US2371205A - Coiled - Google Patents

Description

'1, Patented Mu. 13, 194s COILED FILAMENT OR CATHODE AND` METHOD OF MANUFACTURE William P. Zabel, East Cleveland, Ohio, assignor to General Electric Company, a corporation of New York Application october 3o, 1943, serial No. 508,312

(ci. 17e-4o) 12 Claims.

'I'his invention relates to coil structures for electric lamps and discharge devices, and especially to electric discharge electrodes, including cathodes suitable. for fluorescent lamps or other tubes of low pressure positive column discharge type. The invention is hereinafter explained with particular reference to'coils of refractory metal for use as fluorescent lamp cathodes, a1-

though it may also be found useful for laments of incandescent electric lamps.

It is desirable that a fluorescent lamp cathode should heat up and become fully emissive very quickly. On the other hand, it is also desirable that the active cathode surface should all be at potentials within a restricted range during operation, if not at substantially the same potential. When the cathode is of the activated type (embodying an electron-emissive material of low work 'function, as distinguished from bare metal), it

is desirable that the cathode should hold and retain a plentiful supply of activating material or oxide, such as a mixture of alkaline earth metal oxides including those of barium and strontium. In addition, it is desirable that the metal structure of the cathode should present prominences or points from which the discharge can take 01T readily.

In the effort to meet these requirements, some rather complicated and expensive cathode constructions have been proposed, such, for example, las a coiled coil or even a triple coil; or a coiled coil with an overwind of iiner wire, which may either hug the main wire or surround it loosely. However, I have now devised a coil construction which meets the requirements without necessity for multiple coiling of any component-though a coiled coil might, indeed, be embodied in'it, if found desirable. Accordingly, my coil is much simpler, easier, and more economical to make than the complex structures above referred to, and can be made on ordinary, standard ceiling machines. The invention affords the further advantage that in suitable forms of construction it gives a cathode which is adapted either for cold starting, or for hot starting with preheating by preliminary current iiow through the cathode.

unactivated, because of its relatively low work function.

In the forms of cathode here illustrated and described, there is a coil of main conductor wire associated with other wire that is coiled substantially coaxially and co-ordinately with the main wire, so as to intervene between its convolutions. Thus the wires co-operate to form a tubular openwork basket or grille for holding electron-emissive activating materiaL The size of the main conductor wire may be chosen primarily with reference to the current to be carried, and that of the other wire to assure rapid heating thereof by the discharge-though of course both wires must be taken into account in determining the electrical resistance of the cathode. If desired, the main coil may be a double or other multiple coil of several parallel wound Wires, and the other coil may likewise be a multiple one; but in general the desired stiness, resistance, and other characteristics can be obtained without the complication of multiple coils.

When coiled in the same helical sense (i. e., both as right-hand helices, or vice versa), the main and associated wires may be Wound to difference pitches so as to give a criss-crossing of these wires. But in general, opposite coiling of these wires is preferable. Such a, grillage holds the activating material very effectively and securely; it oiiers many crevices with strong capillary attraction for retaining this material against repeated or continual positive ion bombardment; and the interlocking wires coact and hold together in a thoroughly unified structure. If desired, one

or more additional windings of ne wire might besuccessively wound on over the rst ne wire winding, thus increasing the closeness of the basket or grid texture.

Various other features and advantages of the invention will appear from the description of species and forms of embodiment and from the drawing.

'Ille coils may be made of any metals suitable for Y the conditions of use, including even metals or alloys that might not ordinarily be considered refractory-such as nichrome, for example. Amongst refractory metals, tungsten and molybdenum, which I term tungsten metals, may be preferred for cathode coils that are activated with alkaline earth oxides, while tantalum` may be In the drawing,

Figs. 1 and 2 are somewhat diagrammatic fragmentary side views illustrating the winding of the several associated wires on a mandrel to form -a. cathode coil suitable for the purposes of the invention, and Fig. 3 is a longitudinal sectional view of the resulting coil, after heat treatment and removal of the mandrel.

Fis. 4 is a side view of the coil after mounting as part of a fluorescent lamp cathode mount, but before impregnation with activating material; and Fig. 5 is a fragmentary side view of the impreferred for cathode coils that are used bare and pregnated coil on a larger scale.

Fig. 6 is a view similar to Fig. 2 illustrating a somewhat diii'erent winding of the component wires of a cathode coil; and Figs. 7 to 10 are slmilar views illustrating still different windings.

All the figures are drawn much larger than the actual coils as they would be made for the cathodes of ordinary fluorescent lamps.

Figs. l and 2 illustrate the fabrication of a form of my cathode coil according to a preferred method which I have found convenient and economical as compared with prior methods involving multiple coiling. As shown in Fig. 1, a main conductor wire is iirst wound into a helix I I on a mandrel wire M of suitable metal and size, with its adjacent convolutions suitably separated. Thereafter, another wire of smaller gauge is wound under tension on the same mandrel M into a helix I2 of opposite pitch: i. e., if the helix I I is a left-hand one, the helix I2 is a right-hand one, or vice versa. Accordingly, convolutions of wire I2 -lie against the mandrel M between those of wire II and cross the latter in bends outside of and conformable to Wire II. The windings may be applied to long lengths of mandrel wire by standard ceiling machines used in the manufacture of coiled filaments for incandescent lamps. As shown, the pitch of the right-hand helix I2 is considerably less than that of the left-hand helix II, being onlyabout one-third as great. Accordingly, there are three times as many turns of the wire I2 as of the wire II. As shown in Fig. 2, a couple of turns or convolutions of the helix I2 are seen to lie between adjacent turns or convolutions of the helix II in front of the mandrel M, while another turn I2 is seen crossing each turn Il. Thus the helix I2 is distorted by an outward displace ment of the flner wire at each crossing I that it makes over the helix II, and is, as it were, interlocked with the helix Il against relative displacement. Of course the crossings of wire II by wire I2 behind the mandrel M do not appear in Fig. 2. But as the windings II, I2 are oppositely wound and continuous, each convolution I2 must cross a convolution II once, and each convolution I I is crossed by three convolutions I2.

After winding, the wires II, I2 undergo a heat treatment on the mandrel M to set them. While the prolonged heat treatment known in the tungsten metal wire art as sintering" may be used in treating tungsten wire coils II, I2, I have found it generally suilcient and preferable to use the less severe heat treatment known in the art as annealing," since this removes most of the stresses due to wire drawing and soiling, yet leaves the wires II, I2 ductile, and so reduces loss in handling and shipping coils and lamps. For annealing, the mandrel wire M with its coils II, I2 may be run through an annealing furnace containing an atmosphere of hydrogen, thus cleaning and annealing it at one operation. No special precautions as to drying the hydrogen are ordinarily required. When the mandrel M is of molybdenum, a temperature of the order of about 1550 C. may be preferred, with an exposure of the windings thereto for a period of the order of seconds, or not over half a minute. When the mandrel M is of iron or of mild, low-carbon steel, the temperature may be of the order of 1300 C., and should not substantially exceed this figure, in order-to obviate alloying of iron with the tungsten of the windings II, I2 which would embrittle the tungsten, while the exposure may be of the order of half that above indicated for a molybdenum mandrel. Even at 1300 C..

too long an exposure of the tungsten windings II, I2 in contact with an iron mandrel M may similarly embrittle the tungsten. After the heat treatment, the assembly of coils on mandrel is cut to individual lengths and then the mandrel M is removed from within the windings II, I2, which may readily be done by chemically disintegrating the mandrel, as by dissolving it out with any suitable reagent-such as the usual combination of nitric and sulphuric acids for molybdenum, or hydrochloric acid for iron or mild steel'.

In winding the wires II, I2 on the mandrel M, they are highly stressed elastically, so that they tend very strongly to uncoil. This elastic stress is largely relieved in the heat treatment, bul, not entirely so; hence when the wires II, I2 are released and the mandrel M is dissolved out, the wires readjust themselves and are irregularly displaced.

The annealed coil assembly (of which only a fragment appears in Figs. 1-3) is cut into suitable lengths L which may be mounted in leadwire clamps 2li, 20 of a fluorescent cathode mount' 2i, Fig. 4, according to the usual practice in mounting cathodes or filaments. As shown, the leads 20, 20 have the usual auxiliary anode extensions 22, 22 beyond the clamps. After mounting a coil L in lead clamps 20, 20, the latter may be bent further apart somewhat, so as to stretch the coi-1 L taut between them. This also has the effect of definitely separating the adjacent convolutions of the associated coils II, I2 to a desired spacing. This completes the unactivated cathode mount as shown `in Fig. 4.

The impregnation of the coil with activating material may be carried out with the usual mixture of powdered alkaline earth metal carbonates in binder of nitrocellulose lacquer and diluent, which may be applied with a spoon" in the usual way to cover the coil and ll its interior and its interstices. To take full advantage of the large space afforded by and -Within the tubular basket or grillage formed by the coils II, I2, the liquid activating mix may be thicker than usual for coiled coil cathodes, about of the consistency of very thick cream; but care must be taken to avoid leaving an accumulation of this mix hanging to the exterior of the coil between the leads 20, 20 or on the lead wire clamps. The resulting cathode mount is subsequently sealed into a fluorescent lamp tube, and is treated and activated as usual in the processing of the lamp to break the carbonates down to oxides.

In the impregnated cathode as shown in Fig. 5, none of the convolutions of either coil II or I2 are totally embedded in the activation material, which substantially fills the grillage formed by these coils, but only coats the outer sides of the wires very lightly, as indicated by the stippling. After activation of the cathode shown in Fig. 5, that is, breaking down ofthe carbonates to oxides, the outer sides of the convolutions of coll II and the cross-over portions I5 of coil I2 have a metallic luster. Thus the finer wire I2 heats up quickly and facilitates .starting of the discharge,

fluorescent lamp operable on a voltage of 108 with.

a current of 0.41 ampere are here given, but are not to be understood as defining or limiting the invention:

For the mandrel M may be used round molybdenum or iron wire of 23 mil size, while for the wires Il, I2 may be used round tungsten wires of 2.3 and 0.7 mil sizes, both preferably what is known in the art as non-sag wire. The primary, main conductor wire II of 2.3 mil size may be wound left-hand on the mandrel M at 172 turns per inch, while the secondary associated wire l2 of 0.7 mil size may be wound right-hand at 530 turns per inch. Using a length L of the resulting coil about mm. long after heat treatment and removal of the mandrel M, it may be clamped in lead clamps 20, with a length of 13 mm. of coil in circuit between themcomprising lengths of 174 mm. and 533 mm. respectively of the wires II and I 2-and the clamps 20, 20 may then be bent apart to stretch this 13 mm. length to a length of 15 mm. in circuit between the clamps. The operations not here mentioned or described in detail may be performed as already described.

Fig. 6 illustrates a modification in which wires li, I2 are wound on a mandrel M to equal but opposite pitchesone left-hand and the other right-hand, or vice versa. These pitches may be intermediate between those of the wire II and I2 in Figs. 1 and 2, so that the interval between adjacent turns II, I2 shall be about the same, while the over-all length of the resulting cathode coil is correspondingly shorter. For a watt 108 volt fluorescent lamp operating on 0.41 ampere, the mandrel wire M may be of 35 mil size, the wire I I may be of 2.3 mil size and wound in a left-hand helix at 265 turns per inch, while the wire I2 may be of 1 mil size and wound righthand at 265 turns per inch. The length of coil between the lead clamps 20, 20 (Fig. 4),may be 10.7 mm. and stretched to 11.5 mm.

Fig. 7 illustrates a modification in which the main heavier gauge conductor wire II is wound left-hand, a finer wire. I2 is wound right-hand,Y

and another finer wire I3 is wound left-hand.

In Fig. 8, the finer wire I2 is wound at less turns per inch and in the opposite direction from heavier wire Il.

In Fig. 9, two finer wires I2a, 12b are coiled side-by-side around the mandrel M and heavier wire coil II.

In Fig. 10, the ner wire I2 is wound in the same direction as the heavier wire II but at less turns per inch.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A coiled filament comprising main helically coiled conductor wire; associated finer wire helically coiled substantially coaxially and coordinately with said conductor wire, with portions of its convolutions crossing over and other portions intervening in the space between convolutions of said conductor wire and interlocked therewith against relative displacement of the coils.

2. A llamentary cathode comprising main helically coiled conductor wire; associated finer wire helically coiled substantially coaxially and coordinately with said conductor wire, and having portions of its convolutions crossing over and other portions intervening in the space between convolutions of said conductor wire, whereby the coils are interlocked against relative displacement; and electron-emissive-activating material held in the openwork tubular grill cooperatively formed by the coordinate convolutions of said wires.

3. A coiled filament comprising main helically coiled conductor wire; associated finer wire helically coiled substantially coaxially and coordi` nately with said conductor wire but to a different pitch, with portions of its convolutions intervening in the space between those of said conductor wire and other portions of its convolutions crossing the latter in bends outside of and conformable thereto.

4. A coiled filament comprising a single main conductor wire helix; a single associated finer wire helically coiled to a coil of diierent degree of and opposite pitch coaxial and coordinate with said conductor wire, and having portions of its convolutions intervennigin the space between convolutions of the conductor wire and othenportions crossing the latter in bends outside of and conformable thereto.

5. A. filamentary cathode comprising a single main conductor wire helix; a single associated finer wire helically coiled to a coil of the same pitch but opposite helical sense coaxial and coordinate with said conductor wire, with its convolutions intervening between and crossing those of the conductor wire in bends outside of and conformable thereto, and electron-emissive-activating material held in the openwork tubular grill co-operatively formed by the coordinate convolutions of said wires.

6. A coiled iilament comprising main helically coiled conductor wire; associated finer wire helically coiled substantially coaxially and coordinately with said conductor wire in the same helical sense but to a different pitch, with convolutions intervening between those of said conductor wire and also crossing the latter in bends outside of and conformable thereto.

7. A method for fabricating a complex iilamentary coil which comprises helically winding elastic main conductor wire on a mandrel, with its adjacent convolutions separated, and helically winding finer wire on said mandrel with convolutions of the ner wire lying between and crossing over those of the conductor wire, and subsequently removing the mandrel and permitting the convolutions of main conductor Wire to open up, whereby the coils are interlocked against relative displacement.

8. A method for fabricating a complex filamentary coil which comprises helically winding elastic main conductor wire on a mandrel, with its adjacent convolutions separated, and helically winding finer wire on said mandrel to an opposite pitch, with convolutions of the latter wire lying between and crossing over those of the conductor wire.

9. A method for fabricating a complex lamentary coil which comprises helically winding elastic main conductor wire on Fa mandrel, with its adjacent convolutions separated and elastlcally stressed, helically winding liner wire on said mandrel with convolutions of the latter wire lying between and crossing over those of the conductor wire, partially relieving the elastic stress in said main conductor wire, and releasing said coiled wires and removing said mandrel from the coils.

10. A method oi' fabricating a complex lamentary coil which comprises continuously winding a main conductor wire on a long length of mandrel with its adjacent convolutions separated and elastically stressed, continuously winding iiner wire under tension over said main conductor wire with convolutions of the liner wire crossing over those of the conductor wire and contacting the mandrel between convolutions of the conductor wire, cutting the resultant assembly to individual iilament lengths, and then removing the mandrels from the individual iilaments.

l1. A method for fabricating a complex lamentary coil which comprises helically winding elastic main conductor wire on a mandrel, with its adjacent convolutions separated, helically winding finer wire on said mandrel in the opposite helical sense and at more turns per inch than the main conductor Wire, with convolutions of the nner wire crossing over those ofthe main conductor wire and contacting the mandrel between convolutions of the main conductor wire, and subsequently removing the mandrel and permitting the convolutions of main conductor wire to open up, whereby the coils are interlocked against relative displacement.

12. A fllamentary cathode comprising main helically coiled conductor wire; associated liner wire helically coiled substantially coaxially and coordinately with said conductor wire but in the opposite helical sense and at several-fold the number of turns per inch, with portions of its convolutions crossing over and other portions intervening in the space between and within the convolutions of said conductor wire and interlocked therewith against relative displacement of the coils; and electron-emissive activating material held in the tubular grill cooperatively formed by the co-ordinate convolutions of said wires.

WILLIAM P. ZABEL.

Images