US3089120A

US3089120A - Signalling device

Info

Publication number: US3089120A
Application number: US842864A
Authority: US
Inventors: Aiken William Ross
Original assignee: Ross Radio Corp
Current assignee: Ross Radio Corp
Priority date: 1959-09-28
Filing date: 1959-09-28
Publication date: 1963-05-07
Anticipated expiration: 1980-05-07

Description

May 7, 1963 AIKEN 3,089,120

SIGNALLING DEVICE Filed Sept. 28, 1959 3 Sheets-Sheet 1 f F7 INVENTOR 32 34 William Ross Aiken May 7, 1-963 w. R. AIKEN 3,089,120

SIGNALLING DEVICE Filed Sept. 28, 1959 3 Sheets-Sheet 2 INVENTCR William Ross Aiken W. R. AIKEN May 7, 1963 SIGNALLING DEVICE 3 Sheets-Sheet 3 Filed Sept. 28, 1959 INVENTOR. William Ross Aiken it Sttes 3,089,120 SIGNALLING DEVICE William Ross Aiken, Los Altos, Calif, assignor to Ross Radio Corporation, Los Altos, Calif., a corporation of California Filed Sept. 28, 1959, Ser. No. 842,864 13 Claims. (Cl. 340-44) The present invention relates to signalling devices of the type employing relatively movable components to display, expose or conceal an identifying mark, such as numbers, warning inscriptions, colored areas, and the like. More particularly the present invention relates to adjustable signalling devices of the type referred to, wherein the relative movement of the mark-displaying and concealing components is accomplished by the repelling and attracting effect of electrostatic charges rather than by the conventional cam-controlled actuating linkages.

Signalling devices of the type to which the present in vention relates, are described in my co-pending US. Patent application Serial No. 733,936 filed on May 8, 1958, for an Adjustable Sign now Patent No. 2,912,674, and Serial No. 822,026 filed on June 22, 1959, for a Signalling Device Involving Physically Movable Components now Patent No. 2,952,835. In devices of this type a sheet of conductive material is h-ingedly connected to a stationary plate of conductive material for movement from a position at rest parallel and adjacent to said plate, to an elevated position swung away from said sheet, and such movement is effected by applying an electrical charge jointly to said plate and said sheet, which repels the sheet from the stationary plate and thus swings it into its elevated position in the same manner in which the leaves of an electroscope repel each other and spread apart. In signalling devices of the type disclosed in my above mentioned co-pending applications, the described movement of the sheet is employed either to display a sign marked upon the outer surface of the sheet, to expose a sign marked upon the plate and initially covered up by the sheet, or to conceal from view a sign marked on a separate plate located behind the sheet in its elevated position.

- t is an object of my invention to provide an improved adjustable signalling device of the type referred to.

More particularly, it is an object of my invention to provide an electrostatically controlled adjustable signalling device, of the type referred to, wherein the swinging sheet is dependably pushed from a horizontal into an upright position by the application of electrical charges.

As illustrated and described in the above mentioned patent applications, the hingedly mounted control sheet is usually located within a housing and the bottom of said housing, if made of conductive material and appropriately insulated from other portions of said housing, may serve as the electrode for repelling the control sheet from its position of rest adjacent said bottom into an upright position wherein it conceals an identifying mark from the outside or displays such a mark through a suitable window. I have discovered that the electro static condition of any one of the walls of the housing may significantly influence the operation of the control sheet, and it is another object of my invention, therefore, to control the electrostatic condition of the walls of the housing in such a manner that the performance of the device is improved, i.e. to apply such charges to the various walls of the housing as will aid the intended performance of the device.

Another object of my invention is to provide a control sheet that is of extremely light weight so that it may readily respond to the electrostatic charges employed for its actuation, and which will yet operate satisfactorily 3,689,129 Patented May 7, I963 as a means for displaying or concealing an identifying mark.

Still another object of my invention is to provide effective, yet inexpensive means for hingedly mounting the control sheet in such a manner that it may readily and without hindrance respond to attracting and/ or repelling forces exerted thereon by electrostatic charges applied to said sheet and its surrounding structure.

Furthermore, it is an object of my invention to provide a simple yet effective control circuit for a signalling device of the type referred to.

These and other objects of the present invent-ion will be apparent from the following description of the accompanying drawings which illustrate certain preferred embodiments thereof, and wherein FIGURE 1 is a fragmentary perspective of a signalling device embodying my invention, with one of its side walls partially removed to expose structure in the interior of the device.

FIGURE 2 is a development of the walls of the signalling device, in combination with a circuit diagram illustrating the manner in which the conductive areas on said walls are electrically connected for cooperation in the spirit of the present invention.

FIGURE 3 is an elevation of a side wall of a modified embodiment of the invention;

FIGURES 4 and 5 are fragmentary perspectives illustrating the manner in which two different types of control sheets are hingedly secured to the bottom wall of the housing in which they are located;

FIGURE 6 illustrates an apparatus comprising a plurality of devices of the invention; and

FIGURE 7 is a vertical section through the apparatus shown in FIGURE 6 taken along line 7-7 thereof and viewed in the direction of the arrows associated with said line.

The signalling device of the invention encompasses a housing collectively identified by the reference numeral 10 which may be made of a transparent plastic, such as Lucite and which comprises a bottom wall B, a top wall T, .a front wall F, a rear wall R and two side walls S and S". Glued to the inner surfaces of the bottom, top, rear and side walls of the housing are thin sheets of aluminum foil, 12, 14, 16, 18' and 18", respectively, of somewhat smaller size than the surfaces to which they are secured, so that these sheets are electrically insulated from each other wherever necessary. The inner surface of the front wall F is provided with a transparent conductive coating 29 such as a coating of tin oxide, which may also be of somewhat smaller size than said surface.

Hingedly supported fnom the bottom wall B at points adjacent the front wall 'F of the housing and out of contact with the conductive sheet 12 of the bottom wall, is the control sheet 22 which may likewise be of aluminum foil. The upper or outer surface of the control sheet may be painted red and/ or may bear the inscription Stop, and the free inner surface of the aluminum sheet 16 on the rear wall R may be painted green and/or bear the inscription Go. The total housing may be mounted upon a pedestal 24.

The aluminum sheet 12 on the bottom wall of the housing is permanently connected to one .pole of a source of high voltage such as 4000 volts. In the embodiment of the invention illustrated in FIGURE 2, the sheet '12 is connected to the positive pole 26 of a rectifier represented by the block 28', as indicated by the line 30, and the conductive coating 20 on the front wall of the housing is permanently connected to the opposite pole 32 of the rectifier, as indicated by the line 34. The control sheet 22, however, is initially connected to ground through a high ohmic resistor 36 which may have a resistance of megohms. Spacers of insulating material, such as one or several thin rods of a suitable plastic mounted upon and projecting upwardly from the bottom sheet should be provided as indicated at 38 in FIGURES 1 and 5, to prevent direct contact between the control sheet and the ground sheet for reasons to be presently explained in greater detail. With the electrical connections as described so far, the control sheet 22 lies in a position parallel and adjacent to the ground sheet 12, without contacting said sheet, and the identifying mark on the rear wall of the housing is exposed to view through the transparent front wall F.

In accordance with the invention, I provide a normally open switch 40 by means of which the control sheet may selectively be connected directly to the same pole 26 of the rectifier 23 as the ground sheet 12. When the switch 41) is closed, the presence of charges of equal polarity upon the ground sheet and the control sheet is ordinarily effective to repel the latter from the former and cause the control sheet to swing upon its hinges away from the ground sheet in the direction of the arrow 41 shown in FIGURE 1. To reliably initiate such operation of the control sheet, I have found it important to provide the hereinbefore mentioned spacer rods 38 which prevent direct contact of the control sheet with the ground sheet. It might be assumed that these spacers are unnecessary since charges of the same polarity are applied to both said sheets during operation of the device, and the intended operation of the control sheet would seem an inevitable result of the presence of charges of equal polarity upon said sheets, especially if these sheets are in electrically conductive contact with each other. I have observed, however, that in practice this is not always the case. In certain areas the sheets may adhere to each other to a degree which the repelling electrostatic force established by closure of the switch 40 is unable to overcome; and instead of performing in the intended manner, the control sheet may merely flutter rapidly with parts thereof seemingly tied to the ground sheet. The presence of the described spacer rods 38 prevents such misoperation of the device and renders the control sheet immediately responsive to manipulation of the switch 40.

When the control sheet has properly detached itself from the ground sheet, it swings upon its hinges in the direction of the arrow 41, as pointed out hereinbefore, so as to expose the marking on its outer surface through the transparent front wall of the housing to the outside while concealing the marking on the rear wall of the housing; and as the control sheet folds away from the ground sheet, it is increasingly attracted by the charge of opposite polarity that is permanently applied to the conductive coating 20 on the inner surface of the front wall.

During the initial phases in the described upwardly folding movement of the control sheet, a maximum force is required to actuate the control sheet in the described manner for a reason other than the described tendency of the control sheet to adhere in spots to the ground sheets in spite of the presence, on said sheets, of electrical charges of equal polarity. At the beginning the control sheet is still in a horizontal or nearly horizontal position and in this position the component of the gravitational force which opposes the upwardly folding movement of the control sheet, is at its maximum. In accordance with my invention, the force which repels the control sheet from below, and the force which attracts the control sheet from above, may be substantially increased by applying to the conductive sheet on the rear wall a charge of the same polarity as is applied to the ground sheet and the control sheet, and by applying to the conductive sheets on the top wall and the side walls of the housing charges of the same polarity as the charge applied to the conductive coating on the front wall of the housing. For this purpose, I establish an electrically conductive connection between the ground sheet 12 and the conductive sheet 16 ion the rear wall of the housing as symbolically indicated by the line 42 in FIGURE 2,

and I also establish electrically conductive connections between the coating 20 on the front wall, and the sheets 18' and 18" on the side walls and the sheet 14 on the top wall of the housing, as symbolically indicated by the

lines

44, 46 and 48, respectively. It should be noted that while the electrical connections between the conductive surfaces within the housing 10 are symbolically indicated by connecting lines, the electrically connected conductive surfaces may actually be formed by a continuous sheet of metal foil. For instance, the ground sheet 12. and the sheet 16 on the rear wall of the housing may be formed by a single, appropriately folded sheet, and the top sheet 14 and the side sheets 18' and 18 may likewise be formed by a single sheet.

The charges on the side sheets 18' and 18" exert their maximum attraction upon the oppositely charged control sheet 22, during the initial phases of its actuation, when the control sheet is mostly in need for such attraction, because it is during the initial phases in the upward movement of the control sheet that the larger portions of the oppositely charged side sheets 18' and 18 are situated above the control sheet and thus endeavor to pull the control sheet upwardly. Hence, when a charge is applied directly to the control sheet 22, which is of the same polarity as the charge applied to the ground sheet 12, and the same charge is applied to the rear sheet 16 while charges of opposite polarity are applied not only to the conductive coating 28 on the front wall, but also to the top sheet 14 and the side sheets 18' and 18", the control sheet swings dependably from its horizontal position parallel and closely adjacent to the ground sheet into an almost vertical position adjacent to the front wall F as determined by spacers of insulating material which may be formed by hardened drops of glue as indicated at 50 in FIGURE 1; and whenever the switch 49 is opened, the charge applied to the control sheet is allowed to leak off to ground through the resistor 36 permitting the control sheet to drop back into a position parallel and adjacent to, yet spaced from the ground sheet 12, so that any mankings provided on the rear wall of the housing are ex posed to view through the transparent front wall thereof.

In the embodiment of the invention described hereinbefore, the electrodes that influence the position and performance of the control sheet 22 are formed by thin sheets of metal, such as aluminum foils, that are glued to the walls of the housing which are made of an insulating material. Good results may also be obtained by providing the surfaces of the walls with coatings of a conductive paint such as silver paint. While it is easy to apply such paints at the required areas, silver paint is relatively expensive. I have discovered, however, that it is not necessary to cover the total wall areas of the housing with conductive paint, and that it is possible to obtain practically identical performances of the device at a very substantial saving in cost when only the marginal areas 52 of the wall surfaces are covered with the conductive paint in the manner of a picture mat (FIGURE 3), while the center area 54 is left blank.

In describing the embodiment of the invention illustrated in FIGURES 1 and 2, I have indicated that the control sheet 22 may be made of a thin sheet of an electrically conductive material, such as aluminum foil. I have discovered, however, that excellent performance may be obtained when the control sheet is made of a thin sheet of plastic material, such as the extremely light and durable packaging material known under the trade name Cellophane. While plastic materials such as Cellophane, are usually regarded as insulators rather than conductors, they are in fact conductors of a very high ohmic resistance, and what limited conductivity they have, is sufiicient to enable them to perform as control sheets in the spirit of the present, because after the initial charge has been applied to the control sheets, no current flow takes place through said sheets during performance of the device.

The use of thin sheets of plastic materials, such as Cellophane, for the construction of the control sheets has the added advantage of making it extremely simple and inexpensive to provide the necessary hinge connection. Having reference to FIGURE 4, it is merely necessary to leave a pair of

lugs

56a and 56b on the front edge 58 of the control sheet 60 adjacent the front wall of the signalling device, and glue the end portions of these lugs to the bottom Wall E of the device adjacent the corresponding edge of the conductive bottom sheet 12. The

lugs

56a and 56b Will then perform perfectly as hinges for the control sheet in that they offer almost no resistance to the folding movements of the control sheet and are of practically limitless durability. in fact, I have found it advantageous to use strips of plastic material, such as Cellophane, for hingedly mounting control sheets made of metal foil for movement from a position adjacent the bottom wall of the housing to a position adjacent the front wall thereof as illustrated in FIGURE 5. In FIGURE 5, the strips 62a and 62b of Cellophane are glued to the front edge 64 of a control sheet 22 of metal foil, at points adjacent the opposite ends of said edge, and the projecting ends of said strips are glued to the floor plate B of the device.

If a sheet of plastic material, such as Cellophane is used as the control member in the display device of my invention, the spacers 38 and 56 (FIGURE 1) on the front wall and on the bottom wall of the housing may be made of metal without significantly affecting the proper performance of the device. Thus, in the embodiment of the invention illustrated in FIGURE 4, the spacers are formed by a wire spider 66 composed of two diagonally crossed

wires

68a and 68b that are suitably secured to the bottom wall B of the housing at their center point 70 and the free ends of which are bent upwardly as shown at 72 to form the actual spacer members.

The signalling devices of my invention may be employed individually, for instance as traffic signs to expose alternately the inscriptions Go and Stop, as mentioned hereinbefore. On the other hand, a plurality of such signalling devices may be assembled somewhat in the manner of a mosaic, and adjustment of selected ones of the assembled devices to positions wherein the brightly painted outer surfaces of their control sheets are exposed to the outside through the windows formed by the transparent front walls thereof, may be employed to form large and clearly visible numbers, letters, or like symbols. FIGURES 6 and 7 illustrate an apparatus of this type which is formed by a box 80- of a transparent plastic material that is subdivided in the manner of an egg crate to form four adjacently positioned rows '81 of each six superposed cells 82. Each of said cells constitutes a signalling device of the type described hereinbefore. The inner face of the front wall of each of these cells is provided with a transparent conductive coating 29 (FIGURE 7) and the inner surfaces of the side walls, the top wall, the bottom and the rear wall of each cell are provided with thin sheets of aluminum foil l8, 14, 12 and 16 respectively, as indicated in FIGURE 7. The conductive coatings on the front wall of all the cells comprised in the apparatus and the sheets of aluminum foil on the side walls and the top walls of all the cells are suitably connected to ground as indicated at 83, while the sheets of aluminum foil on the floor and the rear Wall of all the cells are connected to the positive pole of a source of direct current 84, as shown at 85. Each of the cells has a hingedly supported control sheet 22 the upper face of which may be painted black or red or in any other suitable color. By means of a plurality of manually operable switches indicated at 86, any one, or any combination, of the control sheets in the cells may selectively be connected to the positive pole of the source of direct current 84, to raise said control sheets into upright position adjacent the front wall of their respective cells. Thus, by setting the switches 86 to connect the control sheets of appropriate ones of the twenty-four signalling devices comprised in the apparatus illustrated in FIGURE 6, to the positive pole of the source of direct current 84, the visible outer surfaces of the raised control sheets may be made to form the character 2 as illustrated in FIGURE 6.

While I have explained my invention with the aid of certain preferred embodiments thereof, the invention is not limited to the specific constructional details shown and described, which may be departed from without departing from the scope and spirit of my invention. Thus, while I have shown the control sheets of the device as mounted for folding movement about a transversely disposed, horizontal axis parallel and adjacent to the corner formed by the bottom wall and the front wall of the housing, it will be understood that the control sheets may be mounted for movement about vertical axes, if desired.

I claim:

1. In a signalling device, an arrangement for selectively exposing and concealing an identifying mark comprising a sheet of an electrically conductive material, a member having an electrically conductive surface disposed adjacent one edge of and at right angles to said sheet, a thin sheet of plastic material mounted for folding movement between a position adjacent to said sheet of electrically conductive material to a position adjacent to said conductive surface, means applying an electrical charge to said sheet of electrically conductive material, means applying an electrical charge of opposite polarity to said conductive surface, and means including a switch operable to apply an electric charge to said sheet of plastic material.

2. An arrangement according to claim 1 wherein said sheet of plastic material is a sheet of Cellophane,

3. A signalling device comprising a housing formed by a transparent front wall having an electrically conductive surface, a bottom wall having an electrically conductive surface, and side walls having electrically conductive surfaces; a control sheet; means hingedly mounting said control sheet adjacent the corner defined by said front wall and said bottom wall for folding movement from a position adjacent to said bottom wall to a position adjacent to said f-nont wall; means applying an electrical charge to the electrically conductive surface of said bottom Wall; means applying an electrical charge of opposite polarity to the electrically conductive surfaces of said front Wall and said side walls; and means including a normally open switch operable to apply an electrical charge of the same polarity as applied to the electrical surface of said bottom wall to said control sheet.

4. A device according to claim 3 wherein the electrically conductive surfaces of said side walls are strips of conductive paint extending in the form of a picture mat adjacent to and along the edges of said Walls.

5. In a signalling device, a mechanism for selectively exposing and concealing an identifying mark comprising a horizontmly disposed first sheet of an electrically conductive material, a vertically disposed window adjacent one edge of said sheet having an electrically conductive surface, a second sheet mounted for folding mov ment from a position adjacent to said horizontally disposed first sheet to a position adjacent to said window, a number of spacers on said horizontally disposed first sheet to keep said second sheet apart from said first sheet; means applying an electrical charge to said first sheet, means applying an electrical charge of opposite polarity to the electrically conductive surfaces of said window, means including a high ohmic resistor connecting said second sheet to ground, and means including a switch operable to apply a charge of the same polarity as applied to said first sheet to said second sheet, to cause said second sheet to swing from a position adjacent to said first sheet to a position covering said window.

6. In a signalling device, a mechanism for selectively exposing and concealing an identifying mark comprising a horizontally disposed first sheet of an electrically conductive material, a vertically disposed window adjacent one edge of said sheet having an electrically conductive surface, a second sheet mounted for folding movement from a position adjacent to said horizontally disposed first sheet to a position adjacent to said window, means applying an electrical charge to said first sheet, means applying an electrical charge of opposite polarity to the electrically conductive surface of said window, means including a high ohmic resistor connecting said second sheet to ground, and means including a switch operable to apply a charge of the same polarity as applied to said first sheet to said second sheet, to cause said second sheet to swing from a position adjacent to said first sheet to a position covering said Window.

7. In a signalling device, a mechanism for selectively exposing and concealing an identifying mark comprising a horizontally disposed first sheet of an electrically conductive material, a vertically disposed window adjacent one edge of said sheet having an electrically conductive surface, a thin second sheet of a plastic material mounted for folding movement from a position adjacent to said horizontally disposed first sheet to a position adjacent to said window, a number of spacers on said first sheet to keep said second sheet :apart from said first sheet, means applying an electrical charge to said first sheet, means applying an electrical charge of opposite polarity to the electrically conductive surface of said window, means including a high ohmic resistor connecting said second sheet to ground, and means including a switch operable to apply a charge of the same polarity as applied to said first sheet to said second sheet, to cause said second sheet to swing from a position adjacent to Said first sheet to a position covering said window.

8. A signalling device comprising a housing formed by 'a transparent front wall having an electrically conductive surface, a rear wall having an electrically conductive surface, a bottom wall having an electrically conductive surface, a top Wall having an electrically conductive surface, and side walls having electrically conductive surfaces, a control sheet of an electrically conductive material, means hingedly mounting said control sheet adjacent the corner defined by said front wall and said bottom Wall for folding movement from a position adjacent to said bottom Wall to a position adjacent to said front wall, means applying an electrical charge to the electrically conductive surfaces of said bottom wall and said rear wall, means applying an electrical charge of opposite polarity to the electrically conductive surfaces of said front wall, said top wall and said side walls, means including a high ohmic resistor connecting said control sheet to ground, and means including a normally open switch operable to apply an electrical charge of the same polarity as applied to the electrical surfaces of said bottom wall and said rear wall directly to said control sheet.

9. A signalling device comprising a housing formed by a transparent front wall having an electrically conductive surface, a rear wall having an electrically conductive surface, a bottomwall having an electrically conductive surface, a top wall having an electrically conductive surface, and side walls having electrically conductive surfaces, a thin control sheet of plastic material, means hingedly mounting said control sheet adjacent the corner defined by said front wall and said bottom wall for folding movement from a position parallel and adjacent to said bottom Wall to a position parallel and adjacent to said front wall, a plurality of spacers m0unt ed upon said bottom wall to keep said control sheet apart from said bottom Wall, a plurality of spacers provided on said front Wall to prevent contact of said control sheet with said front wall, means applying an electrical charge to the electrically conductive surfaces of said bottom wall and said rear Wall, means applying an electrical charge of opposite polarity to the electric-ally conductive surfaces of said front wall, said top wall and said side walls, means including a high ohmic resistor connecting said control sheet to ground, and means including a normally open switch operable to apply an electrical charge of the same polarity as applied to the electrical surfaces of said bottom wall and said rear wall directly to said control sheet.

A signalling device comprising a member having an electrically conductive horizontally disposed surface, a sheet mounted for folding movement from a first position adjacent and parallel to said conductive surface to a second position removed from and forming an angle with said surface, spacers provided upon said surface to inhibit direct contact between said sheet and said surface when said sheet is in said first position, and means for applying charges of the same polarity to said surface and said sheet to repel said sheet from said first position into said second position.

1 1. A device according to claim 10, wherein said sheet is -a thin sheet of a plastic material of high resistivity.

12. A signalling device comprising a plurality of adjacently positioned rows of superposed signalling units each including a member having a horizontally disposed electrically conductive surface and a sheet mounted for folding movement from a first position parallel and adjaceut to said surface into a second position removed from and forming an angle with said surface, and means for applying charges of the same polarity to the surface and the sheet of selected ones of said units to repel the sheets thereof into said second position.

13. A signalling device comprising a plurality of adjacently positioned rows of superposed signalling units, each including a member having a horizontally disposed elec trically conductive surface, a sheet mounted for folding movement from a first position parallel and adjacent to said surface into a second position removed from and forming an angle with said surface, and spacers provided upon said surface to inhibit direct contact between said surface and its cooperating sheet when said sheet is in said first position; and means for applying charges of the same polarity to the surface and the sheet of selected ones of said units to repel the sheets thereof into said second position.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

9. A SIGNALLING DEVICE COMPRISING A HOUSING FORMED BY A TRANSPARENT FRONT WALL HAVING AN ELECTRICALLY CONDUCTIVE SURFACE, A REAR WALL HAVING AN ELECTRICALLY CONDUCTIVE SURFACE, A BOTTOM WALL HAVING AN ELECTRICALLY CONDUCTIVE SURFACE, A TOP WALL HAVING AN ELECTRICALLY CONDUCTIVE SURFACE, AND SIDE WALLS HAVING ELECTRICALLY CONDUCTIVE SURFACES, A THIN CONTROL SHEET OF PLASTIC MATERIAL, MEANS HINGEDLY MOUNTING SAID CONTROL SHEET ADJACENT THE CORNER DEFINED BY SAID FRONT WALL AND SAID BOTTOM WALL FOR FOLDING MOVEMENT FROM A POSITION PARALLEL AND ADJACENT TO SAID BOTTOM WALL TO A POSITION PARALLEL AND ADJACENT TO SAID FRONT WALL, A PLURALITY OF SPACERS MOUNTED UPON SAID BOTTOM WALL TO KEEP SAID CONTROL SHEET APART FROM SAID BOTTOM WALL, A PLURALITY OF SPACERS PROVIDED ON SAID FRONT WALL TO PREVENT CONTACT OF SAID CONTROL SHEET WITH SAID FRONT WALL, MEANS APPLYING AN ELECTRICAL CHARGE TO THE ELECTRICALLY CONDUCTIVE SURFACES OF SAID BOTTOM WALL AND SAID REAR WALL, MEANS APPLYING AN ELECTRICAL CHARGE OF OPPOSITE POLARITY TO THE ELECTRICALLY CONDUCTIVE SURFACES OF SAID FRONT WALL, SAID TOP WALL AND SAID SIDE WALLS, MEANS INCLUDING A HIGH OHMIC RESISTOR CONNECTING SAID CONTROL SHEET TO GROUND, AND MEANS INCLUDING A NORMALLY OPEN SWITCH OPERABLE TO APPLY AN ELECTRICAL CHARGE OF THE SAME POLARITY AS APPLIED TO THE ELECTRICAL SURFACES OF SAID BOTTOM WALL AND SAID REAR WALL DIRECTLY TO SAID CONTROL SHEET.