Title:
VIBRATION RESISTANT LAMP
United States Patent 3798490
Abstract:
An incandescent lamp having a filament made of tungsten wire containing between 1-2 percent by weight thoria has greatly improved resistance to fracture caused by shock and vibration thereby increasing the service life of the lamp. The thoriated tungsten filament has a recrystallized grain structure which is characterized by approximately equiaxed crystals of tungsten of a smaller and more uniform size and thoria particles which are more uniformly distributed throughout the tungsten matrix than can be obtained by conventional powder metallurgy.
US Patent References:
/3697321.html
Eckert - October 1972 - 3697321
Electric incandescent lamp
Cardell, Jr. - October 1965 - 3211943
/3697321.html
Eckert - October 1972 - 3697321
Electric incandescent lamp
Cardell, Jr. - October 1965 - 3211943
Inventors:
Ackerman, Stanley C. (Cleveland Heights, OH)
Dunham, Thomas E. (Cleveland Heights, OH)
Dunham, Thomas E. (Cleveland Heights, OH)
Application Number:
05/285938
Publication Date:
03/19/1974
Filing Date:
09/01/1972
View Patent Images:
Export Citation:
Assignee:
General Electric Company (Schenectady, NY)
Primary Class:
Other Classes:
313/344, 252/515, 313/578
International Classes:
H01K1/04; H01K1/00; H01J1/38; H01J19/06; H01J1/48; H01J1/05; H01J1/14
Field of Search:
313/218,311,344,222,346 252/515
Primary Examiner:
Rolinec, Rudolph V.
Assistant Examiner:
Chatmon Jr., Saxfield
Attorney, Agent or Firm:
Sos Jr., Emil Truesdell Henry Newhauser Frank F. P. L.
Claims:
What we claim as new and desire to secure by Letters Patent of the United States is
1. An incandescent lamp comprising a sealed vitreous envelope, lead-in conductors hermetically sealed in the envelope and connected to an incandescible filament wherein the improvement comprises that the filament material consists essentially of a tungsten alloy containing between 1 and 2 percent by weight of thoria (ThO2), said tungsten alloy having a recrystallized grain structure of approximately equiaxed tungsten crystals with a grain size smaller and more uniform in size than can be obtained by conventional powder metallurgy preparation.
2. An incandescent lamp as claimed in claim 1 wherein said thoria range is 1.3 to 1.9 by weight percent.
3. An incandescent lamp as claimed in claim 1 wherein said filament is further characterized by the thoria particles being distributed in a slight but uniform concentration gradient with maximum concentration being located at the exterior surface of the filament and with said thoria concentration continuously diminishing with increasing distance to the longitudinal central axis of the filament.
4. An incandescent lamp as claimed in claim 1 wherein said filament is further characterized by the thoria particles being distributed more uniformly throughout the tungsten than can be obtained by conventional powder metallurgy processes.
5. An incandescent lamp comprising a sealed vitreous envelope, lead-in conductors hermetically sealed in the envelope and connected to a plurality of incandescible filaments wherein the improvement comprises that one of the filaments consists essentially of a tungsten alloy containing between 1 and 2 percent thoria by weight, said tungsten alloy having a recyrstallized grain structure of approximately equiaxed tungsten crystals with a grain size smaller than can be obtained by conventional powder metallurgy preparation.
6. An incandescent lamp as claimed in claim 5 wherein the thoria concentration range is between 1.3 and 1.9 by weight percent.
1. An incandescent lamp comprising a sealed vitreous envelope, lead-in conductors hermetically sealed in the envelope and connected to an incandescible filament wherein the improvement comprises that the filament material consists essentially of a tungsten alloy containing between 1 and 2 percent by weight of thoria (ThO2), said tungsten alloy having a recrystallized grain structure of approximately equiaxed tungsten crystals with a grain size smaller and more uniform in size than can be obtained by conventional powder metallurgy preparation.
2. An incandescent lamp as claimed in claim 1 wherein said thoria range is 1.3 to 1.9 by weight percent.
3. An incandescent lamp as claimed in claim 1 wherein said filament is further characterized by the thoria particles being distributed in a slight but uniform concentration gradient with maximum concentration being located at the exterior surface of the filament and with said thoria concentration continuously diminishing with increasing distance to the longitudinal central axis of the filament.
4. An incandescent lamp as claimed in claim 1 wherein said filament is further characterized by the thoria particles being distributed more uniformly throughout the tungsten than can be obtained by conventional powder metallurgy processes.
5. An incandescent lamp comprising a sealed vitreous envelope, lead-in conductors hermetically sealed in the envelope and connected to a plurality of incandescible filaments wherein the improvement comprises that one of the filaments consists essentially of a tungsten alloy containing between 1 and 2 percent thoria by weight, said tungsten alloy having a recyrstallized grain structure of approximately equiaxed tungsten crystals with a grain size smaller than can be obtained by conventional powder metallurgy preparation.
6. An incandescent lamp as claimed in claim 5 wherein the thoria concentration range is between 1.3 and 1.9 by weight percent.
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to incandescent lamps having tungsten filaments. More particularly, the invention relates to a lamp having a thoriated tungsten filament which has a unique grain structure to give greatly increased resistance to fracture caused by vibration and shock.
2. Description of the Prior Art
In many lamp applications, the lamp filament is subject to various forms of vibration and shock. One such application is automobile stop, tail and signal lamps. Failure of these lamps is caused predominantly by road vibration and shock fracturing what is known as the minor filament used for tail lighting. The frequency of these failures and the accompanying inconvenience in charging lamps is well known to the transportation industry. Additionally, there is a safety hazard involved when the vehicle operator is unaware of a tail lamp failure.
In an effort to increase the service life of the minor filament, thoriated tungsten wire was substituted for unalloyed tungsten or, in some cases, for tungsten doped with aluminum, sodium, silicon and potassium known as lamp-doped tungsten. Various percentages of thoria were added to the tungsten to improve strength and fracture resistance in lamps. It was found that thoriated tungsten ingots were difficult to process into filament wire. This difficulty and the accompanying production losses increased proportionately with an increase in the percentage of thoria. Because of the high production losses, it has not been economically feasible to manufacture a thoriated tungsten wire of relatively small diameter with more than approximately 1 percent by weight thoria. This 1% thoriated wire, known in one commercial form as NF wire, although an improvement over unalloyed tungsten and lamp-doped tungsten, continued to fracture and fail at a relatively high rate due to the presence of large crystals of tungsten.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to produce a lamp with long service life and high reliability. A further object of the invention is to increase highway safety by improving lamp service life and thereby diminishing the frequency of failures due to road vibration and shock. Still another object of the invention is to manufacture a lamp with a tungsten filament having more than 1 percent by weight of thoria.
The objects of the invention are accomplished by producing a lamp containing a filament of tungsten alloyed with between 1 and 2 percent by weight of thoria with the recrystallized tungsten particles being approximately equiaxed crystals of a grain size smaller and more uniform in size than can be obtained by conventional powder metallurgy preparation and the thoria particles being more uniformly distributed throughout the tungsten matrix. Increased dampening capacity and associated resistance to fracture caused by vibration is related to grain size and the number of grain boundaries. In lamps of the invention, the filament has tungsten crystals significantly smaller and more uniform in size than that obtained in other thoriated tungsten wire. With the smaller and more uniform tungsten grain wire, the number of grain boundaries are significantly increased which, in turn, gives the wire increased dampening capacity and resistance to fracture caused by vibration.
In order to obtain the wire filament used in lamps of the invention, a particular type of metallurgical processing is necessary. This process and dispersion alloy products made by it are more fully described in copending applications entitled "Metal Products and Processes of Preparation," Ser. No. 186,143, filed Oct. 4, 1971 now U.S. Pat. No. 3,741,734 "Tungsten Alloy Products," Ser. No. 248,933, filed May 1, 1972, and "Tungsten Filament", Ser. No. 285,939, filed Sept. 1, 1972. Both of these copending applications are assigned to the assignee of the present invention.
Briefly stated, the process for making the filament wire comprises the making of a porous compact of tungsten particles, soaking the compact in water, then soaking the water-saturated compact in a solution of aqueous Th(NO 3 ) 4 having a concentration of approximately 520 grams ThO 2 per liter of solution for about 30 hours. The compact is removed and then vacuum dried until the greater part of the solvent has evaporated thereby leaving a thorium-rich additive in the tungsten. After the evaporation process, the compact is presintered in hydrogen at approximately 1200°C to convert thorium nitrate to thorium oxide. This compact or ingot is then rolled, swaged and drawn by conventional means to form the diameter wire required for the filament.
It has been found that this processing will yield thoriated tungsten wire with between 1 and 2 percent by weight thorium and small approximately equiaxed crystals of tungsten of a size smaller and more uniform than that obtainable through conventional powder metallurgy methods. With the smaller more uniform grain sizes and increased number of grain boundaries, the dampening capacity and resistance to fracture of the tungsten wire is greatly increased. This construction, in turn, increases the service life of the lamp.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevation view partially in section of an automotive stop, tail and signal lamp;
FIG. 2 is a graph illustrating test data of conventional thoriated filament lamps and thoriated tungsten filament lamps of the invention;
FIG. 3 is a photograph taken at 1000X magnification of a 2.2-mil wire of the prior art; and
FIG. 4 is a photograph taken at 1000X magnification of a 2.2-mil wire used in the lamp of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1 of the drawings, lamp 1 is comprised of envelope 2 to which is sealed re-entrant stem 3. Pairs of lead-in conductors 4 and 5 are hermetically sealed to re-entrant stem 3. The lamp 1 of the invention may contain a fill of inert gas such as argon or may be a vacuum lamp.
The two-filament lamp illustrated in FIG. 1 is commonly used in automobiles for stop, tail and turn signal lighting. However, the invention is not limited to a two-filament lamp since the lamp may have a single filament or two or more filaments. Filament 6 is known as the major filament and is used for stop and turn signal lighting purposes. This filament is connected to lead-in conductors 4, one of which is attached to the base shell 8 and the other of which is attached to contact tab 9. Since major filament 6 requires a higher color temperature, it is usually made of a lamp-doped tungsten.
Minor filament 7 is connected to lead-in conductors 5 which are, in turn, connected to contact tab 9 and base 8 which serves as a common contact for both filaments. Contacts 9 and 10 are electrically insulated from each other by plastic insulator 11. Also shown as part of base member 8 is one of two alignment lugs 12 which serve to locate the lamp 1 in an automobile socket, not shown.
Filament 7 burns with a color temperature of approximately 2260°K at a design voltage of 14 volts and generally has a smaller diameter than that of major filament 6. Because of the smaller diameter of filament 7, necessitated in part by candle power and wattage requirements, the lamp 1 has a tendency to fail predominantly through fracture of filament 7. Some experimental data show that 60 percent of the two-filament lamp failures are due to failure of minor filament 7.
One of the methods used to test lamps subject to vibration in various applications, such as automobiles, is known as the "Michigan Tester." This test device holds a lamp in contact with a rotating cam which has four equally spaced steps on the circumference of the cam. As the cam rotates, the lamps drop from step to step. This movement imparts a force or a bounce which simulates the vibrational forces exerted on a lamp when an automobile hits a bump. Lamps on the Tester were in electrical contact with a power source which would cycle the lamps on and off to simulate user conditions in which tail, stop and signal lamps are used only a portion of the time.
Michigan test results of lamps using 1% by weight thoriated tungsten wire of the prior art are shown in FIG. 2 as circles. The triangles on the graph indicate service life of lamps manufactured with wire having the uniquely small tungsten grain structure and the more uniformly distributed thoria. Four of these lamps are shown as light triangles in the upper left portion of the graph because they failed at about the same time the four lamps illustrated by the circles failed. The horizontal axis of the graph of FIG. 2 is the number of hours of lamp service life as measured by the electrical continuity of minor filament 7, and the vertical axis indicates the percentage of lamps that survived for a given number of hours.
Twenty lamps of each type were tested to produce the data illustrated in FIG. 2. Eighteen of the lamps or 90 percent using the thoriated wire of the prior art failed on or before the expiration of 300 hours. The remaining two lamps were removed from the Tester. By comparison, 14 of the lamps of the invention failed on or before the expiration of 980 hours. Six lamps, not shown on the graph, were still running at the 980-hour mark.
A further illustration of improved service life can be seen at the 50 percent survivor line. Half of the prior art lamps failed after 150 hours, whereas, half of the lamps of the invention can be expected to fail at 600 hours. This is an improvement in lamp service life of the magnitude of 4.
An explanation of this improved performance is the increased dampening capacity and strength which is related to the grain structure of the thoriated tungsten alloy wire and the percent of thoria in the wire.
Initial tests were conducted on lamps with filaments having approximately 0.6 percent by weight of thoria and the smaller grain sizes of tungsten and uniformly distributed thoria particles. The results from the Michigan Tester indicated that the 0.6 percent thoria wire with the unique crystal structure did not increase the lamp service life because it lacked sufficient grain size stability at lamp operating temperature. In some instances, the 0.6 percent wire lamps had a shorter life than the 1 percent thoria wire of the prior art.
By increasing the percentage of thoria in the filament to between 1 and 2 percent and by maintaining the unique crystal structure of the thoriated tungsten wire, a marked improvement in lamp service life was obtained. Unlike the 0.6 percent thoria, wire with 1-2 percent thoria exhibits grain size stability at lamp operating temperatures. Other test results have indicated that the preferred range within the 1 to 2 percent range is from 1.3 to 1.9 percent thoria. This preferred range takes into consideration the balancing between increased service of the lamp and manufacturability of the tungsten wire.
FIG. 3 shows the grain size of the outer diameter surface of thoriated wire of the prior art after it has been tested on the Michigan Tester. The photograph of FIG. 3 is a 1000X magnification of a 2.2-mil wire which is generally used for minor filament 7 of stop, turn and tail lamp 1. As can be seen from the photograph, there are large size grains of tungsten with the largest being located at the upper right side of the photo. Large grain size microstructures have fewer grain boundaries through which to absorb the vibrational force. Accordingly, the wire shown in FIG. 3 has a greater likelihood of fracture.
By contrast, the 1.7 weight percent, 2.2-mil wire of the present invention magnified 1000 times as shown in the photograph of FIG. 4 has many small grains of tungsten. This, in turn, gives a greater number of grain boundaries which can absorb or dampen the vibrational force. In order to obtain the small grain size and more numerous grain boundaries, the lamp filament is prepared by way of a new wire processing method as is more fully described in copending applications, Ser. Nos. 186,143 and 248,933, assigned to the same assignee as the present invention. Several other wire properties which are believed to contribute to increased resistance to fracture from vibration are a more uniform size and spacial distribution of thoria particles throughout the tungsten matrix and a slight but uniform thoria gradient which decreases with increasing distance to the longitudinal central axis of the filament.
In accordance with the patent laws, one embodiment of the invention has been described. It will be understood, however, that other variations of the lamp of the invention such as a single-filament lamp of the double- or single-ended variety are also within the scope of the invention and the appended claims.
1. Field of the Invention
This invention relates generally to incandescent lamps having tungsten filaments. More particularly, the invention relates to a lamp having a thoriated tungsten filament which has a unique grain structure to give greatly increased resistance to fracture caused by vibration and shock.
2. Description of the Prior Art
In many lamp applications, the lamp filament is subject to various forms of vibration and shock. One such application is automobile stop, tail and signal lamps. Failure of these lamps is caused predominantly by road vibration and shock fracturing what is known as the minor filament used for tail lighting. The frequency of these failures and the accompanying inconvenience in charging lamps is well known to the transportation industry. Additionally, there is a safety hazard involved when the vehicle operator is unaware of a tail lamp failure.
In an effort to increase the service life of the minor filament, thoriated tungsten wire was substituted for unalloyed tungsten or, in some cases, for tungsten doped with aluminum, sodium, silicon and potassium known as lamp-doped tungsten. Various percentages of thoria were added to the tungsten to improve strength and fracture resistance in lamps. It was found that thoriated tungsten ingots were difficult to process into filament wire. This difficulty and the accompanying production losses increased proportionately with an increase in the percentage of thoria. Because of the high production losses, it has not been economically feasible to manufacture a thoriated tungsten wire of relatively small diameter with more than approximately 1 percent by weight thoria. This 1% thoriated wire, known in one commercial form as NF wire, although an improvement over unalloyed tungsten and lamp-doped tungsten, continued to fracture and fail at a relatively high rate due to the presence of large crystals of tungsten.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to produce a lamp with long service life and high reliability. A further object of the invention is to increase highway safety by improving lamp service life and thereby diminishing the frequency of failures due to road vibration and shock. Still another object of the invention is to manufacture a lamp with a tungsten filament having more than 1 percent by weight of thoria.
The objects of the invention are accomplished by producing a lamp containing a filament of tungsten alloyed with between 1 and 2 percent by weight of thoria with the recrystallized tungsten particles being approximately equiaxed crystals of a grain size smaller and more uniform in size than can be obtained by conventional powder metallurgy preparation and the thoria particles being more uniformly distributed throughout the tungsten matrix. Increased dampening capacity and associated resistance to fracture caused by vibration is related to grain size and the number of grain boundaries. In lamps of the invention, the filament has tungsten crystals significantly smaller and more uniform in size than that obtained in other thoriated tungsten wire. With the smaller and more uniform tungsten grain wire, the number of grain boundaries are significantly increased which, in turn, gives the wire increased dampening capacity and resistance to fracture caused by vibration.
In order to obtain the wire filament used in lamps of the invention, a particular type of metallurgical processing is necessary. This process and dispersion alloy products made by it are more fully described in copending applications entitled "Metal Products and Processes of Preparation," Ser. No. 186,143, filed Oct. 4, 1971 now U.S. Pat. No. 3,741,734 "Tungsten Alloy Products," Ser. No. 248,933, filed May 1, 1972, and "Tungsten Filament", Ser. No. 285,939, filed Sept. 1, 1972. Both of these copending applications are assigned to the assignee of the present invention.
Briefly stated, the process for making the filament wire comprises the making of a porous compact of tungsten particles, soaking the compact in water, then soaking the water-saturated compact in a solution of aqueous Th(NO 3 ) 4 having a concentration of approximately 520 grams ThO 2 per liter of solution for about 30 hours. The compact is removed and then vacuum dried until the greater part of the solvent has evaporated thereby leaving a thorium-rich additive in the tungsten. After the evaporation process, the compact is presintered in hydrogen at approximately 1200°C to convert thorium nitrate to thorium oxide. This compact or ingot is then rolled, swaged and drawn by conventional means to form the diameter wire required for the filament.
It has been found that this processing will yield thoriated tungsten wire with between 1 and 2 percent by weight thorium and small approximately equiaxed crystals of tungsten of a size smaller and more uniform than that obtainable through conventional powder metallurgy methods. With the smaller more uniform grain sizes and increased number of grain boundaries, the dampening capacity and resistance to fracture of the tungsten wire is greatly increased. This construction, in turn, increases the service life of the lamp.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevation view partially in section of an automotive stop, tail and signal lamp;
FIG. 2 is a graph illustrating test data of conventional thoriated filament lamps and thoriated tungsten filament lamps of the invention;
FIG. 3 is a photograph taken at 1000X magnification of a 2.2-mil wire of the prior art; and
FIG. 4 is a photograph taken at 1000X magnification of a 2.2-mil wire used in the lamp of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1 of the drawings, lamp 1 is comprised of envelope 2 to which is sealed re-entrant stem 3. Pairs of lead-in conductors 4 and 5 are hermetically sealed to re-entrant stem 3. The lamp 1 of the invention may contain a fill of inert gas such as argon or may be a vacuum lamp.
The two-filament lamp illustrated in FIG. 1 is commonly used in automobiles for stop, tail and turn signal lighting. However, the invention is not limited to a two-filament lamp since the lamp may have a single filament or two or more filaments. Filament 6 is known as the major filament and is used for stop and turn signal lighting purposes. This filament is connected to lead-in conductors 4, one of which is attached to the base shell 8 and the other of which is attached to contact tab 9. Since major filament 6 requires a higher color temperature, it is usually made of a lamp-doped tungsten.
Minor filament 7 is connected to lead-in conductors 5 which are, in turn, connected to contact tab 9 and base 8 which serves as a common contact for both filaments. Contacts 9 and 10 are electrically insulated from each other by plastic insulator 11. Also shown as part of base member 8 is one of two alignment lugs 12 which serve to locate the lamp 1 in an automobile socket, not shown.
Filament 7 burns with a color temperature of approximately 2260°K at a design voltage of 14 volts and generally has a smaller diameter than that of major filament 6. Because of the smaller diameter of filament 7, necessitated in part by candle power and wattage requirements, the lamp 1 has a tendency to fail predominantly through fracture of filament 7. Some experimental data show that 60 percent of the two-filament lamp failures are due to failure of minor filament 7.
One of the methods used to test lamps subject to vibration in various applications, such as automobiles, is known as the "Michigan Tester." This test device holds a lamp in contact with a rotating cam which has four equally spaced steps on the circumference of the cam. As the cam rotates, the lamps drop from step to step. This movement imparts a force or a bounce which simulates the vibrational forces exerted on a lamp when an automobile hits a bump. Lamps on the Tester were in electrical contact with a power source which would cycle the lamps on and off to simulate user conditions in which tail, stop and signal lamps are used only a portion of the time.
Michigan test results of lamps using 1% by weight thoriated tungsten wire of the prior art are shown in FIG. 2 as circles. The triangles on the graph indicate service life of lamps manufactured with wire having the uniquely small tungsten grain structure and the more uniformly distributed thoria. Four of these lamps are shown as light triangles in the upper left portion of the graph because they failed at about the same time the four lamps illustrated by the circles failed. The horizontal axis of the graph of FIG. 2 is the number of hours of lamp service life as measured by the electrical continuity of minor filament 7, and the vertical axis indicates the percentage of lamps that survived for a given number of hours.
Twenty lamps of each type were tested to produce the data illustrated in FIG. 2. Eighteen of the lamps or 90 percent using the thoriated wire of the prior art failed on or before the expiration of 300 hours. The remaining two lamps were removed from the Tester. By comparison, 14 of the lamps of the invention failed on or before the expiration of 980 hours. Six lamps, not shown on the graph, were still running at the 980-hour mark.
A further illustration of improved service life can be seen at the 50 percent survivor line. Half of the prior art lamps failed after 150 hours, whereas, half of the lamps of the invention can be expected to fail at 600 hours. This is an improvement in lamp service life of the magnitude of 4.
An explanation of this improved performance is the increased dampening capacity and strength which is related to the grain structure of the thoriated tungsten alloy wire and the percent of thoria in the wire.
Initial tests were conducted on lamps with filaments having approximately 0.6 percent by weight of thoria and the smaller grain sizes of tungsten and uniformly distributed thoria particles. The results from the Michigan Tester indicated that the 0.6 percent thoria wire with the unique crystal structure did not increase the lamp service life because it lacked sufficient grain size stability at lamp operating temperature. In some instances, the 0.6 percent wire lamps had a shorter life than the 1 percent thoria wire of the prior art.
By increasing the percentage of thoria in the filament to between 1 and 2 percent and by maintaining the unique crystal structure of the thoriated tungsten wire, a marked improvement in lamp service life was obtained. Unlike the 0.6 percent thoria, wire with 1-2 percent thoria exhibits grain size stability at lamp operating temperatures. Other test results have indicated that the preferred range within the 1 to 2 percent range is from 1.3 to 1.9 percent thoria. This preferred range takes into consideration the balancing between increased service of the lamp and manufacturability of the tungsten wire.
FIG. 3 shows the grain size of the outer diameter surface of thoriated wire of the prior art after it has been tested on the Michigan Tester. The photograph of FIG. 3 is a 1000X magnification of a 2.2-mil wire which is generally used for minor filament 7 of stop, turn and tail lamp 1. As can be seen from the photograph, there are large size grains of tungsten with the largest being located at the upper right side of the photo. Large grain size microstructures have fewer grain boundaries through which to absorb the vibrational force. Accordingly, the wire shown in FIG. 3 has a greater likelihood of fracture.
By contrast, the 1.7 weight percent, 2.2-mil wire of the present invention magnified 1000 times as shown in the photograph of FIG. 4 has many small grains of tungsten. This, in turn, gives a greater number of grain boundaries which can absorb or dampen the vibrational force. In order to obtain the small grain size and more numerous grain boundaries, the lamp filament is prepared by way of a new wire processing method as is more fully described in copending applications, Ser. Nos. 186,143 and 248,933, assigned to the same assignee as the present invention. Several other wire properties which are believed to contribute to increased resistance to fracture from vibration are a more uniform size and spacial distribution of thoria particles throughout the tungsten matrix and a slight but uniform thoria gradient which decreases with increasing distance to the longitudinal central axis of the filament.
In accordance with the patent laws, one embodiment of the invention has been described. It will be understood, however, that other variations of the lamp of the invention such as a single-filament lamp of the double- or single-ended variety are also within the scope of the invention and the appended claims.