US3691373A

US3691373A - Corona device

Info

Publication number: US3691373A
Application number: US29926A
Authority: US
Inventors: Robert G Compton; Ray S Richmond; Robert A Tracy
Original assignee: FIRST CITY NATIONAL BANK OF
Current assignee: FIRST CITY NATIONAL BANK OF
Priority date: 1970-04-20
Filing date: 1970-04-20
Publication date: 1972-09-12
Anticipated expiration: 1989-09-12

Abstract

A corona device for charging insulating surfaces comprising an electrically nonconductive U-shaped base having a corona generating member mounted in the central slot. The corona generating member comprises two electrically conductive side strips and an electrically conductive central strip having a number of projections along the top edge, the member being held together and fastened to the base by a number of transverse pins fitted into matching holes in the three strips.

Description

United States Patent Compton et al.

[ 51 Sept. 12,1972

[54] CORONA DEVICE [72] Inventors: Robert G. Compton, Albuquerque; Ray S. Richmond, Placitas; Robert A. Tracy, Albuquerque, all of N. Mex.

[73] Assignee: First City National Bank of Houston,

as Trustee of the Francis A. Callery 1968 Trust [22] Filed: April 20, 1970 [21] Appl.No.: 29,926

[52] US. Cl. ..250/49.5 ZC, 250/49.5 GC [51] Int. Cl. ..G03g 15/00 [58] Field of Search ..250/49.5 GC, 49.5 ZC, 49.5

TC; 313/326, 351, 352; 317/262 A; 355/3 [56] References Cited UNITED STATES PATENTS West ..250/4 9.5 ZC I 3,369,152 2/1968 Spengler ..250/49.5 GC 2,934,650 4/1960 De Witt ..250/49.5 ZC 3,581,149 5/1971 Tanaka ..250/49.5 ZC

Primary ExaminerJames W. Lawrence Assistant ExaminerC. E. Church 7 Attorney-George N. Hibben and Harry E. Burke [5 7] ABSTRACT A corona device for charging insulating surfaces comprising an electrically nonconductive U-shaped base having a corona generating member mounted in the central slot. The corona generating member comprises two electrically conductive side strips and an electrically conductive central strip having a number of projections along the top edge, the member being held together and fastened to the base by a number of transverse pins fitted into matching holes in the three strips.

1 1 Claims, 14 Drawing Figures PATENTED 12 I97? 3 6 91 3 73

sum

1 or 3

sum

2 or 3 PATENTEDSEP 12 I912 CORONA DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to corona devices for charging insulating surfaces, and more particularly to an improved corona device for charging the surface of a photoconductive element in an electrostatic copier to a substantially uniform potential.

2. Description of the Prior Art Corona devices are known which are used in electrostatic copiers employing a photoconductive element to produce visible copies of documents to be reproduced. In typical known copiers, each visible copy is produced by applying a uniform electrostatic charge to the surface of the photoconductive element, dissipating selected portions of the surface charge by exposing the surface to a light image corresponding to the document to be copied and developing the latent electrostatic image resulting from this exposure by applying visible toner particles to the exposed surface.

In such copiers, the function of the corona device is to provide the electrostatic charges which are distributed uniformly over the surface of the photoconductive element. Known devices, such as the corona charging unit disclosed in U.S. Pat. No. 3,075,070, customarily employ one or more wire strands which are stretched taut between anchoring end pieces and connected to a source of high voltage of sufficient magnitude to ionize the air about the strands. The ions so produced are then attracted to the surface of the photoconductive element by any suitable means. For example, in copiers employing a zinc oxide photoconductive element, ions are attracted to the surface of the zinc oxide element by connecting the back of the element to ground potential.

In known single or multiple wire corona devices, the wire strands employed are very delicate and, as a consequence, are extremely susceptible to breakage under normal operating conditions. This susceptibility is aggravated by the fact that the wire strands are normally mounted under tension. Since replacement of a broken wire is a precision operation requiring a skilled technician, such breakage often requires that the copier be left idle for relatively long periods of time while the corona unit is being repaired, which is highly unsatisfactory. Attempts to solve this problem by replacing the entire corona unit whenever a wire breaks have been found to be equally unsatisfactory because of the relatively great cost of an entire corona unit and because such a replacement also normally requires the services of a skilled technician in order to properly align the new unit in the machine.

Further, in many copiers the corona devices are physically located in positions so arranged with respect to the other units (e.g., the toner developer unit) that toner particles and other foreign substances fall into or otherwise find their way into the corona devices and foul the active elements therein. This fouling progressively reduces the uniformity of the corona and finally renders the corona device ineffective to produce sufficient ions to satisfactorily charge the surface of the photoconductive element. This results in deterioration of copy quality and, finally, no copy at all. In extreme cases, falling clusters of toner particles have been known to break the strands in a wire-type corona device, thereby rendering the copier useless until the broken unit is repaired or replaced. Attempts to solve this problem, such as placing protective shields about the corona device, have frequently introduced more problems, have proven expensive, and have met with little success.

SUMMARY OF THE INVENTION The invention disclosed herein comprises a corona device which is extremely inexpensive and simple to construct and which is highly reliable. More particularly, the invention comprises a generally U-shaped, electrically nonconductive base and a corona generating member which is mounted in the central longitudinal slot in the base. The corona generating member comprises three electrically conductive strips: two keeper side strips and a central strip having a number of spaced projections along the top edge thereof. The three strips are fastened together by a number of pins which are fitted into matching, aligned transverse apertures in the three strips. The corona generating member is secured in the base by a number of electrically nonconductive pins fitted into matching, aligned transverse apertures in the base and the strips.

The central strip projections, which generate the corona when connected to a source of high voltage, are alternatively formed in the shape of isosceles triangles or scalloped portionsdefined below-having an apex angle preferably in the range from 5-30. The height of the projections is sealed in relation to the depth of the central base slot so that the projections are confined within the slot in an assembled device in order to protect these projections from falling toner particles or other foreign materials circulating past the corona discharge device when it is mounted in its operating environment, and also to focus the ions produced near the tips of the projections.

The above-described construction provides a rugged corona device which, in addition to the above-mentioned advantages, can be easily and quickly mounted in, and removed from, the associated apparatus.

For a fuller understanding of the nature and advantages of the invention, reference should be had to the following detailed description, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded view in perspective of a preferred embodiment of the invention;

FIG. 2 is a sectional view taken along the lines 2-2 of FIG. 1;

FIG. 3 is a sectional view taken along the lines 3--3 of FIG. 1; v

FIG. 4 is a sectional view taken along the lines 44 of FIG. 1;

FIG. 5 is a plan view of a worksheet containing a number of pin strip elements of the invention;

FIG. 6 is an enlarged sectional view showing a portion of a pin strip element;

FIG. 7 is an enlarged sectional view taken along the lines 7-7 of FIG. 6;

FIG. 8 is an enlarged view of an alternate pin strip having scalloped edges;

FIGS. 9ll illustrate an alternate embodiment of the invention having a tubular keeper; and

FIGS. 12-14 illustrate an alternate embodiment of the invention having triangular keepers.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to the drawings, FIGS. 1-4 show a preferred embodiment of a corona device constructed according to the invention which is suitable for use in an electrostatic copier and which comprises an electrically nonconductive, generally U-shaped base member and an electrically conductive corona generating member 20. As best shown in FIGS. 3 and 4, base 10 is provided with a central longitudinal slot 11, having a narrow lower portion 12 which is preferably dimensioned substantially equal to the thickness of member 20 to provide a snug fit therebetween. Slot 11 also has a wide upper portion 13 to allow ions produced by the corona device sufficient freedom to migrate to the photoconductive surface to be charged. Running lengthwise along each side of base 10 is a mounting groove 14 adapted to mate with a suitable holder of a type known to those skilled in the art and having parallel rails adapted to slidingly accommodate grooves 14. A plurality of spaced transverse apertures 15 are formed in base 10 to provide mounting holes for mounting pins 16 which are used to firmly secure corona generating member to base 10, as described below. An end portion 17 of slot 11 is enlarged to accommodate connector 18, also described below. Base 10 may be formed from any suitable durable electrically nonconductive substance, such as TEFLON, by extruding, milling or any other method known to those skilled in the art. After formation of base 10, slot 11 is cut away to form enlarged end portion 17. Next, apertures 15 are formed by drilling or any other suitable means known to those skilled in the art. Alternatively, apertures 15 may be formed after corona generating member 20 is positioned in narrow slot portion 12.

Corona generating member 20 comprises two supporting or

keeper side strips

21, 22 and a center strip 24 having a plurality of integrally formed tapered projections 25 along the top edge thereof, the latter shown in section in FIG. 2. Keeper side strips 21, 22 can be formed from any good electrical conductor known to those skilled in the art, e.g., brass, and are preferably rectangular in cross-section, as shown in FIGS. 3 and 4. Side strip 21 has an extended end portion 26 which receives connector 18 for electrically connecting center strip 24 to a suitable source of high voltage (not shown). Center strip 24 can be formed by stamping, cutting, or milling from any good electrical conductor which is corrosion resistant, e.g., nickel-iron, stainless steel, or Monel. All three

strips

21, 22 and 24 are provided with two sets of aligned

apertures

28 and 30. Apertures 28 serve to receive keeper pins 29 which temporarily hold together and align the

strips

21, 22 and 24 during assembly of the device, as more fully described below. Keeper pins 29 can be fashioned from any suitable durable conductive material, e.g. brass. Apertures 30, which may be formed either before or after corona generating member 20 is assembled and inserted in slot 1 l, are aligned with apertures 15 of base 10, and are adapted to receive mounting pins 16. Mounting pins 16 can be constructed of any suitable substance known to those skilled in the art which can be fashioned into pins and is durable and electrically nonconductive. Pins constructed of TEFLON were found to provide excellent results in the preferred embodiment.

To assemble corona generating member 20, the three

strips

21, 22 and 24 are positioned with apertures 28 aligned, and keeper pins 29 are inserted therein. Although keeper pins 29 may be precut to the proper length (equal to the combined thicknesses of the three

strips

21, 22 and 24), in the preferred method of as-. sembly, electrically conductive dowel stock of the proper diameter (equal to the diameter of apertures 28) is inserted into each aperture 28 and then cut. If desired, each end of each keeper pin 29 may be slightly burgeoned or burred to lock the pin in place. However, this is not absolutely necessary since the side walls of narrow slot portion 12 serve this purpose when corona generating member 20 is positioned in slot 1 1, as shown in FIG. 3.

After assembly, corona generating member 20 is positioned in slot 11 of base 10 by sliding the member 20 from either end of base 10 along the bottom of slot 11 until apertures 30 are aligned with apertures 15. Alternatively, if either apertures 15 alone or both

apertures

15 and 30 have not been previously formed, corona generating member 20 can be slid along the bottom of slot 11 to a predetermined position, after which the required apertures can be formed by any suitable means, e. g., drilling. A mounting pin 16 is then inserted into each one of the aligned aperture pairs 15, 30 and secured in place by burgeoning each end. As with keeper pins 29, mounting pins 16 may be either precut to size or sized during the assembly process from dowel stock. The assembled device is now ready to be inserted into place in the copier by fitting grooves 14 of base 10 onto the rails (not shown) of the holder and sliding the device into position, after which high-voltage connector 18 is fitted onto extended end portion 26 of keeper side strip 21.

As is clearly shown in FIG. 4, in an assembled device, the tips of projections 25 are recessed within the wide portion 13 of slot 11. This recessing has been found to be highly effective in preventing damage to projections 25 during handling and also during operation of the copier from falling toner particles or other foreign substances circulating past slot 11. Further, recessing of the tips of projections 25 also results in a focusing of ions generated by the corona near each tip, which is highly desirable. Recessing is achieved by selecting the proper dimensions for the depth of slot 11 and the height of projections 25.

FIG. 5 illustrates a worksheet 50 which contains a number of center strips 24 and which is particularly well suited for the rapid assembly of corona-discharge members 20 requiring extremely fine projections 25. Such center strips 24 are ideally suited for coronadischarge devices used in copier applications. In such applications, extremely fine projections are highly desirable since they produce a highly uniform corona. However, central strips having extremely fine projections must also be extremely thin and are thus very difficult to handle, being very flexible and fragile-particularly when being used in the mass production of large numbers of corona-generating members. The worksheet 50 shown in FIG. 5 provides an optimum solution to the problems attendant upon extremely thin central strips and is produced by a chemical milling process.

Chemical milling techniques have been employed for various purposes heretofore, such as in the manufacture of miniature electronic printed circuits. One such chemical milling process suitable for manufacturing a worksheet 50 is known in the art as photoetching and, briefly described, proceeds as follows.

A highly detailed and accurate master pattern of a predetermined number of center strips 24 having the desired contour is prepared on a greatly enlarged scale and photoreduced to produce a master mask of the desired size. A thin sheet composed of an inert, slowsputtering metal such as stainless steel, nickel-iron or Monel is then coated with a matching photoresist compound, such as Kodak KMER, KPR-Z, KPR-3. A light image of the master mask is then projected onto the photoresist coating. During exposure of the photoresist to the image, those portions of the photoresist subjected to the light polymerize, while the other portions remain unaffected. The remaining unexposed portions of the photoresist are then washed away and the entire sheet of metal is brought into contact with a suitable etching solution, e.g., a ferric chloride solution of 3642 Baume, or a solution of one volume concentrated hydrochloric acid (37 percent), one volume concentrated nitric acid (70 percent) and three volumes water. During the etching step of the process, exposed portions of the metal are etched away by the etching solution, leaving the desired pattern. After the exposed portions of the metal sheet have been completely etched away, the assembly is removed from contact with the etching solution, washed, and the polymerized photoresist is stripped from the now formed worksheet 50.

In the above process, the exposed metal may be etched by either spraying the surface with the etching solution or dipping the metal sheet in the etching solution. If spray etching is performed, only one side of the sheet need contain the polymerized photoresist pattern. If dip etching is used, however, both sides of the metal sheet must contain the polymerized photoresist pattern.

The resulting worksheet 50 produced by the above photoetching process contains a predetermined number of center strips 24 having identical projections 25, each strip 24 being connected at each end to a frame 52 by a thin web 54.

For clarity, only two complete center strips 24 are shown in the worksheet 50 of FIG. 5, it being understood that many more than this number can be included. In one such worksheet constructed, fifty center strip 24 were included. It has been found that the provision of frame 52 enables the entire worksheet to be handled extensively without damaging the delicate projections 25.

After production of the worksheet 50, apertures 28 and 30 (FIG. 2) are normally formed by punching or drilling. The worksheet 50 is now prepared for assembly of the corona generating members 20.

During assembly of the corona-generating members 20, the center strips 24 are individually cut, as required, from worksheet 50 and combined with keeper side strips 21, 22. In a preferred method of assembly, side strip 21 (see FIG. 1) is placed in a recessed holder, after which worksheet 50 is maneuvered by frame 52 until a center strip 24 is located in the correct position over side strip 21. Center strip 24 is then severed from frame 52 by cutting webs 54 and side strip 22 is placed in position, after which keeper pins 29 are inserted into place in apertures 28. Further assembly of the corona device then proceeds as described above. As is evident from the above description, the assembly of corona generating members 20 is greatly facilitated by the arrangement of a number of center strips 24 mounted in a single worksheet 50 produced by the photoetching process.

FIG. 6 shows an enlarged portion of a single center strip 24 constructed according to the invention and illustrating certain critical parameters. Each projection 25 is formed in the shape of an isosceles triangle having an apex angle a, the tips of projections 25 being separated by a distance D. It has been found that uniform coronas are obtained for apex angles in the range For best results, the tip separation distance for the FIG. 6 embodiment should be kept in the range 0.200 D 0.400 inch A corona generating device constructed according to the FIG. 1 embodiment and having a 0.010 inch thick center strip 24 with a contour as shown in FIG. 6 was found to produce optimum results, measured in terms of uniformity of corona and uniformity of surface charge produced, with a 10 and D 0.300 inch at a separation of 0.425 inch between the tips of projections 25 and the surface of a zinc oxide coated photoconduc-. tive web, with a potential of 5,000 BC volts applied to the center strip 24. It was further found that excellent results were obtained over a voltage range from 4,500 to --5,500 VDC and a projection tip-web surface separation distance in the range from 0.300 to 0.600 inch.

FIG. 7 shows an enlarged cross-section of a portion of a single projection 25 of a center strip 24 produced according to the dip etching process. It has been found that dip etching produces projections 25 which each have a sharp-edge tip 70, which is separated from the main body of projection 25 by contoured portions 72. This configuration has been found to produce exceptionally uniform coronas. Accordingly, center strips having projection tips produced by the dip etching technique are preferred over those produced by the spray etching method of photoetching.

FIG. 8 shows an enlarged portion of a single center strip having an alternately contoured upper edge. The upper edge of strip 80 has a scalloped contour consisting of a number of alternate crests 82, separated by a distance D, and generally arcuate-shaped troughs 84, with each crest having an apex angle a. It has been found that the optimum ranges on the angle a and the distance D for a center strip 80 having a scalloped configuration are the same as those given for the FIG. 6 embodiment. The FIG. 8 embodiment offers the added advantage, however, of having less fragile projections than the triangular projections 25 of FIG. 6. As with the FIG. 6 embodiment, center strips 80 may be either spray etched or dip etched, with the latter technique being preferred.

FIGS. 9 to 11 illustrate an alternate embodiment of the invention having a tubular keeper 91 for center strip 24. As best seen in FIG. 10, keeper 91 has a longitudinal slot 92 from which the upper portion of center strip 24 and projections 25 protrude. Additionally, tubular keeper has an extended end portion 93 similar to the extended end portion 26 of keeper side strip 21 in the FIG. 1 embodiment and for the same purpose. As with the FIG. 1 embodiment, tubular keeper 91 is provided with

apertures

28 and 30 for receiving keeper pins 29 and mounting pins 16 (the latter shown in phantom in FIG. 11). As best seen in FIG. 11, base 94 is identical with base of the FIG. 1 embodiment, with the exception of having a lower slot portion 12 with a partially circular cross-section 95 to accommodate tubular keeper 91.

FIGS. 12 to 14 illustrate another alternate embodiment of the invention having a triangular keeper for center strip 24. The triangular keeper comprises a pair of keeper side strips strips 96 and 97, each having a triangular cross-section, as best seen in FIG. 14. Keeper side strip 96 has an extended end portion 98 which is rectangular in cross-section and adapted to receive a suitable high voltage connector similar to connector 18 of FIG. 1. As with the FIG. 1 embodiment, triangular keeper side strips 96 and 97 are provided with

apertures

28 and 30 for receiving keeper pins 29 and mounting pins 16 (shown in phantom in FIG. 14). As best seen in FIG. 14, base 99 is identical with base 10 of the FIG. 1 embodiment, with the exception of having a lower slot portion 100 with a triangular cross-section to accommodate the triangular keeper.

Both keeper 91 and triangular keeper comprising triangular side strips 96 and 97 can be formed from any good electrical conductor known to those skilled in the art, e.g., brass. Further, the FIGS. 9-11 and FIGS. 12-14 embodiments are assembled in a similar fashion to the embodiment of FIGS. 1-4. In addition, all three embodiments disclosed may employ either center strip 24 or center strip 80 with equally effective corona generating results.

Corona devices constructed according to the abovedescribed invention are more rugged and durable than prior art devices, and less susceptible to interference from toner particles and other foreign substances than other known devices. In addition, as has been shown, corona devices constructed according to the invention are simple and inexpensive to assemble and install, while replacement is accomplished by merely removing the high voltage connector, sliding out the old device, and sliding in the new replacement. Further, extremely uniform coronas are obtained from devices constructed according to the invention, particularly when extremely thin center strip projections, manufactured by the photoetching technique, are employed. I

While the foregoing provides a full disclosure of the preferred embodiment of the invention, it is understood that various modifications, alternate constructi'un, and equivalents may be employed without departing from the true spirit and scope of the invention herein. For example, center strips with projections having different contours from those disclosed can be designed and employed without departing from the scope and spirit of the invention. Therefore, the above description and illustrations should not be construed as limiting the scope of the invention which is solely defined by the appended claims.

What is claimed is:

1. A corona device for applying an electrostatic charge on an insulating surface comprising an electrically nonconductive base member;

an electrically conductive corona generating member removably carried by said base member, said corona generating member including a first elongated thin strip adapted to be coupled to a corona generating potential potential and having a plurality of integral coplanar projections extending from one edge portion thereof and spaced along the length thereof;

said corona generating member further including at least one electrically conductive supporting strip secured to said thin strip and extending along the other edge thereof;

said base member having an elongated slot therein,

said other edge of said thin strip and said supporting strip being fitted in said slot; and

. removable mounting pins extending laterally of and through said base member, said other edge of said thin strip and said supporting strip to removably secure said corona generating member to said base member.

2. The apparatus of claim 1 wherein said base member comprises a generally U-shaped block, the depth of said slot being greater than the height of said projections when said strip is mounted in said base.

3. The apparatus of claim 1 wherein the spacing between said projections is in the range from 0.200 to 0.400 inch.

4. The apparatus of claim 1 wherein each of said projections is formed in the shape of an isosceles triangle having an acute apex angle, the bases of said triangles being spaced apart along said one edge portion.

5. The apparatus of claim 4 wherein said angle is in the range from 5 to 30.

6. The apparatus of claim 1 wherein said strip has a scalloped upper portion and said projections comprise the apices of said scalloped upper portion.

7. The apparatus of claim 6 wherein said scalloped upper portion comprises a series of convex crests and concave troughs, said convex crests having an apex angle in the range from 530.

8. The apparatus of claim 1 wherein said supporting strip comprises a cylindrical tube having a longitudinal slot adapted to receive said first elongated strip, said cylindrical tube and said first elongated strip are provided with a first set of transverse apertures spaced longitudinally in matching alignment, and further including a plurality of keeper pins fitted into said apertures to fasten together said cylindrical tube andsaid first elongated strip.

9. The apparatus of claim 1 wherein said supporting strip comprises second and third elongated strips, said first, second, and third strips being provided with a first set of apertures spaced longitudinally thereof in matching alignment, and further including a plurality of keeper pins adapted to be fitted into said apertures to fasten together said first, second, and third strips.

10. The apparatus of claim 9 wherein said second and third elongated strips each have a triangular crosssection.

ll. The apparatus of claim 9 wherein said base member is provided with a plurality of transverse apertures spaced longitudinally thereof, said first, second, and third stripsbeing provided with a second set of second set of apertures to firmly secure said corona generating member to said base.

Patent No. 3,691,373 Dated September 1972 Inventor(s) Robert G, Compton et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 5, line 56,: "strip" should read--strips-. Col. 6, line 24, between "5 and "a" should appear and between "oz" and "30" should appear Col, 6, line 28, between "0.200 and "D" should appear and between "D' and "0.400 inch" should appear Col, 7, line 23, "strips strips" should read st'rips-. col. 7, line 36, "keeper" should read tubula keeper Col. 8, line 14, Claim 1, "potential potential" should read -potential--.

Signed and sealed this 10th day of April 1973.

(SEAL) Attest:

EDWARD M.PLETCHER,JR. ROBERT C SOTTSCHALK Attescing Officer Commissioner of Patents

Claims

1. A corona device for applying an electrostatic charge on an insulating surface comprising an electrically nonconductive base member; an electrically conductive corona generating member removably carried by said base member, said corona generating member including a first elongated thin strip adapted to be coupled to a corona generating potential potential and having a plurality of integral coplanar projections extending from one edge portion thereof and spaced along the length thereof; said corona generating member further including at least one electrically conductive supporting strip secured to said thin strip and extending along the other edge thereof; said base member having an elongated slot therein, said other edge of said thin strip and said supporting strip being fitted in said slot; and removable mounting pins extending laterally of and through said base member, said other edge of said thin strip and said supporting strip to removably secure said corona generating member to said base member.

5. The apparatus of claim 4 wherein said angle is in the range from 5* to 30*.

7. The apparatus of claim 6 wherein said scalloped upper portion comprises a series of convex crests and concave troughs, said convex crests having an apex angle in the range from 5*-30*.

8. The apparatus of claim 1 wherein said supporting strip comprises a cylindrical tube having a longitudinal slot adapted to receive said first elongated strip, said cylindrical tube and said first elongated strip are provided with a first set of transverse apertures spaced longitudinally in matching alignment, and further including a plurality of keeper pins fitted into said apertures to fasten together said cylindrical tube and said first elongated strip.

10. The apparatus of claim 9 wherein said second and third elongated strips each have a triangular cross-section.

11. The apparatus of claim 9 wherein said base member is provided with a plurality of transverse apertures spaced longitudinally thereof, said first, second, and third strips being provided with a second set of apertures spaced longitudinally thereof in matching alignment with said transverse apertures, said mounting pins being fitted into said transverse apertures and said second set of apertures to firmly secure said corona generating member to said base.