US2254957A - Electric source of light - Google Patents

Description

Sept. 2, 1941.

Z. BAY ET'AL ELECTRIC SOURCE OF LIGHT Filed Nov. 12, 1940 inventors Zoltan Bag L5 SE@ eti GHPQ heir` Attorney Patented Sept. 2, 1941 2,254,951 ELECTRIC soUnCE oF LIGHT Zoltan Bay, Ujpest,.and Gyrty Szilreti, Budapest,

Hungary, assigner-s to General Electric Company, a corporation of New York Application November 12, 1940, Serial No. 365,374 In Hungary October 23, 1939 11 Claims.

The invention relates to an electric source of light of such kind as converts electrical energy in a direct manner into luminous energy, and in which it is not necessary to cause the light-radiating solid body employed for the production of the light to be heated by means of the current to the temperature required for the emission of any thermal radiation, the radiation of light being obtained instead by utilizing, on the basis of a new phenomenon, certain special properties of the light-radiating body.

The electric sources of light employed up to now for the most part utilize the thermal radiation of solid bodies heated by means of electric current; further, increasing use is being made of the luminous phenomena set up during the passage of electric current through gases, and of the phenomenon of so-called fluorescence, which latter, as well known, consists in the fact that certain substances, under the iniiuence of ultraviolet light or of cathode rays, emit visible light.

All these sources of light possess certain drawbacks, the most important among which are the following: With electric incandescent lamps or thermal radiators comprising solid incandescent bodies temperatures around 3000 C. were the `highest which it has been possible to reach so as to ensure at the same time a satisfactory term of life. Now at this temperature the colour of the luminous radiation is still very far from the colour of the white light corresponding to the colour of sunlight, seeing that the temperature of the sun is about 6500 C. In addition thereto the luminous efficiency of these lamps is, in view of the present degree of development of technical science, unsatisfactory and thus it is mainly to their simplicity, ease of handling and other practical advantages that the wide use made of them is due. As to lamps based on a discharge through gases or vapours, it is a well-known fact that their characteristic is generally negative, because their terminal voltage diminishes with the increase of the intensity of the current passing through them. In order to compensate the negative characteristic, it is, in case of feeding the lamps from sources of energy of constant voltage as generally employed today, e. g. from mains systems, necessary to connect series resistances, choke-coils, reactive transformers, or condensers in series with the lamps, which fact results in a substantial increase of the iirst cost of the lighting equipment. Moreover, in case of making use of a series resistance the latter will consume unnecessary current, whilst the emwill exert a detrimental iniluence on the phase conditions of the equipment, and condensers can, for practical reasons, not always be employed. A further drawback is constituted by the fact that the ignition voltage of gas discharge lamps is as a rule substantially higher than their operation voltage and that it is therefore either necessary to make provision forl special ignition apparatus, or to keep the operation voltage substantially below the mains voltage, either of which arrangements is disadvantageous from an engineering as well as from an economic point of view. Against these drawbacks, however, must be set'the advantage, that 15 'with lamps oi this kind it is possible to obtain luminous emciencies which are substantially better than those of incandescent lamps. Incase oi flllploying lnuor'esc'ent light the ultra-violet ravdiation producingsuch light is as a rule generated by means of a gas-discharge lamp and.,

thus all the drawbacks enumerated above ofthe latter present themselves. In case of making use of cathodejrays it is asa rule necessary to employ very high voltages, in order to ensure the necessary amount of vacceleration of the electrons,

which circumstance represents a serious drawback in practice and it is only by means of voltages exceeding 1000 volts, with the aid of complicated apparatus and as a rule with an unsatisfactory term of life that satisfactory luminous emciencies can be obtained.

In addition to these methods actually employed in practice for the production of light, other methods of producing light by means of electric voltages are also known, which methods, however, have, for the reasons which will be clear from what follows, not found any practical employment for the purposes of lighting technique.

Faint luminous phenomena have already been observed on the carborundum crystals of crystal detectors at the point of contact between the needle and the crystal, where minute luminous points have appeared. This phenomenon was interpreted by its rst observer as representing a cold electron discharge, the very high field inten'sity set up at the apex or at the edges of the crystal being suiilcient for releasing electrons from the solid substance, which cause an electric gas discharge in the ambient gas or air.

It is also known that the layer formed in certain cases on the surface of electrically semiconductive bodies, as for instance, on .the eiective surfaces of dry rectiflers, crystal detectors ployrnent of a choke-coil or reactive transformer or condensers of the electrolyte type, will emit semi-transparent electrically semi-conductive tion analogous to X-ray radiation, that is to say the explanation of the phenomenon is considered to consist in the fact that the electrons.' when reaching the solid substance, are decelerated and their 'kinetic energy becomes partly converted into luminous radiation. This phenomenon has long been known, but its practical application has not even been thought of, because it was known on the basisof theoretical considerations` that when emitting visible light the luminous efiiciency of braking radiation is very low. Thus for instance, W. Flnkelburg, when discussing these pehenomena says, on page 91 of his work Kontinuierliche Spektren" (published by J. Springer, 1938) substantially what follows:

No use has been made up to now of optical brakingradiation for the purpose of lighting technique, and its application in any substantial extent can. owing to its poor eiilciency, hardly be expected in the future either.l

Finally, luminous phenomena were observed also in insulating liquids, if the latter were sub- Jected to the action of a very intense electric iield; thus, for instance, a mixture of diphenyl oxide and diphenyl placed between two elec-*' trodes situated* at a distance of about one millimetre from each other has supplied light of sufiicient intensity to enable a well-visible photographic picture to be taken The source of light according to the invention based on our discovery of the fact that the whole mass of certain transparent or at least crystals. becomes luminous when electric current passes through them, notwithstanding the fact that their temperature remains far below the temperature at which they would emit the thermal radiation corresponding to the light emitted by them. Thus, for instance, a transparent crystal of carborundum the dimensions of which are approximately between 0.1 and 1 millimetre and which is made from raw materials which are entirely pure, and particularly devoid of any contents of iron, will, when held fast between two electrodes, between which the voltage amounts in order of magnitude to volts, in which case a current of about 0.1 milliampere passes through the crystal, become luminous so as to emit vivid white light, notably not only at the points of contact with the electrodes but in the whole mass of the crystal. 'I'his light can evidently not be considered to represent any thermal radiation,V because it is only when employing a substantially higher current intensity and voltage that the crystal would begin to enter into red incandescence.

This new phenomenon can best be compared to fluorescence set up under the action of cathode rays, but with the substantial difference that it is not necessary to accelerate the electrons specially in a vacuum, the said electrons probably becoming accelerated in the crystal itself or in the closing layer existing on the surface of the crystal, and using their energy for generating the fluorescence of the crystal. The diierence as compared to generation by means of cathode rays is, apart from the nature of the light-emitting substances, also substantial from the point of view that the voltage suflicient for generating the light is smaller by a whole order of magnitude than the one required in case of cathode rays.

oi' the phenomenon.

Notably, in the oase of generation by means of cathode rays, a voltage of at least a few hundred. and possibly of several thousand volts is required. whereas the phenomenon constituting the basis of the invention presents itself already at voltages below 10 volts. Nor can the phenomenon be considered identical with the braking radiation described in what has preceded, because in braking radiation the radiation is developed by deceleration of electrons in a layer of less than 104 mm. thickness whereas in the present phenomenon the luminosity can be observed through the whole mass of the crystal. Accordingly what takes place probably is not that the energy of the electrons is being braked with concurrent emission of radiation, but that it causes the crystal to become fluorescent. Accordingly, if this assumption is correct, the difference between the radiation obtained in connection with the new phenomenon and that obtained in braking radiation is of a character similar to that of the difference between the fluorescent radiation generated by cathode rays produced with the aid of voltage of several thousand volts and the X-ray radiation generated by cathode rays of the same kind. Furthermore, the new phenomenon differs substantially also from the electro-luminescence set up in insulating liquids at high field intensities which has been described above, because this electro-luminescence is only set up at very high field intensities whereas a few volts are sufllcient for producing the luminous phenomenon observed by us. This substantial difference is not only of quantitative but also of qualitative importance, seeing that the insulating substance contains no free electrons, and that it is therefore necessary to release them by the employ-` ment of high voltages, whereas in a conductor or semiconductor such electrons are available. We would, however, emphasize, that the correctness or incorrectness of the theoretical explanation attempted above of the new phenomenon discovered by us does not aii'ect the substance of our invention, as it is a fact of experience that, in case the measures mentioned in the present speciilcatiion are strictly observed, it is possible to produce luminous radiation by means of the source of light according to the invention.

What ischaracteristic for the source of light according to the invention is therefore, according to what has been said above, that is lightemitting body is a transparent or semi-transparent, electrically conductive or semiconductive solid body radiating light under the direct effect of the current owing through it. By direct effect we mean that, it is not in consequence of the thermal effect of the current flowing through it that the solid light-emitting body of the source of light according to the invention radiates light, the situation being thus contrary to the one existing with the incandescent bodies of the usual kinds of electric incandescent lamps, in which it is the thermal effect that represents the cause directly producing the radiation, and in which it is only indirectly, owing to the production of this thermal eifect, that the the luminous radiation, it being in fact immaterial for the light radiation of the .incandescent body whether it is by means of electric current or by any other ymeans that it is brought into a condition of incandescence. 'I'he solid light emitting body of the source of light according to the invention, on the other hand, is kept by the'current flowing through it at a temperature lower than the one necessary for emitting the thermal radiation corresponding to the light radiated by the electric current producessaid light-emitting body, the said body being preferably kept at a temperature lower than 1000 C., e. g. atv a temperature lower than 500 C.

In case of employing carborundum crystals as light-emitting bodies in the source of light described above, we found that the purity of the material of the crystal is a very essential requirement for ensuring good utilization of light. Notably, on carborundum crystals which are contaminated by iron and non-transparent, we have not been able to observe the phenomenon of luminosity. In the case of transparent crystals of greenish colour, containing only a very slight amount of impurities, luminosity was well observable. However, the current flowing through them had, in the case of a crystal having dimensions identical to those of the crystal mentioned above and with identical voltage, a value of about 20 to 60 milllamperes. In the case of crystals of the same size, and with an identical voltage, luminosity of an identical intensity was obtained by us at a current intensity of about 0.1 to 0.5 mllliampere when the content of iron of the material was so low that the crystal grains presented only a quite faint yellowish hue. The latter condition (crystal with faint yellowish) corresponds to a content of ironof approximately less than 0.1 per cent. We also found that among crystals of entirely similar dimensions obtained simultaneously fromV the same raw material there were some which emitted quite pure white light, others which emitted white light of a blueish hue, and others again which emitted white light of a greenish hue. Viewed under the microscope, these crystals seemed to be quite similar to each other. Accordingly, the radiations of different hues may probably be attributed to the effect of quite small quantities of activating impurities-e. g. of manganese or copper or silver. In the case of employing crystals of other materials,

' as e. g. of boron carbide or of aluminium-borocarbide, likewise the purity of the material proved to be of substantial importance for the radiation of light.

We also found that the conduction of the current through the crystals can also be effected by very different methods, without any substantial influence being exercised hereby on the phenomenon of luminosity and a few of these methods will be described in greater detail below in connection with the descriptions of a few embodiments, serving as examples, of the source of light according to the invention.

It is a substantial advantage of the source of light according to the invention that with the increase of the intensity of the current passing through it, its terminal voltage increases, and that, accordingly, its characteristic is positive and that it can be connected to its source of current without employing any choke-coil, reactive transformer or condenser, and further that the crystals begin to emit light immediately upon the flow of current, i. e. without any practically appreciable time lag, as soon as current passes through them. Further advantageous properties of this source of light are that it can be fed by a low voltage, that its design is simple and inexpensive, that it is safe against vibrations and that in case of its being constructed properly it also possesses a very long term of life.

In what follows, the source of light according to the invention will be described in greater detail in a few of its embodiments serving as examples, with reference to the annexed diagrammatical drawing, in which Fig. 1 represents a source of iight comprising an evacuated bulb.

Fig. 2 represents a source oi light comprising a bulb iled with liquid, and

Fig. 3 represents a source of light comprising a bulb lled with air or gas.

The lay-out of the source of light according to Fig. 1 is similar to that of an electron tube.

It is into the base 2 of the bulb I that the incanthem on the superiicially melted metal, and caus- Y ing the melted metal to solidify rapidly, that these crystals may be fixed on the anode plate in such a manner as to project from its surface. The free surface of the anode plate may be insulating. Already in case' of an anode voltage, connected to the leads 3a and 5a, as low as 20 to 50 volts, the incandescent cathode will emit the anode current of a few mlliiamperes suiilcient for causing the crystals 6 to emit vivid white light.

'I'he lay-out of the source of light according to Fig. 2 is similar to that of a cell of the electrolyte type. The casing 1 made of any suitable insulating material suchv as Bakelite or transparent plastic is fitted with a covering plate t made of glass and inserted in a leakage-proof manner. The casing may be of any suitable shape such as a parallelepiped. 'Ihe current leads 9a and l0a of the cathode 8 and of the anode l0 are led through the wall of the said casing, this also being done in a manner preventing any leakage of liquid. The cathode 9 is a conductor of annular shape, which is preferably made of precious metal, e. g.'of platinum, at

f least on its surface, whilst the anode I0 is an aluminium plate and the casing is filled with a liquid electrolyte of the kind usually employed in condensers of .the electrolyte type. The carborundum crystals 6 are rolled into the surface of the plate i0 in themanner mentioned above and following that the plate is shaped so as to represent the anode of a condenser of the electrolyte type so as to ensure that it should become coated with an insulating layer on its free parts of surface. In consequence hereof it is only through the crystals 8 that the current of the cathode 8 can enter from the electrolyte into the anode plate. With a voltage of 10 to 50 volts connected between the cathode and the anode, the crystals will emit vivid white light and their light will radiate outwards through the glass pane B.

In the case of the source of light according to Fig. 3 the casing l is fitted with two glass panes 8a. and 8b which are fixed into it in a dustproof manner, or in the case of a casing lled with gas in a manner preventing any leakage of gas. On the inner side of the glass panes metal closemeshed wire nets ila and Hb, made of very thin wire are provided, through which the current introduced through the current leads I2a and I2b is led into the 'crystals 6 in contact with the wire nets. In the case of this embodiment it is also possible to substitute for the wire nets a transparent layer of metal provided on the glass pane, e. g. a layer of gold produced by means of cathodic dispersion, but the introduction of the current Amay also be effected by means of thin coats of metal applied on the crystals. In the case of employing crystals of larger size the latter may also be clamped one by one between resilient electrodes, or the good i contact of the electrodes with them may be assured by means of drops of solder consisting of a metal of low melting point, e. g. of Woods metal. In the case oi' crystals of larger size the crystal may also be connected in series with each other, so as to obtain electric sources oi light also capable of being direct-connected by themselves to the usual mains voltages.

We wish to emphasize that our invention is far from being exhausted by the examples enumerated above, because the particular nature of the sources of light enables them to be constructed in very different types, and also in types suitable for many special purposes, without thereby deviating from the invention characterized in the claims enumerated below; thus, for instance lt is possible, making use of the source of light according to the invention, to construct luminous bodies of very low surface luminosity and having areas of very great magnitude, possibly of several square metres.

We claim:

1. A source of light comprising a light-permeable body of highly puried crystalline carbide capable of radiating light under the direct influence of current flowing therethrough and means for supplying current to said carbide, said current having a value materially below that necessary to produce incandescence.

2. A source of light comprising a light-permeable body of highly purified crystalline conducting material capable of radiating light under the direct influence of current flowing therethrough and means for supplying current to said material, said current having a value materially below that necessary to produce incandescence.

3. A source of light comprising a light-permeable body of highly purified crystalline conducting material capable of radiating light under the direct influence of current flowing therethrough, and means for supplying current to said material, said material having an operating temperature less than 1000 C.

4. A source of light comprising a body of one or more light-permeable carborundum crystals, and means for passing through said crystals a current insufficient to produce incandescence but sufficient to produce a substantial amount of visible radiation.

5. A source of light comprising a body of one or more light-permeable .carborundum crystals .having an iron content of less than 0.1% and means for passing through said crystals a current insufllcient in magnitude to produce incandescence but sufiicient to produce a substantial amount of visible radiation.

6. A source of light comprising a pair of electrodes and a bodyof one or more carborundum crystals interposed therebetween so as -to be traversed by electric current flowing between said electrodes, and means for supplying current to said electrodes, said current being too small to produce incandescence in said crystals, yet sufficient to produce a substantial amount of luminescence therein.

7. A source of light comprising a pair of electrodes and a body of one or more carborundum crystals having an iron content of less than 0.1% interposed therebetween so as to be traversed by electric current flowing between said electrodes, and means for supplying current to said electrodes, said 'current being too small to produce incandescence in said crystals, yet sumcient to produce a substantial amount of luminescence therein.

8. A source of light comprising a pair of electrodes in vacuum and a body of one or more carborundum crystals interposed therebetween and positioned on the surface of one of said electrodes. andA means for supplying current to said elecf trodes, said 'current'being too small to produce incandescence in said crystals, yet sufflcient to produce a substantial amount of luminescence therein.

9. A source of light 'comprising a pair of electrodes immersed in a transparent conducting liquid, a body of one or more carborundum crystals interposed therebetween and positioned on the surface of one of said electrodes, and means for supplying current to said electrodes, said current being too small to produce incandescence in said crystals, yet sufficient to produce a substantial amount of luminescence therein.

10. A source of light comprising a pair of spaced meshed electrodes embedded in a body of one or more carborundum crystals in such a manner that current passing between said electrodes traverses said crystals, and means for supplying current to said electrodes, said current being tco small to produce incandescence in said crystals, yet sufficient to produce luminescence therein. i

11. A source of light comprising a sealed envelope containing a pair of electrodes and a body of highly purified light-permeable crystalline carbide capable of radiating light under the direct influence of current owing therethrough, said body of carbide being associated with said electrodes so as to be traversed by electric current owing therebetween, and means for supplying current to said electrodes in an amount too small to produce incandescence in said carbide but suiiicient to produce a substantia amount of luminescence therein.

zoLTAN BAY.

GYRGY szIGE'rr.

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