US2333052A - Electrical discharge device - Google Patents

Description

Oct. 26, 1943. c. G. SMITH 2,333,052

ELECTRICAL DISCHARGE DEVICE Original Filed March 18, 1931 Chg/[ 5- Gf 59mm INVENTOR- ATTORNEY.

Patented Oct. 26, 1943 I 2,333,052 ELECTRICAL DISCHARGE DEVICE Charles G. Smith, Medford, Mass.,

mesne assignments, to Raytheon 7 ing Company, Newton, Mass.,

Delaware Original application Mar 523,411. Renewe 11 Claims.

The present invention relates to an electrical discharge device of the type which utilizes an excited gas or vapor as a source of light.

Among the objects of this invention is to produce a lamp in which most of the light emitted is concentrated in a particular part of the radiant energy spectrum, for example, in that portion of the ultra violet spectrum which has the most beneficial therapeutic effects. The foregoing and various other objects will be best understood from the following description of exemplifications thereof, reference being had to the accompanying drawing wherein- Fig. 1 is a view of one form of lamp embodying my invention; and

Figs. 2 and 3 are similar views showing two further modifications of such a lamp.

Rays of light in the region near the wave length of 3000 Angstrom units are very beneficial therapeutically, especially in the formation of vitamin D in various animal organisms, including the human organism. Light in this region is also beneficial in various other processes. Light which varies to a considerable degree from the wave length is either useless or in some cases actually detrimental. This detrimental action is particularly true of wave lengths considerably shorter than specified. Lamps which emit ultra violet light are well known, but in all previous lamps the light emitted has consisted of wave lengths which are distributed over a wide range of the ultra violet spectrum and therefore the light in the region near the wave length 3000 has formed a comparatively small part of the total light emitted. Thus the efllciency of these lamps for therapeutic purposes has been rather low.

The spectrum of zinc contains a line whose wave length is 3076. This line is a resonance line of zinc. Since the wave length of the resonance line is near the desired 3000 radiation it is desirable to use zinc vapor as source radiations of wave length 3076.

Previous to my present invention, the 3076 line has always appeared as one of minor intensity in zinc vapor, other lines appearing with greater intensity. As far as I am aware it was not known heretofore how the 3076 line could be made more intense, nor in fact was it known that the intensity of this line could be increased.

In accordance with my invention I produce a lamp in which zinc vapor is utilized as an emitter of light, said zinc being specially treated so as to become a particularly effective emitter of the radiation of wave length 3076. Such assignor, by Manufactura corporation of ch 18, 1931, Serial No. d February 24, 1939 treatment consists in the removal of certain particular impurities.

I have discovered that, if the purity of the zinc used in such a lamp be extended sufliciently, the intensity of the 3076 line is increased to a far greater extent than the corresponding increase in purity. In fact with zinc of the purity which I use, about of the radiant energy emitted by the zinc vapor is concentrated in radiation of wave length 3076.

Zinc which is ordinarily obtainable in commerce as pure zinc contains minute amounts of hydrogen and cadmium. The spectrum of this commercially pure zinc contains the characteristic lines of hydrogen and cadmium. If commercially pure zinc is purified still further the hydrogen and cadmium lines decrease in intensity but as long as there is even a faint trace of any of these lines the line 3076 does not increase in intensity to any considerable degree. If the purity of the zinc is still further increased and special precautions taken to avoid the presence of impurities the cadmium and hydrogen lines will disappear and the 3076 line will be enormously increased in intensity.

This surprising phenomena can be explained upon a spectrum analysis of zinc. According to modern spectral notation, the resonance line of zinc may be represented as 1S2 P1. The symbol 18 represents the normal, unexcited state of atom, while the 2 P1 symbol represents an excited state of the atom in which an electron has been displaced to a point further removed from the center of the atom than in the unexcited state. This electron in dropping to the 13 state emits the resonance line of zinc which is 3076. In addition to the 2 P1 state the zinc atom also can be put into a number of metastable states in which the energy content of the zinc atom differs by only a very smallamount from that of the desired 2 P1 state. The zinc atom cannot pass directly from these metastable states to its unexcited state by radiation, and therefore, the resonance line of zinc can only be emitted from an atom in the metastable state if the atom first passes into the 2 P1 state. The ordinary thermal agitation is sufiicient to cause an atom to pass back and forth between its 2 P1 state and its metastable states. If a zinc atom in its metastable state collides with an atom or molecule of such a kind that the metastable zinc atom can transfer energy to that foreign atom or molecule, the zinc atom will transfer the energy which it possesses due to its metastable state to the foreign atom or molecule.

Thus, the energy which has gone into changing an unexcited zinc atom into its metastable state is transferred to the foreign atom or molecule and is lost as far as the radiation of wave length 3076 is concerned.

In order that the foreign atom or molecule can absorb energy from the metastable zinc atom, certain conditions must be true. Some changed state must exist into which the foreign atom or molecule can be put by the absorption of a definite amount of energy, and the amount of energy so absorbed must be of a value less than that which the zinc has absorbed in changing from its normal state to its metastable state.

The metastable zinc atom has the greatest tendency to transfer its energy to the foreign atom or molecule when the amount of energy necessary to put said foreign atom or molecule into its changed state is very close to but less than the amount of energy which has been absorbed in changing the zinc atom from its normal state into its metastable state.

, This changed state into which an atom or molecule may be put may be of various kinds. Some of these states may be excited states of an atom in which an electron of that atom may be considered as being displaced from its normal orbit. Each atom has a definite number of such excited states, and, each of these states is reached by the absorption of a definite quantity of energy. Atoms or molecules may be put into other kinds of states by the absorption of definite amounts of energy. For example, a molecule having :a number of atoms may be broken up into its component atoms; or a molecule of some compound may be disassociated into the molecules or atoms of its component parts. Each of these changes also requires the absorption of a definite amount of energy. I

In some cases the dissociation of a molecule either of a compound or of a simple substance may take place in two or more steps.

Any state such as, for example, those cited above into which a normal atom or molecule of a substance can be put by the absorption of a definite quantity of energy can be considered an energy sta of that substance. The energy content of such'energy state can be considered as the amount of energy necessary to change an atom or molecule from its normal state into its energy state. According to the above explanation, if a substance has an energy state of energy content lower than the energy content of the metastable state of zinc, a zinc atom in its metastable state will in general lose the energy of its metastable state to such substance upon collision with an atom or molecule thereof.

When an atom or molecule is in one of its energy states, it can lose energy due to said state in various ways such as, for example, by emission of light or heat radiation, collision with other bodies, and the like. When a foreign atom or molecule of the kind described above absorbs energy from a metastable zinc atom, said. foreign atom will soon lose the energy so absorbed in some such manner, as stated above. However, since the energy states of zinc are of higher energy content than that of the energy state existing in the foreign substance, none of the energy of the foreign substance will be transferred to the zinc. In losing this energy, the foreign atom returns to its normal state and is again ready to absorb energy from a metastable zinc atom. Thus a comparatively few foreign atoms of this kind in an atmosphere containing zinc atoms in aasaosa excited states will in time absorb energy from a large number of metastable zinc atoms. These few foreign atoms will, therefore, cause a considerable decrease in the radiation of the wave length 3076 emitted from the zinc vapor.

What is true of the metastable zinc atom is true also of the zinc atom in its 2 P1 state. If a zinc atom in the 2 state collides with an atom or molecule having an energy state of lower energy content than the 2 P1 state of zinc, said zinc atom will transfer its energy to that foreign atom or molecule. While an atom in its 2 Pi state tends to lose its energy by radiation, yet such a collision may occur before the radiation takes place, and thus an appreciable loss of the desirable radiation occurs through the 2 Pi. atoms of zinc in the presence of the kind of impurities mentioned. Since the zinc atoms can exist in their metastable states a considerable length of time, which time is much longer than the life of 2 P1 atoms, the possibility of collision between the foreign atoms and molecules and the metastable zinc atoms is much greater than for the 2 P1 atoms of zinc. Thus the loss of effective radiation through the metastable zinc atoms is much larger than through the 2 Pl. zinc atoms.

Since the energy contents of the metastable zinc atoms and the 2 P1 zinc atoms differ very slightly, foreign atoms or molecules which can absorb energy from the metastable zinc atoms can usually absorb energy from the 2 P1 zinc atoms. Cadmium and hydrogen, for example, the

two impurities ordinarily found in zinc, can ab- Lil sorb energy from zinc in both the 2 P1 and the metastable states.

The 2 P1 state and the metastable state of zinc can be considered as the excited states of the zinc atom. Therefore it can be seen that, if a zinc atom in one of the excited states collides with an atom or molecule which may possess an energy state of lower energy content than that of the excited state of zinc, the zinc atom will lose its energy to that foreign atom or molecule.

As stated above, cadmium and hydrogen are the impurities which are usually found in commercially pure zinc. Molecular hydrogen, which consists of two atoms, has an energy of dissociation higher than the energy content of an excited zinc atom. However, according to my present understanding of the phenomena involved, upon collision of an excited zinc atom and a hydrogen molecule, zinc hydride will be formed and a hydrogen atom liberated.

The formation of the zinc hydride liberates an amount of energy which, when added to the excitation energy of the zinc atom, is in excess of the energy necessary to dissociate the hydrogen molecule. The zinc hydride, having a low energy of dissociation, will be dissociated by subsequent collisions, leaving a normal zinc atom and an atom of hydrogen. The two atoms of hydrogen will tend to recombine, and the energy which has been absorbed in dissociating the hydrogen molecule will be liberated in some form such as heat. The reformed hydrogen molecule is then ready to absorb energy from another excited zinc atom.

Cadmium possesses an excited state in which the energy content is slightly less than that of an excited zinc atom. Upon collision with an excited zinc atom, a cadmium atom will absorb energy and become excited. The excited cadmium atom will return to the normal condition by losunder operating conditions.

ing the energy it absorbed, for example, by emitting a light radiation, by colliding with the walls of the container surrounding the vapor or by colliding with other impurities. The radiation so .emitted, however, is not in the desired wave length region. The normal cadmium atom is then ready to absorb energy from another excited zinc atom.

Cadmium is particularly detrimental to the emission of the radiation of wave length 3076 due to the fact that it is more volatile than zinc. If zinc which contains but a trace of cadmium is placed in a lamp and part of the zinc is vaporized, practically all of the cadmium in the entire body of zinc will be vaporized due to lower vaporization temperature of cadmium. Thus a larger percentage of cadmium will exist in the vapor than existed in the original body of zinc.

Since the proportion of impurities in the zinc must be kept extremely small in order to take advantage of the beneficial results explained above, the production of a lamp containing zinc vapor of this requisite purity presents many difficulties.

First, care must be taken that no impurities are introduced into the vapor from the walls of the container. In order to take care of this factor the walls of the container are thoroughly freed of all impurities and all occluded gases. I use as a material for my container preferably quartz inasmuch as it can be heated red hot during evacuation of the container whereby all occluded gases are driven off.

Second, if electrodes are used to maintain an electrical discharge in the vapor special precautions must be taken to purify them. I prefer to avoid the possibility of impurities being so introduced by maintaining a discharge without the use of electrodes in the vapor, such as, for example, by inducing a discharge in the vapor by a coil fed by high frequency currents.

Thirdly, the zinc itself must be so treated and introduced into the container in such a manner that the proportion of impurities is kept at the requisite low value. I obtain zinc of the required purity preferably by fractionally distilling commercially pure zinc. This zinc is heated and the zinc vapor is allowed to condense in a chamber maintained at about 350 C. The cadmium and other materials more volatile than zinc are drawn off from this chamber. It is sometimes necessary to repeat this process one or more times before zinc of a purity which can be satisfactorily used to practice my invention is obtained.

As an added precaution against the introduction of too much cadmium into the lamp I introduce just enough zinc so that all of it is vaporized Thus, if the zinc as originally introduced is of the requisite purity, there will be no greater proportion of impurity in the vapor under operating conditions than there was in the zinc. It is a comparatively simple matter once the operating conditions are decided upon to calculate the exact amount of zinc which should be introduced into the lamp from the dimensions of the lamp itself.

Referring to Fig. 1, I construct a lamp by thoroughly evacuating a transparent container, the walls of said container being purified of all occluded gases during said evacuation. A small piece of zinc treated in accordance with my invention is introduced into the container I. The amount of zinc so introduced is just enough so that all of it is vaporized under operating condltions. The container is mounted within an evacuated envelope 2 and is supported therein by spring arms 5 and 6, carried by a clamping ring 4. The ring 4 is clamped around a reentrant stem 3 within the envelope 2. The envelope 2 and the container I are constructed of a material transparent to the wave length 3076, the container I being preferably of quartz and the envelope 2 being preferably of thin Pyrex.

In order to produce an electrical discharge in the envelope 2, I surround the envelope by a coil I0 adapted to be fed by a source of high frequency current I0. At ordinary room temperatures the vapor pressure of zinc is so low that no discharge would be induced by the coil I0 unless some means were provided to raise the temperature and consequently the vapor pressure of the zinc to a point where the coil II) will induce a discharge therein. In this exemplification I fill the envelope I with some very pure rare gas such as argon, neon and the like, at a pressure of about 2 mm. When the coil I0 is energized from the source III a discharge will be induced in the rare gas filling. This discharge will cause the temperature within the container I to rise to a point at which the zinc is vaporized. Upon the appearance of the zinc vapor within the envelope, a light of which about 70% consists of a wave length 3076 will be emitted. Since the ionization voltage of zinc is lower than that of the rare gas filling, the discharge will be maintained under operating conditions, primarily by the zinc vapor.

Since argon and other rare gases do not contain energy levels which are lower than that of the metastable states of zinc, there will be no transfer of energy by a zinc atom in a metastable state upon collision with such gas atoms. Therefore, the presence of the rare gas within the envelope I produces no detrimental effect upon the emission of the 3076 line of zinc.

However, when using such gases care must be taken to obtain very pure gases in order not to introduce impurities of the kind described.

It is desirable to operate the zinc at a vapor pressure of .001 mm. more or less, and, since the vapor pressure is dependent on the temperature of the zinc, it is advantageous to prevent an excessive loss of heat from the container I. By mounting the container in a vacuum, as described, the container is insulated against loss of heat, except by radiation.

The rare gas is used in the embodiment illustrated in Fig. 1 to start a glow discharge within the envelope I in order to raise the temperature suificiently to evaporate enough zinc vapor to carry an electrical discharge. However, the temperature of the zinc may be raised to its vaporization point by heating it externally, in which case the gas filling may be omitted. By the omission of the gas, the purification of the contents of the lamp is correspondingly simplified. In this case, it is necessary only to purify the zinc while the possibility of impurities which may exist in the gas being introduced is eliminated. Two arrangements in which the gas filling is omitted are illustrated in Figs. 2 and 3.

Fig. 2 discloses, as does Fig. 1, a quartz envelope II, a container I2, a reentrant portion I3, a ring I4, and spring arms I5 and I6, supporting members I! and I8, and an exciting coil 20. In Fig, 2, however, the envelope II is provided with a reentrant portion 2| within which is placed a heating filament 22 which is provided with lead-in wires 23 and 24 sealed in a press 25 formed at the upper end of the reentrant portion E3. The filament 22 is supplied with heating current from some suitable source of current such as a battery 22'. The container I2 is filled with an inert gas such as nitrogen, argon, helium or the like at a fairly high pressure which may be about atmospheric pressure. A shield 27' is provided, which shield forms a partition across the container l2 and may be of suitable material such as mica or a highly polished metal. This shield completes a restricted pocket around the envelope I l. The gas in the pocket is heated by the filament and circulates around the container ll, giving it a uniform temperature throughout. The envelope H is thoroughly evacuated of all gases, and a piece of very pure zinc i9 is placed therein. The heating filament 2| is so designed that it raises the temperature of the envelope l l preferably to about 350 C. At this temperature the zinc I9 is vaporized, and the envelope It is filled with zinc vapor. When the coil is energized from a high frequency current ource 20, a discharge will occur in the zinc vapor and a brilliant zinc spectrum about 70% of which is concentrated in the 3076 line is emitted. The pressure of the gas in the container I2, being :at approximately atmospheric pressure, is sumciently high so that a discharge does not occur outside of the envelope H.

Although the application of external heat is contemplated when zinc alone is used, yet even when a rare gas is also used, external heat may be applied as shown, for example in Fig. 2. This is due to the fact that the heat liberated within the envelope by the discharge may not be sufficient to raise the temperature, and consequently the zinc vapor pressure in the envelope to the desired point.

Fig. 3 is another arrangement in which external heat is applied to the container. Fig. 3 shows a heating filament 26 which instead of being placed within a reentrant portion of the container is placed at a point away from the container and is provided with a reflector 21 supported by the spring arms 28 and 29. Instead of a battery a transformer 26' may be used as a source of heating current for the filament 26. The shield 21 contains a bright polish on its upper surface which may be formed by chromium plating said surface. This shield reflects the heat generated by the filament 26, which falls upon said reflector, onto the container 30. The container 30 is made of some material which is transparent to the radiations of wave length 3076 but which also can absorb the heat raditions from the filament 26 such as for example, thin Pyrex glass. The reflector 21 may be provided with radial slots 28' in order to prevent eddy currents from being produced in it by the exicting coil. As set forth for Fig. 2, the envelope 30 may contain either pure zinc or zinc in an atmosphere of some rare gas such as argon or xenon.

There are many materials in addition to zinc which have a 2 P1 state and metastable states from which energy may be lost by collisions with atoms or molecules of some impurity. In each of thes materials the radiation emitted from the 2 P1 atom in returning to the normal state of the atom may be considered the resonance radiation of that material. In accordance with my invention the resonance radiation in each of these materials may be greatly increased. Such materials may b the vapors of such substances as, for example, mercury and cadmium or such gases as, for example, helium and neon.

Some of the impurities which absorb energy from metastable mercury atoms are hydrogen, zinc, cadmium, carbon dioxide and oxygen. Hydrogen and carbon dioxide are examplespf impurities which absorb energy from metastable cadmium atoms. Sinc helium has metastable atoms of very high energy content neoniiand a great number of other materials will cause a loss of the helium radiation through the metastable atoms. Argon is one of the impurities which has a like efiect on neon.

In practicing my invention those radiation emitting materials of which the abov are examples, are purified and maintained at the requisite purity by exercising the precautions as specified in describing my invention as applied to the use of zinc.

This invention is not limited to the particular details of construction, materials, or processes described above, as many equivalents will suggest themselves to those skilled in the art.

It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

Having described my invention, I claim:

1. A vapor discharge device comprising a sealed envelope, a sealed container within said envelope, an easily vaporizable material within said container, a heating element within said envelope adjacent said container, an inert gas within said envelope, and high frequency inducing means for maintaining an electrical discharge within said container, said inert gas being at a sufiiciently high pressure so that a discharge is not induced therein.

2. A vapor discharg device comprising an evacuated envelope, a sealed container within said envelope, an easily vaporizable material within said container, a heating element within said envelope, a reflector for reflecting the heat generated within said envelope onto said container, and means for maintaining an electrical discharge within said container.

3. A vapor discharge device comprising an evacuated envelope, a sealed container within said envelope, an easily vaporizable material within said container, means within said envelope for heating said container, and means for maintaining an electrical discharge within said container.

r 4. A vapor discharge device comprising an evacuated envelope, a sealed container within said envelope, an easily vaporizable material within said container, means for heating said container, and means for maintaining an electrical discharge within said container.

5. A gaseous discharge device comprising a sealed envelope, a sealed container within said envelope, an ionizable'atmosphere within said container, a gas within said envelope, andhighfrequency inducing means for maintaining an electrical discharge within said container, said gas being at a sufiiciently high pressure so that a discharge is not induced therein.

6. A vapor discharge device comprising an evacuated envelope, a sealed container within said envelope, a vaporizable material within said vcontainer, a resistance heating element within said envelope external to said container and adjacent thereto for vaporizing said material, and means for maintaining an electrical discharge within said container through said vapor.

7. A vapor discharge device comprising an evacuated envelope, a sealed container within said envelope, a quantity of zinc within said container, means for heating said container, and means for maintaining an electrical discharge within said container.

- 8. A vapor discharge device comprising an evacuated envelope, a sealed container within said envelope, an easily vaporizable material, the vapor of which radiates therapeutically-active radiations when excited by an electrical discharge, within said container, means for heating said container, and means for maintaining an electrical discharge within said container.

9, A vapor discharge devic comprising an evacuated envelope, a sealed container within said envelope, an easily vaporizable material, the vapor of which radiates ultraviolet light when excited by an electrical discharge, within said container, means for heating said container, and

means for maintaining an electrical discharge within said container.

10. A vapor discharge device comprising an evacuated envelope, a sealed container within said envelope, a limited quantity of vaporizable material of an amount to be completely vaporized during normal operation within said container, means for heating said container, and means for maintaining an electrical discharge within said container.

11. A vapor discharge device comprising an evacuated envelope, a sealed container within said envelope, a limited quantity of zinc of an amount to be completely vaporized during normal operation within said container, means for heating said container, and means for maintaining an electrical discharge within said container.

CHARLES G. SMITH.

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