DE2748322A1 - CATHODE SYSTEM - Google Patents

Description

RCA 68973 / 68973A

U.S. Ser. Nos. 737,098 / 784,365

from 10/29/76 and 4/4/77

RCA Corporation, New York, N.Y. (V.St.A.)

Cathode system.

Known cathode systems comprise in their simplest form a cathode, i.e. an electron source, and several electrodes spaced from the cathode. The electrodes are with suitable electrical potentials applied to control the flow of electrons emitted by the cathode. A hot cathode requires an additional structure for heating it to a temperature that induces electron emission sufficient.

A common cathode system is described, for example, in U.S. Patent 3,772. This cathode system comprises three discrete ones Cathodes and is widely used for color picture tubes. Despite its widespread use, it has several Disadvantages »One problem is that the control grid, i.e. the first grid in front of the cathodes, is carefully must be aligned with each individual cathode. However, since this control grid, normally referred to as grid plate G1 If it has a freestanding structure, aligning it is a difficult job. The alignment is made even more by it makes it difficult that the three cathodes belonging to the beam system themselves do not necessarily lie in a single plane. In addition, the heat developed by the cathodes can be so great that it causes a slight displacement of the grid G1 in relation to

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caused by one or more cathodes or opposite other grids and thereby the correct alignment influenced.

The disadvantages of conventional cathode systems become even more serious when a long cathode source is used. So the problems associated with alignment and heat generation are exacerbated in the case of a linear one Electron source, which extends over a distance greater than that of the three cathodes, with increasing Length of the linear source. In itself, such a linear source would be a cathode in a large, flat display device based on cathode luminescence is particularly well suited. With such a system the linear source would have to emit electrons selectively along its length. In other words, the linear cathode source would have to have the function of several discrete sources, the sources each having a small section would correspond to the linear source in the longitudinal direction. The electrons emitted by the source would become one Phosphor screen passed to make a display.

According to the invention comprises a cathode system, in particular a Cathode system with a linear cathode or a line cathode, an insulating substrate which has several discrete electrode pads or elements on one surface. On one side of the surface is a hot cathode arranged. The cathode extends over one surface of each electrode pad so that separate parts of the Cathode are each assigned to a different electrode pad. At a distance from the cathode and the electrode pads a device for sucking off the electron emissions from the cathode is arranged.

In the following the invention with further advantageous details based on schematically shown embodiments

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examples explained in more detail. In the drawing show:

Figure 1 - a plan view of a cathode system according to the invention,

Figure 2 - a section along the line 2-2 in Figure 1,

FIG. 3 is an isometric view, partially broken away, of the cathode system according to FIG 1 and 2,

FIGS. 4 and 5 - schematic views of the electrical potential profile in the cathode system according to FIG the invention during on and off operation,

FIG. 6 is an isometric view, partially broken away, of a modification of the cathode system according to Figures 1 to 3,

Figure 7 - a cross section along the line 7-7 in Figure 6,

FIG. 8 - a schematic representation of the electron concentration brought about by the cathode system according to FIGS. 6 and 7 or collimation,

Figures 9 and 10 are isometric views of parts of others Modifications of the cathode system according to the invention,

FIGS. 11 and 12 - top views of modifications of the perforated electrode in the cathode system according to the invention,

FIG. 13 shows a cross-section according to FIG. 2 through a different embodiment of the cathode system according to the invention,

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Figure 14 - a cross section through an indirectly heated cathode, which is used for the cathode system according to the invention is suitable,

FIG. 15 is an isometric view, partially broken away, of a further modification of the FIG Cathode system according to Figures 1 to 3,

Figure 16 - a cross section along the line 16-16 in Figure 15,

FIGS. 17 and 18 - cross-sections made in accordance with FIG. 16, respectively to illustrate typical electrical connections required for the cathode system according to Figures 15 and 16 are suitable.

Figures i to 3 show a first embodiment of a inventive cathode system 10. The cathode system 10 comprises an electrically insulating substrate 12, for example made of quartz, having a cavity 14 therein. The surface at the bottom of the cavity 14 is provided with a plurality of discrete electrode pads 16. Preferably, each electrode pad 16 has a surface 16a, which lies in the same plane as the surfaces 16a of the other electrode pads 16. Any electrode pad 16 can have or be a tantalum coating. The thickness of the tantalum coating is not critical; Typical values are between 0.3 and 0.5 μm.

A filament 18 serving as a cathode is stretched in the cavity 14 and extends over the surfaces 16a of the electrode pads 16 extends in such a way that separate, longitudinally directed parts of the filament 18 are assigned to different electrode pads 16 are. The filament is typically between 1cm and about 1m in length. It can be directly heated Be a filament, e.g. a tungsten body that

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was cataphoretically coated with an emission carbonate. A suitable emission carbonate comprises, for example, approximately 13% CaCO ₃ , 31% SrCO ₃ and 56% BaCO ₃ . The diameter of the filament 18, including the emission coating, is approximately 0.25 mm. The filament 18 is kept in the cavity 14 in place, that it is set at both ends by means of springs 20 under tension _o The springs 20 may be _0. 25 having a diameter of 100 to come out of the Haynes alloy No.. The springs 20 can be set to a tensile force of 3.7 Newtons, as a result of which a tensile stress of 470 Newtons «mm is generated in the filament.

An electrode 22 with an opening 24 therein is spaced apart arranged from the cathode 18, wherein the cathode 18 lies between the electrode pad 16 and the perforated electrode 22. the The opening or the hole 24 has the shape of a single slot. The electrode 22 can be made of any material with good electrical conductivity exist that can be easily edited. For example, a perforated electrode 22 can consist of beryllium copper, that is plated with nickel. In relative terms, the ratio of the distance between the perforated electrode is 22 and the cathode 18 to the distance between the cathode 18 and the electrode pads 16, typically at least 10: 1. For example, in one embodiment, the distance between the cathode 18 and the electrode pads 16 is 100 + 25 .mu.m and between the cathode 18 and the hole electrode 2500 + 25 .mu.m.

During the operation of the cathode system 10, the cathode 18 is at an elevated temperature of 760 ° C., for example held so that electron emission occurs. In order to achieve passage of electrons through the opening 24, the cathode 18 and the electrode pads 16 are held at ground potential,

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the following is defined as 0 volts, while the Hole electrode 22 held at a DC potential in the range of + 10 volts to about + 100 volts depending on the respective dimensions and the desired maximum emission level. Under these conditions, current flows through the hole electrode 22 through the entire length of the cathode. This is considered to be the "on" state of the cathode. The passage of electrons through the perforated electrode 22 can be done simply by changing the electrical potential on one or more electrode pads 16 can be changed, i.e. by setting the electrode pad 16 to a negative position with respect to the cathode 18 Brings potential. For example, if the perforated electrode 22 is at + 100 volts DC voltage, a DC voltage leads of about -90 volts on an electrode pad to the effect that the electrons emitted from the cathode 18 to be caught there. This is called the "off" state View cathode. In general, the reverse voltage has the same magnitude as that on the perforated electrode 22 applied voltage. The effect of the electrode pads in controlling the cathode is such that the Electrode pads 16 can also be referred to as control pads 16.

In the cathode system 10, each electrode pad 16 has a fixed position relative to the cathode 18 so that the control discussed above can be achieved by appropriately applying the desired electrical potential to one or more electrode pads. Correspondingly, the effect of the cathode, namely the continuous filament, is made into a plurality of small cathodes, each of which is influenced by an individual, associated electrode or control pad. The control cushions can be set photo-lithographically and are on a thermally stable insulating

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lations substrate applied so that their alignment with respect to the overall cathode and / or the opening exactly and can be easily reached »As a result, the control cushions are also with each one belonging to the cathode small cathode aligned. It is essential for this cathode system that the control pads on the back the cathode are arranged, represent the equivalent of the usual control grid, daä typically on the Front of the cathode is arranged. In other words, affect the control grid of a conventional electron beam system and the control pads of the cathode system according to the invention both control the flow of electrons from the cathode. On the other hand, however, there is an essential difference; In the cathode system according to Figures 1 to 3, the alignment difficulties are which are unavoidable in conventional cathode systems, largely eliminated.

It should also be noted here that the tax effect discussed above is quite unexpected. When according to the invention The cathode system is namely arranged behind the cathode by increasing the negative potential Electrode decreases the hole current. As such, one would expect the result that by increasing the negative potential at an electrode arranged behind the cathode, the hole or opening current would be increased, namely due to the fact that the electrons emitted from the cathode are repelled by the negative potential of this electrode will.

Nevertheless, the unexpected control behavior of the cathode system be explained according to the invention with reference to the representation of the electrical potential curve. The electric Potential curves are shown in simplified form in FIG. 4 and on the basis of a rubber model. The use

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Generation of a rubber model to illustrate potential curves is described in "Electron Optics and the Electron Microscope", Zworykin et al., John Wiley, New York 1945, Pages 418-442 explained.

It can be seen from FIG. 4 that in the "on" state, in which the cathode and control pad are at ground voltage (0 volts) are located, the electrical potential curve is such that electrons emitted by the cathode from the Hole electrode are attracted and pass through it. On the other hand, it can be seen from FIG. that in the "off" state, in which the control pad is sufficiently negative with respect to the cathode (-5 volts =) / the electrical The potential curve is such that the electrons emitted at the cathode are enclosed in a potential well are. The potential well has such a size that electrons emitted at the cathode leave the Cathode are practically hindered »

It should be pointed out that there is a condition which must be met in order to achieve the "off" state as explained necessary is. This necessary condition is that the cathode, with the potential 0, is sufficiently close must be arranged on the negatively biased control cushion, so that the space in which it is arranged is, would be negative in the absence of a cathode. If this condition is met, the negatively pre-stressed will be generated Control pad a potential minimum in the local area of the cathode at ground potential, i.e. at 0 volts = held cathode. So as long as this condition is met, the distances between the elements and the stresses can be freely can be varied without this standing in the way of maintaining the explained "on" and "off" states.

In addition to the basic on and off states the cathode system according to the invention also a mo-

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dulation. For example, pulse width control provides a convenient means of changing the amount of electrical charge which goes through the hole electrode »In the case of pulse width control, the" on "time of the cathode varies according to the desired change in charge. In this form of charge control, magnification is used the "on" time increases the amount of charge that passes through the hole electrode. On the other hand, by downsizing the "on" time of the cathode decreases the charge passing through the hole electrode. It is noteworthy that that the emission in the "on" state is fairly uniform because of the uniform cathode-opening spacing and the space charge effects "

A modification of the cathode system explained above is shown in FIGS. 6 and 7. The cathode system is essentially the same as the cathode system discussed above, except that there are two spaced parallel ones Has filter plate 26. The filter plates 26 are arranged on the walls of the cavity 14 and have surfaces 26a, which are oriented orthogonally to the surfaces 16a of the control pads 16 and parallel to the longitudinal axis of the cathode 18 are. With a suitable design, the filter plates can consist of the same material as the control cushions 16. During operation of the cathode system according to FIGS. 6 and 7, the filter plates 26 can be at a low positive potential of, for example, + 5 volts = compared to the ground potential (0 volts) of the cathode 18 are kept «If this Operating parameters serve the filter plates 26 to remove unbundled electrons (e-) from the perforated electrode 22, As shown schematically in FIG. 8, "Alternatively, the filter plates can also be operated with a negative potential thereby improving the focusing of the beam passing through the hole electrode. The voltage the filter plates 26 can be adjusted so that a certain focusing or bundling of the exiting beam

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is achieved. This possibility of influencing is for adaptation of the cathode system to the system that is used to guide the electrons to the screen.

In the cathode system according to FIGS. 6 and 7, the high cathode temperatures require activation and operation of 1100 ° C and 760 ° C a careful selection of the material for the filter plates and the substrate. Besides, it's over Reasons explained below to improve the performance of the cathode desirable that the filter plates have a low sheet electrical resistivity, e.g., about 0.1 ohm / square unit. Though how previously stated. Tantalum can be used for the control pads, leads the use of this material for the filter plates because of its relatively high specific resistance, not necessarily the best mode of operation of the cathode »A filter plate material, which has the desired low specific resistance and the desired thermal compatibility, includes a 0.25 µm thick tantalum layer that acts as a buffer layer is used and on which conductive material is deposited comprising 95% molybdenum and 5% steatite. The conductive material is heated at 1300 ° C in a water-saturated, Fired 10% forming gas atmosphere. The resulting conductive layer has a sheet resistance of about 0.1 ohm / square unit.

In order to maintain the ability to control the cathode by means of the individual discrete control cushions, be ensured in the cathode system according to the invention that no deposits from the emitting cathode electrically conductive lines between adjacent control cushions form. Such deposits caused by evaporation are generally formed during the operation of oxide cathodes. One measure to prevent the formation of conductive paths is that grooves 28 in the substrate surfaces between adjacent control pads can be provided, as shown in

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Figure 9 is shown. These grooves 28 represent irregularities between adjacent electrode pads 16. Grooves 28 with walls 28a that are perpendicular to the surfaces 16a of the electrode pads 16 are preferred because such Grooves are a particularly large obstacle to the formation of conductive paths between adjacent control pads. In general, grooves 28 having a depth in the range of about 0.13 mm to about 0.25 mm have been found to be satisfactory proven.

Occasionally, there is an undesirable interaction between neighboring electron sub-beams, which is caused by the Control pads are formed along the cathode. This undesirable interaction is due to the fact that the electrical Potential of a control cushion affects the area of an adjacent control cushion. The unwanted interaction can be reduced, inter alia, by arranging insulation electrodes 30 between the control cushions 16, as shown in FIG. The control cushions 16 are expediently withdrawn with respect to the insulation electrodes 30 or arranged deeper to the formation of electrical short circuits between the insulation electrodes 30 and the Control pad 16 to prevent. Because of the withdrawal, the isolation pads 30 are closer to the cathode (not shown) than the control cushions.

When operating the arrangement according to Figure 10, the isolation electrodes 30 biased negatively with respect to the cathode, e.g. to -30 volts =, whereby negative potential blocking zones along the cathode to be erected. This negative potential is superimposed with the potential that surrounds the cathode, so that the net potential is transformed into alternating segments of higher and lower field strength in the longitudinal direction of the cathode will. In this way, the control pads 16 are effective from each other through zones of approximately constant field strength insulated created by the isolation electrodes 30

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will. The alternating segments of negative potential in the longitudinal direction of the cathode are used to decouple the neighboring electron beams that are sucked off by the cathode.

Although the perforated electrode 22 has been shown as having a single slot 24 therethrough, there are also modifications possible here. In any modification, however, it is necessary that the hole electrode is capable of the suitable course of the positive potential relative to the cathode to provide, so that electron suction can be done. For example, the perforated electrode 22 can have several openings practically arranged on a line 24 as shown in FIG. Alternatively, the perforated electrode can also be formed by a row of wires, as Figure 12 shows. The perforated electrode can therefore be in the form of any electrically conductive material, which has openings through which electrons can pass.

Although the cathode systems discussed above have control cushions, which are arranged on the side of the cathode facing away from the perforated electrode (FIGS. 1 to 3, 6 and 7) Variations are also possible here. With any modification, however, it is necessary that the relative distances and orientations of the elements are such that the control pads can generate a potential well in the vicinity of the cathode. For example, in the cathode system according to FIG. 13, the control pads 16 are on one side of the cathode 18, however arranged closer to the perforated electrode 22.

In order to have a practically uniform current in the direction of the longitudinal extension of the cathode systems explained above

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away

To achieve cathode, it is necessary that the potential difference between cathode and hole electrode in the longitudinal direction the cathode is also practically uniform. However, as explained above, the cathode by means of a If the current passing through is heated, a potential gradient is created in the longitudinal direction of the cathode. This potential gradient is undesirable because it represents the value of the exit potential changes in the longitudinal direction of the cathode and thus also the size of the leakage current.

The voltage gradient that arises in the longitudinal direction of the cathode can be determined by using the previously explained Filter plates are eliminated as heating elements. This in turn is possible because of the cavity in the substrate the control pad and the perforated electrode is enclosed in such a way that it can be used as an oven with good efficiency works.

Another measure to solve the problem of the stress gradient due to longitudinal heating the cathode is created, consists in the use of an indirectly heated line cathode with a low cathode resistance. A corresponding structure 31 comprises a heating element 32, for example a tungsten wire, which is concentric with a body 34 made of insulating material is coated, as FIG. 14 shows. The body 34 of insulating material is in turn covered with a conductive body 36, which in turn is covered with a layer 38 of emission material is. The conductive body 36 has the function of providing the indirectly heated cathode with a desired electrical potential admit. The voltage gradient arising in the longitudinal direction of the heating element 32 is from the emitting surface the cathode separated by the insulating body 34. Accordingly, it can be assumed that the indirectly heated line cathode 31 how the directly heated cathode works, however

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with the exception that it shows a practically constant tension in its longitudinal direction.

According to the preceding explanation, the cathode systems with electrode pads 16, which are behind the Cathode (Figures 1 to 3) or on one side of the cathode (Figure 13) are arranged, the size of the lock required voltage is a pronounced function of the cathode diameter. In other words, it increases Reverse voltage with increasing cathode diameter. For some applications this strong dependency can be between Cathode diameter and reverse voltage can be inexpedient.

A modified cathode system 110 in which the The dependence between the reverse voltage and the cathode diameter is kept as low as possible Figures 15 and 16. The cathode system 110 can include all elements of the previously explained cathode system according to FIGS. 1 to 3, but different modifications compared to this to have. The most important modification relates to the number and arrangement of the electrode pads 116. In this embodiment, the linear or line cathode 18 is between two groups of electrode pads 116 sandwiched, with the electrode pads 116 of each group respectively are arranged opposite one another. As with the previous embodiments, the line cathode are separate parts associated with the various pairs of opposing electrode pads 116. Each electrode pad 116 has two end portions 116e which extend over the circumference of the Survive cathode 18. Preferably, a conductive back plate 120 is positioned behind the cathode 18 to ensure secure to ensure that the electrical potential in the area behind the cathode is well defined.

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Examples of electrical connections that are suitable for the cathode system according to the invention are shown in the figures 17 and 18. The two opposing electrode pads 116 of each pair are preferably electrical connected with each other; the reasons for this are given below in connection with the description of the mode of operation of the cathode system explained. According to FIG. 17, the cathode system has push-through contact connections 122 which extend through the substrate 12 and each touch an electrode pad 116. The contact terminals 122 comprise sections 122a which lead to a common input terminal 122b. According to Figure 18 show spaced apart and opposed insulating substrates 212 and 214 are two spaced and opposed on which the electrode pads 116 are arranged. A third insulating substrate 216 is perpendicular between substrates 212 and 214 arranged to these. The third substrate 216 has a surface 216a on which a conductive back plate rests 120 is located. In this cathode system, contact terminals 218 are connected to electrode pad sections 116e, which reach behind the cathode 18. When operating the cathode system 110 according to FIGS. 15 and 16, separa te pairs of opposing electrode pads 116 according to the desired cathode output with modulation potentials applied. This is different to a certain extent from the previously explained cathode system to which the individual electrode pads were subjected to modulation potentials. Since the cathode 18 in FIG 15 and 16 is surrounded by the electrode pads 116 are the voltages required for blocking for a comparable cathode diameter are relatively smaller than for the cathode system according to FIGS. 1 to 3. This is advantageous for various reasons. One reason is that the

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reduced dependence of the reverse voltage on the cathode diameter allows the use of cathodes with a larger diameter, which operate with a lower emission current density and therefore for the specific application a comparatively longer service life can be expected. In addition, a larger cathode diameter means one lower cathode resistance and therefore a reduced potential gradient due to the filament voltage at a directly heated cathode.

The cathode system according to FIGS. 16 to 18 can be modified will. For example, electrode cushions can be provided which only protrude beyond one side of the cathode, namely the exit side. You can also contact the previously with the cathode system according to FIGS. 1 to 3, 9 to 12 and the modifications explained. For example the perforated electrode 22 can be varied freely and even omitted as long as other suitable suction means the electron emission from the cathode are provided.

The invention thus provides a cathode system available, are fixedly aligned in the control pad relative to a cathode. The cathode system according to the invention is well suited as an electron source in a large, flat, cathode-luminescent display device.

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Claims (11)

Expectations 1. Cathode system with an insulating substrate, one arranged on one side of a surface of the substrate Incandescent cathode and a device for sucking emitted electrons from the cathode, characterized in that on several discrete electrode pads (16) on the surface are provided that the cathode (18) extends over a surface (16a) of each electrode pad in such a way that that separate parts of the cathode are each assigned to a different electrode pad, and that the suction device (22) is arranged at a distance from the cathode and the electrode pads. 2. Cathode system according to claim 1, characterized in that the cathode is a line cathode (18) that the Suction device is formed by a perforated electrode (22), and that the cathode is between the perforated electrode and the electrode pad (16) is arranged. 3. Cathode system according to claim 2, characterized in that the ratio of the distance between the perforated electrode (22) and the cathode (18) to the distance between the cathode and the electrode pads (16) at least 10: 1. 4. Cathode system according to claim 2 or 3, characterized in that discontinuities between the electrode pads (16) (28) to prevent the formation of electrical 809819/0750 ORIGINAL INSPECTED conductive lines from cathode deposits are provided ο 5. Cathode system according to claim 2 or 4, characterized in that that at least one insulation electrode (30) is arranged between adjacent electrode pads (16) is. 6. Cathode system according to one of claims 2 to 5, characterized characterized in that at least two spaced apart filter plates (26) are provided, each of which at least has a surface (26a) perpendicular to the surfaces (16a) of the electrode pads and parallel to the Longitudinal axis of the line cathode (18) is aligned, and that the filter plates between the cathode and the perforated electrode (22) are arranged in such a way that electrons migrating from the cathode to the hole electrode fill the space between cross the filter plates. 7. Cathode system according to claim 6, characterized in that the filter plates (26) on the insulation substrate (12) are arranged. 8. Cathode system according to claim 7, characterized in that the insulation substrate (12) comprises quartz that the Filter plates (26) have a buffer layer made of tantalum, and that a conductive layer made of molybdenum steatite is arranged on the tantalum buffer layer. 9. Cathode system according to one of claims 1 to 8, characterized characterized in that the substrate (12) has two spaced apart and opposing insulating surfaces each of which has a plurality of discrete electrode pads (116) arranged opposite one another in pairs, 809819/07 5 0 and that the cathode (18) between the opposite one another is arranged lying surfaces and extends perpendicularly over a surface of each electrode pad in such a way that that separate parts of the cathode each have a different pair of opposing electrode pads with at least a portion of each electrode pad extending beyond the perimeter of the cathode enough. 10. Method for generating free electrons by means of a Cathode system in particular according to one of claims 1 to 9, which has an insulating substrate with at least one electrode pad on a surface thereof, furthermore a hot cathode, which is on one side of the surface is arranged and extends over a surface of the electrode pad such that a portion of the cathode is assigned to the electrode pad, and finally a perforated electrode, which is at a distance from the cathode is arranged, characterized in that one to the electrode pad, the cathode and the hole anode electrical Applies potentials, the potential at the electrode pad being the magnitude of the electron flow from the cathode controls through the perforated anode. 11. The method according to claim 10, characterized in that the electrical potential on the electrode pad toggles between two values, at least one of which leads to the formation of a potential well, which the The cathode surrounds or circumscribes in such a way that electrons emitted at the cathode practically leave the cathode are prevented. 12c method according to claim 11, characterized in that that one chooses the other value of the electrical potential on the electrode pad in such a way that from the cathode emitted electrons are caused to leave the cathode and pass through the perforated anode. 809819/0750