US2034572A - Electric lamp and method of producing light - Google Patents

Description

March 17, 1936. Q G, FOUND 2,034,572

ELECTRIC LAMP AND METHOD OF PRODUCING LIGHT Original Filed-Aug. -16, 1953 2 Sheets-Sheet l Inventor": Clifton G. F um by 2.

His AttOFfieg.

March 17, 1935. G, FOUND 2,034,572

ELECTRIC LAMP AND METHOD OF rnonucme LIGHT Original Filed Aug. 16, 1935 2 Sheets-Sheet 2 4 Fig.6.

42 Fig. 7.. 40

I I I Inventor:

Clifton G. Found,

Patented Mar. 17, 1936 UNITED STATES ELECTRIC LAIMP AND METHOD OF PRODUCING LIGHT Clifton G. Found, Schenectady, N. Y., asaignor to General Electric Company, a corporation of New York Application August 16, 1933, Serial No. 685,377 Renewed September 20, 1935 18 Claims. (Cl. 176-124) The present invention relates to electric lamps of the type in which light is produced by an electric discharge taking place in a gas or vapor, or in various mixtures of gas and vapor.

It is the object of my invention to improve the lighting efllciency of such lamps and to render them more regulable. As another feature of my invention a desired modulation of light output, or a change of color of light output, may be obtained.

In an ordinary electric discharge in a lamp, or other form of gas ionization device, there is a reciprocal dependence between the value of the electron current which emanates from the oathode and passes to the anode and the current of positive ions which passes in an opposite direction to the cathode. There must be in the ordinary gas ionization device a drop of voltage at the cathode which automatically so fixes itself in accordance with the conditions, as for example, character and pressure of the gas, the character of the electrodes, and so forth, that enough ions are produced to permit the desired electron current to flow. This mechanism of ionization will be explained in greater detail hereinafter.

In accordance with my present invention the drop of voltage at the cathode required to produce the positive ions which are necessary for the operation or the luminosity-producing dis- 30 charge is brought under control by producing the positive ions by a mechanism independent of the main discharge with the result that the color of the light may be modified, and materially higher lighting eficiency than was attainable heretofore is made available.

The novel features of my invention which includes both a method and apparatus will be pointed out with greater particularity in the appended claims and explained in connection with the accompanying drawings.

In the drawings, Figs. 1 and 2 illustrate somewhat conventionally simple embodiments of my invention; Fig. 3 illustrates a positive column type of lamp; Fig. 4 is a longitudinal section of a cathode structure for such a lamp; Fig. 5 shows conventionally a lamp and electrical connections adapted for producing variable or modulated light; Fig. 6 illustrates in side elevation a lamp with a single cathode and two anodes together with electrical connections for alternating-current operation; and Fig. 7 illustrates a modified circuit for operating the lamp shown in Fig. 6 with direct current; and Fig. 8 shows a double cathode lamp connected to an alternating-cur- 5 rent circuit.

Fig. 1 of the drawings shows conventionally an embodiment of my invention in a lamp containing a gaseous flllingat low pressure and containing in addition to the main electrodes, also auxiliary electrodes, including a thermionic cath- 5 ode. In this illustrative example the lamp envelope 2 commonly consists of glass, fused quartz, or other suitable materials. It is provided with a main thermionic cathode 3 which is here shown as being of filamentary form although, as will 1 presently appear,,other forms may be used. The main anode 4 is here shown as a simple disc, although in this case also other forms may be used.

A heating means for the cathode is represented in the drawings conventionally by a battery 5. Cur- 15 rent supply conductors 8, I lead from a suitable source of energy, such as a battery 8, to the main electrodes of the lamp.

Preliminary to the sealing of the lamp the envelope 2 is freed from water vapor and other dele- 20 terious gas by methods known in the preparation of high vacuum tubes. After evacuation the envelope is charged with a desired gas which for the purpose of illustration may be assumed to be neon. Other gases of the rare or noble gas group 5 may be employed and also ordinary gases such as carbon dioxide, or nitrogen, or vapors such as titanium tetrachloride, as well understood in the art. In some cases metallic vapors may be used as the luminosity-producing agency, for example, 30 vapor of mercury, sodium, thallium or cadmium.

As will presently appear, it is advantageous in some instances to use a mixture of gas and vapor. All such luminosity-producing media are referred to herein generally as gaseous fillings. The pres- 35 sure of the gaseous filling which is best suited for the production of light is different for difierent gases and vapors as is well known. In the case of neon, pressures may be used within a range of one-tenth to ten millimeters of mercury.

Electrons emitted by the cathode 3 passing through the gaseous filling to the anode 8 under the influence of the impressed voltage collide with gas molecules. It the electrons have been accelerated by a voltage exceeding a certain critical voltage diiiering for each gas, then the collision may result in ionization thereby producing from the gas molecule additional electrons and also positive ions. When some of the original eleco trons, and in some cases also the additional electrons produced when ionization occurs, collide with gas atoms, a so-called excitation" of the atom rather than ionization takes place. When the excited atom returns to the no state light Anelectron must travel a certain distance after leaving the cathode under the influeneeof the electric field before it acquires the critical energy necessary to ionize a gas atom or molecule. There is a region close to the cathode throughout which electrons are being accelerated, but have not yet reached the critical value required to produce ions. The negative charge of the electrons in this region can be neutralized only by positive ions passing into.it from the plasma. There is a narrow region between the cathode and the plasma called cathode sheath in which the applied voltage must overcome both the negative space charge of the electrons near the oathode and the adjacent positive space charge of the ions leaving the plasma and headed for the cathode. The voltage drop under these conditions automatically adjusts itself to establish and maintain these space charges, which are termed double sheath, at the cathode. This voltage drop however rarely has a value which is best suited for the efficient production of light in the lamp.

There are certain critical or resonance voltages at which the atoms or molecules being excited by the electrons emit light most efiiciently, the efliciency of light production being a maximum when the speed of the electrons is slightly higher than the resonance voltage. In other words, when the speed of the electrons reaches a certain critical value, which is different for different gases, transfer of energy'takes place from the electrons to the gas atoms causing the atoms to be raised to a higher energy state or level. The energy thus acquired by the atoms is converted into light, ultra-violet or other radiation when the atoms return to the normal state. As is well known, the color of the light produced is dependent on the energy transfer when the atom falls from a state of higher energy to one of lower energy. A discussion of this phenomenon may be found in Transactions, American Institute of Electrical Engineers, vol. 47, (1928) pa es 147 to 752. When the speed of the electrons which is dependent on the voltage drop is materially higher than the resonance voltage, the efficiency of transfer of energy from the electrons to the gas atoms or molecules decreases rapidly and hence the production of light decreases.

In many gaseous atmospheres the production of light is obtained by a multi-stage process, the

I amass transition from either the resonance or mean stable state to higher, radiation-producing states must be produced by collisions with other electrons having the same relatively great energy as those which originally produced the resonance atoms. This is a relatively ineffectual process however due to the fact that the necessary energy transfer occurs most emciently when the energy of the colliding electrons only slightly exceeds the difference. between the energy of the resonance or metastable atoms and the energy of the atoms at the radiation-producing state. For example, in neon the atoms are first excited to either of two resonance states or levels of approximately 16.5 volts. From either of these levels, or from the two adjacent metastable levels, the atoms must be raised to one of a series of levels in the neighborhood of 18.5 volts, that is, 2 volts in excess of the resonance voltage if the characteristic neon radiations are to be produced. For efiicient production of atoms having these radiation-producing levels, (having in mind the rule hereinbefore mentioned,) it is obvious that electrons having an energy of approximately two volts are necessary. But in the usual discharge the energy of the electrons which are available to collide with the atoms in the metastable or resonance condition are determined by the cathode drop which in turn is determined by the conditions governing the production of the ions necessary for the conduction of the discharge 'current. This condition may or may notbe suitable for the efficient production of light. I have discovered a means of producing large numbers of low energy electrons, such as the two volt electrons required for the eflicient excitation of neon independently of the conditions governing the so that many unsuccessful collisions, either elastic or exciting, are required on the average before an electron can produce ionization. Each of these unsuccessful collisions results in a slight loss of energy, and as a result of a number of these unsuccessful collisions a large proportion of the electrons have their energy reduced by the unsuccessful collisions to a value less than that required to produce ionization, so that the over-all efficiency of ionization--that is, the ratio of the energy used in ionization to the energy imparted to the electrons-is correspondingly low. If the voltage margin is progressively increased however the average number of collisions required decreases, with the result that this over-all efliciency rises extremely rapidly, reaching a maximum when the cathode fall is considerably in excess of the ionizing potential.

Hence I find it desirable to produce the ionization required for a given main discharge by means of an auxiliary discharge so adjusted as to have a cathode fall of the optimum value for the production of ionization.

When the necessary ionization is produced by an auxiliary discharge in the foregoing manner a small voltage drop or cathode fall in the sheath Cal about the main cathode suifices to produce a large current in the main discharge. This potential or cathode fall is adjusted to produce electrons of the desired potential, in neon, for example, two volts being employed.

Again referring to Fig. 1 of the drawings, and assuming the gas filling to consist of neon, an auxiliary discharge between the electrodes 9 and at a potential above ionization potential of neon causes ionization of the neon gas which renders the gas conductive and at the same time causes excitation of the neon. This excitation results in a slight glow close to the cathode 9 and also in the production of metastable neon atoms. A property of these metastable atoms, which difiuse throughout the bulb, is their ability to absorb energy from an electron accelerated by a potential of two volts. When a potential of two volts is applied between the main electrodes 3 and 8, the entire bulb is filled with a strong red glow which is due to the passage of electrons possessing the required two volt energy. Heretofore, in neon tubes, no light was'produced until a voltage of at least it volts was applied. In the present case, more light is obtained with a given current at two volts than was formerly obtainable at sixteen volts to twenty volts. Moreover, the light thus produced is much redder than light from the former type of neon discharge. The change in color is due to the excitation of metastable neon by the low velocity electrons which are able to excite the metastable neon predominantly to an energy level from which red light only is radiated. Thus the red radiation of neon has been enhanced by a selection of voltage of the main discharge and the energy distribution in the resulting neon spectrum is different from that obtained from neon lamps heretofore commonly in use.

When a discharge takes place in mercury vapor,- metastable mercury atoms are produced. These metastable atoms also may be excited by electrons having a velocity insuflicient to produce ionization or metastable atoms. As a result of this excitation, radiation in the yellow-green lines may be made predominant. In this way, the relative energy in the various radiation lines of the mercury spectrum may be controlled. The same result may be obtained from any gas or vapor in which metastable atoms can be produced.

The auxiliary or additional cathode 9 is, shown as being of the thermionic type and being heated by a battery i i (as representative of any suitable form of heating agency). However the auxiliary cathode also may be non-thermionic as shown at l2, Fig. 2. The thermionic cathode 9 preferably consists of timgsten, tantalum, or other suitable uncoated'material. However, coated cathodes of the Wehnelt type (such as nickel coated with oxide of barium or other alkaline earth material) or cathodes of the thoriated type (described in Langmuir Patent 1,244,216) may be used in some cases.

When the cathode 9, due to its size, temperature, and other characteristics, has a relative low electron emissivity, it may be so operated that all of the available electrons are carried to the anode or anodes, a condition known as saturated electron emission. When operated under such conditions, the voltage applied to the emissionlimited cathode is under control and may be made any desired value above ionization voltage.

The ionizing voltage for the auxiliary discharge is furnished by a battery I3, which is connected to the electrodes 9, II), respectively, by the conductors ll, IS, the latter including a switch it. when the auxiliary discharge between the electrodes 9, 10, has a current value of onlyabout 50 milliamperes at a potential of 30 volts, the main discharge may have a value of several amperes. .5 In the absence of ions generated by the auxiliary discharge, negative space charge would make it impossible to transmit an effective current through the gaseous filling upon the application of only two volts. The ionizing discharge pro- 10 duces enough ions to permit the flow of large currents between the main electrodes at a low voltage, below the ionization voltage of the gaseous filling, and thus permits the voltage drop at the cathode of the main discharge to be adjusted to the value at which a high efficiency of light production results. It is preferable to have the main cathode 3 also operate under a condition of saturated emission in order to be certain that the applied voltage will be consumed close to the cathode 3 thereby accelerating the electrons as soon as they leave the cathode.

The first critical voltage at which transfer 02 energy takes place to sodium atoms is about 2.1- volts. At this voltage, the characteristic yellow light (5890-A.) of sodium is radiated. In accordance with my invention, ionization of sodium vapor may be produced by an auxiliary discharge of low current and a voltage considerably in excess of the ionization potential, and the energy of the electrons emitted by-the main cathode may be made the value at which the greatest transfer of energy of the main discharge having a higher current value may be utilized most emciently to produce the yellow light of sodium, this value being slightly greater than 2.1 volts.

When light from sodium or other metal vapor is desired, I prefer to employ in the envelope a mixture of such vapor and an attenuated gas, such as argon, neon, or the like. When employ- 49 ing neon gas in connection with an alkali metal, such as sodium, an advantage is obtained due to the-fact that metastable atoms of neon are excited to emit 'visible radiation by collision with electrons having substantially the same velocity as those required for the excitation of sodium, so that with an applied voltage of 2.2 volts between the main electrodes, light in the visible range can be obtained from the neon metastable atoms as'well as from the sodium atoms. Some- 59 whatmore energy is required for the ionization of neon gas than for argon or krypton gas, but the energy, thus, employed is more than offset by the advantage resulting from the production of light from the excited neon gas.

As the current in the auxiliary discharge .is increased the light from the neon gas increases in the same manner as in an ordinary neon tube with a thermionic cathode. As the current in the main .discharge is increased the light from the sodium vapor increases more rapidly than the light from the neon gas. Thus, the ratio of the amount of light from the neon gas to the light of the sodium vapor can be varied by changing the ratio of the current of the auxiliary discharge to the current of the main discharge.

Should a different radiation be desired under other conditions, or, with a gaseous filling other than the above, the proper voltage can be chosen.

If the ultra-violet radiation of mercury corresponding to 2537" A. is desired in a tube containing mercury vapor, and embodying my invention, the chosen voltage would be about 5 volts.

It is especially advantageous in some cases to employ in a device embodying my invention a mixture 0! sodium vapor and mercury vapor. At a temperature at which a sodium lamp operates emciently, that is a temperature of about 230 C., the pressure of mercury vapor will be several thousand times that of sodium vapor. when the entire ionization and light production is due to a discharge between two electrodes only in such mixture of vapors, the mercury vapor spectrum is obtained largely to the exclusion of the sodium spectrum. When an auxiliary ionizing discharge is employed and the impressed voltage between the main electrodes is low, say 2.2 volts, then the light is emitted substantially exclusively by the excited sodium atoms. The: only light emitted by the mercury vapor in that case is due to the auxiliary discharge. In that case also by varying the relative magnitudes of the auxiliary and the main discharge currents the relative amounts of mercury light and sodium light can be varied at will. In this manner the color of the light produced by the lamp may be controlled.

In Fig. 5 is shown a modification of the arrangement of Fig. l, which is adapted for producing variations of the light from a device embo'dying my invention. For this purpose,,the secondary winding of a transformer I 8 is inserted in the conductor l4, thus superimposing its voltage on the voltage of the direct-current source l3. When the alternating current voltage of the primary winding of the transformer I8 is varied by any appropriate means, then the voltage impressed on the electrodes 9, l0, varies according- 1y. Other means for producing the desired result readily will suggest themselves.

The benefits of my invention may be obtainedin lamps in which the drop of voltage between the main electrodes is very much greater than the ionization voltage of the gaseous filling. Such a lamp is shown in Fig. 3. In this lamp, the main cathode 3 and the anode 4 are located a relatively long distance from one another in an elongated envelope 2', so as to produce a luminous positive column in which the drop of voltage varies with its length and diameter. As shown in detail in Fig. 4, the cathode 3' is a hollow cylinder I! which is supported by heat-conserving shells 20, 2| and is provided with a heater 22. The cylinder l9 may be supported by the conductor 23. The heater 22 for the cathode is connected at one end to a conductor 25 and at the opposite end to the cylinder IS; The interior wall of the cathode cylinder I9 is coated with thermionically active material, such, for example, as barium oxide. The auxiliary cathode 26 is located here within the main cathode cylinder l9, being supported by the conductors 21, 28. The auxiliary cathode in this location is capable'of assisting the heater 22 in maintaining the cathode cylinder I! at an electron-emitting temperature. The envelope 2' contains a luminosity-producing gaseous filling; for example, I may employ a filling of neon gas at a pressure of the order of about one-half millimeter to three or four millimeters of mercury, or a mixture of neon and argon or krypton, or various other gases of the rare or noble group; or I may employ mercury vapor at-a pressure corresponding to a bulb temperature within a range of to 100 C., or a mixture of mercury, so'dium, or other metal vapor and a so-called permanent gas, such as argon, may be employed, such metals becoming vaporized during the operation of the lamp.

The electrical connections shown in Fig. 3 are similar to those already described in connection with Fig. 1. The main circuit between the oathode 2' and the anode 4 comprises the conductors C and I, the latter containing a ballasting resistance 3|. These conductors are supplied by a some of energy not shown. The heater22 for the main cathode 3' is supplied with current through the conductors 23 and 25 from the secondary of a transformer ll, although, of course, a directcurrent source may be employed as shown in connection with Fig. 1. The auxiliary cathode 26 is supplied with current through the conductors 21 and 28 by a battery I I (or other suitable source).

A regulating resistance 32 and a switch 24 are in- I cluded in the circuit. The energy for the auxiliary discharge is supplied by a battery l2, through the conductors I4 and I5 containing a switch I. In this case, the return lead I 5' is connected to the main cathode instead of being connected as in Fig. 1 to a separate anode.

During the operation of such a lamp, a discharge of relatively small current value and small energy consumption, say ten milliamperes and thirty volts, operates between the auxiliary cathode 26 and the main cathode cylinder l9. Between the main electrodes I9 and 4 is operated the luminosity-producing discharge which may have a current value of the order of one or several amperes, the voltage consumption of which will vary with the length of the positive column. Such lamps may be readily constructed to operate at ordinary distribution voltages of 110 or 115 volts, the lamps being started byknown mechanism not here illustrated. Because of the positive ions produced in the gas by the auxiliary discharge, the drop of potential in the cathode is lowered, thereby increasing the efliciency of light production. A hollow cathode unprovided with an auxiliary source of positive ions tends to overheat, due to energy received from the discharge, even when during operation the heater is deenergized. This effect, which is particularly marked in discharges in neon and helium, less so in argon and mercury, is much reduced in devices constructed in accordance with my invention, because of the lowered cathode drop. Also, the ions and the radiations furnished by the discharge from the cathode 26 facilitate the starting of the main positive column discharge.

In Figs. 6 and 7 are shown embodiments of my invention in what may be called cathodic types of lamps in which there is no substantial voltage increase throughout the plasma in the lamp. The lamp of Fig. 6 is shown with a source of alternating-current energy supply for the main electrodes. In this case, a single cathode and two anodes are employed, the discharge passing alternately to the different anodes during alternate half wage impulses. In Fig. 'l is shown a lamp containing two anodes which however are connected together to constitute an efiect of a single anode. This lamp is shown as deriving energy from a directcurrent source of supply. In both of the lamps of Figs. 6 and "l, the auxiliary cathode surrounds the main cathode.

Referring in particular to Fig. 6, the main cathode consists of a cylinder of nickel, or other suitable material, which is externally coated with active material such as barium oxide or the like, and is provided with an internal resistance heater 3, this heater being connected at 31 to the coated cylinder and being connected at the opposite end to a supporting wire 28. The cylinder 35 is connected to a supporting wire 39. The anode 40 is connected to a supporting wire 4| and the anode 42 is connected to a supporting wire 43. The auxiliary cathode 4' is connected to supporting wires 41 and 48 which are sealed into a stem 49 at the opposite end of the envelope. Each of the supporting wires 38, 39, 4|, and 43 is surrounded by a refractory insulating tube which may consist of alumina or magnesia. The wires 38, 39, ll, and 43' are all sealed into the usual pressed glass stem 44 of the envelope l5, and are connected to external conductors bearing the same numerals, these being primed. The cathode heater 36 is supplied with current through the conductors 38' and 39' from a transformer secondary 50. The energy for heating the auxiliary cathode is supplied through the conductors 41' and 48' by the secondary winding of the transformer 5|, an adjacent resistance 52 being provided in the conductor 58'. The anode conductors 4i and 43 are connected to the terminals of an autotransformer 53, the primary being supplied by the conductors 5t, 55 from a source not shown The cathode 35 is connected to the midpoint of this transformer through an inductance 55 by means of the conductor 39'. Energy for the auxiliary discharge between the auxiliary cathode 4B and the main cathode 35 is supplied by a battery 51 which is connected respectively to the auxiliary cathode by conductors 58 and 67' and to the main cathode by conductors 59 and 38'. The gaseous filling for the envelope 55 may be similar in character to the gaseous filling already described in connection with Fig. 3.

The connections for the modifications shown in Fig. 7 are similar to the connections above described, except that direct-current source instead of alternating-current source is supplied. In place of the transformer 5! for heating the auxiliary cathode, there is employed a battery 6!. In place of the autotransformer 53, there is employed a battery 62 which is connected in common by the conductor 63 to the respective anode conductors 4i and 43'. The heater 36 for the main cathode is heated by current derived from a battery 68 which is connected thereto by the conductors 38' and 39'. The cathode 35 is connected by the conductor 39 to the negative terminal of the battery 62. The discharge between the auxiliary cathode Q6 and the main cathode 35 is maintained by a battery 57 which corresponds to the same element in Fig. 6. The operation of these lamps it is believed will be obvious from what has been said in connection with Fig. 1.

The modification shown in Fig. 8 is adapted for full wave alternating-current operation. Two main cathodes 66 and 67 are provided respectively with adjacent auxiliary cathode 68 and 69. The portions of the conductors I and H which are exposed to the discharge within the tube are surrounded respectively by refractory insulators I2 and I3. The main cathodes, asis the case in the modifications of Figs. 6 and 7, are provided with internal heaters 14 and 15. The external members of the cathodes 66 and 67 respectively are connected to conductors I6 and 11. One end of the heater 14, I is connected to the external member as shown at 80 and BI respectively. The free ends of the heaters M and 15 are connected respectively to conductors l8 and I9. The conductors l6 and 18 are connected to a winding 82 and the conductors H and 19 are connected to a winding 83. These two secondary windings are wound in common with the main secondary upon a transformer 84, the primary of which is supplied with energy by the conductors 85 and 86- from a source not shown. The terminals of the main secondary winding are connected by the conductors 81 and 88 to the auxiliary cathodes 89 and 68 in series with inductance coils 89 and 90. The conductors I6 and 11 are also each connected to taps on the main transformer secondary by conductors 9| and 92.

Initially, the main cathodes 66 and 61 are heated to an electron-emitting temperature by the energy furnished by the transformer windings 82 and 83. The voltage between these cathodes 66 and 61 will be made less than the voltage required to sustain a discharge between them. However the voltage between the main cathodes and the auxiliary cathodes being greater and being above the ionization voltage, a discharge occurs between one or both of the main cathodes and the auxiliary cathode which is positive in any particular half wave and hence can function as an anode. The current value of this discharge is limited by the reactor (89 or 99 as the case may be) in series with such auxiliary cathode to the value which will maintain such auxiliary cathode at an electron-emitting temperature. On the reverse half cycle, this auxiliary electrode, which now is hot, will function as a cathode for a discharge between it and the main cathode (functioning as anode), the current being limited by the temperature of the auxiliary cathode. In other words, the current value of this discharge is determined by the saturation'emission of the auxiliary cathode. It may be of the order of ten milliamperes. This discharge creates a plasma through which amperes of current may flow between the main electrodes 66 and 6'11 at a voltage below ionization voltage, the particular value of voltage being chosen to be most efiective in exciting the desired radiation. During this opposite half cycle, the other auxiliary electrode becomes heated in the same manner so that it may function as auxiliary cathode during the next half cycle. It is desirable that the current value of the auxiliary discharge is sufiiciently high to permit all of the available electrons to leave the main electrode which during any particular half wave is acting as cathode. This insures the voltage applied between the main electrodes will be consumed close to the electrode functioning as cathode thereby permitting the emitted electrons to excite the gas or vapor throughout substantially their whole path.

In my application Serial No. 685,376 filed concurrently herewith claims have been made generic to lamps (such as herein described) and also other embodiments of my invention, such as rectifiers for alternating current.

What I claim as new and desire to secure by Letters Patent in the United States is J l. The method which consists in producing metastable atoms in a gas by an electrical discharge and exciting said atoms by another electrical discharge having a voltage and current value sumciently great to cause the emission of light but insuflicient to cause ionization of said gas.

2. The method of producing light which consists in passing an electrical discharge through an attenuated gas at voltage sufiiciently high to form a plasma and passing through said plasma a second discharge at a luminosity-producing voltage below ionization voltage of said gas.

3. The method of producing light by an electrical discharge in a gas which consists in producing ions in said gas by an auxiliary discharge at a voltage considerably exceeding the ionization voltage of said gas and maintaining the voltage of said first-mentioned discharge at approximately the resonance voltage of said gas.

4. The method of producing luminous color eiiects which consists in operating a plurality of thermionic discharges in a gaseous mixture comprising components of unlike luminous spectrum, one of said discharges being regulated to operate at a potential above the ionization voltage of the least easily ionizable component and the other discharge being operated at a potential below the ionization voltage of the most easily ionizable component and varying the relative current values of said discharges to vary the color of the light produced.

5. The method oi. producing light eifects in a gas which consists in forming a. plasma by an electrical discharge conducted through said gas, producing a second discharge at a lower voltage suiilciently high to produce luminosity but below ionization voltage and varying said plasma-forming discharge to vary the light produced by said second discharge.

6. The method of producing radiations which comprises the following steps: producing positive ions within an attenuated gas by means of an electrical discharge, producing a second discharge through said ionized gas with a voltage drop in the vicinity of the cathode of said second discharge at such value that the electrons emitted by said cathode receive an energy which only slightly exceeds that necessary to raise atoms of said gas from an existing energy state to the state from which the atoms are capable of emitting the desired radiations.

7. The method of producing radiations which comprises the following steps: producing positive ions within an attenuated gas by means of one discharge having a cathode fall considerably in excess of the ionizing potential of said atmosphere, producing a second discharge through said ionized gas, and maintaining the voltage drop in the vicinity of the cathode of said second discharge at such value that the electrons emitted by said cathode receive an energy which only slightly exceeds that necessary to raise atoms 01' said gas from an existing level to a higher level.

8. The method of producing radiations which comprises the following steps: producing positive ions within an attenuated gaseous atmosphere by means of an electrical discharge having a cathode fall considerably .in excess of the ionizing potential of said atmosphere, passing a stream of electrons through the space containing the ions, and maintaining the energy of such electrons only slightly higher than necessary to raise atoms of said gas from an existing level to the level from which the desired radiation is emitted.

9. An electric lamp comprising an envelope, a gaseous filling therefor, electrodes therein, including a thermionic cathode, means cooperating with said cathode for supporting in said gas an electrical discharge at a voltage above ionization voltage of said gas, and means associated with said first-mentioned means for producing at higher current value and with a fall of potential at the cathode belowionization voltage.

11. An electric lamp comprising the combination of an envelope, a luminosity-producing gastherein, means for supporting a discharge of at least about an ampere at a voltage below ionization voltage or said gas. and supplementary means for supporting a discharge at materially lower 6 current value and at a voltage above ionization voltage of said gas.

12. An electric lamp comprising the combination of an envelope, a gaseous filling therefor including a charge of sodium, a main thermionic cathode, a main anode, and means for operating an electric discharge in said gaseous filling at a voltage above the ionization voltage for said gaseous flllingto generate positive ions by collision whereby a discharge may be operated between said main electrodes at a voltage below such'ionization voltage.

13. ,An electric lamp comprising the combination of an envelope, a gaseous filling therefor, a main thermionic cathode, a main anode spaced 20 away from said cathode, and an auxiliary cathode spaced more closely to said main cathode than said main anode.

14. An electric lamp comprising, in comblna- '4 tion, a sealed envelope, a gaseous atmosphere 2:;

, therein, a plurality oi electrodes within said envelope, at least one of said electrodes being a thermionic cathode, means to produce a discharge between a pair oi said electrodes to produce a plasma about said thermionic cathode, and means to maintain in said plasma a discharge between said thermionic cathode and another electrode with a voltage drop in the vicinity 01' said cathode adjusted to such a value that the electrons emitted by said cathode receive an energy which only slightly exceeds that necessary to raise atoms of said gas from an existing level to a higher level from which desired radiations 15. An electric lamp comprising, in combination, a sealed envelope, a gaseous atmosphere therein, a plurality of electrodes within said envelope, at least one of said electrodes being a thermionic cathode, means to produce a discharge between a pair of said electrodes with a cathode fall considerably in excess 0! the ionizing potential of said atmosphere to produce a plasma about said thermionic cathode, and means including a thermionic cathode to maintain a discharge in said plasma between said thermionic cathode and another electrode with a voltage drop inthe vicinity of the cathode adjusted to such a value thatthe electrons emitted by said cathode receive an energy which only slightly exceeds that necessary to raise atoms of said gas from an existinglevel to a higher level from which desired radiations are emitted.

16. An electric lamp comprising a sealed envelope, a gaseous atmosphere therein, a thermionic cathode at one end of said envelope, an anode at the opposite end thereof, means including an auxiliary cathode in theneighborhood of said thermionic cathode to produce a plasma ther'eabout, and means to produce a discharge between said thermionic cathode and said anode, said discharge being adjusted to have a voltage fall in the vicinity of said cathode of such a value that the electrons emitted by said cathode receive an energy, which only slightly exceeds. that necessary to raise atoms of said gas from an existing level to a higher level from which desired radiations are emitted.

17. An electric lamp comprising an elongated envelope, a charge therein 01 luminosity-producing gas at a pressure of about one-half to several 76 millimeters of mercury, a. main thermionic cathode, a main anode spaced far enough from said main cathode to permit of a positive column type of discharge therebetween, an auxiliary thermionic cathode spaced closely adjacent said main cathode and having a materially smaller electron emissivity than said main cathode, and means for producing between said auxiliary cathode and said anode an electric discharge above the ionization voltage of said gas.

18. An electric lamp comprising a sealed envelope, a charge of luminosity producing gas therein, an anode, a main hollow thermonic cathode coated upon its inner surface with thermionically active material, a heater therefor, and an auxiliary cathode located within said main cathode having materially smaller electron-emissivity than said main cathode and means for producing between said auxiliary cathode and said main cathode a discharge at a sufllciently high voltage to lower the drop of voltage at the main cathode and to increase the luminous em- 10 ciency of said lamp.

CLIFTON G. FOUND.

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