US2143907A - Photoelectric screen for cathode ray tubes - Google Patents

Description

Jan. 17, 1939. A. D, BLUMLEIN ET AL 2,143,907

' PHOTOELECTRIC SCREEN FOR CATHODE RAY TUBES Filed Oct. 10, 1935 2 Sheets-Sheet 1 flvemibm A00. BLUMLE/A/ ME. HOLMA/V Jan. 17, 1939.

A. D. BLUMLEIN ET AL 2,143,907

PHOTOELECTRIC SCREEN FOR CATHODE RAY TUBES 2 Sheets-Sheet 2 Filed Oct. 10, 1955 Patented Jan. 17, 1939 UNETED STATES PATENT OFFIQE PHOTOELECTRIC SCREEN FOR CATHODE RAY TUBES Application October 10, 1935, Serial No. 44,316 In Great Britain October 26, 1934 8 Claims.

The present invention relates to improvements in insulated electrical conductors, and to the manufacture of grids for photo-electrically active screens for cathode ray tubes and embodying such conductors.

In co-pending application Serial No. 34,304 filed August 21, 1935, there is described television transmitting apparatus comprising a cathode ray tube having disposed within its envelope a photoelectrically active mosaic screen. The mosaic screen comprises a metal signal plate, which henceforth will be called a grid, pierced with a multiplicity of closely spaced small holes; this grid is coated all over with a thin layer of insulating material and the holes are filled with metal rivets which pass right through the grid; the surfaces of the rivets on one side of the grid are coated with photo-electrically active material.

A mosaic screen is also known which comprises a grid in the form of a wire mesh, the mesh being covered with insulating material and the holes in the covered mesh being filled with photo-electrically active rivets of the kind described above.

During the operation of transmitting apparatus of this kind an optical image of the object to be transmitted is projected upon the photo-electrically active side of the mosaic screen, the opposite side is scanned with a cathode ray and picture signals are generated in an external circuit associated with the grid.

Diificulties have been experienced in the construction of mosaic screens of this kind arising out of the fact that the distance between the centres of adjacent holes in the grid must be very small. In the first place it is difiicult to coat the grid with insulating material without filling up the holes and this dificulty is enhanced by the fact that, since the insulating material has subsequently to withstand being baked in vacuo and also must be a very good insulator, the insulating materials available are very few. Glass or vitreous enamel has been found suitable, but even with these materials it is extremely difiicult to avoid filling the holes. Secondly, if a grid in the form of wire gauze is used, the weaving of the wires gives a rough final surface which is detrimental to the operation of the mosaic screen.

It is an object of the present invention to provide a simple method of producing insulated electrical conductors suitable for use in a mosaic screen of the kind described above.

According to the present invention there is provided a method of coating wire with insulating material wherein a tube of insulating material is drawn whilst surrounding the wire to be coated.

The improved method preferably includes the step of evacuating the tube while it surrounds the wire. The tube may be evacuated after drawing, and it may be redrawn while it is in an evacuated condition.

According to the present invention in a further aspect, a method of coating wire with glass comprises the steps of passing a part of a length of wire through a glass tube, sealing the wire to one end of said tube, heating a part of the tube until it becomes plastic, drawing the tube, evacuating the drawn tube, and reheating and redrawing the evacuated tube.

This method of coating wires with insulating material is clearly not limited in scope by the use to which the coated wire is subsequently put and the grid of wires, formed as hereinafter described, may be used for purposes other than the formation of a mosaic screen.

According to the present invention in another aspect, a grid is formed of crossed wires, the wires being coated with insulating material and being held together by adhesion between the insulating material covering the wires.

Preferably two sets of wires are used, each set consisting of substantially parallel lengths of wire, the wires of one set being disposed transversely to those of the other set and the wires of one set lying wholly on one side of the wires of the other set. Alternatively a small amount of weaving of the wires may be utilized, however, in order to hold the Wires together during the construction of the grid.

In the preferred method of constructing such a grid, the wires required to lie in one direction are placed in grooves in a plate, the wires required to lie in the other direction are placed in grooves in another plate, the two plates are juxtaposed face to face in such a manner that the wires on one plate lie transversely of those on the other plate, and the wires on one plate are secured to those on the other plate by adhesion between the insulating coverings. Thus where the insulation is of glass, the wires may be joined at the intersection by fusion of the glass coverings.

Where the finished grid is required to be as flat as possible, each plate is provided with a second set of grooves disposed transversely of the set in which the wires are placed, the plates, after being loaded with the wires, are so juxtaposed that the empty grooves of each plate register with the loaded grooves of the opposite plate, and the plates are thereafter forced together.

The invention will be further described with reference to the accompanying diagrammatic drawings, in which Fig. 1 is a side elevation of a draw bench,

Fig. 2 is a side elevation of a part of the draw bench of Fig. 1, with an additional element,

Fig. 3 is a sectional elevation of two grooved plates, loaded with insulated wires, and nearly juxtaposed,

Fig. 4 is an elevation of a part of a finished grid, with some of the insulation broken away,

Fig. 5 is a section on the line 5-5 of Fig. 4, and

Fig. 6 is an elevation, to a smaller scale, of a finished grid.

In Fig. 1 there is shown a draw bench I having an endless belt 4 which passes over pulleys 2 and 3, the pulley 3 being driven by a motor 5 through a clutch mechanism housed in the pulley 3 whereby the belt may be started and stopped by the operator, or stopped automatically at a predetermined point, by control means not shown.

On the left hand end of the bench (as viewed in Fig. 1) is fixed a cylindrical glass chamber 6 hav ing a neck portion 7. A cover 3 makes an airtight closure over the base of the chamber '6. A slide tube 9 connects the chamber 6 with a vacuum pump l and a manometer l l is also provided.

The belt 4 is provided with'a stud l8 which can engage loosely with a slot in a wooden lath ll which lies on the belt 4.

The operation of covering a tungsten wire with glass sold under the registered trade-mark Pyrex is asfollows. A spool l2 of the wire, which in this example is 0.052 mm. in diameter, is rotatably mounted in the left hand end of the chamber 6 on a clip ii. A rubber stopper I3, which is so bored as to be able to receive as a tight fit a piece of the glass tubing [4, is pushed into the end of the neck 1. Suitable dimensions for the glass tubing are 5.5 mm. external diameter and about 4 mm. internal diameter. The left hand end of the glass tubing I4 is about cm. from the spool l2, and its right hand end projects some distance from the cork. A piece of stout copper wire is passed down the tube and the tungsten wire 28 is hooked on to it, thus enabling the tungsten wire to be drawn through the tube. The tungsten wire is then sealed into the right hand end of the tube by means of a suitable burner, for instance a Marshall burner 19, having finely adjustable needle valves for regulating the supply of air and gas. The sealed end of the tube M, while still plastic, is fixed to the lath I] by means of a clip [6.

The burner is now shifted along the tube l4 slightly away from the sealed end, and the tube is heated until it becomes soft; an indication of the right temperature may be had from the degree of incandescence of the tungsten wire 28 within the tube.

When the tube is hot enough, the burner is removed, the clutch in the pulley 3 is engaged and the belt 4 carries the lath I! tothe right hand end of the bench, and leaves it in a trough provided for this purpose. The clutch is then automatically released.

As the lath moves to the right, it draws out the glass tube into a fine capillary, having inside it the tungsten wire which has unwound from the spool l2. The diameter of the drawn glass is about 0.2 to 0.4 mm.

Although the wire is completely covered with glass, this covering is loose on the wire, owing to the film of air which adheres to the wire during the drawing process. The diameter of the glass covering is also uneven.

The glass covered wire is now examined and the end is once more sealed if broken. The vacuum pump it is now started to exhaust the chamber 6, and to remove air from inside the glass covering of the wire. The sealed end of the covered wire is fixed to a winding drum 2| mounted on the draw-bench l. The winding drum is rotated by any suitable means such for example as by a handle 22. The wire is moved clear of the belt 4, for example by displacing the drum 2! and the chamber 6 transversely.

When the chamber 5 has been suificiently evacuated as indicated by the manometer l l, the wire is redrawn in a manner illustrated in Fig. 2. A Marshall burner 23 is arranged to heat a nickel through 2 2. The whole is fitted with a handle 25, so that it may be held in the hand and passed along the glass covered wire, thus heating it locally.

As the glass, here denoted by Mia, becomes heated and softened, it collapses on to the tungsten wire owing to the external atmospheric pressure. The winding drum is rotated, and the glass is extended, the thickness of the glass covering decreasing. Tungsten wire unwinds from the spool l2 and advances through the tube Ma to allow for this extension, which usually amounts to between 4 and 6 times the length of the tube Ma after the first drawing. The tension required to draw out the glass covering is governed by its thickness, because a thick wall will extend lengthwise more readily than a thin one, since with a thick tube a'smaller change in molecular arrangement is required for a given extension than in the case of a thin tube.

Hence if the torque on the drum is constant, the drawing will cause a greater reduction of the thick parts than of the thin parts of the covering, I

thus largely removing variations in thickness which arose during the preliminary drawing. The external diameter of the glass is of the order of 0.08 to 0.11 mm. The stresses set up by this extension also assist the collapsing of the glass on to the tungsten wire.

The redrawn wire, denoted by Hlb, most of which is now coiled on the drum is cut at its junction with the parent tube M. The tube I4 is resealed at the right hand end, and pulled out The controls for the clutch mechanism are preferably taken to the left hand end of the bench so that one operator is able to control the whole process.

The covering process may be carried out with various sizes of wire, and various thicknesses of covering may be given by suitably choosing the size of the tube i l and the extent of drawing. It is found however that as the thickness of covering increases, the variations in size of the finished insulated wire become greater.

In coating wires of larger diameter, the step of evacuating the glass tube may be dispensed with if desired.

The whole process of drawing may be carried out in a vacuum, and the stage of redrawing may be dispensed with, when the unevenness of the coating of the wire is not objectionable.

Before the wire prepared in the way described is used for making wire mesh grids it is cut into the lengths needed for the grids, and graded, by means of a dial micrometer, into two sizes: 0.07 to 0.09 mm, and 0.09 to 0.11 mm., larger and smaller sizes being rejected. The diameter of the Wire is taken at 2 cm. intervals. The wire may be inspected under the microscope for faults, and tests may be made on the adhesion of the covering. It is found, however, that faulty adhesion is rare.

In order to form an insulated grid a number of lengths of the graded wire may be arranged side by side on a metal plate. A further set of wires are then laid on top of them at right angles, and another plate is laid on top of the crossed wires. The combined pile is then heated and pressed until the glass insulation is firmly welded at the intersections.

If the insulated grid so formed is to be used in a mosaic screen the coated wires should preferably be pressed together with sufficient force to make the resultant surfaces substantially flat, as this facilitates the subsequent operation of forming photo-electrically active rivets in the holes in the grid.

In an alternative method of forming an insulated grid which will be described with reference to Figs. 3 to 6, the two sets of coated wires are laid respectively in grooves in specially prepared plates 26 and 21 (Fig. 3). Each plate has part-round grooves of the required radius and pitch formed in it. For instance, to form a 10- cm. by 10-cm. grid with C.25-mm. pitch, the plate may be about 14 cm. by 14 cm. large and has one set of 4:00 grooves, such as 33a and 331) (Fig. 3) extended across it in one direction and a second set of 400 grooves, such as 34a and 3% extended across it transversely ofpreferably at right angles tothe first set. The outer 2 cm. at the edges is grooved in one direction only and the corners are plain, that is to say the plate bears grooves which correspond to the position of the wires in the finished grid. The elevation of the finished grid shown in Fig. 6 indicates the position of the grooves on the plates. Such a plate may be prepared from an original wax master which is suitably grooved and plated and. from the master so obtained many duplicate plates may subsequently be made by further plating, the whole process following the technique well known for producing gramophone record matrices. The final plates 26 and 2? may be of thin copper, nickel faced. Before use, location holes are put in the corners of the two plates so that later they may be clamped together with the grooves in one plate exactly registering with the grooves in the other. The glass clad wires are cut into 15 cm. lengths and the 400 north-and-south wires 28a. are laid in the grooves 33a in the plate 26. These wires are held in position by cement placed near the edges of the plate, a suitable cement being wax. The plate 27 is similarly charged with 400 east-andwest wires 2% placed in the grooves 341). The two plates are then juxtaposed face to face with the wires in contact and at right angles, and the plates are located so that the grooves register. The plates are then mounted between thick metal plates which are heated and pressed together so that the glass welds at the intersections. The pressing is preferably continued until the two plates are in contact so that the grid assumes the form shown in Figs. 4 and 5. In this Way there is produced a substantially flat grid, the north-and-south glass surfaces being almost, or quite co-incident with the east-andwest glass surfaces. In order that this result may be obtained, the initial radius of the glass-clad wires must be less than the radius of the grooves, in order to give space to receiver the excess glass pressed out of the intersection points, where the two glass-clad wires will be pressed into the space of one. After cooling the thin mould plates may be stripped off or dissolved away in acid if they refuse to separate from the glass. Since the resultant grid is nearly or quite flat, rivets can be inserted in the holes (by electro-plating for example) without contact being made between adjacent rivets.

When finished, the wires of the grid shown in Fig. 6 may be stripped of insulation at their ends and connected, for instance by welding, to a suitable frame, which may be in the form of a metal plate having a square hole to receive the grid.

We claim:

1. An electrode structure formed of crossed wires covered with insulating material, said wires comprising two sets, each set consisting of substantially parallel lengths of wire, the wires of one set being disposed transversely to those of the other set, the wires of one set lying wholly on one side of the wires of the other set, and said sets being held together by adhesion between the insulating material covering the wires of said sets.

2. An electrode structure formed of crossed Wires forming interstices therebetween, said wires being covered with an isulating covering of glass, and being held together by welding the glass coverings at every intersection.

3. An electrode structure formed of crossed wires coated with an insulating covering of glass, said Wires comprising two sets, each set consisting of substantially parallel lengths of wire, the wires of one set being disposed transversely to those of the other set, the wires of one set lying wholly on one side of the wires of the other set therebetween forming an interstitial arrangement, and said sets being held together by fusion of the glass coverings at every intersection.

4. A method of constructing a mosaic electrode structure of crossed wires covered with insulating material comprising placing the wires required to lie in one direction in grooves in a plate, placing the wires required to lie in the other direction in grooves in another plate, juxtaposing said plates face to face in such a manner that the wires on one plate lie transversely of those on the other plate, and causing the wires on one plate to adhere to the Wires on the other plate by adhesion between their insulated coverings.

5. A method as claimed in claim 4 wherein each of said plates is provided with a second set of grooves disposed transversely of the set in which the wires are placed, and said plates, after being loaded with said wires are so juxtaposed that the empty grooves of each plate register with the loaded grooves of the opposite plate and said plates are thereafter forced together.

6. A method of constructing a mosaic electrode structure of crossed wires covered with an insulating coating of glass, comprising placing the wires required to lie in one direction in grooves in a plate, placing the wires required to lie in the other direction in grooves in another plate, juxtaposing said plates face to face in such a manner that the wires on one plate lie transversely of those on the other plate, heating said plates so that said insulating coatings of glass become plastic, and forcing said plates together,

to cause the coatings of the wires on one plate to become welded to the wires on the other plate at the points where said wires cross.

'7. A method as claimed in claim 6, wherein each of said plates is provided with a second set of grooves disposed transversely of the set in which the wires are placed, and said plates, after being loaded with said wires are so juxtaposed that the empty grooves of each plate register with the loaded grooves of the opposite plate and said plates are thereafter forced together.

8. A method of constructing a mosaic electrode structure of crossed wires covered with insulating material, comprising laying a plurality of insulated wires side by side on a plate, arranging a further plurality of insulated Wires side by side on top of said first mentioned wires, and across them, placing on top of said wires a second plate, rendering plastic the insulated material covering said wires, and pressing said plates together so that said wires become held together to form a grid by adhesion between the insulating material. 10

ALAN DOWER BLUMLEIN. HERBERT EDWARD HOLMAN.

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