable ionisation coupling screening means between adjacent discharge paths from the cathode to each pair of anodes.

An embodiment of the invention will be described with reference to the accompanying drawings, in which:

Fig. 1 shows a longitudinal cross-section through a tube according to the present invention;

Fig. 2 shows a transverse cross-section of a tube taken through the line IIII of Fig. 1; and

Fig. 3 shows a circuit diagram illustrating one mode of use of the tube of Figs. 1 and 2.

Fig. 4 shows a portion of a cross-section through a tube containing another embodiment of the invention.

Fig. 5 shows'a transverse cross-section of the tube of Fig. 4 taken through the line V-V.

The discharge tube of Figs. 1 and 2 is enclosed within a conventional glass envelope 1 provided with a glass base 2 carrying an exhaust tubulation 3 and circuit connecting leads 4, sealed through the base 2. The electrode structure, which is mounted upon theleads 2, comprises a hollow cylindrical cathode 5 mounted between a pair of annular mica washers 6 and 7. Radially disposed sheets of insulating material 8 arelocated in slots in the mica washers 6 and 7, respectively, as indicated at 9, to subdivide the space within the cathode 5 into a set of compartments 10, each communicating with a central space 11. Each compartment contains a pair of anodes 12 in the form of metal strips mounted edgewise-on to-one another adjacent the cathode. These strips are cut with projecting lugs at their ends which pass through suitable slots in the respective top or bottom mica washers, the anode strips being fixed in position by twisting over the projecting lugs, as indicated at 13. Each pair of anodes is equally spaced from the cathode so that the individual anode-to-cathode discharge gaps have the same characteristics. The subdivision of the cathode space into compartments is such as to reduce coupling between different pairs of gaps sufiiciently to prevent cross-fire or cross-talk between the several transmission paths provided between each pair of anodes. We use the expression cross fire in the sense that cross-fire is said to occur when a discharge in any one compartment influences the maintaining or striking potentials of the gaps of any other compartment to an extent which will appreciably eifect the tolerances of circuit components of the supply voltages, taking for example, the circuit which is to be described with reference to Fig. 3.

Before specifying dimensions and characteristics of a typical tube according to the invention we shall describe a simple example of the use to which embodiments of the invention may be put.

In Fig. 3 there is shown a circuit diagram representative of part of a switching system having a limited number of signal outlets each of which if not already busy, may be connected, as required, to any one of a larger number of signal inlets. In Fig. 3 we indicate three outlets labelled Out 1, Out 2 and Out 3 respectively, and ten inlets of which the first three and the tenth are labelled respectively In 1, In 2, In 3 and In the remaining inlets not being shown. The inlet terminals are the respective terminals of the low impedance primary windings of transformers T which are provided with balanced-to-ground high impedance secondary windings. Similarly the outlets are from the respective low impedance windings of transformers T which transformers are similar to the transformers T One pair of three pairs of discharge tubes, 14, 14', 15, 15' and 16, 16 is associated with each of the three outlets. Each tube may be constructed as described above with reference to Figs. 1 and 2 and one of each pair of the anodes 12 of Fig. 2, is connected to a common lead. In Fig. 3 this is indicated, for simplicity, by representing those anodes which are connected together as a single anode such as formers T are connected to the respective sets of commoned anodes a a' a a and a a;,, of pairs of the corresponding tubes 14, 14', 15, 15' and 16, 16. The connection together of one anode of each of the pairs of anodes 12 to form a set such as a is preferably internal of the tube envelope so that, from the external circuit point of view each set of commoned anodes can be considered as one electrode, as explained above and as indicated in Fig. 3.

The high impedance winding of the transformer T associated with Inlet 1 has its ends connected respectively to the independent anode A of tube 14 and to the anode A of tube 14'; similarly the high impedance wind- .ings of the transformer associated with Inlet 2 is connected at one end to the independent anode A of tube 14 and A'2 of tube 14'. In the same way Inlet 3 is connected between anodes A and A;.; and so on, Inlet l0 being connected between anodes A and A The corresponding anodes of the tubes 15 and 15 are identified by the reference letters B and B', respectively, their order being indicated by the numerical suifixes 1 to 10; in the same way the tubes 16 and 16' have corresponding anodes labelled C and C with similar suflixes. The three anodes A B and C are connected together as are, similarly the three anodes A' B' and C and the remaining corresponding anodes, so that each inlet is multipled to corresponding anodes of each of the three pairs of tubes associated with the three outlets.

The centre point of each high impedance winding of the outlet transformers T is connected to ground through a resistor R.- The centre point of each of the high impedance windings of the inlet transformers T is connected to a respective resistor R conveniently of the same value as the resistors R and thence to ground through respective rectifiers W which are connected so as to provide low impedance for current flowing from ground to the circuit. The junction point between each The ends of the high impedance winding of trans-- .anodes of the discharge tubes; if current does flow through any rectifier W the pulse connection to the respective anodes is short-circuited.

The cathodes of the several discharge tubes are respectively denoted by thereference numerals K K K K and K K The pairs of cathodes having the same suffix numbers are joined together through respective centre-tapped chokes L, each centre tap being connected through a respective resistor R and associated rectifier W to negative-supply-potential terminals 27, the rectifier W being poled so as to present a low impedance for current flowing through R to terminals 27. The junction point between each resistor R and its associated rectifier W is connected to respective pulse terminals 28, 2.9 or 30 through a respective resistor R The resistors R and rectifiers W2 should be chosen so that negative pulses applied to terminals 28, 29 or 30 are transmitted to the respective cathodes unless a steady discharge current is flowing through the resistor R to the terminals 27, in which case the pulses are rendered inoperative.

The terminals 27 are connected to a source of potential 200 volts below earth. Negative going 250 volt pulses PH PH;, and PH;, of, say, milliseconds duration, are applied repetitively in succession to the respective terminals 28, 29 and 30. The characteristics of the discharge tubes 14, 14'; 15, 15' and 16, 16' are such that, while the 200 volt negative potential applied to the terminals 27 will maintain discharge between any pair of anodes such as A a or B' a etc. and the cathode of a tube, this voltage is insuflicient to initiate discharge, a striking voltage of the order of 300 volts being required. Thus the pulse PH applied to terminal 28 every 300 microseconds will not alone cause the tubes 14 and 14' to fire. If, however, during the time interval in which the pulse PH is applied, a 50 volt positive pulse PL is applied to terminal 17, then discharge will be initiated between the cathode K and anode A and also between cathode K and anode A' The coupling between the discharge A K and the gaps a K then causes discharge also to pass to anode a similarly discharge occurs at anode a' Thus in Fig. 3 we have assumed that a 50 volt position pulse PL has been applied to terminal 18 during the presence of a pulse PH at terminal 28, so causing a discharge current I to pass from ground through rectifier W and resistor R dividing equally to the anodes A and A' and thence, via the cathodes K and K through the associated resistor R and rectifier W to the negaitve-supply-terminal 27. 1 At the same time a current I of the same magnitude, will flow through the resistor R and divide between the anodes a and a' Thus a current I will continue to flow through rectifier W of Inlet 2 and render inoperative any further application of the pulses PL while the combined current 21 flowing through rectifier W renders inoperative the subsequent pulses P-I-I applied to terminal 28. The pair of tubes 14 and 14, and Outlet 1 have thus been seized by Inlet 2. In similar way we' have indicated that Inlet 10 has seized Outlet 2; a current I passing through the rectifier W associated with Inlet l0 and nullifying the effect of pulses PL applied to terminal 26, and a combined current 21 passing through the rectifier W associated with the tube pair 15, 15' nullifying the effect of pulses PH, applied to terminal 29. The voltage drops across the resistors R, due to the currents I, and I associated with Outlets 1 and 2, can be for monitoring or other control purposes if required.

We will now assume that his desired to connect Inlet 1 with the remaining free outlet Out 3. This connection is initiated by permitting the 50 volt positive pulse PL to appear at terminal 17. It should be explained that a pulse train comprised of similar pulses PL PL PL occurs during each of the pulses PH only the pulses PL PL and PL appearing at the respective inlet pulse terminals under the conditions postulated. If the pulses PH are each of one hundred milliseconds duration the pulses PL may be of '10 milliseconds duration. The first pulse PL of the train of pulses PL may be applied only to the terminal 17, the second pulse PL to the terminal 18 and so on. We shall assume that the pulse PL is first applied to terminal 17 during the time in which pulse PH is applied to terminal 28. of the fact that this pulse appears on each of the anode pairs A A,, B B; and C C; no discharge can occur from the anodes A and A; because the pulse PH, does not reach the cathodes K and K due to current 2I flowing through the rectifier W to terminal 27, while the screening from the discharge of anode pairs A a and A' a; is sufiicient to avoid cross-fire. Similarly the screening in tube pair 15, 15', and also the absence at the moment of any pulse PH ensures that no connection is established from Inlet l to Outlet 2. There being no discharges nor any pulse PH in tube pair 16, 16', no connection can yet be made to Outlet 3. Thus the pulse PL has been ineffective. Similarly, 100 milliseconds later, when pulse PH is present at terminal 30, the recurrence of pulse PL is also ineffective. 200 milliseconds after the first application of pulse PL however, this pulse is again applied to terminal 17 and this time pulse PH is present on terminal 30 and is transmitted to cathodes K and K' discharge is initiated at anodes C and O and, by coupling from their discharges, is also initiated from the adjacent sections of common anodes a and a;,. A transmission path is thus established and is maintained between Inlet l and Outlet 3. All three outlets are now busy and are rendered inaccessible to any of the other inlets.

When it is required to disconnect the path between an inlet the supply voltage at the appropriate terminal 27 is reduced below that needed for maintenance of discharge in the associated pair of tubes. On re-establishment of the normal supply voltage the outlet is rendered accessible to any disengaged inlet.

The circuit of Fig. 3 described above is purely illustrative of the mode of use of tubes according to the present invention and is based upon analogous circuits contained in the co-pending application No. 458,934 filed September 28, 1954 on behalf of F. H. Bray, R. G. Knight, G. C. Hartley.

It will be evident that in tubes for use with a circuit such as described above with reference to Fig. 3, firing of the tubes must occur immediately coincidence of pulses occur. On the other hand, in common with all cold cathode discharge tubes, in the absence of any ionisation whatever within the tube breakdown of a gap cannot occur. Such initial ionisation, unless otherwise provided for, will be dependent upon the incidence of light on a cathode, passage of cosmic radiation through the tube or other random efiect, whose uncertainty can cause very considerable delays between the application of a voltage sufficient to break down a gap and breakdown taking place. This delay is known in the art as statistical delay of firing of a discharge gap. It may be eliminated by providing a controlled source of ionisation within the tube, eifective at each gap, but insufiicient to effect the general electrical characteristics of the gaps.

In Figs. 1 and 2, provision for the elimination of this statistical delay of firing in each of the discharge gaps of the tube is provided by means of a tungsten wire 31, suspended between transverse members 32 which are connected to a respective pair of lead wires 4 by means of which current may be passed through the wire to irradiate the cathode in each of the compartments.

As an alternative to the tungsten wire mentioned above In spite Diameter of cathode cylinder 32 millimeters. Length of cathode cylinder 12 mm.

Anode strips 17mm. x 3.5 mm. Filling Helium at a pressure of 60 mm. mercury. Breakdown voltage, each anode to cathode 260-310 volts. Maintaining voltages each anode to cathode 120 volts i 5 volts. Resistance of transmission path between anodes :10 ohms.

For eliminating the statistical delay of firing in the above tube a 0.003 inch diameter tungsten. wire one inch long is maintained at a temperature of 2,100 K. a power of 0.75 watt being required for this purpose. Alternatively, as stated above, an auxiliary discharge gap may be provided as shown in Fig. 4 in the same position as the tungsten wire, in which case a pair of rods 31a, 31b, 0.040 inch in diameter, spaced 0.040 inch apart end to end and defining a gap 31c therebetween passes a current of 0.002 ampere at a maintaining voltage of 120.

It will be evident that various other alternative constructions of the tube are possible within the scope of the present invention. Thus, to mention one varient, the number of pairs of anode-to-anode gaps provided in a single envelope is largely a matter of economics, the cost of tube complexity and possible replacement being weighed against the cost of external wiring; in a telephone exchange system such considerations may be decisive in deciding on the optimum number of gap cornbinations in a single tube. Again, in most applications, such as that of Fig. 3, a balanced arrangement is required, for which, if the cathodes can be commoned it may be convenient to use a construction involving, for example, a single annular cathode with groups of anodes on either side of it, leads being brought out at either end of the tube. On the other hand, in the particular application of Fig. 3, the cathodes of each pair of tubes should be maintained separate in order that the anode-to-ground impedance may be made high by the insertion of the chokes L. In such cases, the cathode cylinder 5 of Figs. 1 and 2 could be split diametrically into two halves 5a, 5b, as shown in Fig. 5. The cathode halves 5a and 5b are separated by spaces 50 diametrically opposite each other and result in a pair of tubes such as shown in Fig. 3 arranged in one envelope with all the anode leads being brought out at one end of the tube so as to enable twisted pair wiring to be used right up to the base of the tube.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What we claim is:

1. An electric glow discharge tube comprismg an envelope containing a gaseous medium, a plurality of electrodes including a plurality of pair of anodes each pair of anodes co-operating with a cathode common to the pair to define a pair of discharge gaps having l1ke electrical characteristics and a signal transmission gap between said anodes, with the cathode current dlvlded between the two anodes in such manner that a trans mission path for signals over said transmisslon gap is established between the said two anodes only during a discharge between each of the two anodes of a pair and the cathode, and means isolating from other like gaps the discharges at separate of said pairs of gaps to prevent undesirable cross-fire or cross-talk between the several transmission paths. 1

2. A tube according to claim 1 in which a single cathode is common to each said pair of discharge gaps.

3. A tube according to claim 2, in which said cathode is a hollow cylindrical cathode mounted between upper and lower sheets of insulating material, said isolating means comprising a set of radially disposed partitions of insulating material dividing the cathode space between the sheets of said insulating material into compartments, and in each said compartment one of said pairs of anodes being mounted side by side and equally spaced from the said cathodes.

4. A discharge tube according to claim 3, in which the said anodes are formed of metal strips mounted edge- 8 on to one another between top and bottom of said sheets of insulating material. p 1

5. A discharge tube according to claim 3, in which each said compartment opens on to a central space and- 10 other said discharge gaps.

References Cited in the file of this patent UNITED STATES PATENTS 2,103,022 Senauke Dec. 21, 1937 2,419,485 Desch et a1. Apr. 22, 1947 2,565,103 Toulon Aug. 21, 1951

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