CA1172303A - Apparatus for producing a directed flow of a gaseous medium utilizing the electric wind principle - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides an apparatus wherein charged particles are generated by a discharge wire electrode and are collected on a plurality of spaced-apart collector plates at a potential different from that of said discharge wire elec-trode, the improvement which comprises intermediate collector plates not connected to any voltage source and disposed between but insulated from the first-mentioned collector plates and posi-tioned in the electric field between said first collector plates and said discharge wire electrode to have induced on said inter-mediate collector plates potentials between those of said first collector plates and said discharge wire electrode.

Description

The present invention relates to a me-thod of and to an apparatus for producing a directed flow oE a gaseous medium, e.g.
an air stream utilizing the electric wind or static breeze prin-ciple. More particularly the invention relates to an apparatus for the bulk displacement of a gasl especially air, by electric field forces without moving parts.
This application is a divisional application of co~
pending application No. 365,068 filed November 20, 1980.
For the displacement of air at standard, ambient or normal atmospheric pressure, mechanical means are practically exclusively used today although a number of physical phenomena are known to be effective in the displacement of gases.
For example, it is known to pump gases by adsorption processes, by cooling and heating processes, by diffusion pro-cesses and by the use of electric fields or charges in so-called electric wind or static breeze processes or in accordance with the principles thereof (see Bergmann-Schaefer, Vol. 2, pp. 10-474).
At atmospheric pressures, however, these processes have found little utility to date and the concentration has been upon the use of mechanical blowers with impellers or rotors or on the use of water jet pumps.
For higher pressures, both rotary and linearly reci-procating mechanical devices may be used and for reduced pres-sures the concentration is upon the use of adsorption pumps of various constructions, ion-getter pumps, diffusion pumps, cryo-pumps and the like. Electric wind has also been utilized at reduced pressures, generally in the range between so-called pre-vacuums and high vacuums for gas transport as described for example, in German Patent No. 265,534.
In general, the mechanical systems used at atmospheric pressure or above for the displacement of air are noisy and of low energy efficiency while the techniques utilized for the dis-~q ` ! 11 '~ 23 ~ 3 placement of gases at low pressures have seldom been considered effective at atmospheric pxessure or for the high volume dis-plaeement of gases such as air.
: The present invention p:rovides a method of displaci.ng air which allows the utilization o:E the electric wind prineiple or statie breeze prineiple at higher pressures than those with whieh electrie wind systems have hitherto been contemplated, e.g.
normal atmospherie pressures, for t:he effieient displacement of air or other gases.
The present invention also provides an apparatus operating under eleetrie wind prineiples but which can be used for the displacement of gases at atmospheric pressure and has eomparatively high energy effieieney while being free from the disadvantages of earlier gas displaeement systems.
In eopending applieation No. 365,068 there is diselosed and elaimed an apparatus for generating a direeted stream of a gas, eomprising: means forming a duct having an inlet side and an outlet side; at least one discharge wire elec-trode in said duct for producing ions therein; and a counterelec-trode spaced in said duet from said discharge wire electrodetoward said outlet side, said electrode being positioned and energized to establish a potential field between them and generate a glow-discharge at said discharge wire electrode with a discharge current between substantially 10 6 A/cm2 and 10 1 A/cm , said counterelectrode having guide surfaces Eor air traversing said duet having edges parallel to the discharge wire electrode and disposed such that ions are emitted from said diseharge wire electrode over an apex angle of less than 180 whereby an ion drift between said discharge wire electrode and said countereleetrode displaees said gas through said duct.
According to the present invention there is provided an apparatus wherein charged particles are generated by a dis-~ 1 ~2303 charge wire electrode and are collec-ted on a plurality of spaced apart collector plates at a poten-tial different from that of said discharge wire electrode, the improvement which comprises intermediate collector plates not connected to any voltage source and disposed between but insulated from the first-mentioned collector plates and positioned in the electric Eleld between said first collector plates and said discharge wire electrode to have induced on said intermediate collector plates potentials between those of said first collector plates and said discharge wire electrode.~
Thus the present inven-tion is based upon the discovery that electric wind or static breeze can be effective even at atmospheric pressures for energy-efficient gas transport.
According to the invention, ions are formed by an electrode in a high electric field, for example a wire which is operated in the region of a Townsend discharge or glow discharge, the discharge current being preferably between 10 6 A/cm sq. and 10 1 A/cm sq., a flow passage, preferably formed by at least one conductive surface of a counterelectrode, being provided such that the edges of the conductive surfaces are parallel to the discharge electrode wire so that the ions move in a direction of the counterelectrode at least in a sector of an imaginary sphere surrounding the emission point of the discharge electrode with an aperture of generating angle of less than 180.
When the field strength on the wire is dimensioned such that only a Townsend or glow discharge is formed, the anode currents are between 10 6 A/cm sq. and 10 1 A/cm sq.
When minimal noise evolution is required for the air pumping processes and ozone generation is to be minimized or e~cluded, the discharge current is held between 10 5 A/cm sq. and 10 3 ~ ~ ~230 3 A/cm sq.
Surprisingly, under -these condi-tions, the bulk displacement of air at atmospheric pressure can be generated with high efficiency since the ion movement direction coincides with the direction of movement of the air entrained thereby and turbulence is excluded. In mechanical blowers, of course, such turbulence cannot be excluded even at low impeller vel-ocities.
Experimental investigation has shown that an ion blower in accordance with the present invention can displace approximately 1 liter of air per second with an energy input of 1 watt as long as excessively high air velocities and pressure differentials are not required.
According to a feature of the invention, the side turned away from the discharge electrode of the counter-electrode is juxtaposed with an auxiliary electrode which is - held in the region of the potential of the discharge electrodes.
The ions a,re generated by the discharge electrode which is preferably a wire held at high potential and drift toward the counterelectrode which can be grounded and which can ~, consist of one or more air-guide plates whose edges are parallel to the discharge wire while the auxiliary electrode can also comprise a guide plate with edges parallel to the discharge wire.
, The blower effect produced by the ion drift results .,~ from an internal friction of the ions within the gas such ' that the ions entrain a sufficient number of air molecules in an electrically induced movement to establish a bulk flow of air in the direction of the ion drift or movement.
When this process is analyzed it is found that the initial friction is between the ions and the gas mole-cules and thereafter water molecules in the gas are entrained.

According -to a further feature of the invention through one or more nozzles at high potential, a liquid aerosol of unipolar charged droplets (volatile or nonvolatile) can be forrned in the gas which is displaced in the manner described over a stretch or path downstream of the aerosol-generating region.
The geometry of the nozzle and may counterelectrode provided for the formation of the unipolar aerosol is such, in accordance wi-th the invention, -that the aerosol forms or tends to form a cone with an apex angle less than 90 operating in the direction of the displacement stretch, this being assured by control].ing the voltage level at the nozzles or the charging electrode and by grounding an appropriate counterelectrode.
The length of the aerosol path can be at least 20 to 30 mm with a potential difference thereacross of 10 to 30 kv, preferably 15 to 25 kv, for the generation of a compara-tively large flow in the direction of the counterelectrode by entrainment with the aerosol.
When the liquid to he atomized in forming the aerosol is conductive and/or it is desirable on other grounds to apply a high potential to the nozzle or nozzles, it is possible in accordance with the present invention, to operate the nozzle or nozzles at ground potential and to bring the counterelectrode to the high potential.
The power consurnption of an aerosol blower according to the invention will depend upon the energy required for producing a stream of droplets subjected to the high voltage field and hence for producing the requisite number of ions which, by adsorption or other pick-up are collected by each droplet in the formation of the electroaerosol, thereby bringing about complete charging of the droplets and a ~ 1 72303 maximum air entrainment with the electrically displaced droplets of the aerosol.
The invention is applicable to the displacement ln air in filter devices for household use and, in this embodiment of the invention, the apparatus should comprise an ionization part and, downs-tream thereof, a collector part. The ioniza-tion part comprises at least the discharge electrode means, preferably a plurality of discharge electrodes, most advantage-ously in the form of thin wires of a corresponding high poten-tial, and counterelectrodes which in general can be at theground potential and thus define with the discharge electrodes the electric field imparting a drift to the ions formed at the discharge electrodes in the downstream direction.
Depending upon the polarity of the discharge elec-trodes a plurality of electrically conductive mutually parallel plates which can be alternately of high potential and ground potential so that electric fields are established between each pair of plates oE different potentials so that dust particles which are charged can be attracted to one oE the plates of each pair and collected thereon.
The disadvantage of such an air filter apparatus which may result from the fact that the various components require different potentials can be obviated by appropriate selection of a transformer and a rectifier system. For example, the discharge electrodes, generally in the form of wires, require a potential between 12 and 15 kv while the collector plate voltages may be between 3 and 6 kv~ Since the 6 kv of the collector plates are only required when the 12 kv are applied to the ionization zone, a 6 kv transformer may be used in accordance with the present invention together with two diodes connected in a voltage doubling circuit. Thus, a single transformer with a peak-to-peak voltage of 6 kv can suffice ! I ~ 2 3 0 3 to supply bo-th the ionization potential and the collector potentials. Of course, collector plates which are at a voltage of 6 kv require effective electrical insulation and must be spaced apart by relatively large distances to prevent electri-cal break-down. This can increase the cost of the unit and the space required.
It has been found that separate ionization of the collectable electrodes in an air filter according to the inven-tion may not be necessary, i.e. a separate electrical source or the aforementioned transformer need not be connected to the collectable electrodes, the latter need not be insulated with expensive high voltage insulation and the spacing between the plates need not be as great as the case when the pla-tes are separately energized. More particularly it has been found that the intermediate collector plates can be positioned between and insulated from the other collector plates so that the plates develop appropriate collector charges in the field.
The best results are obtained for the movement of gases, by the generation of positive ions. However, there is a significant disadvantage when positive ions are released by any electrically operated unit and this applies to systems of the present invention and to prior art electrostatic devices which have a tendency to charge the air with positive ions or reduce the level of negatively charged particles. In practice it is found that negatively charged particles tend to be pre-ferentially captured by collector plates.
The emission of positively charged particles and the predominance of positive ions in an air stream has been found to have biological effects upon animal life and people. For e~ample, in addition to causing discomfort, headaches and other disorders and leading to a general reduction in the resistance to colds and microbial infections, similar to those which are ~ ~ 72303 attributed to weather inversions and hot winds and like atmos-pheric lnfluences, the high posi-tive ion con-tact of an air stream has been associated with a level of disquiet and dis-comfort whieh is both annoying and capable of preventing people from being able to concentrate or work properly.
To overcome this disadvantage, the invention provides an apparatus capable of enriching the air with negative ion or eharged particles, i.e. raising the level of such negative charged particles in displaced air above the level characteriz-ing earlier electrostatic devices such as electrostatic filters.
To this end, ground collector plates are providedwith recesses or channels in which are disposed thin wires which are at ground potential but lie parallel to portions of eollector plates overhanging these cutouts and at high positive potential.
This construction appears to give rise to a discharge which increases the concentration of negative particles or ions in a gas stream.
While the apparatus described previously, utilizing unipolar charging of ions and their entrainment of air, for air displacement over mechanical blowers and the like, has signilicant advantages as previously pointed out, it has been found that it is possible to eliminate the high cost of generat-ing the high direet-current potentials by substituting therefor under eertain eonditions a high voltage alternating current.
Thus, according to another aspect of the invention, an apparatus for producing a direct stream of a gaseous medium, e.g. air by electrostatic forces which operate upon gas ions, comprises the eountereleetrode, discharge electrode and pre-ferably also an auxiliary electrode in the manner originallydescribed and means for applying a high-voltage alternating current to those electrodes which are spaced apart in the flow 1 ~7~303 directlon. The flow velocity of -the gas or driEt velocity of the ions in the gas at a given alternatin~-current frequency and the distance between charging counter electrodes for a given frequency are established in a predetermined relationship~
More particularly, the smaller of these velocities is greater or equal to a coefficient times the product of the frequency and spacing.
The apparatus of the present invention can thus be operated with a high voltage alternating current and ions of either charge can be utilized to generate the blower effect mentioned previously.
In a preferred embodiment of the invention, a plural-ity of ion blower stages are provided in a succession in a flow passage without spacing of the electrodes and establish-ment of the velocities s-tepped so as to provide a resonance effect to increase blower output or produce a greater blower pressure differential.
The features and advantages of the present invention will become more readily apparent from the following descrip-tion, reference to be made to the accompanying drawings inwhich:-Fig. 1 is a diagrammatic axial section illustratingan electric wind apparatus for displacing air in accordance with the principles of this invention;
Fig. lA is a diagrammatic section along the line lA-lA of Fig. l;
Fig. lB is a section corresponding to Fig. lA but illustrating another embodiment;
Fig. lC is a diagrammatic section taken along line lC-lC of Fig. l;
Fig. 1~ is a diagram illustrating a feature of the invention as applied to a wire ionizing electrode and a counter-g electrode consisting of a group of electrode wires;
Fig. 2 is a diagrammatic view similar to Fiy. 1illustrating a series arrangemen-t of discharge electrode and counterelectrode whereby the output of the device can be in-creased utilizing a plurality of stages;
Fig. 3 is a similar view of another embodiment of the invention using a plurality of stages in series;
Fig. 4 is a fragmentary perspective view, partly broken away, showing an embodiment utilizing discharge stages in a cascade and adjacent one another;
Fig. 5 is a diagram illustrating a circuit arrange-ment in accordance with an embodiment of the invention;
Fig. 6 is a detailed view of a portion of the system of Fig. 5 but showing electric field lines;
Fig. 7 is a perspective view of a portion of an air filter embodying the invention;
Fig. B is a diagram of an embodiment of the inven-tion illustrating concentrations of positive and negative ions with reference to the timing of variations in the field strength in a series of stages; and Fig. 9 is a diagram illustrating the electrode wire arrangements for this embodiment.
Referring first to Fig. lD, it can be seen that within a conduit represented at 103 open at one end 103a to a supply of air and open at the opposite end 103b to a region to be supplied with air, there is provided a discharge wire electrode 102 which ls maintained at glow discharge potential, i.e. a Townsend discharge potential, by a high voltage direct current power supply 112.
Downstream`of the discharge wire in the direction of arrow 105, there is provided a counterelectrode means 104 which, in this ernbodiment, comprises two planar arrays of wires 1 ~72303 104a at ground poten-tial. Each point along the discharge wire 102 thus functions as an emission point for ions which at least in a sector 102b of the imaginary sphere 102a around this point define an angular apert:ure ~ for the ions which is less than 180 and preferably even less than 90, i.e. substan-tially all of the ions produced by the field between the elec-- trode 102 and the counterelec-trode means 104 is concentrated in the region defined between the limbs 102c and 102d, defining ~`~ the angle ~. The electrode ~-is operated such that the dis-charge current is between 10 5A/cm2 and 10 A/cm2 while the edges of the counterelectrode means 104, represented by the generatrices 104b are parallel to the generatrices or edges 102e of the wire 102. This ensures that the ions are emitted and moved in a direction or at least a sector having the aper-ture angle less than 1~0 previously mentioned.
Naturally, other means can be provided with an equi-valent purposeO
For example in Fig. 1 the ions are generated within the guide tube 3 for the gaseous stream by a radioactive alpha particle emitter, e.g. tritium, a composition of which is applied as repre-sented at 1 on the discharge electrode 2. The unipolarity is assured by the discharge electrode 2 which can be a pair of plates as sh~n in Fig.
lA or a tube 2a as indicated in Fig. lB.
The plate pair 2 is spaced from a preferably ground ring- or plate-shaped counterelectrode 4 defining a directed field with field lines which generally extend in the direction of arrow 5 and along which the unipolar gas ions drift and entrain neutral gas molecules by reason of molecular impact also in the direction of this arrow so that, macroscopically, a bulk movement of gas occurs in the field direction.
As can be seen in Fig. lD, moreover, the diameter d of the electrode 102 is less than the diameter D of the ~ 1 ~2303 electrodes 104a/ the latter electrodes being cooled, e.g. b~
the circulation of liquid through them so as to have a lower temperature than that of the electrode 102.
In Fig. 2 the ions are generated by point discharge electrodes 1' at a high potential such tha-t only ions of the same polarity are injected into the gas stream. The electrode points 1' are so formed that the field lines extend generally from left to right in the direction of the annular counter-electrodes 2', thereby causing a displacement of air within the insulated tube 3 in the direction of arrow 4'. In the embodi-ment shown two stages A and B are provided in succession or cascade to increase the pressure differential which can be generated between the inlet and outlet side of the tube 3 by the electric wind.
In the embodiment of Fig. 3 a similar arrangement has been shown in which the discharge wires 1" cooperate with auxiliary electrode 2" of the same voltage and potential to ensure that the field lines ex-tend to the two pairs of ground plates 3" associated with the electrode 1' of each stage, two stages A' and B' being provided in succession. The auxil-iary electrodes 2" of each stage serve to collect oppositely charged ions which might migrate in the direction opposite the ion drift direction 5' within the tube 4".
The embodiments of Figs. 1 through 3 and lD can be used as room ventilators and can be provided as the charging stages of air filters which can have collectors as subsequently described formed downstream of the charging stages.
Fig. 4 illustrates diagrammatically a practical em-bodiment of the ion-drift blower utilizing the principles of the blow of Fig. 3.
In this embodiment the ion-drift paths are disposed in a pluraltiy of superposed planes.

~ 172303 The grounded counterelectrodes 1''' and the auxiliary electrodes 2''' can be stamped from sheet metal and discharge electrodes can be in the form of wires which are stretehed across the paths and are held with spot welds 7 to the auxiliary electrodes.
The assembly thus comprises the successive layers 3''', 4''', 5''', 8 disposed one below the other.
A terminal or final electrode 6 along each path con-trols the quantity of residual ions which can emerge from the blower in the direction of the arrow.
An insulating partition 9 enables the assembly to be subdivided into two blowers for displacing air with ions of different polarities, thereby precluding static charging of the pumped gas and fault currents in~the supply network.
Fig. 5 shows, in highly diagrammatic form, a section in whieh the ion-produeing discharge wires ~1- are connected to a high voltage source 12 which can be, for example, at a po-tential of 12 kV. The downwardly flowing air stream passes between the grounded plates 13 which are insulated from col-lector plates 1~ which pick up the aerosol, dust particles andexcess ions and which are at an induced potential of about 3 kV, the eollector plates 14 alternating with the grounded counter-electrode plates'l3.
The field relationship is better seen in Fig. 6 which represents the field lines in broken line form running from the discharge wire 11 to the grounded plates 13 and intercepted by the insulated plates 14 which are not directly connected to the supply circuitry. The latter term is used to describe not only the supply 12 but also ground and any direct electri-eal conn~ction with the supply 12 or the ground.

Since the end 15 of the plate 14 projects beyond theeorresponding edges of the plates 13 into the field lines, a 1 1 7~30 3 lower potential is induced on plate 14 than appears at dis-charge electrode 11, although the potential on plate 14 is higher than that which appears on plates 13, i.e. ground poten-tial.
Thus any charged particle between the plates 13 and 14 will be a-ttracted, depending upon its polarity, to one of these plates or the other.
Since the potential on plates 14 will depend upon the potential at the point 15 in the electric field, by selec-tion of the spacing of the plate 14 from the wire 11, the platevoltage can be readily selected to lie practically anywhere within the range of 2 kV to 10 kV.
When the system of Figs. 5 and 6 is utilized as an electrostatic air filter, it can also function as a wet filter into which a liquid aerosol is fed. For example, an array of nozzles 201-203 can be provided upstream of the electrodes 11 and can be connected to the same potential as these electrodes and, via a pipe 204 to a source of liquid, e.g. water, at high pressure to generate atomized sprays as represented by the broken lines 205 in Fig. 5, the spray cone having an apex angle ~ which is preferably less than 90. The length of the path L between the nozzles 201 and the counterelectrodes 13 should be at least 20 to 30 mm and the potential difference between the nozzles 201-203 and the plates 13 should be between 10 and 30 kV, preferably 15 to 25 kV. The air flow duct is here represented at 206 and has an inlet side 207 and an outlet side 208.
-The filter of Fig. 5 thus can operate as a wet electrofilter in which the ions are picked up by the aerosol drople-ts and electrically charge these droplets so that the bulk movement of air in the direction of arrow 208a through the filter is improved.

; - 14 -! 172303 When the plates 14 are to operate at a po-tential of about 2 kV or below, additional vol-tage stabilization can be obtained by providing a spark gap 210 be-tween the plate 14 and ground, e.g. by connecting threaded conductive member 211 to ground and adjusting a conductive screw 212.
The principles of the invention are also applicable to potential symmetry or mirroring, thereby enabling the dis-charge electrodes to be at ground potential while the field is generated by maintaining the separator p]ates which form or contain the counterelectrodes at the high potential.
To provide the desired high field strength at the discharge electrodes, the latter should be of relatively small area confronting the counterelectrodes, i e. in the form of wire, points or the li~e. In this case, the ions are of the opposite polarity and the intermediate plates 14 should have potentials adjusted accordingly to permit the above-described collection operation to be effective.
Fig. 7 shows a system in which the grounded collector plates 21 are formed with cutouts 21' spanned by wires 22 which are spot-welded at 23 to the plates and thus are also at ground potential. The wires 22 thus lie parallel to and are juxta-posed with -the collector plates 24 which are at high potential.
The electric fields around the wires 22 are the same whether the wires are of negative high potential and juxtaposed with grounded plates or are grounded and juxtaposed with plates of high positive potential. In both cases, the wires 22 gen-erate negative ions at the downstream sides of the collector plates to introduce such negative ions into the air stream entering the room. Preferably, the wires 22 are juxtaposed with the edges of the plates 24, i.e. the plates 24 terminate at the same level as the wires 22. In Fig. 7, the ionization part of the apparatus has not been illustrated.

- ~ ~7~303 The quantity of negative ions thus introduced into the air entering the room can be controlled either by varying the wire diameter or the width of the cutout7 or both. It is also possible to control the proportion of ions introduced into the air stream by varying the position of the edge 2~' of each collector plate 24 with respect to the wires. Preferably the wires are thin, high-potential wires.
Fig. 8 shows another embodiment of the invention in highly diagramrnatic form, the ion path being provided with alternating thick wires 25 and thin wires 26 spaced apart by distance a in the flow direction.
Electrically and in pairs, each thick and thin wire is provided at the same potential tFig. 9) being connected to the same side of a high voltage alternating current source 32.
In this case, the counterelectrode 27 for preceding stage in the direction of arrows 50 forms the auxiliary electrode of the subsequent stage, a series of stages being represented at X, Y and Z.
In this embodiment, the ions move only in the direc-tion indicated by the arrows 50 and thus entrain air therewithin the manner previously described. For each stage, the thin wire 26 constitutes the discharge electrode generating the ions while the preceding thick electrode 25 or 27 constitutes the auxiliary electrode while the succeeding thick electrode 27 constitutes the counterelectrode of opposite polarity.
The discharge electrodes 26 alternate in polarity and thus alternatingly generate positive and negative ions. The line 28, having the configuration of a sine curve, represents somewhat diagrammatically the concentrations of positive ions (above the axis 29) and the concentration of negative ions (below the axis 29) after long-term operation, the wave moving to the right as represented at 30 with generation of new ions ~ 1 72~ 3 at each polarity reversal of the source.
Fig. 9 shows that a blower of the type illustrated inFig. 8 can comprise a duct 51 having an inlet 52 and an outlet 53 and, within this duct, a plurality of paths 54, 55, 56, etc.
formed by the array 57 of wires, each path being constituted.
In pro~ection upon the flow direction 31, all of the wires have the same spacing a. When a is given in centimeters, the alternating current frequency f in Hertz, the flow velocity ; of the gas will be V = 4 af.
When the drift velocity of the ions in the gas dif-fers from the flow velocity thereof sharply in the same co-ordinate system V naturally will represent the drift velocity.
This is not the same for all ions, since the ions are picked up by various particles, they can be hydrated to different degrees or can be absorbed by dust particles. As a consequence, the velocity V will represent a mean~ drift velocity.

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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In an apparatus wherein charged particles are genera-ted by a discharge wire electrode and are collected on a plurality of spaced-apart collector plates at a potential different from that of said discharge wire electrode, the improvement which comprises intermediate collector plates not connected to any voltage source and disposed between but insulated from the first-mentioned collec-tor plates and positioned in the electric field between said first collector plates and said discharge wire electrode to have induced on said intermediate collector plates potentials between those of said first collector plates and said discharge wire electrode.

2. An apparatus as defined in claim 1, including nozzle means for generating an aerosol of a vaporizable or non-volatile liquid to form said particles.

3. The apparatus as defined in claim 1, including means forming a spark gap between the intermediate plates and ground for stabilizing the potential on said intermediate plates.

4. The apparatus as defined in claim 1, wherein said discharge electrode is at ground potential and said first collector plates are at a high potential.

5. The apparatus as defined in claim 4, wherein said intermediate plates are spaced from said discharge wire electrode by a smaller distance than said first collector plates.

6. The apparatus as defined in claim 5, further compris-ing stabilizing means forming a spark gap between said intermediate collector plates and ground.