US1933329A - Electric discharge device - Google Patents

Description

Oct. 31, 1933. A. w. HULL I ELECTRIC DISCHARGE DEVICE Filed Feb. 28, 1930 inventor:

Albert \x/. Hull,

(4 2601) HLS Attorney;

Patented Oct. 31, 1933 UNITED STATES PATENT OFFICE ELECTRIC DISCHARGE DEVICE Albert W. Hull, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York The present invention relates to electric discharge apparatus, more particularly to those devices which are subjected to high voltage, such as lightning arresters, X-ray tubes, cathode ray tubes, high voltage rectifiers, glow lamps and the like. This application is a continuation-in-part of my application Serial No. 328,759, filed Dec. 27, 1928, and entitled Lightning arresters.

In the art of translating high voltage electrical energy there may be employed vacuum devices which utilize an envelope of insulating material such as glass and conductors sealed at the ends of the envelope through which energy is conducted to electrodes positioned within the tube. In the case of highly evacuated tubes, it has been found that a dischargefrequently occurs over the surface of the glass, either inside or outside, even though the glass be clean and dry, at a voltage which is much less than that which should cause a similar discharge through air. It is probable that such discharges are started from the vicinity of a terminal by corona effect from the exterior of the tube or by glow discharge on the inside surface of the glass, the discharge apparently progressing along the surface of the tube from some initial point. Moreover, when the tube is operated at exceedingly high voltage, for example of the order of many thousands, the glass is subjected to intense electrical stress which may give rise to structural defects, such as checks or minute cracks, along the interior surface of the tube. In order to minimize some of the deleterious effects, the section of glass between the charged conductors has been lengthened heretofore but envelopes of this character are often unduly long when designed for extremely high voltages.

I have discovered and in accordance with my invention, that it is possible to obtain protection in a substantial degree from the harmful effects which normally accompany the use of high voltages without resorting to large dimensions of envelope. In brief, this result is accomplished by corrugating or otherwise changing the surface continuity of the envelope thereby obtaining the advantage of long insulation distances in tubes of reasonable size. It has further been found that in deforming the configuration of the normally smooth envelope whereby the interior as well as the exterior surface is rendered discontinuous, the checking or cracking of the glass which takes place on the interior surface, is completely avoided. The invention thus presents a sharp line of demarcation over the common form of solid insulator provided with transverse corrugations. Moreover, it is well known that with solid insulators whose surfaces are never wetted there is little advantage to be gained in preventing flash-over in increasing the surface distance between terminals by the use of corrugations and the like.

While the principles of my invention apply to all high voltage tubes, gas-filled or highly evacuated, wherein large electrical stresses would normally be present in the envelope material, the invention is illustratively exemplified in the form of an improved lightning arrester and rectifier. Thus, in Fig. 1, there is shown a lightning arrester or two-way conducting device for protecting high voltage transmission lines from abnormal voltages and currents; Fig. 2 illustrates a unilaterally conducting device for rectifying high voltage alternating current, and Fig. 3 shows an application of the invention to an X-ray tube. It will be understood that these illustrations are to be considered generic of all types of high voltage devices which employ vitreous envelopes.

Referring to Fig. 1, numeral 1 designates a pair of non-thermionic or cold electrodes spaced a sufficient distance apart to constitute a substantially open circuit for normal voltages but to allow excess current to drain off when subjected to abnormal voltages as when lightning strikes the line to which it is connected. These electrodes are supported by rigid conductors 2 and are contained within a highly evacuated glass envelope 3 which terminates at each end in a reentrant stem 4. A pant leg 5 surrounding a portion of the rod 2 is preferably fused to each stem in order to prevent long discharges from the stem due to gas that may be liberated by the discharge of the tube. In accordance with my invention, the envelope 3 is provided with a plurality of corrugations 6 which extend transversely of the electrical discharge. These corrugations, preferably formed by molding, are curvilinear and have a sufficiently gradual slope to avoid regions of excessive mechanical stress and also have optimum depth without unduly increasing the diametral dimension of the tube.

As is stated in my copending application referred to hereinbefore, a device of this character may be employed as a by-pass for excessively ions. The electrodes should therefore be constituted of a metal which readilyevolves vapor upon the arrival of a steep wave front so that breakdown of the tube may begin without delay. The metal should also be such that its vapor instantly condenses upon cessation of the electrical surge so as to remove the source of positive ions. I have found that materials such as graphite and particularly copper, tungsten and aluminum serve these purposes satisfactorily. In order to withstand the normal high voltage on the transmission line, it is necessary that no residual gas be present in the envelope and hence the metal members must be thoroughly ole-gassed in any well known and suitable manner. The envelope must also be evacuated to the greatest possible extent. The electrodes 1, 1, are made as large as may be conveniently slipped past the corrugated portion of the tube. For the protection of an 11 kv. line, I have successfully used a tube such as is shown in which the electrodes are aluminum discs 1" in diameter, spaced 1 mm. apart, the tube having a total length of approximately 6" and exhausted to the extent that the internal pressure is materially less than 1 micron.

I have found that a discharge tube provided with corrugations will withstand a much higher voltage applied to its terminals without breakdown than is possible with a tube not so provided. It has been noted that when a tube is not provided with corrugations and is subjected to extremely high voltage, there may be formed on the interior surface of the envelope a number of minute fissures. The latter represent failures of the glass and in time may give rise to air leakage and possibly tube rupture. However, when corrugations or other surface discontinuities are provided these cracks are entirely avoided. It is possible that the interior surface of the corrugations serves to break up the strong electrical lines of force in the region of electrodes and hence prevent excessive stresses being set up within the body of the glass. These corrugations also tend to prevent continuous layers or regions of condensed material from forming on the interior surface of the envelope which might blacken the bulb and in certain cases, may cause short-circuit. The effect of the corrugations on the exterior surface of the envelope is to provide long creepage distances between the ends or terminals of the conductors 2, 2, thereby precluding in a substantial degree flash-over and at the same time avoiding the necessity for an unduly elongated tube. Inasmuch as the thickness of glass remains the same throughout the corrugated section, the insulation between the conductor terminals is also increased.

It is apparent that my invention finds particular utility in high vacuum tubes which are designed to conduct currents when voltages of the order of many tens of thousands are applied to the electrodes. Examples of this type of device in addition to the lightning arrester described hereinbefore are X-ray tubes, both selfrectifying and non-rectifying, high voltage rectifiers for energizing X-ray tubes or similar apparatus, also cathode ray tubes and the like. However, as will be explained in connection with Fig. 2 which illustrates a rectifier, certain of the advantages of the invention are also applicable to tubes which operate at relatively lower voltages and which may contain gas or vapor. In this figure, numeral 8 designates an envelope which may be highly evacuated or may contain gas or vapor at suitable pressure. The cathode shown is the usual type of shielded construction suitable for gas as well as high vacuum conditions. The filament 7 constitutes a heater for rendering the cylindrical cathode 8 electronically active which may be coated with an active metal oxide. The anode 9 also is of well known construction and is provided with a large heat radiating surface. Combined lead-in conductors and supports 10 sealed in reentrant stems 11 position the electrodes within the envelope. It will be observed that these stems are not provided with pant legs as in Fig. 1 but with one or more corrugations 12 similar to those in the envelope. If such a tube were energized by voltage of the order of 40,000 or more for example, the corrugations would serve advantageously in preventing surface creepage during the inverse voltage cycle, also would present adequate insulation between the charged conductors 10, 10 without necessitating an unduly long envelope. It will further be evident that the improved conditions are also present in a grid-controlled arc discharge device made in accordance with my invention such as would be exemplified in Fig. 2 by positioning an eleetrostatically controlled grid (not shown) in the region of the anode and bringing out a lead through the side of the envelope in the well known manner. A gas or vapor at a pressure between 1 and 100 microns may constitute the source of positive ions in a device of this sort. As in the case of the rectifier the corrugations in the envelope and stem would serve to prevent electrical breakdown when high voltages are applied between the anode and cathode members. While in this figure I have shown the corrugations or other surface discontinuities as extending over the entire length of the envelope, it is apparent that the corrugations may be provided only at those places where excessive electrical stresses are found to exist.

In Fig. 3, which illustrates, partly in section and partly diagrammatic, an X-ray tube improved in accordance with my invention, the envelope 14 is shown in bulbous form and has corrugated extensions 6 at each end. The cathode end is provided with an elongated stem 15 which may be provided with corrugations 12 and which may serve as a support for the usual filament 16 and focussing cup 17. The combined anode-target 18 may be supported from the other end of the envelope in the usual manner and if desired, the reentrant stem 19 may also be corrugated as at the cathode end. It is obvious that the corrugated section in the envelope and stem serves to increase the creepage and insulation distances between the high voltage terminals considered from the useful voltage as well as the inverse voltage standpoint and aids in the prolongation of the operating life of the device. 1

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An. electric discharge device adapted to withstand. relatively high voltage, said device comprising -a highly evacuated envelope of insulating material terminating in reentrant stems, electrodes mounted in the envelope, the terminals for said electrodes extending through the reentrant stems, said envelope and stems being provided with corrugations of insulating mateevacuated cylindrical envelope of insulating material, electrodes mounted therein and terminals for said electrodes extending through the envelope at opposite ends thereof, said envelope having its walls of uniform thickness and corrugated transversely of the longitudinal axis of the envelope.

3. A lightning arrester comprising a highly evacuated cylindrical envelope of insulating material, electrodes mounted therein and terminals for said electrodes extending through the envelope at opposite ends thereof, said envelope having its walls of uniform thickness and corrugated transversely of the longitudinal axis of the envelope, said electrodes being constituted of a material which readily vaporizes when subjected to heat and the vapor of which immediately condenses when the source of heat is removed.

4. A lightning arrester comprising a highly evacuated cylindrical envelope of insulating material, electrodes of copper mounted therein and terminals for said electrodes extending through the envelope at opposite sides thereof, said envelope having its walls of uniform thickness and corrugated transversely of the longitudinal axis of the envelope.

5. An electric discharge device comprising a highly evacuated envelope, electrodes mounted therein, terminals for said electrodes extending through said envelope at opposite ends thereof, said envelope being of uniform thickness and corrugated over substantially its entire length, said corrugations extending transversely of the shortest line drawn over the envelope between said terminals whereby the electrical leakage distance over the interior and exterior surfaces of the envelope is increased.

ALBERT W. HULL.

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