US2898502A - Cold cathode electric discharge tubes - Google Patents

Description

Aug. 4,

A. H. W. BECK ETAL COLD CATHODEZ ELECTRIC DISCHARGE TUBES Filed May 3, 1956 2 Sheets-Sheet 1 Fig.2.

A Item e y A g 1959 A. H. w. BECK ETAL 2,898,502

COLD CATHODE ELECTRIC DISCHARGE TUBES Filed May 3, 195's 2 Sheets-Sheet 2 Fig.4.

VO/ts Inventors H. B o M. JACKS N- LYTOLL At omey tiaes atentfiice Patented Aug. 4, 1959 p 2,898,502 coLn CATHODE ELECTRIC DISCHARGE TUBES Application May 3, 1956, Serial No. 582,596 I I Claims priority, application Great Britain May 13, 1955 6 Claims. (Cl. 313-192) The present invention relates to the elimination of the statistical delay of firing in cold cathode gas-filled electric glow discharge tubes.

It is well known that when a voltage is applied between a pair of cold electrodes in an ionisable gas, the voltage being normally sufiicient to initiate glow discharge between the electrodes, discharge cannot occur until at least one charged particle is present in the gaseous atmosphere between the electrodes, which particle is accelerated to one or other of the electrodes, depending on its sign, and produces further ionisation along its path by collisions with gas molecules. If no other agency is at work, the passage of cosmic rays will sooner or later provide such an initial ionised particle, but there will be a delay of indefinite duration between the application of the said voltage and the commencement of breakdown in the gap between the electrodes; this delay is known as the statistical delay of firing, and in many circuit applications cannot be tolerated. There are several methods known to be effective to overcome the statistical delay by ensuring that at all times during which a discharge tube may be required to operate electrons or ions are present in the gap which it is desired to fire.

One method commonly used in glow discharge tubes to eliminate the statistical delay of firing is that of photoelectric priming, in which light from a source external or internal to the tube liberates photo-electrons from the cathode of the discharge gap in question. The efliciency of this method depends on the wavelength of the irradiating source and its relation to the photoelectric threshold of the cathode material. With many pure metal cathodes, the work function of the metal is so high that irradiation by ultra-violet light is required, which light is so attenuated by the glasses normally used for tube envelopcs, that the light source must be placed inside the envelope; an appreciable amount of power is then required for its production, While precautions may have to be taken to prevent excessive ionisation which would unduly lower the striking voltage of the discharge gap.

There have been various trialsand proposals in the past concerning the introduction of radioactive material into the envelopes of glow discharge tubes. Little quantitative data on the efliciency of radio-active priming has been published, however. The present applicants have made experiments with tritium, the radio-active isotope of hydrogen, which is a low energy fl-particle emitter of half life approximately twelve years. Besides difiiculties in inserting known quantities of tritium inside the tube envelope, the use of this gas involves the known troubles of hydrogen-filled tubes-namely clean up of the hydrogen. Q

A third, generally known method of eliminating statisticaldelay'is b'ythe use of a separate glow discharge within the tube envelope, the auxiliary dischargebeing arranged to be continuous during operation, including stand by quiescence, of the discharge tube. While this method is immediately efficacious, its use may not only involve discharge in a large number of quiescent tubes, but also needs especial care in electrode and circuit design to en sure that there is neither so great an ionisation coupling between the auxiliary and working discharge gaps that the striking voltages of the latter are reduced, nor that discharge transfers in undesired manner between an electrode of the auxiliary discharge gap and an electrode of a working gap.

In the present invention an auxiliary discharge gap is arranged in such manner that, not only is there little danger of encountering the difficulties just mentioned, but it serves the further function of controlling the potential at the inner envelope wall of a discharge tube. During the processing of tubes having bare metal cathodes, it is usually necessary to condition the electrodes by the passage of discharge current much larger than will be used during operation. Cathode material is thereby deposited on the tube envelope and, during use, wall charges on this coating may adversely affect the operation of the tube; it is, in any case, desirable that the potential of such an internal envelope coating be properly controlled.

According to the present invention there is provided a cold cathode gas-filled electric glow discharge tube comprising an envelope of insulating material enclosing discharge gap electrodes spaced therefrom, a conductive coating on the inside of the envelope, and a separate lead connected to the coating for setting up a discharge through the gas between the coating or the said lead and one of the aforementioned discharge gap electrodes or a lead connected thereto.

An embodiment of the invention will be described with particular reference to the use of the invention in the type of gas-gap switching tube disclosed in US. Patent No. 2,775,722 and reference will be made to the accompanying drawings in which:

Fig. 1 shows a front elevational cross-section through a tube embodying the invention;

Fig. 2 shows a side elevation, in partial section, of the tube of Fig. 1; a

Fig. 3 shows a typical circuit using a pair of the above tubes; and

' posed symmetrically one on each side of and parallel to an excessive power drain in maintaining a permanent Fig. 4 is a graph showing the voltage-current characteristic of the auxiliary discharge gap in the tubes of Figs. 1 and 2.

The tube of Figs. 1 and 2 has a glass envelope 1 formed at one end into'a press 2 through which are sealed electrodeleads 3. Mounted on respective ones of the leads 3 are a cathode 4 in the form of a flat plate, and two pairs of anode rods, 5 and 6, disposed symmetrically with respect to the cathode, the rods of each pair being disthe cathode plate. The lead 7 carrying the cathode is surrounded within and above the press 2 by a glass head 8 sealed into the press and, immediately below the cathode plate, by a glass sleeve 9. Each pair of anodev rods is formed of a single length of rod bent into the shape of a U. A portion of the interior of the glass envelope is coated with silver, as indicated at 10 in Fig. 2. A metal ring 11 (Fig. -l-) is a spring fit against the coating 10 and surrounds the upper ends of the leads carrying the anode rods.

One of the leads 3, indicated at 12 in Fig. 1, makes contact with the ring'll. I The cathode plate 4 is formed of molybdenum and the envelope is filled with helium at a pressure of 60 mm. of mercury. After processing, a film,

of'molybdenum forms over the envelope wall and contacts the coating ll) and ring 11. a

In the circuit diagram of Fig. 3 a pair of tubes as justdescribed is indicated at 13 and 13. The electrodes of these tubes are indicated by the same reference numerals as in Figs. 1 and 2. Except for the auxiliary priming gap the circuit and its operation is essentially the same as that described with reference to Fig. 2 of the aforementioned patent. The transformers 14 and 15- couplethe circuit with an audio frequency line. In the presence of glow dischargebetween the cathodes 4 and the respective anodes Sand 6, a low-loss path-isestablished between-the centre-tapped windings of transformers 14 andlSthrough the gap between each set of anodes.

cathode 4 of each tube is connected to earth through a choke 16 and rectifier 17 and each cathode is also connected through aresistor 18 to a pulse input terminal 19. A source of positive supply voltage is connected to terminals 20 from which current is fed through-therespective transformer windings to the pairs ofanodesthereafter maintained by the supply source connected to terminal-19. The speech path is broken, when desired, by raising the potential of both cathodes 4 and/ or lowering the potential of all anodes so that the anode-cathode voltage isheld below the maintaining potential long enough for deionisation of the gas to prevent refiring on re-establishing the maintaining voltage.

For the priming function, with which the present invention is concerned, each ring 11, and hence the associated envelope coating 10, is connected through a high value resistor 24 to a source of negative potential applied to terminal 25. Care must be taken that the stray capacity across this circuit is kept to a minimum so that a continuous discharge takes place between each ring- 11 and the adjacent lead supporting the anode rods 6; It ispreferred that this priming discharge pass to the pair of anodes to which the circuit-making starting pulse is applied. In the tubes of Figs. 1 and 2 this is achieved by mounting the cathode and anode rod system-slightly eccentrically with respect to the longitudinal axis of the envelope 1, so bringing the lead carrying one pair of anode rods somewhat closer to the ring 11 than the otheranode lead and the tube is connected in circuit so that starting pulses are taken to the preferred anode pair. In all other respects the electrode system is symmetrical.

With the supply potential value indicated in Fig. 3, resistors 24 are each 50 megohms and in a typical tube, the priming current to ring 11 is one rnicroampere, the voltage drop across the auxiliary discharge gap being 260. From the graph of Fig. 4 which shows-the voltagecurrent characteristic of the auxiliary gap in such a tube, it will be seen that for the region A of normal glow discharge, the maintaining voltage is about 154. In the region B the discharge is sub-normal, i.e. as the discharge current is decreased the interelectrode voltage must be increased to maintain the discharge. The operating point of 260 volts, 1 ,uA. is seen to be Well in the sub-normal discharge region. At this discharge current the potential of the coating 'is 60 volts.

An auxiliary discharge operating in the manner described above has proved to be very satisfactory in elimi: nating statistical delays without causing any substantial reduction in main gap breakdown voltage. As has been mentioned above, care must be taken to minimise stray capacity across the auxiliary cathode circuit as this pro-- duces a discontinuous discharge and prevents correct operation of the tube. p While the principles of the invention have been described above in connection with specific'embodiments,

In the absence of such discharge the line circuit is virtually broken. The

and particular modifications thereof, it is to be clearly understood that this descriptionis made-only by way of example and not as a limitation on the scope of the invention.

What we claim is:

1. A cold cathode electric discharge tube comprising an envelope of insulating material filled with an ionizable gas, a plurality of electrodes within said envelope defining a main discharge gap therebetween, conductive current carrying means for carrying space discharge current to given of said main gap electrodes, a conductive coating on the inside of said envelope and defining a priming discharge gap with said conductive carrying means, and additional current carrying means coupled to saidcoating for carrying space discharge current to said coating, said primary, discharge gap adapted to provide an ionization coupling with said main discharge 7 gap thereby to eliminate statistical delay in firing of said main gap upon application of current to said first current carrying means.

2. A glow discharge tube according to claim 1 wherein said main gap is defined between comprising a main 7 cathode in the form of a flat plate and two pairs of anode rods symmetrically disposed with respect to the said plate,

the rods of each said pair being disposed symmetrically one on each side of and parallel to the plate, said separate lead comprising a resilient metal ring contacting the said conductive coating on the inside of the envelope, the said cathode and the anode rods being mounted slightly eccentrically with respect to the said ring so that one of the said anode rods is closer to the said ring than any other discharge gap electrode.

3. A glow discharge tube according to claim 2 in which each pair of anode rods is mounted on a lead sealed through a glass press, the said cathode is mounted on a rod sealed through the said press in substantially the plane containing the said further rods and the said ring is positioned about the said further rods.

, discharge from said separate lead.

5. A glow discharge tube according to claim 1, wherein said main gap is defined between a main cathode in the form of a flat plate and two pairs of anode rods, each pair mounted on a separate connecting lead symmetrically disposed with respect to the said plate, the rods of each said pair being disposed symmetrically, one on each side of and parallel to the plate, said separate lead comprising a resilient metal ringcontacting the said conductive coating on the inside of the envelope, the said cathode and 3 the anode rods being mounted slightly eccentrically with respect to the said ring so that one of the connecting leads of said anode'rods is closer to the said ring than another discharge gap electrode.

6. A circuit arrangement comprising a glow discharge tube according to claim 2, in which the statistical delay of firing of a gap or gaps, between the said discharge gap electrodes, is eliminated by passage of a steady sub-normal discharge from said metal ring.

References Cited in the file of this patent UNITED STATES PATENTS

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