US2112719A - Electric discharge device - Google Patents

Description

March.29, 1938. R. M. soMERs 2,112,719

ELECTRIC DSCHARGE DEVICE Filed Nov. l0, 1934 ATTORNEY Patented Mar. 29, 1938 UNITED STATES 2,112,719 ELECTRIC DISCHARGE DEVICE Richard M. Somers, Orange, N. J., assignor to Thomas A. Edison, Incorporated, West Orange, N. J., a corporation of New Jersey Application November 1o, 1934, serial No. '152,401

Claims.

This invention relates to electric' discharge devices of the typethroughout the normal operation ol which a small quantity of included metal is vaporized, providing within the device a metal I vapor at low pressure through which an arc discharge takes place. tion relates to those low pressure, metal vapor discharge devices which also contain an inert gas, or combination of gases, the gas providing a medium for a temporary or initial arc discharge whose 'function is to heat the device from an initially cold condition to substantial metal Vaporizing temperature. thereto, the invention is particularly applicable "Y to discharge devices having appreciable discharge column lengths, and especially to illuminating discharge tubes. A

It is an object of my invention to provide an improved device of the type above outlined which tion from the current supply.

Still another object of my invention is to provide a device of the type outlined in which the temporary arc discharge will be dependably produced at any current sup'ply voltage within sub-.

stantially the full range o f supply voltages appropriate to the maintenance of the normal vapor arc discharge.

Other and allied objects will more fully appear from the following description and the appended claims.

In the description reference is had to the accompanying drawing, of which:

Figure 1 is an illustration of a typical discharge device of the type above outlined, together with an appropriate associated circuit through which the device may be connected to the current supply;

Figure 2 is a typical curve of a function herein- 45 after described, plotted against gas pressure; and

Figure 3 is a curve of optimum gas pressure, as hereinafter explained, plotted against discharge column length.

Before outlining my invention proper I refer to 50 and describevin detail the typical system illustrated in the ligure. The discharge device has been illustrated as one adapted for illuminating purposes, having two anodes, and arranged for operation on an alternating current supply; it 55 will hereinafter be obvious, however, that my in- More particularly my invenv While not in al1 aspects limited acterized by a very high efficiency-i. e., ratiooi'- power in the discharge column to power consump- Vention is equally available for employment with other forms of discharge devices and current supplies, such for example as single anode devices and vdirect current supplies, or double element tubes for alternating current operation in which each element is alternately anode and cathode. In the figure will be seen the tube l having the glass or other envelope 2, the space 2' within the envelope having been evacuated of air and containing an inert gas, or combination' of gases, at pressures as hereinafter more fully set forth; the gas may for example be krypton. Also within the space 2 is provided a small amount of the metal whose vapor is to form the medium for the normal arc discharge; and while I intend no limitation to any specific metal, the small pool 2", within the space 2 in the ligure may be considered as a pool of mercury. 'I'he elements of the tube are the cathode 3, the anodes 4a and 4b, and the auxiliary starting electrode 5. The cathode 3 is of the thermionic type, and by way ofexample has been illustrated as a folded filament which may be coated with suitable oxides or the like to increaseits emission. The anodes 4a and lb are located near the opposite end of the tube from the' cathode 3, and may for example lbe of the vusual carbon variety. The starting electrode 5 may be a conductive ring surrounding and in slight spaced relationship to the cathode 3.

The discharge current for the device may be obtained from an alternating current line through the transformer 6,'which by way of illustration has' been shown-as an auto-transformer. One side of the line is connected through the on-off switch 9' to the primary terminal 6p of the transformer, while the -other side of the line is connected selectively to the primary terminalsI 6p', 6p and Gp'" through the selecting switch 92. 'I'he transformer may have the extreme secondary terminals 6a and 6b and the secondary center-tap 6c; the first two are connected respectively to the anodes la and 4b through the ballast resistances or lamps 8a and 8b. while the center-tap 6c is connected to Athe cathode 3 through the inductance coil or choke I0 both the lamp 8a-8b and the choke I 0 form ballast orcurrent-limiting impedance means for the system.4 A tertiary winding 'l forming a part of the transformer 6 may be connected across the extremities of -the filament or cathode 3 for heating the same. For starting thel temporary arc discharge through the gas I have illustrated herein, though purely by way of example, the starting system shown inthe Figure 3 of U. S. Patent 2,013,974 to myself, which system is described in that patent with reference to quite similar designating numerals. With this system the armature I2" is attracted by choke l0 to open the closelybiased switch I2, and to break the circuit I0-6c-6d-l2-53, when the cathode 3 has attained substantially normal operating temperature; a high voltage transient is thus produced across the choke and causes the initiation of the temporary arc discharge. This-discharge heats the device, gradually vaporizing more and more metal; and the arc discharge transfers itself (usually gradually) from the gas as a medium to the vapor as a medium, forming the normal vapor are discharge.

It is well understood that to maintain the normal vapor are discharge in a given system of the type described the supply voltage must be maintained above a minimum limit. It is also true, neglecting the purely theoretical case of a perfect ballast, that there is a maximum value which the supply voltage must not be permitted to exceed. This upper limit of supply voltage appears to be occasioned as follows: With an increase of supply voltage .there will occur some increase of discharge current and hence of temperature within the device. This temperature increase causes vaporization of more of the included metal (e. g., 2"), increasing the vapor pressure, and occasions an increase of drop in the discharge column. As the supply voltage and hence the temperature increase, not only does the discharge column drop rise, but the rate of its rise progressively increases; presently a temperature is reached at which the drop tends to exceed the value which the supply voltage can maintain between anodes and cathode, and extinction of the discharge results. Thus the vapor arc discharge when in progress will be maintained only so long as the supply voltage remains within a definite range; this may lconveniently be termed the vapor voltage range and its limits respectively the upper and lower vapor limiting voltages.

As to the temporary arc discharge through the gas, there is in general no significant upper supply voltage limit. There is, however, a lower permissible supply voltage limit which may be termed the gas extinction voltage; if the supply voltage falls below this limit while the temporary arc discharge is in progress,l extinction will result. For the low vapor pressures the lower vapor limiting voltage is'always exceeded by the gas extinction voltage. Accordingly the system, if it is to be used without the requirement for manipulation of its parameters during operation, has a net permissible supply voltage range whose upper limit is the upper vapor limiting voltage, but whose lower limit is the gas extinction voltage.

It will of course be understood that the absolute values of the limits of both the vapor and the net permissible voltage ranges may be together adjusted upwardly or downwardly, as by changing of the transformer ratio by adjustment of tap-selecting switch 9"; and that the most important characteristic of either range is the ratio of its upper to its lower limit, or its logarithmic width. For a given ballast, this ratio or width may be increased (as to both vapor and net permissible voltage ranges) by shortening the discharge column; again, for a given tube or other device this ratio or width may be increased (as to both ranges) by increasing the ballast of the system. In either case the ratio or width increase, representing increased tolerance to supply voltage fluctuation, is obtained by increase of the relative ballast`-i. e., by increase of the power in the ballast relative to that in the discharge column. It is common practice to impart to the system a sufciently wide net permissible voltage range for operation with any particularv range of supply voltage uctuation..

simply by a suflleient progressive increase of this relative ballast. But for conventional discharge devices with any ballast the gas extinction voltage is materially higher than the lower vapor limiting voltage, and the width of net permissible voltage range therefore very mucl less than that of the vapor voltage range; accordingly in common practice a relative ballast is employed which provides a width of vapor voltage range very much greater than will be needed for supply voltage fluctuationsA during continuance of the 'vapor arc discharge, and a wholly useless tolerance thus obtains during this continuance, i. c., throughout the actually useful operation of the system.

According to my invention I employ a relative ballast suilcient to make the vapor voltage range only a little wider than the range of supply voltage iiuctuations, and yet maintainv the net permissible voltage range just as wide as the supply range by reducing to a very low value the excess of gas extinction voltage over lower vapor limiting voltage. By this procedure I am able to secure dependable operation with materially higher than usual eiliciencies. The manner in which I reduce the difference above mentioned may be outlined as follows:-

If for a device and system of the type described there be plotted the excess of gas extinction voltage over lower vapor limiting voltage as a function of the pressure of the included gas, it will pressure the function attains a minimum value; the curve of such a function has been illustrated as A in Figure 2, taken in connection with the left-hand set of ordinates. While for devices otherwise similar different gasesv will cause the corresponding functions to have their respective minima at diierent gas pressures, I have' found that for any particular gas thel vpressure at which the function minimum occurs is essentially independent of ballast parameters and of parameters of the discharge device properotber than the length of the discharge column` or anodecathode separation. I have further found that the gas pressure for minimum function value is for practical purposes similar to the gas pressure for minimum gas extinction voltage: been indicated in Figure 2 by the right-hand set of ordinatesapproximate gas extinction voltageslikewise applying to the curve A. According to my invention I establish the gas within the device at a pressure which causes the function above mentioned to assume substantially its minimum value, or, as a satisfactory approximation, at a value which renders the gas extinction voltage a substantial minimum for example at the pressure which is the abseissa of the point P on curve A.

I have further found that the function (or extinction voltage) will remain invested with a minimum value upon variation of the discharge column length if the gas pressure be varied substantially inversely with this length. Therefore, in convenient terms, I establish the gas pressure P at a value given by the expression P K/L wherein L is the length of the discharge column in convenient units) and K is a constant debe found that at some particular gas.

this has pressure in atmospheres for the gas krypton,

plotted against discharge column length in centimeters. It will be understood that this curve B represents the locus of the minimum point P of a family of the curves A of Figure 2, expressed in terms of pressure for various column lengths (the locus of the point P, expressed in terms of excess voltage for various lengths, exhibiting a variation, from constancy of that excess, which is small enough to be disregarded).

The constant K for any gas is readily determined by adjusting that gas in a representative discharge device to the pressure yielding the minimum value of the function above discussed, and multiplying that pressure by the discharge column length in that tube. Because the gas pressure for minimum function value is approximated by the pressure for minimum'gas extinction voltage, a satisfactory constant for practical purposes may be arrived at by adjusting the gas in a representative discharge device to the pressure yielding minimum gas extinction voltage, and multiplying that pressure by the` discharge column length. In the latter case the representative device may be a simple discharge devicei. e., one from which vaporizable metall has been omitted. Y

I may illustrate the application of my invention by an example. Let there rst be assumed a mercury-containing tube intended for operation with a mean discharge current of approximately 2 amperes and at a mercury pressure of approximately .000133 atmosphere, the tube -having a discharge column length of 100 centimeters and a diameter of 2 centimeters. Let this tube be assumed lled with krypton at a pressure of approximately .00133 atmosphere, which is repre-'- sentative of vconventional practise. With asuitable ballast, the vapor are discharge may be maintained in this tube by a fluctuating supply voltage which maintains between terminal 6c and either of the terminals 6a. and'sb a voltage ranging from 80 to 115 volts; the lower and upper vapor limiting voltages are accordingly 80 and 115 respectively. The gas extinction voltage, however, will be of the order of 100 volts, so that the net permissible voltage range is only 100 to 115 volts. If the system is to function on a cur rent supply .whose voltage fiuctuates between limits having the ratio oi" 115:100, it is obvious that neither can the ballast -be decreased, nor can the 4tube be lengthened. The excess of gas extinction voltage over lower vapor limiting voltage is approximately 20 volts, and is quite typical.

I have found' that by the application of my invention to such a tube I may decrease the excess of gas extinction voltage over lower vapor limiting voltage to between 2 and 5 volts, the precise value within this small range depending on many factors such as normal tube operating temperature, room temperature, and associated circuit parameters. Using the maximum difference figure of 5 volts, I may lengthen the discharge column to such a length that the vapor voltage' range is 95 to 115 volts, while still maintaining the net permissible voltage range at the assigned 100 to 115 volt value. Since the tube at a voltage just above its original vapor limiting voltage would operate with an arc drop of very approximately .4 volt per centimeter, I may lengthen i the discharge column by or 37.5 centimeters. In round figures I may increase the discharge column length by IA-i. e.,

from 100 to 133 centimeters. A lower magnitude of ballast impedance will now serve to keep the upper limiting voltage at 115 volts. With the reduced ballast impedance the current through the lengthened tube, for any supply voltage throughout the 100 to 115 volt range, will not be far different from the current at that supply voltage through the original tube and ballast. Since the voltage drop in the discharge column is increased by 33%, an eilciency increase of very nearly 33% is obtained.

'I'he lengthening of the tube is rendered permissible by the employment of the proper gas pressure. To determine this pressure for the lengthened tube I divide .4, the constant for krypton above mentioned, by 133, the length of the revised discharge column in centimeters, the' quotient of approximately .003 being the krypton pressure in atmospheres which I employ for the lengthened tube. It will of course be obvious that instead of lengthening the tube in this example I might, upon establishment of the optimum gas y pressure of .004 atmosphere for the tube with its original length, have reduced both the ballast and the mean supply voltage. 'Ihe same result of eiciency increase without impairment of dependability of operation would be secured.

It will be understood that the voltages to which I have referred herein, except as otherwise spe- 'cially qualified, are voltages measured across the primary of the transformer 6, to whichv voltages there is proportional the sum of the voltages across the discharge device (I) and vacross `the ballasting means (I0 and 8a).

It will be understood that the broader aspects of my invention are not intended to be limited by the examples set forth herein, but that the scope of the invention is intended to be expressed in y the following claims:

and thermionic cathode elements spaced apart to provide a substantial positive column space therebetween, and containing lkrypton at a pressure ,of approximately .4/L atmosphere, L being the anode-cathode separation in centimeters.

2. A low pressure metal vapor discharge device including anode and thermionic cathode elements spaced apart to provide a'substantial positive column space therebetween, and containing krypton at a pressure of approximately .4/L atmospheres, L being the anode-cathode separation in centimeters. A

3. A low pressure mercury vapor discharge device including anode and thermionic cathode elements spaced apart to provide a substantial positive column space therebetween, and containing krypton at a pressure of approximately .4/L atmosphere, L being the anode-cathode separation in centimeters.

4. An electric discharge device including anode and thermionic cathode elements spaced apart to provide a substantial positive column space therebetween; ballast means adapted for serial connection with said elements to a current source;

between; ballast means said elements and forming therewithv a load cirvaporizable metal within said device for providing a low pressure vapor discharge medium; and an initial heating discharge medium within said device, comprising krypton at a pressure of approximately .4/L atmosphere, L being the anodecathode separation in centimeters.

5. An electric discharge device including anode and thermionic cathode elements spaced apart to provide a substantial positive column space therebetween; ballast means adapted for serial connection with said elements to a current source; vaporizable mercury within said device for providing'a low pressure mercury vapor discharge medium; and an initial heating discharge me- 'dium within said device, comprising krypton at a pressure of approximately .4/L atmosphere, L being the anode-cathode separation in centimeters.

6. An electric discharge device including anode and thermionic cathode elements spaced apart to provide a substantial positive column space therebetween; ballast means adapted for serial connection with said elements to a current source; vaporizable metal within said device for providing a low pressure vapor discharge medium; and an initial heating discharge medium included in said device, comprising an inert gas at a pressure adjusted for substantially minimum gas discharge extinction voltage.

'ls An electric discharge device including anode and thermionic cathode elements spaced apart to provide a substantial positive column space thereserially lconnected with cuit; means for applying voltages across said circuit; vaporizable metal within said device for providing, when hot and with voltages across said circuit above a lowerlimitirig4 voltage, a low pressure vapor discharge medium between said elements; and an inert gas included in said device at a pressure adjusted to provide a gas discharge medium between said elements with substantially 8. An electric discharge device including anode and thermionic cathode elements spaced apart to provide a substantial positive column space therebetween; ballast means serially connected with said elements and forming therewith a load circuit; means for applying voltages across said circult; vaporizable metal within said device for providing, when hot and with voltages across said load circuit above a lower limiting voltage, a low pressure vapor discharge medium between said elements; and an inert gas included in said device at a. pressure adjusted -to provide between said elements a gas discharge medium characterized by an extinction voltage, measured across said load circuit, of within ve volts above said lower limiting voltage.

9. An electric discharge device including anode and thermionic cathode elements spaced apart to provide a substantial positive column spaced therebetween; ballast means adapted for serial connection with said elements to a current source; vaporizable m'etal within said device for providing a low pressure vapor discharge medium; and an initial heating discharge medium included in said device, comprising an inert gas at a pressure of approximately K/L, L being the anode-cathode separation in said device, and K being the product of anode-cathode separation and gas pressure in a simple purely gas discharge device containing similar gas at pressure adjusted for minimum discharge extinction voltage.

l0. A low pressure metal vapor discharge device including anode and thermionic cathode elements spaced apart to provide a substantial positive column space therebetween and containing an inert gas at a pressure of approximately K/L, L being the anode-cathode separation in said device, and K being the product of anode-cathode separation and gas pressure in a simple purely gas discharge device containing similar gas at pressure adjusted for minimum discharge extinction voltage.

' RICHARD M. SOMERS.

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