DE2656949A1 - ELECTRODE-LESS FLUORESCENT LAMP - Google Patents

Description

Electrodeless fluorescent lamp

(Addition to German patent application P 26 01 587.2)

The invention relates to fluorescent lamps that can be exchanged directly for existing incandescent lamps. In particular the invention relates to fluorescent lamps in which ionization is caused by a transformer contained in the lamp envelope.

The incandescent lamp is the main lighting for household and residential lighting. This lamp generally contains a filament in a specific non-oxidizing atmosphere contained in a teardrop-shaped envelope or a corresponding flask

709827/0264

is included, the filament for example in an Edison socket is attached, which is screwed into a permanent attachment or into a movable socket.

Despite their widespread use, incandescent lamps are relatively low in efficiency, producing only 15 to 17 lumens / watt of input power, and have relatively short, unpredictable maintenance times. Fluorescent or fluorescent lamps, which have efficiencies of around 80 lumens / watt, are an attractive alternative to incandescent lamps. However, conventional fluorescent lamps require a long tubular envelope which, together with the required ballast, has somewhat limited their inclusion in house lighting. Increased use of fluorescent lamps for household purposes, with the associated energy savings, can be achieved with the development of fluorescent lamps that are directly compatible with existing sockets and incandescent lamp fixtures.

Electric lamp technology has long sought electrical discharge devices that produce visible light for general lighting purposes without the use of electrodes as the base of a glow or arc discharge. Although the concept of the electrodeless discharge lamp is very old, such lamps always contained the principle of coupling electrical energy into a hermetically sealed gas-containing envelope by means of a ferromagnetic or air-core transformer in order to avoid the use of electrodes. Such devices have never proven practical or commercially feasible because it has been impossible to obtain reasonable light emission efficiency. This was due to the use of iron or air core transformers due, for example, to their core losses.

It is already known to excite electrodeless gas discharge lamps using electromagnetic induction to electrical energy in the Entlädungskoiben t \ x Transfer. Experiments in this direction show that such agents inflate

709827 / 02S4

have been highly impractical. If an air core transformer is used, leads to the ineffectiveness of the coupling process, which leads to the achievement of reasonable input power for the Gas discharge is required, resulting in a loss of power from radiation, which is detrimental and can be dangerous. That's why such devices have never worked successfully at any reasonable efficiency for useful periods of time.

Another known alternative is the use of ferromagnetic or iron cores. However, such cores can only be used at very low frequencies so that the eddy current heating the iron does not damage the core. When using alternating current, it is extremely difficult to operate an iron-core transformer for the transmission of energy of this type at frequencies greater than 5 or 10 kHz Core range from about 80 to 90 % . Therefore it becomes clear in a simple manner that air-core and iron-core transformers cannot be used from a practical point of view at the high frequencies which are required for efficient operation of the gas discharge lamps according to the invention.

Fluorescent lamps are disclosed in U.S. Patents 3,500,118 and 3,521,120 described, which use a magnetically induced high frequency electric field to a gaseous radiation medium to ionize. The elimination of discharge electrodes within these lamp envelopes extends their life essential and allows lamp shapes to match household lighting needs are more easily compatible.

US Pat. No. 3,500,118 describes an improved electrodeless fluorescent lamp with a high-frequency power supply. This arrangement, although quite useful, was quite bulky and comprised a large tubular discharge ring, several ferrite

709827/0254

'44.

cores and a remotely mounted power feed that made the arrangement unsuitable for use in many industrial and household applications.

U.S. Patent No. 3,521,120 describes a more compact lamp configuration, but this lamp still had a high frequency magnetic field in the air surrounding the bulb and thus provided an unpleasant source of electromagnetic radiation and interference

It is therefore an object of the invention to provide a To create fluorescent or fluorescent lamp, which is an economical replacement for a conventional incandescent lamp suitable is.

This object is achieved by a fluorescent lamp that can be accommodated in a spherical or teardrop-shaped structure, as is typical for household incandescent lamps. A ring-shaped magnetic core, which is contained within the lamp bulb, is excited by a high-frequency magnetic field. The field induces an ionizing electrical discharge in a gaseous medium within the piston or the envelope. The radiation from the gas excites a conventional lamp phosphor on the inner surface of the envelope and / or on the outer surface of the core to produce visible light. The core can be coated with a glass layer in order to maintain the vacuum and to facilitate the coating with the phosphor. Furthermore, conductive means are provided in order to dissipate heat developed within the core from the lamp envelope.

The invention will now be further features and advantages with reference to the following description and drawings Embodiments of the invention explained in more detail.

709827/0254

Figure 1 and la are complete fluorescent lamp assemblies.

Figure 2a is a front view of one embodiment of the lamp with one perpendicular to the base conductors Transformer axis.

Figure 2b is a front partial view of the plasma within the lamp of Figure 2a.

FIG. 2c is a side view of the plasma according to FIG. 2b.

Figures 2d and 2e are sectional views of the plasma according to Figures 2b and 2c.

Figures 3a and 3b are sectional views of a transformer core with a circumferential heat transfer belt.

Figure ¹ I is a front view of a lamp according to an embodiment of the invention with a base to the conductors extending parallel transformer axis.

Figure 5 is a sectional view of a transformer core with an axial heat transfer ring.

FIG. 6 is a schematic representation of the operating circuit for the lamps according to the invention.

The operating principles of electrodeless fluorescent lamps are described in the aforementioned US Pat. Nos. 3,500,118 and 3,521,120.

Figure 1 is an external view of a lamp structure, the parallel or may contain perpendicular transformer core orientations, as described in more detail below. One with phosphorus

709827/0254

coated translucent envelope 11 contains an ionizable Gas and an excitation transformer (not shown). One Pest body power supply and ballast circuit are included in a base assembly 12 that is associated with the lamp envelope 11 is connected. A conventional Edison screw socket 13, which is inserted into the base assembly 12, can power of conventional Take up light bulb sockets. The complete structure is similar to a conventional incandescent lamp with, for example, a bulb diameter 7.6 cm (this is sometimes referred to as an A-24 lamp size) and it is compatible with lighting equipment, designed for such a configuration. The base arrangement can be permanently connected to the casing 11 (Figure 1) or have a detachable shape 12A (Figure la) to a separate replacement or repair of individual lamp components to allow.

A preferred orientation for the inner lamp components is shown in the front sectional view according to FIG. 2a. A substantially spherical or teardrop-shaped evacuated lamp envelope 11, which may be made of glass, is shaped using known techniques. A portion of the lamp envelope forms a base 11a which is pierced by two metal support rods 15 which are connected to the glass in a known manner in order to form vacuum seals 16. A winding made of electrically conductive material 17, which can be insulated with glass fiber material, for example, is arranged between the metallic holding rods 15 and connected by a toroidal transformer ferrite core 18, which is thereby held within the lamp bulb 11. In this exemplary embodiment, the winding ends 17 a are oriented in such a way that the axis of the core 18 is arranged perpendicular to the holding rods 15. The specific winding configuration is determined by the input operating voltage of the lamp. Typically, the windings can be chosen so that one turn is taken on the core for every 5 volts input voltage.

709827/0254

The space within the piston or the envelope contains an ionizable one Gas 19, which can be chemically identical to that used in conventional fluorescent lamps and which is a mixture of a noble gas such as krypton and / or argon with mercury vapor and / or cadmium vapor. The inner surface of the glass envelope 11 and the outer surfaces of the transformer core 18 are colored with a suitable ultraviolet / visible Light phosphor 20 coated, such as a calcium haloapatate, as they are well known. These phosphors are able to absorb the ultraviolet radiation from mercury vapor, which generally peaked at about $ 2537 and emit radiation within the visible spectrum when excited as a result, in order to achieve a highly efficient and to generate pleasant light output. In this embodiment of the invention, the generation is not based on the ionized gas the emission of light, but rather the generation of radiation that causes light emission from a fluorescent phosphor. As is well known, this system allows a relatively efficient use of power, since the required light emission is not based on the gas itself, but only the radiation to excite the Phosphorus. A source 21 of high frequency electrical power which is outside the lamp envelope and preferably inside the base assembly is attached, causes a current flow through the support rods 15 and the primary winding 17 of the transformer, whereby the core 18 is excited with a magnetic field. The core induces an electric current flow in the gas 19, ionizes this gas and stimulates the emission of ultraviolet radiation at about 2537 S.

In a manner typical of conventional discharge lamps, the ionized gas represents an electrical load with negative impedance, the would destroy an unprotected low impedance power source. A pres ehalt impedance 24 can, for example, with the Le; L-

connected in series stungsquelle 21 and a support rod 15 in the usual way to provide sufficient positive impedance to offset the negative impedance of the gas. This will make the Power source is given a load with positive impedance, thereby ensuring stable operation. Alternatively, a current

709827/0254

-Sf-

• / foT

Limitation be built into the power source 21 in order to exercise an active ballast function. Obviously, the choice of core material is an important factor in the operation of this lamp. While similar lamp configurations with air or iron cores have been described in the literature so far, it has now been found that the losses inherent in the operation of these known cores prevent the construction of a practical lamp. As indicated in the patents already mentioned, the Perritmaterial must be selected to provide high permeability and low internal heat loss at the frequency of operation. As is known, a ferrite is a ceramic-like material which is characterized by ferrimagnetic properties and usually has a spinel structure with a cubic crystal lattice and which, for example, has the generalized form MeFe ₂ O ^, where Me represents a metal atom.

According to the present invention, it is necessary that the cores used are made of such a material and have a configuration that the core losses are not greater than 50 % in order that the electromagnetic energy can be effectively coupled into the light source. Similarly, lower core losses reduce core heating and minimize the possibility of damage and maximize core efficiency. Preferably, the core losses are kept to less than 25 % of the total input power at the operating temperature of the lamp.

A core material with high permeability is required to ensure adequate coupling of the high frequency energy with the gas with minimal electromagnetic radiation. A core with a relative permeability of at least 2000 is advantageous. Suitable ferrites are available with these characteristics over the frequency range of 25 kHz to 1 MHz. From the standpoint of reducing core losses, an operation is at

709827/0254

high frequency desirable; but the cost of currently available semiconductors for use in the high frequency Power source 21 limits the maximum frequency at which a practical economic lamp can be operated to about 50 kHz. Among other materials, it was found that Ferrite Type 8100 manufactured by Indiana General Corp. Keysbee, New Jersey, manufactured and characterized by losses of less than 120 mW cm "'at 1000 Gauss peak flux density (at operating temperature) for operation at 50 kHz, for one use is suitable in the lamp according to the invention.

It is also possible to make the core from other materials with smaller ones build magnetic permeability, for example, a composite powdery polyimide resin with a small Permeability of 40. With such a lamp, the ampere turns of the excitation winding must of course be proportional be enlarged.

For example, in a typical 40 watt lamp, the transformer core is 18 has a thickness of 1.3 cm, an inner diameter of 3 cm and an outer diameter of 5 cm. The magnetic flux rod inside the core is about 1000 Gauss.

Although it is possible to use a bare ferrite core within the vacuum envelope, it has been found desirable to glaze the core 18 with an impermeable glass layer 25 prior to coating with the phosphor 20. The glass layer ensures a minimal degassing of the ferrite ceramic and equips conventional heating techniques (Lehring) when applying the phosphorus coating. The core may wish into two or more sections may be divided, allow an economic installation of the lamp.

709827/0254

- vtr-

In operation, the ionized gas forms a link between the transformer or coupling plasma. The shape of this plasma can be changed over the area by changing the gas pressure within the lamp from about 0.2 to about 3.0 Torr. A gas pressure of about 1 torr has been found to generate a gas plasma j that illuminates the lamp uniformly. Unless otherwise stated, all gas pressures refer to room temperature.

The shape of a typical plasma 22 is shown in FIGS. 2b and 2c together with the sectional views according to FIGS. 2d and 2e. In these views, all of the lamp components except the transformer core 18 have been shown for clarity omitted.

Despite the improved efficiency of the ferrite materials specified above, heat above 10 watts must be dissipated through the transformer core of a kO watt lamp. Since the core of this lamp works essentially in a vacuum, heat dissipation is a critical problem. For example, it is known that the Curie points of ferrite materials, which are suitable for applications of the present type, may be below 150 ⁰ C and that further decrease the efficiencies of phosphorus at temperatures above 120 ^0C. As a result, these temperatures require lamp designs that allow lamp operation below these temperatures.

709827/0264

Figures 3a and 3b illustrate a heat dissipating from the core structure, 1st allocated for use with the lamp of FIG suitable. A metal band 29 is connected to the outer periphery of the core 18 and welded to the support rods 15. A glass layer 25 is applied over the core 18 and the metal strip 29, to provide thermal contact and a base for the phosphor 20th The support rods 15 conduct heat through the base 11a of the lamp envelope. An electrical connection to the primary winding 17 of the transformer can be made by two electrical connecting rods 15a or in the manner shown in FIG. 4 .

An embodiment of the lamp according to the invention, in which the heat dissipation is increased and which provide a uniform phosphor lighting, is shown in Figure k . In this lamp , a glass bulb 11 with a base 11a is produced in a manner similar to that in the exemplary embodiment according to FIG . 2a. The base is pierced by two support rods 15 and an electrical contact rod 15a with vacuum seals 16. One end face of an annular core 18 is connected to a heat-dissipating metal ring 23, which in turn is welded to the support rods 15 and held by them. A primary winding 17 is wound on the core and connected at 15b between the electrical contact rod 15a and one of the support rods 15. In a manner similar to that in the exemplary embodiment according to FIG. 2a, the primary winding is connected via bars 15 and 15a to a reserve impedance 24 and a high-frequency power supply 21 which excites the core 18. The heat generated within the core is thereby effectively conducted to the ring 23 and from there to the support rods 15 through which it is conducted out of the vacuum flask 11.

Figure 5 illustrates the details of what is used in this lamp The metal ring 23 is connected to one end face of the core 18. The ring 28 can be made of copper,

709827/0254

Aluminum, beryllium or another material with a high thermal conductivity compared to the core. the Support rods 15 are welded to the ring 23 and form a heat conduction path from the core to the outside of the vacuum envelope (Not shown). A glass layer 25 is applied over the core 18 and the metal ring 23 to provide a good thermal bond ensure to reduce the degassing to a minimum and to form a basis for the phosphorus 20.

According to a further embodiment of the present invention an instantly starting lamp is created. It is well known that a relatively low voltage is sufficient, to keep a gas discharge operating once an arc has occurred, but generally a high one Tension required to produce a first flashover. This is even in the presence of an easily ionizable inert Gas the case, such as argon, to cause a first or initial flashover to ionize To facilitate mercury, the common metal vapor that carries the discharge and is used in gas vapor discharge lamps. In In many cases this high voltage is used to start by a mechanical starter with capacitive or inductive elements which suddenly disconnects and thereby delivers a high voltage surge to the electrodes of the device to initiate an initial To effect ionization. Alternative arrangements involve the use of ballast transformers to make the required Supply voltage. According to the present invention, this problem is solved in a quick, inexpensive and simple manner by that an auxiliary secondary winding on the core with enough turns to operate as a step-up secondary winding, is used to tap a very high voltage that can be applied to the lamp in order to induce a high starter voltage to cause that quickly causes the initial flashover required for the operation of lamps according to the present Invention is necessary.

709827/0254

Figure ¹ J provides a method for starting the ionization in the gas 19. A secondary Hochspannungswicklunp 27 is applied to the core l8 and covered with a glass layer. When excited for the first time, the magnetic field within the core induces a high voltage across the winding ends 27a, thereby forming a glow discharge therebetween and thereby ionizing the gas 19 first. The plasma 28 in this lamp is in the form of a hollow toroid which surrounds the core structure of the lamp.

The saturation flux density in core 18 must be high enough for the starter voltage to develop. For the lamps of the described For example, the saturation flux density must be at least 1500 Gauss and 750 Gauss for operation at 50 and 100 kHz, respectively be.

FIG. 6 shows schematically an electrical circuit arrangement which can be used in the operation of these lamps. The high-frequency power source 21 receives power at line voltage and line frequency from the Edison screw base 3 * J and supplies an output variable at about 50 volts at 50 kHz for exciting the transformer core l8 via the primary winding 17. The winding ends 27a are excited by the secondary high-voltage winding 27, to create a glow discharge in the lamp. The plasma 22 thus formed within the lamp then forms a secondary winding which has a single turn and which draws power via the transformer core 18. A ballast impedance 2k, which is connected in series with the high-frequency power source 21 and the primary winding 17, limits the plasma current in order to ensure stable operation.

The high frequency power source can have a known structure. For example, that described in U.S. Patent 3,521,120 Inverter circuit be suitable for use in lamps that work in this power range.

709827/0254

From the foregoing description it is clear that a lamp has been described which physically and electrically co-exist Household incandescent lamps is compatible. This fluorescent lamp can be used in existing incandescent lamp sockets and delivers up to three times the light output of comparable existing units. The lamp does not contain any electrodes inside of the vacuum bulb and thereby avoids one of the main reasons for errors in common known fluorescent lamps.

The high frequency magnetic field created with the induction of gas ionization connected is limited to a closed ferrite path, which minimizes electromagnetic interference is reduced.

Constructed in accordance with the preferred embodiment of the invention Lamps are particularly useful because the electrodes have been eliminated, which are the source of many constraints of known fluorescent lamps currently in use. Thus, for example, an electrode can never be burned off 'be the reason for a failure in a lamp according to the present invention. The spraying of electrode materials is similar on the inner surface of the lamp walls completely eliminated, whereby a darkening of the lamp walls was effected.

709827/0254

Blank page

Claims (1)

Expectations 1. "fluorescent lamp with an evacuable, translucent one Envelope with a substantially spherical upper capsule, characterized by a closed one Loop-forming magnetic core (18) contained in the envelope (11), means (17) for exciting the core (18) with a high-frequency magnetic field, a gaseous medium (19) within the envelope (11) which has an electrical Discharge due to an electric field induced therein by the magnetic core (18) maintains and thereby radiation emitted at a first wavelength, and a luminous phosphor (20) at least on the surface of the core (18) and on the interior of the envelope (11), the visible light when excited by the radiation with the first wavelength emitted. 2. Lamp according to claim 1, characterized in that the core has a magnetic permeability of at least 40 and a power loss of not more than 50 % at the lamp operating temperature. 3. Lamp according to claim 2, characterized in that the lamp has a power loss of not more than 25 % at the lamp operating temperature. 4. Lamp according to claim 3 »characterized in that the core has ferrite. 5. Lamp according to claim 1, characterized in that that the means for exciting the core numerous substantially parallel metallic support rods (15), the pierce the sheath (11) at its base section (lla), a winding (17) with numerous turns which concatenate the core (18) and are connected to the rods (15) for Supply of high frequency energy to the core, and means for generating the high frequency energy include. 70 9 827/0 254 originally inspected 6. Lamp according to claim 5, characterized in that the means for creating the high-frequency Energy include a power feed (21) that converts the input energy into a high-frequency output energy converts. 7. Lamp according to claim 6, characterized in that the power supply (21) in one cylindrical part 12 is attached, one end of which with the outside of the base portion of the casing (11) is connected and the other end is attached to a lamp base (13) which can be inserted into a lamp socket, wherein the power supply (21) with the lamp base (13) for receiving input energy and with the support rods (15) is connected to the transmission of the high-frequency output energy. 8. Lamp according to claim 7, characterized in that that the cylindrical part (12) is releasably attached to the base portion of the envelope. 9. Lamp according to claim 6, characterized in that the power supply (21) has a high frequency Provides output energy to the winding at a voltage equal to approximately 5 volts multiplied by the number of turns the first winding (17). 10. The lamp according to claim 6, characterized in that the high frequency energy has a frequency between about 25 kHz and about 1 MHz. 11. The lamp according to claim 10, characterized in that the high frequency energy has a frequency of about 50 kHz. 12. Lamp according to claim 11, characterized in that that the magnetic core has a magnetic saturation 709827/0254 -Af- has a flux density of at least 1500 Gauss at the operating temperature of the lamp. 13. Lamp according to claim 5 _S, characterized in that the core (l8) is coated with a glass material which is impermeable to gas and suitable for receiving the phosphor (20). 14. Lamp according to claim 13 _}, characterized in that the core (18) is connected to a metallic, heat-transferring part (23). 15. Lamp according to claim 14, characterized in that the metallic thermally conductive part (23) made of copper, beryllium or aluminum. 16. Lamp according to claim 14, characterized in that the magnetic core (L8) is ring-shaped and the thermally conductive member (23) is a metal tape that is attached to the outside periphery of the core and to one or more the metallic support rods (15) is connected. 17. Lamp according to claim 14, characterized in that that the magnetic core is ring-shaped and the heat-conducting part is a flat ring with a flat one Surface of the core and one or more of the metal support rods is connected. 18. The lamp of claim 5 _S characterized in that further at least one pair of sealed with glass metallic Starterelek ^ broden disposed within the enclosure (11), and means are provided for forming a glow discharge between the starter electrodes. 19. The lamp according to claim 18, characterized in that the glow discharge forming 709827/0254 Means is a glass-coated winding (27) on the magnetic core (18), the winding ends of which are the starter electrodes form. 20. Lamp according to claim 1, characterized in that the gaseous medium is a mixture is made up of a noble gas and mercury vapor, cadmium vapor, or mixtures thereof. 21. Lamp according to claim 20, characterized in that the noble gas krypton, argon or a Mix of it is. 22. Lamp according to claim 20, characterized in that the pressure of the gaseous medium (19) is between about 0.2 torr and about 3> 0 torr. 23. The lamp according to claim 20, characterized in that the pressure of the gaseous medium is marked is about 1 Torr. 24. Lamp according to claim 5, characterized in that the magnetic core (L8) is ring-shaped and the axis of the magnetic core is substantially parallel to the metallic support rods (15). 25. Lamp according to claim 5, characterized in that the magnetic core (L8) is ring-shaped and its axis is substantially perpendicular to the metallic support rods (15). 26. Lamp according to one or more of claims 1 to 25, characterized in that the Envelope (11) has a substantially spherical upper shell portion and a flat circular base portion (11a), the circular magnetic core (18) has a right-angled Cross-section, with the core axis essentially 709827 / 02B4 borrowed perpendicular to the plane of the base section (lla), the flat metal ring (23) is connected to the core surface facing the base section (Ha), the ends of the support rods (15) piercing the base section (Ha) are connected to the metal ring (23), furthermore at least one metal connecting part (15a) the base section (Ha) pierced, a first winding (17) is arranged on the core (18), one end of which is connected to one of the support rods (15) and the other end is connected to one of the metallic connecting parts (15a), and means for inducing a high-frequency voltage is provided between the one support rod (15) and the one connecting part (15a), so that a high-frequency magnetic field in the core (18) and the electric field in the gaseous medium (19) can be induced is. 27 · Lamp according to claim 26, characterized in that the flat metal ring (23) and the Core (18) are coated with a glass material (25). 8. Lamp according to claim 27, characterized in that the glass material (25) is impermeable to gas is. 29. The lamp according to claim 26, characterized in that a second winding (27) around the Core (18) is wound, the ends of which maintain a glow discharge for ionizing the medium (19). 30. Lamp according to claim 29, characterized in that the second winding (27) with glass is covered. 31. Lamp according to claim 27, characterized. That the high-frequency voltage inducing Means comprise a high frequency power source, 709827/0254 ■ C- which is arranged outside the envelope (11) and the high-frequency Provides voltage between the one support rod and the one connecting part. 32. Lamp according to claim 31 »characterized in that a substantially cylindrical Base element is provided, which surrounds the high-frequency power source and one end of which with the flat circular base part is connected and from the the other end of electrical energy from a utility power source receives and transmits to the high frequency power source. 33 · Lamp according to claim 32, characterized in that the other end of the cylindrical Base element can be used in a lamp socket for receiving electrical network power. 34. Lamp according to one or more of claims 1 to 32, characterized in that the gaseous medium (19) has a mixture of noble gas and mercury vapor. 35. Device for maintaining an electrical discharge in a gaseous medium contained in an evacuable envelope by a magnetic core forming a closed loop (18) within the sheath (11), a core (18) concatenating winding (17) for exciting the core with a high-frequency magnetic field and means for generating a high-frequency electric current within the winding (17). 36. Apparatus according to claim 35, characterized in that the magnetic core has a magnetic permeability of at least 40 and a power loss of not more than 50 % at the operating temperature of the lamp. 709827/0264 37. Apparatus according to claim 36, characterized in that the power loss is not more than 25 % . 38. Apparatus according to claim 37, characterized in that that the magnetic core has ferrite. 39. Apparatus according to claim 37, characterized in that the magnetic saturation flux density at least 150 gauss at the operating temperature of the lamp amounts to. 40. Apparatus according to claim 35, characterized in that the means forming the stream numerous electrically isolated metal elements (15), which penetrate the sheath (11), one end of the winding (17) with an element and the other end of the winding connected to another element, and a source of high frequency voltage that is outside the envelope is connected between the two metal elements. 41. Apparatus according to claim 35, characterized in that the core (18) with a gas-impermeable Material is covered. 42. Apparatus according to claim 35, characterized in that the gaseous medium (19) has a Pressure between about 0.2 torr and about 3.0 torr. 709827/0254