US2079784A - Plating by thermal evaporation - Google Patents

Description

May 11, 1937. R. c. WILLIAMS PLATING BY THERMAL EVAPORATION Original Filed Jan. 19, 1933 3 Sheets-Sheet 1 y 1937- R. c. WILLIAMS 2,079,784

PLATING BY THERMAL EVAPORATION Original Filed Jan. 19, 1933 3 Sheets-Sheet 2 y 1937- R. c. WILLIAMS 2,079,784

PLATING BY THERMAL EVAPORATION Original Filed Jan. 19, 1953 s Sheets-Sheet 5 [All ATTORNEY 1 Patented M ay ll, 1937 PATENT OFFICE PLATING BY THERMAL EVAPORATION ltobley C. Williams, Rockefeller Hall, Cornell University, N. Y.

Application January 19, 1933, Serial No. 652,529 Renewed August 8, 1936 Claims.

This invention relates to apparatus wherein coatings of various materials may be deposited Another object is the provision of apparatus of it the character specified above and wherein means is provided to enable coatings to be deposited on surfaces of different size and contour.

Another object is to provide apparatus of the character specified and wherein the filaments 00 from which the coating material is evaporated can be located in predetermined relation with respect to the surface to be coated so that the coating throughout the surface will be uniform in character.

Another object is to provide apparatus as above specified wherein the filaments from which the coating material is evaporated may be adjusted toward or away from the surface to be coated, whereby the filaments can be located at uniform b distances from a non-planular or irregular surf ace, as, for example, a concave surface or'a convex surface.

A further object is to provide apparatus of the character specified wherein the filaments can be 3:, so located and spaced with respect to each other and to the surface to be coated that all of said surface can be coated by a uniform thickness of the material evaporated from the filaments.

' Another object is the provision of apparatus of such character that with but one evacuation of the apparatus a plurality of coating materials may be deposited upon the surface either simultaneously or successively.

A further object is to provide apparatus as spec- 45 ified wherein the filaments may be heated separately, in groups, or all together.

A further object is to provide apparatus of the character referred to wherein provision is made for selectively increasing or decreasing the num- 50 her of filaments whereby surfaces of different areas and contours can be coated in the apparatus.

In order to make clear the manner in which the apparatus is employed and the uses to which 55 it may be put, reference will now be made to certain of the more important uses of the invention.

In astronomy, in experimental and demonstrative physics, and in other fields as well, devices are employed which include light reflecting surfaces, and the apparatus of the present invention possesses great utility in connection with the coating of such surfaces. As illustrative of the surfaces referred to and which can be coated in the apparatus of the present invention, reference is made to telescope mirrors, search-light reflecm tors, spectrohelioscopes, electronometer suspensions, interferometer plates, ruled gratings, prisms of Littrow spectographs, and other instruments where a reflecting surface having unchanging characteristics is required. While many of these 1 surfaces are of small size, many of them, such as the reflecting surfaces of telescope mirrors, may be of large size.

Inasmuch as the coating of reflectors and telescope mirrors constitutes fields wherein the present invention is particularly useful, reference to such fields will be made herein in detail and the invention described in connection therewith, it being understood, however, that such description is purely by way of illustration.

The coatings secured with some evaporated metals are especially useful in the making up of front surface telescope mirrors, that is, mirrors in which there is no glass between the reflecting surface and the air or other medium through which the light rays approach the mirror. This type of mirror eliminates the effects introduced by the optical properties of glass and it permits the use of metals not generally used in the formation of reflecting surfaces. For instance, chromium has been found to provide a very satisfactory surface when it is properly coated on glass. Chromium gives to the surface two especially valuable properties: it reflects light over a wide range of wave lengths having a reflectivity of 4 65% in the region of the ultra violet, for instance, where silver drops to 4% reflectivity; and, second, the surface does not tarnish quickly, if at all, as compared with silver. Aluminum, also, has been found to provide an advantageous coating where high reflectivity is required.

The apparatus used by me comprises an evacuable enclosure provided with means giving access to the interior thereof, means cooperating with the enclosure for evacuating the latter, means within the enclosure for removably supporting an object a surface of which is to be coated, one or more filaments within the enclosure, said filaments carrying in contacting relation a solid to be vaporized, which solid has a boiling point below the melting point of the material constituting the filaments, means for removably supporting the filaments within the enclosure, and means for passing an electric current through said filaments. The enclosure may be defined by an endor baseplate and separable means cooperating therewith to form an enclosure adapted to be evacuated. Through this endor base-plate there may be provided, within that part encompassed by the enclosing means, a plurality of conductors for the passage of electric current into and through the said filaments. Preferably, I make use of an endor base-plate, provided with a plurality of openings closed by removable plugs through one or more of which are positioned suitable conductors for providing the filament or filaments within the enclosure with heating current from a source without the enclosure.

In those cases wherein the endor base-plate is composed of electrically conductive material at least those plugs carrying the aforesaid conductors are composed of insulating, or electrically non-conducting, material surrounding the conductor or conductors (to be described more particularly hereinafter) each of which latter mediately or immediately supports, in electrically conductive relation within the said enclosure, an end of at least one filament whose other. end may be supported by another conductor likewise positioned within the said plug or may be supported by a conductive supporting member secured in conductive relation to the said endor base-plate. That is to say, in the case of an apparatus whose endor base-plate is composed of electrically conductive material, the filament is apart of an electric circuit which includes one conductor insulated from said endor base-plate and another conductor which may be grounded directly to the endor base-plate or may be insulated therefrom: in the former embodiment I provide means for electrically connecting the endor base-plate to a conductor without the enclosure and insulated from said plate, while in the latter embodiment I simply connect both conductors without the enclosure with the proper terminals of an electric circuit. As will be apparent, the said endor base-plate may be composed wholly or in part of electrically non-conductive material, in which event it is notnecessary to insulate either conductor from the said plate, or, in fact, to provide closable openings in the latter, so long as provision is made for removably supporting the filament or filaments within the enclosure in conductive relation to said conductors. Thus, the endor baseplate when composed of electrically non-conductive material may be provided with one or more groups of inwardly facing sleeves or sockets of conductive material in conductive relation with suitable conductors without the enclosure, into which sleeves or sockets there may be removably mounted conductive supports for the filament or filaments.

Each of the aforesaid filaments carries thereon a plating of the solid to be vaporized, or at least carries the said solid in contact therewith, the filament or filaments being so arranged or disposed, with respectv to the area of the surface to be coated, and with respect to the contour thereof, that the vapors leaving it or them are condensed on the said surface in an even manner.

The arrangement of filaments is fiexible, permitting them to be placed so that the metal vapors are produced only in prosimity to the surface being coated. Different filaments. coated with different metals, can be used: certain of the filaments can be heated first, causing the vapors of one metal to be evolved and condensed on the surface, and then other of the filaments can be heated whereby a second metal can be evaporated and condensed on the first metal to make up a two-layer, or plural-layer, coating.

The manner in which the foregoing, and other, objects of invention are realized my be more particularly described with respect to specific embodiments thereof with the aid of the appended drawings in which:

Fig. 1 shows, in sectional elevation. one embodiment of an apparatus for accomplishing the objects of the invention;

Fig. 2 is a partial plan view of the supporting plate and filaments which are comprised in the apparatus shown in Fig. 1;

Fig. 3 is a plan view of an assembly of a filament made up of two elements;

Fig. 4 is a side view of the assembly shown in Fig. 3;

Fig. 5 is a plan view of a leading-in-plug which holds the filaments used in the apparatus shown in Fig. i;

Fig. 6 is a sectional view of the plug shown in Fig. 5;

Fig. 7 shows a plug with elevated filaments which illustrates how the invention is adapted to special problems;

Fig. 8 is a side view, in section, of another embodiment of the invention;

Fig. 9 is a fragmentary view of Hg. 8 taken along AA;

Fig. 10 is a detail, shown somewhat enlarged, taken at 8-18, Fig. 9;

Fig. ii is a sectional view taken substantially on line il-li of Fig. 12, looking in the direction of the arrows, and is similar to Fig. 8 and shows a mirror having a concave surface to be coated, as shown in Fig. 8, while the filaments are located in such manner with respect to the concave surface that all the filaments are placed equal distances from the surface to be coated; and

Fig. 12 is a sectional view taken substantially on line l2-i2 of Fig. 11, looking in the direction of the arrows.

In Fig. 1, reference numeral i designates a supporting plate-like member which supports the object to be coated through uprights 2 and which serves as the bottom of the enclosure or composite vessel which latter is maintained at very low pressure during the coating period. Bell jar 3 constitutes the top and sides of the enclosure and it joins with l at I where a joint that is tight to atmospheric presure has been made by sealing with a plastic in the manner commonly used with bell jars. Member I is preferably a flat plate of metal, e. g., of cold rolled steel, finished with a smooth and clean top and supported upon base members l. Its thickness depends upon several factors such as the extent of the enclosure of which it forms a part and the need of holding leading-in-plugs I. Plate i has an outlet 0 for air. This is connected by pipe nipple I0 and tube II to one or more vacuum pumps, shown diagrammati ally at p and p Figs. 1 and 2.

Jar l, as shown, is of the type commonly made of glass. Glass is recommended for the vacuum chambers because of the visibility which this material affords; but in large siaes where greater strength is required, metal chambers with glass windows have been used.

A mirror I2 is shown to illustrate the recommended position in which objects of medium size, say, up to 24 inches in diameter, are placed while being coated. The surface i3, which is to be used as a reflector, is placed facing downwards to prevent any dust present in the air of the vessel or portions of burned out filaments, from settling upon it. The reflector surface is held a suitable distance above the filaments I.

Fig. 2 shows the spacing of the filament assemblies I. They are spaced symmetrically so that each will furnish vapors for a proportionate area of the surface which is to be coated. In Fig. 2, a total of nine filament assemblies are indicated, a number which has been found adequate for coating a disc up to 18 inches in diameter. In general it can be said that a coating of chromium deposited to an average depth of 1 millimeter on a filament made up of 6 inches of No. 10 tungsten wire, wound in the form of a helix, will, upon evaporation, coat from 25 to 30 square inches of surface, which is held approximately 4 inches above it.

A form of filament assembly that has been used effectively is shown in Figs. 3 and 4. A supporting wire or pin has a T-shaped member l5 of conducting material fastened to it and, to the branches of the T are fastened one end of each of the two heating elements H; which make up the filament. The other ends of the elements are fastened to pins sleeves I6 and i9, respectively, in the leading-inplug 6 shown in detail in Figs. 5 and 6. Sleeves l9 are connected to sealed-in conductors 6 which permit these sleeves and consequently pins H and elements l6 to be connected to a source of electric energy without the enclosure. Sleeve l3 and pin II are merely to give mechanical support to the filament. The body 29 of plug 6 can be made of various non-conducting compositions; glass with a. low coefficient of cubical expansion is a satisfactory material but other suitable non-conducting compositions are not excluded. Conductors 6 may be copper-covered iron-nickel wires, or other conductors may be used which will seal into 29 in air-tight manner. Plug 6 may be formed with any satisfactory taper and makes an airtight, but removable, closure, when sealed in with wax, into the tapered holes 20 in plate I.

The apparatus shown in the figures is adapted for very flexible operation and can be used to plate surfaces of varying area and contour. If a very large mirror, say, several feet in diameter, is to be plated, the base or end-plate, may be made of the necessary size and may contain sufficient holes 20 to receive the plugs and filament assemblies that are required to evaporate the amount of vapor that will evenly coat such a mirror. The holes 20 are shown grouped radially around the center of the plate but the holes in larger plates could to advantage be grouped in rectangular formation. A number of shallow holes 2' are drilled into plate I and tapped for the reception of supports 2. These permit these supports to be variously positioned for holding objects of different size and shape.

Fig. '7 illustrates one manner in which the invention is adapted to the coating of a surface of great curvature, for instance, the parabolic re- -fiector indicated by numeral 2|. In this problem the filament elements l6 are held much higher above the surface of supporting plate I than are The pins I4 and I! fit in' those shown in Fig. 1. a condition which results from making pins l4 and I1 longer than the corresponding pins l4 and I1 shown in Figs. 3

the surface to be coated whatever may be the contour of the surface.

Figs. 8, 9 and 10 show an embodiment of the invention which has proved very flexible and which is very useful when surfaces of large size are to be coated because of the ease with which it can be handled and because of the resistance it offers even in large sizes to atmospheric pressure. Numeral 30 designates an evaporation space or chamber which is cylindrical in shape being bounded by the short right cylinder 3|, which is of glass or other suitable material, fiat end plate 32 and the opposing flat end plate 33. Cylinder 3| is supported upon the base or cradle 5|. Tapered holes 31' have been made in plate 32 to hold tapered plugs 31 which may be of glass or other suitable non-conducting Through each plug, and sealed therein, is a rod or wire 39, a 50-100 mil tungsten wire for example; and, a member 40, which holds one end of filament 36, is fastened to each of these wires by screws 4 Threaded holes 42 are provided in end plate 32 for threaded members 42 and 43. These threaded members hold the opposite ends of the filaments 33 and, being of conducting metal, connect these ends of the filaments, electrically, to the end plate 32.

An arrangement of filaments for plating a large surface is shown in Fig. 9. The number of filaments and their position can be readily changed by selecting various shapes and sizes of holding means, of which 42 and 43 are merely examples. and by variously positioning the holding means 40 on wires 39. In this embodiment all the filaments have one common electrical connection, the late 32, and wire 34 connects this plate to one term nal of the source of current, not shown, for heating the filaments. Wire 35, connected to the other terminal of the said source of current, can be connected by means of pinch clamp 44 to any wire 39 for heating the filament joined electrically to that wire. Ordinarily, only one filament is heated at a time; but, by using suitable electrical connections, the current can be sent through several filaments in series. A resistance, not shown, may be placed in the electric circuit for furnishing control over the filament temperatures during the evaporating operation. The object 45. upon which the vapors are to be condensed. is secured to end plate 33 in any suitable manner, e. g., by fastening at 46, 46. with some adhesive, and it is further supported in its vertical position by means of one or more members 41.

It is a simple matter to assemble the apparatus. Cvlindrical member 3' is mounted in cradle 5| and the ends of the cylinder, which are true and smooth, are coated with wax of proper consistency. End-plate 32, with the desired number and arrangement of plated filaments upon it, is pushed up against one end of the cylinder, and end-plate 33, with the object to be coated adhering to it, is placed against the other end of the cylinder. Upon exhaustion of the air within the cylindler, atmospheric pressure forces the endplates firmly against the cylinder 3| making joints 36, 36, air tight. Air is exhausted from space 30 through tapered hole 48 in end-plate 33.

composition.

A connection is made to suitable vacuum pumps not shown, by means of tube 54. This tube has a tapered end 49 that fits into hole 4| and provides, when coated with suitable wax, an air tight but easily broken joint.

Right cylinder Il may be fabricated from steel or other equivalent metal, and may be joined to the end-plates I2, 33 by rubber-gasket joints. According to this embodiment, holes are tapped into the ends of the cylinder and similarly placed holes are bored in end-plates 32, ll. Annular rubber-gaskets provided with openings in alignment with the aforesaid holes are placed between the end-plates and the cylinder ends, and the end-plates are bolted to the cylinder. For visibility, peep-holes covered with glass plates may be provided in one or the other of the end-plates or in the cylinder itself. Glass plates may be waxed over the holes 20 not used by filament plugs in any one arrangement.

The apparatus shown in Figs. 11 and 12, while similar to the apparatus shown in Figs. 8 and 9, differs therefrom in that the filament supports are different and the filaments are not connected to the negative side of the circuit through the end plate but are connected thereto exteriorly of the apparatus.

The apparatus shown in Figs. 11 and 12 comprises a glass cylinder 52 supported upon a suitable base or cradle 83. One end of the cylinder is closed by an end plate 54 provided with a suitable support 55 for the mirror 58 the front concave surface of which is to be coated. The end plate 54 is provided with a tapered opening receiving the end of a suitable conduit connected to the pumps p and p which act to evacuate the cylinder. The opposite end of the cylinder from that having the end plate 54 is sealed by an end plate 50 that is provided with a plurality of tapered openings into which are fitted tapered plugs I! formed of suitable insulating material, such as glass. These plugs have cast or molded therein electrically conductive rods 60, andit will be noted that the plugs are provided with rods of dlfi'erent lengths for a purpose later to be explained. Flla ment supports it are adjustably mounted on the rods 00 and can be held in adjusted position thereon by suitable means, such as set-screws 42. The filament supports are formed of electrically conductive material, and the filaments D have their opposite ends connected to said supports, wherefore the circuits can be completed through the filaments by connecting the outwardly projecting ends of certain of the rods 40 to a suitable source of electrical energy.

It will be noted that the mirror 56 has a deeply concaved front surface to be coated and that by selecting the correct plugs 69 for rods of proper length the filaments can be so located with respect to the concave surface of the mirror to be coated that they are each placed an equal distance from the portion of the surface that is to be coated by the evaporation of the coating material from that filament. This arrangement is desirable and important inasmuch as the vapors evaporated from the filaments should travel uniform distances to the surface upon which they are to be condensed in order to provide a coating of uniform thickmess on the surface. If the coating condensed on the surface was not of uniform thickness throughout its area, then the optical accuracy of the refiecting concave surface would be destroyed due to the inequalities of thickness in the coating arranged thereon. The area of the surface that is coated by each filament can be determined, and

hence the total area of the surface and its outline will determine the arrangement that the filaments should have in order that they will produce upon the surface a coating completely covering the same and of uniform thickness throughout. It will be understood that if the filaments are not located properly unequal thicknesses will result in the coating which will reduce if not destroy the optical accuracy of the surface. By using the tapered plugs as illustrated in Figs. 11 and i2 and selectively positioning the same in the openings in the end plate, the filaments can be so located with respect to the surface to be coated and with respect to each other that the resultant coat on the surface will be uniform throughout and will not affect the optical accuracy of the surface, it being understood that such openings in the end plate as are not used can be closed by solid tapered plugs or other suitable means. The filaments can be heated separately, in groups, or all simultaneously, as hereinbefore described.

A typical coating cycle consists of the following operations: All elements which are to be exposed to high vacuum are carefully cleaned so that they will not "gas" at low pressure and so that liquid or solid particles will not separate from them. Referring to Figures 1 to 6 inclusive, the object II, which is to be plated or coated, is carefully cleaned, especially any surface which is to be used as a reflecting surface. and temporarily placed upon three or more of supports 2 which have been screwed into plate I at suitable positions. The surface which is to be made highly reflecting is preferably placed facing downwards. Note is made of the positions of the filaments which will be required to furnish the vapors for coating after which the object is removed. Plugs 0 are now inserted into the holes 20 at these positions and filaments, previously electroplated with the metal or metals of which the surface is to be coated, are assembled in the respective plugs. Object II is returned to the coating position on supports 2, and bell jar 3 is put over the object and filaments and sealed to plate I at joint 4 with a mastic-like wax from which gases will not evolve when it is subjected to reduced pressure. A primary vacuum pump connected to tube II is started and the greater mass of the air under bell jar 3 is removed. A secondary vacuum pump p is next connected to tube It and the absolute pressure of air under the bell jar is reduced to less than 0.0001 mm. of mercury. The pumps may, if desired, be connected in series, as shown, with the high vacuum pump next to the evaporation vessel. While maintaining the pressure at the low value mentioned, one of the filaments is connected by conductors I, extensions 22, and clips 23, to a source of electric current, indicated by wires a and b. Ordinarily it requires but a short time after the filament has become incandescent for the electroplated metal to evaporate but this must be finally determined by experiment as different metals and the different thicknesses to which they may have been electroplated on the filaments cause this factor to vary. The different filaments are separately, simultaneously, or successively, heated until all those chosen for the coating operation have had the electroplated metal evaporated from them, the high vacuum being maintained during this period. Air can now be introduced until atmospheric pressure exists in the vessel, bell jar I can be removed, and the object is available for use. The coated surface need not be polished or otherwise conditioned before use.

The invention permits mirrors and the like to be coated with more than one metal. This is important where a mirror is to possess certain specific properties that cannot be obtained with a single metal. For instance, it is often desirable to make a mirror with a removable reflecting surface because exposure of the surface to corrosive or abrasive conditions will cause it to deteriorate, making renewal advisable. Now chromium is especially valuable for reflecting surfaces because of its optical properties and resistance to corrosion. But it is desirable at times to renew chromium coatings and, when it has been previously attempted, it has been found that chromium, deposited on certain kinds of glass by thermal evaporation, was imbedded into the glass and could not be removed with hydrochloric acid or aqua regia. With the apparatus shown, some of the filaments can be electroplated with gold and this gold can.first be plated upon the glass. The chromium is then deposited upon the gold, making a coating which can be readily removed with aqua regia.

Another valuable coating of two materials has been accomplished with the apparatus described wherein a layer of quartz has been placed over a reflecting surface of silver in order to protect the silver from tarnish. Alloys, also, can be deposited on surfaces by evaporating two or more metals simultaneously from two or more filaments, and allowing them to condense simultaneously upon the surface to be coated.

The procedure above described has proved to be a practical way of carrying out my invention but I do not commit myself to the details of structure and method set forth. For instance, the desired vapors need not be evaporated from electroplated filaments: gold, which is easily procured in a relatively pure state, has been evaporated directly by placing pieces of the metal within the filament coils, and my invention therefore includes the concept of positioning pieces of metals or other evaporatable solids within or upon a filament coil or coils.

Among those solids which may be coated onto a surface by the practicing of the present invention are the following: silver, gold, chromium, aluminum, quartz, silicon, antimony, beryllium, copper, magnesium, mercury, nickel, palladium, platinum, rhodium, zinc and lead.

I claim:

1. Apparatus for coating a surface of an object with a solid by thermal evaporation of the latter at low absolute pressure which comprises an evacuable enclosure provided with means giving access to the interior thereof, means cooperating with the enclosure for evacuating the latter, means within the enclosure for removably supporting an object a surface of which is to be coated, a plurality of filaments within the enclosure each of said filaments carrying in contacting relation a solid to be vaporized, means for removably supporting the filaments within the enclosure, and means for supplying electric current to any one of said filaments, said apparatus being additionally characterized in that that portion of the surface of said enclosure which is opposite to the object whose surface is to be coated is provided with a plurality of openings closed by removable and interchangeable plugs a plurality of which carry insulated conductors whose inwardly projecting ends support ends of said filaments.

2. Apparatus for coating a surface of an object with a solid by thermal evaporation of the later at low absolute pressure which comprises an evacuable enclosure provided with means giving access to the interior thereof, means cooperating with the enclosure for evacuating the latter, means within the enclosure for removably supporting an object a surface of which is to be coated, a plurality of filaments within the enclosure carrying, in contacting relation, solids to be vaporized, means for removably supporting the filaments within the enclosure, and means for supplying electric current to any one of said filaments, said apparatus being additionally characterized in that the enclosure structure includes a plate of electrically conductive material, which plate is opposite said object-supporting means, said plate containing a plurality of openings closed by a plurality of removable plugs each of a plurality of which plugs is insulating and carries an electrical conductor insulated from the said end plate and supporting on its inwardly projecting end one end of one of said filaments, the other ends of the filaments being collectively supported by a conductive supporting member mounted directly on said plate, and means for passing an electric current through any one of said conductors and its associated filament, the said supporting member and said conductive plate.

3. An apparatus for coating a surface by thermal evaporation comprising a chamber adapted to be evacuated and in which the article whose surface is to be coated is mounted, a plurality of filaments in said chamber and from which the coating material is to be evaporated, and means for supporting said filaments so that they will be located at equal distances from the surface to be coated when the latter is a nonplanular surface.

4. An apparatus for coating surfaces by thermal evaporation comprising a closed chamber in which the article whose surface is to be coated is mounted, a plurality of filaments from which the coating material is g to be evaporated in said chamber, and means for adjustably supporting said filaments whereby they may be individually adjusted toward or away from the surface to be coated so that all of the filaments can be located at equal distances from a non-planular surface.

5. In an apparatus for coating surfaces by thermal evaporation, a closed chamber adapted to be evacuated and to receive the article whose surface is to be coated, a plurality of filaments in said chamber and carrying the material to be evaporated, and means for supporting said filaments, said means being of different lengths whereby all of said filaments can be positioned at equal distances from a non-planular surface.

6. An apparatus for coating surfaces by thermal evaporation comprising a chamber adapted to be evacuated and to receive the article whose surface is to be coated, a plurality of filaments in said chamber, and means for supporting said filaments and including a member provided with devices of different lengths which can be selectively positioned on said member and with respect to the surface to be coated, whereby the filaments can be located at equal distances from a non-planular surface.

7. An apparatus for coating a surface by thermal evaporation comprising a chamber adapted to be evacuated and to receive the article the 6 amm surface ofwhichisto becoated, aplurality of filamentsinsaidchamberandcarryingthe coating material to be evaporated, and means for supporting said filaments with respect to the gsurfacetobecoatedsothateachfilamentwill coatapredeterminedareaofthe surfaceandall of the filaments combined will coat the entire surface with a coating of uniform thickness.

8. An apparatus for coating a surface by ther- 10 mal evaporation comprising a chamber adapted tobeevacuatedandtoreceivethearticlethe surface of which is to be coated, a plurality of filaments in said chamber-rand carrying the coating material to be evaporated, and means for se- 15 lectively positioning said filaments with respect to the surface to be coatedso that each filament willcoatapredetermined areaofthesurface and all of the filaments combined will coat the entire surface with a coating of uniform thickness. 20 9. An apparatus for coating a surface by thermal evaporation comprising a chamber adapted to be evacuated and to receive the article the of the surface and all of the filaments combined 10 will uniformly coat the entire surface.

10. An apparatus for coating a surface by thermal evaporation comprising a chamber adapted to be evacuated and to receive the article the surface of which is to be coated, a plurality of filaments in said chamber and carrying the coating material, a pair of electrical conduits. and means for electrically connecting each filament with said conduits and including a contact element.

ROBLIY C. WILLIAMS.

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