[0001] This invention relates to granule or particle cleaning, and more particularly relates
to an apparatus and method for separating shavings, fines, dust and debris from granular
material, pellets and the like or small components or parts which have been or are
susceptible to electro-static charge build-up as a result of exposure to electro-static
fields or by tumbling against one another or by virtue of triboelectric charges produced
on such materials by way of friction.
[0002] The present invention is derived from a process, known as "elutriation" wherein a
volume of random-sized particles or pellets are blown into a vertical column or tube
such that they become fluidized and temporarily suspended therein, the lighter weight
particulate consisting of dust, fines and debris being drawn off by suction or other
means for disposal while the larger size fall by gravity where they are collected
for re-cycling or re-use. In the instant invention, balanced bipolar ionized air is
used to neutralize the particles before they are thrust into the elutriation column
for separation of the electro-statically adhered particles from each other prior to
assortment. Preferably, thereafter the falling granules which are to be re-claimed
are again subjected to a balanced bipolar ionized air static neutralization as they
drop through the lower portion of the elutriation column prior to collection.
[0003] The bipolar air stream acting on the granules prior, during and subsequent to separation
from the entrained fines preferably employs pointed static eliminators which are coupled
to opposite sides of a high voltage AC power supply and including a reference electrode
(adjacently spaced from the points to define an air gap with respect thereto) which
is isolated from ground, whereby the reference electrode floats and provides a bipolar,
dual phase balanced ion emission. The reference electrode preferably comprises the
injection tube or the lower portion of the elutriation column in which the static
eliminators are contained, both being conductive to enable ion emission from the points.
[0004] U. S. Patent No. 4,895,642 (Frei) shows a process for separating particles of non-conductive,
plastic material from comminuted waste by first subjecting the ground waste material
to electro-static charging and then fluidizing the charged particles on an upwardly
directed stream of air. While the charged particles are in a state of suspension,
they are electro-statically attracted to a transverse conveyor whereupon they are
separated from the noncharged material and conveyed away.
[0005] U.S. Patent No. 4,299,693 shows a fluidized bed wherein an inlet conduit injects
fines and product entrained in a propellant gas at the bottom of a housing whereby
the fines are drawn off at the top while the product falls to the bottom. An electro-static
charge neutralizing magnetic field envelops the adulterated product before it impinges
against the impact surface of a baffle.
[0006] In U.S. Patent Nos. 4,631,124 and 5,035,331 to Paulson, dust and impurity-laden particulate
material is fed by gravity through a tortuous path and a linear-kinetic, magnetic
field, energy cell which is said to generate an electric field to neutralize static
charges causing dust to adhere to such particles.
[0007] In U.S. Patent No. 3,475,652 to Levy, there is shown a dual or bi-phase static eliminator
system for neutralizing a stream of powdered material through a conduit. Pairs of
multi-pointed bars having AC high voltage applied to each set of points but at 180°
out-of-phase whereby the output of one of the pair is one-half cycle behind that of
the other. Thus, one of the paired static bars delivers ions of a given polarity toward
the conveyed materials during any portion of the cycle while the other supplies ions
of the other polarity. The static bars of this patent utilize a grounded casing adjacently
spaced from the points, as a result of which an unbalanced predominance of negative
ions is produced because of lower negative corona offset voltages and the greater
mobility of negative ions.
[0008] In U. S. Patent No. 5,008,594 (Swanson et al), there is set forth a self balancing
circuit for convection air ionizers wherein the points of the ion emitter are capacitively
coupled to the AC high voltage power supply while the adjacently spaced collector
plate is similarly connected to ground through a capacitor. It is said that the capacitive
isolation of the external charge sources or sinks maintain a zero average balance
and produce a charge balanced system.
[0009] In U.S. Patent No. 5,055,963 (Partridge), the ionizing points of the pairs of the
dual phase static neutralizers are coupled to the opposite sides of the AC high voltage.
These emitter points and the high voltage power supply are completely isolated from
any direct current path to ground. The electrodes acquire a DC bias that maintains
an equal output of positive and negative ions without need for air ion sensors or
feedback for balancing.
[0010] In our prior U.S. Patent Application Serial No. 07/751,093, filed August 28, 1991,
we show and describe a self-balancing ionizing circuit for static eliminators wherein
pointed electrodes were directly coupled to one side of an AC high voltage transformer
while the other side of the transformer is directly coupled resistively to an ungrounded
conductive band supported within an insulative sheet peripherally spaced from the
discharge electrodes to define a floating reference electrode with respect to the
points. Grounding is only effected by way of an external conductive chassis which
is shielded from the internal ionization system by way of the dielectric of the insulative
casing. It is taught that isolating the reference electrode from ground permits substantial
voltages to be developed on said reference electrode without creating the ionization
imbalance normally produced by adjacent grounded components. Balancing of positive
and negative ion production is shown to be independent of capacitors or other electrical
components, and no mechanical adjustment is required to compensate for changes in
environmental factors or contamination conditions.
[0011] U.S. Patent Nos. 4,046,492 and 4,195,780 show and describe air flow amplifiers for
directing a stream of pressurized fluid through a nozzle.
[0012] The instant invention is particularly useful for recycling plastic components, such
as rejects, or re-use of plastic materials previously contemplated for throw-away.
The reject or throw out plastic components are first ground in a mill to a particular
size, such as granules or pellets whose dimensions range from about 0.125 inch to
0.375 inch (3.1 to 9.3 mm), and then introduced into a charge load or feedstock for
moulding or extrusion. However, these ground materials not only contain entrained
dust, and fine particulate or other debris, but also this fine material as well as
the granules themselves are statically charged as a result of the grinding operation
such that they adhere to each other by electro-static attraction. Foreign particles
and other fines in the mix tends to cause burning, charring and other discolouration
in the re-moulded articles.
[0013] In accordance with an embodiment of the present invention, there is provided means
for injecting granules and fines (termed adulterated or dirty granules) on a stream
of ionized air into an elutriation column to define a fluidized bed wherein the granules
and fine particulate entrained therewith are temporarily maintained in floating suspension.
The injection means includes a pair of opposed dual polarity electro-static eliminators
subjecting the dirty granules to bipolar ions and serving to neutralize the charges
which bind any dirt, debris, dust or particulate material to the granules themselves.
At the same time, a high velocity air jet propels the mixture into the elutriation
column and strips the fine particulate from the granules. Inside the elutriation column,
a suction or vacuum above the floating suspension draws the lighter weight particulate
from the upper portion of the fluidized bed while the heavier weight granules are
separated by gravity and fall through a second set of diametrically opposed dual polarity
static eliminators where they are neutralized of any residual charges and collected
for re-cycling. The static eliminators are pointed electrodes which are adjacently
spaced from a conductive reference electrode to produce localised electrical breakdown
of the air or other gaseous medium brought about by high surface charge density at
the points. The corona produced on each set of needle electrodes on the oppositely
arranged static bars causes the electrodes of the respective bars which are coupled
to the opposite ends of an AC high voltage source to emit ions of each polarity on
successive half cycles of the alternating voltage waveform.
[0014] Dual phase power sources are used to drive the adjacently opposed bars so that positive
and negative ions are produced simultaneously. A centre-tapped-to-ground transformer
power supply has its high voltage terminals connected to the bars of opposing polarity
at any particular time. By avoiding any adjacent ground to the reference electrodes
enables the ion emission to be balanced without independent mechanical or electrical
balancing devices.
[0015] A particular embodiment of a method and apparatus in accordance with this invention
will now be described, with reference to the accompanying drawings, in which:-
Figure 1 is a front elevational view of a granule cleaning apparatus embodying this
invention;
Figure 2 is a sectional view taken generally laterally through Figure 1 on a plane
parallel to the plane of the paper;
Figure 3 is a sectional view taken along lines 3 - 3 of Figure 2.
Figure 4 is a sectional view taken along lines 4 - 4 of Figure 2.
Figure 5 is an electrical schematic diagram of the prior art circuit coupled to single
phase static neutralizer means;
Figure 6 is a schematic diagram of the circuit of the electro-static neutralizer means
of the present invention;
Figure 7 is a schematic diagram of the test circuitry for measuring the reference
electrode voltage with respect to ground of the present invention.
[0016] Referring now in greater detail to the drawings in which similar reference characters
refer to similar parts, there is shown in Figures 1 and 2 an apparatus for cleaning
granules comprising a hollow elutriation column, generally designated as A, into which
adulterated ("dirty") granules 10 are injected in a generally upward direction on
a high velocity air stream from a propulsion tube B whereby both the granules and
fine particulate released therefrom become temporarily suspended and transiently float
within a medial portion of the elutriation tube. A feed hopper C loads B where they
are transported into communication with an air amplifying nozzle B1, (termed a Transvector,
manufactured by Vortec Corporation of Cincinnati, Ohio) located immediately before
a port 4 between the upper portion of the injector tube and the lower portion of the
elutriation column A. Positioned at diametrically opposite zones of the injection
tube B are a pair of elongated static neutralizer bars D1 and D2 which are coupled
to an AC high voltage power source G so as to subject the granules 10 and any electro-statically
adhered particles of dust, dirt and debris whereby they are released from contact
with each other and electro-statically neutralized.
[0017] The elutriation column A comprises a vertically disposed tube 2, preferably of transparent
plastic such as "Lexan" polycarbonate made by General Electric, to permit observation
of the fluidized bed at the medial portion of the tube 2. A port 4 enables the dirty
granules with entrained dirt and dust to be squirted from the injection tube B against
an impact pad 6 in column A and thereafter for flotation within the medial portion
of the tube 2. A U-shaped bend 8 at the upper part of the elutriation column descends
vertically on tubular conduit 12 into communication with a conventional vacuum or
suction device, generally designated as E. The vacuum is sufficient to draw off the
lighter weight particulate material (dust, fines and debris) exhausting from the upper
zone of the suspended fluidized cloud. Air ports 14 and 15 enable the suction or vacuum
from device E to draw off the lighter weight particulate without disturbing the flow
in the elutriation column particulate without disturbing the flow in the elutriation
column A set up by the air amplifying nozzle B1. The heavier cleansed granules fall
to the bottom of the elutriation column A for deposit into a clean granule reservoir
or collector 16.
[0018] Prior to dumping into the collector 16, the cleansed granules 10A fall through a
restricted lower portion 18 of column A where they are again subjected to bipolar
ions emitted by opposed AC static bars F1-F2 mounted in the tube to maintain the clean
granules electro-statically neutral. The lower portion 18 of column A is conductive,
for example, any suitable metal such as steel, to act as a reference electrode with
respect to the discharge electrodes of the static bars F1-F2.
[0019] The injection tube B constitutes a high velocity air nozzle in the form of an inclined
metal tube 20 which draws the dirty granules 10 from hopper C and feeder tube 19 for
propulsion by way of Transvector B1 through orifice 4 into the elutriation column
A. The hopper C is a generally funnel-shaped bowl which empties its contents on demand
into feeder tube 19 and thence into the lower portion of inclined tubular member 20.
A compressed air conduit 22 first draws the dirty granule feed through the lower bend
25 of the feeder tube into the inclined tubular member 20. Ambient air enters tubular
member 20 through openings 27 and inter-mixes with the dirty granules. Then, the transvector
B1 squirts the granules 10 and entrained ambient air into the vertical tube 2.
[0020] As best shown in U.S. Patent No. 4,046,492, the Transvector B is an air flow amplifier
that increases air flow without any moving parts by means of an energy transfer process.
The transfer process induces motion in the free surrounding air within a duct or conduit
by employing a small amount of compressed air as a power source to move large volumes
of air in said duct. An impulse principle of the Transvector accelerates a large mass
of air at relatively low velocity to feed air from compressor conduit 24 into the
injector tube B to carry the entrained dirty granules 10 into position for being acted
upon by the transvector B1. Amplification ratios up to 20 times greater than the compressed
air supply allows the ducted air flow to run about 100 CFM (2.83 cubic m/min).
[0021] Referring to Figure 2, a jet of clean compressed air drawn through injector conduit
24 is forced into annular space 26 within Transvector housing 28 to propel the clean
air passed annular lip 30 and thence through barrel 32 of housing 28. This causes
the low velocity air with its dust entrained granules to be blasted through the Transvector
B1 and against polyurethane pad 6 mounted on the opposite wall of the elutriation
tube 2.
[0022] Referring next to Figures 3 and 4, the electro-static neutralizers D1 and D2 (mounted
in injector tube B) and F1 and F2 (mounted in the lower portion of the elutriation
column A) each comprise a longitudinally extending bar 36 of insulative material,
such as polystyrene or polytetrafluroethylene, Teflon (registered trade mark), having
a flanged portion 38 defining slots 40 which attach to the cylindrical wall of the
injection tube 20 or correspondingly to that of the down tube 18 of the elutriation
column. The conductive metal injector tube 20 and the conductive metal down tube 18
of the elutriation column A act as spaced reference electrodes for the points 46 and
47 of the respective electro-static neutralizers D1-D2 and F1-F2 to enable AC ion
emission from said points.
[0023] A general description of said electro-static neutralizers static eliminators D1,
D2, F1 and F2 is shown in U.S. Patent No. 3,137,806 to Schweriner, except that the
prior devices employ a grounded housing whose flanges are adjacently spaced from the
pointed electrodes thereof. An insulated conductive cable 42 is threaded through a
bore in bar 36 of static neutralizers D1 and F1 while an insulated conductive cable
44 longitudinally extends through the bore of the oppositely disposed static neutralizers
D2 and F2. Longitudinally spaced conductive needles 46 pass through complementary
apertures in the bars 36 of neutralizers D1 and F1 into engagement with the central
conductor 42 and extend through a medial slot 48 therein into the hollow space defined
by the walls 18 or 20. Longitudinally spaced conductive needles 47 pass through complementary
apertures in the elongated bars 36 of neutralizers D2 and F2 into engagement with
the central conductor of cable 44. When the neutralizers D1 and D2 are mounted in
diametrically opposed disposition in the walls of injection tube 20 the needle electrodes
46 and 47 face one another. Similarly when the neutralizers F1 and F2 are mounted
in diametrically opposed disposition within the walls 18, the needle electrodes 47
project through the slot 48 of the corresponding bar 36 into the interior of the down
tube 18 and face the pointed electrodes 46 thereof.
[0024] Referring now to Figure 5, the mode of operation of the prior art electro-static
elimination system embodies a pair of diametrically opposed static neutralizers H1
and H2 oriented on the opposite walls of a metal tube 49 and whose discharge electrodes
46 and 47 are both connected to one side of an AC high voltage power supply G. That
is, the high side of the transformer secondary 51 is connected to common lead 53 branching
off to lines 55 and 57 coupled to the points 46 and 47 of each of the static neutralizers
H1 and H2 while the low side of the transformer secondary is connected to ground.
The metal tube 49 is also coupled to ground. It has been found that a slight excess
of negative ions are produced by this system and a predominant negative charge is
produced on particulate material passing through the tube 49.
[0025] Referring next to Figure 6, the opposed static neutralizers D1-D2 and F1-F2 of the
present invention are coupled to the opposite sides of secondary coil 50 of the AC
high voltage power supply G and the transformer secondary 50 itself is centre tapped
to ground via lead 52. In this hook-up, a positive half cycle of voltage imposed on
the primary 54 of the power supply G will appear as a positive half cycle of voltage
on cable 42 and points 46 while a negative half cycle of voltage is imposed on the
points 47 via cable 44. When a negative half cycle of voltage is applied to emitter
points 46 via cable 42, a positive half cycle of voltage is applied to points 47 by
way of cable conductor 44.
[0026] It is to be observed that the points 46 and 47 of static neutralizers D1-D2 and F1-F2
are not close to or adjacently spaced from ground and tube 18 of the elutriation column
A (and tube 20 of the injector tube B) remain ungrounded. By energizing the points
46 and 47 with reverse polarity dual phase AC high voltage and allowing the tubes
18 and 20 to float, the granular material passing between the bipolar ionizers D1-D2
and F1-F2 can assume a balanced state of ionization leading to a zero charge disposition.
[0027] In Figure 7, there is shown a test set up in which one ionizer, for example F1,is
coupled to voltage V1 of phase A from one side of the secondary coil 50 by way of
cable 42 while the other ionizer F2 arranged in diametrically opposing disposition
is coupled to voltage V2 from phase B of the other side of the secondary coil via
conductor 44. The tube 18 was allowed to float (no ground connection).
[0028] Material in the form of pellets carrying a charge of approximately +2.5 nC/g (nano
Coulombs per gram) was passed through the tube 18 by gravity, for example. The pellets
emerged with a level of charge of approximately 2 pC/g (pico Coulombs per gram). This
is an extremely low residual charge, the charge level having been reduced by a factor
of one thousand from the original charge. It was also observed that the polarity of
the residual charge on repeated tests was not consistent, and the mathematical sum
of the various tests (taking polarity into consideration) approached zero thereby
indicating balanced ionization within the tube 18.
[0029] The AC and DC voltage on the tube 18 was measured using a non-contact electrometer
type field meter 60 adjacently spaced from the tube 18 by way of an sir space of about
1/4 inch distance (i.e. - designated by capacitance 62). The voltage on the wall of
tube 18 with respect to ground provided one input 58 to the field meter 60 and field
meter output was connected to an oscilloscope 64. This provided a very high impedance
arrangement for measuring voltages on the tube 18.
[0030] Using this set-up, a non-sinusoidal AC waveform in the order of approximately 400
V
p-p was observed. The DC offset under the foregoing conditions was approximately -230
V
o. Theoretically, with V1 equal to V2 but 180° out of phase and the voltages on F1
equal to F2, the AC voltage on the tube 18 will be zero (0). This would indicate that
the measured AC voltage of 400 V
p-p on the tube is primarily due to ionization currents. Similarly, the measured DC offset
of 230 V would also be due to ionization currents. However, because we know that the
emerging pellets upon exiting from the dual-phase static neutralization floating-conduit
system are basically neutral, the ionization within said tube is balanced. This leads
to the conclusion that the DC offset on the tube counters the effects of the difference
in ionization offset voltage and difference in ion mobility, thereby producing a balanced
ion emission.
[0031] In contra-distinction when a "hot" ionizer is used, one with the ionizing points
connected directly to the transformer high voltage output and the outer case connected
to ground, for example, as shown in Figure 5, a negative ion imbalance is the result.
This negative ion imbalance is due primarily to the fact that the negative corona
onset voltage is lower than the positive corona onset but also because the difference
in negative ion mobility plays a role in ion imbalance. In the case of the Figure
5 arrangement, when pellets having a charge of approximately +2.5 nC/g (nano Coulombs
per gram) were passed through the grounded metal tube, the charge on the pellets was
reduced to approximately -0.31 nC/g. Thus, the charge level was reduced only by a
factor of about ten and the polarity of the charge reversed to become negative. This
would indicate that where single-phase static neutralizers are employed (i.e. the
same AC high voltage being applied to the emitter points at the same time) and the
metal tub grounded (i.e. adjacent ground), that neutralization would occur but a low
level of residual static charge would be deposited on the pellets due to imbalance
of the air ionizers.
[0032] Suitable air pressure gauges 70 and 72 on the face of the cleaning apparatus enable
the operator to feed air at a pre-determined pressure from compressed air sources
(not shown) to the compressed air conduits 22 and 24 in order to inject the dirty
granules through the injection tube B and into the elutriation column A to provide
a fluidized bed at the desired level. The vacuum applied to the tubular conduit 12
is monitored by vacuum gauge 74 to ensure that adequate vacuum level is maintained
and assure complete collection of the lighter weight particulate.
1. Apparatus for cleaning granules and the like having lighter weight particulate material
entrained therewith, comprising:
(a) a hollow elutriation column;
(b) injector means for squirting dirty granules on a stream of air into said elutriation
column to form a fluidized bed therein at a medial portion thereof;
(c) electro-static ionizer means coupled to said injector means and subjecting the
dirty granules to bipolar ionization before said dirty granules enter said elutriation
column whereby the granules are neutralized and release entrained particulate therefrom;
(d) suction means for drawing off the lighter weight particulate material from the
upper portion of the fluidized bed while allowing the heavier granules to separate
therefrom and fall to the bottom of the elutriation column by gravity; and
(e) means to collect the falling clean granules for re-use.
2. The apparatus of claim 1,wherein said injection means comprises a conductive tubular
member supporting said electro-static ionizer means and constituting a reference electrode
therefor, said electro-static ionizer means comprising a pair of diametrically opposed
static bars having pointed electrodes facing one another within said conductive tubular
member, and a high voltage A.C power supply having a centre-tapped-to-ground secondary
coil whose opposite sides are coupled to the facing pointed electrodes of the diametrically
opposed pair of static bars.
3. The apparatus of claim 2, wherein said reference electrode is isolated from ground
whereby said electrostatic ionizer means electrically floats with respect to ground
in reverse phase disposition to effect balanced ion emission.
4. The apparatus of any preceding claim, including second electro-static ionizer means
positioned in said elutriation column below the fluidized bed.
5. The apparatus of claim 4, wherein the lower portion of the elutriation column tapers
down to an elongated narrow zone in which the second ionizer means are disposed, said
lower portion of said elutriation column being conductive to constitute a reference
electrode for said second ionizer means.
6. The apparatus of any preceding claim, wherein said injection means comprises a hollow
tube inclined with respect to said elutriation column and having a port communicating
with a lower interior portion thereof.
7. The apparatus of any preceding claim, wherein said injection means includes an air
amplifier for transporting the dirty granules on a high speed stream of air into said
elutriation column.
8. The apparatus of any preceding claim, wherein said injection means propels the stream
of dirty granules against an impact pad on a wall of said elutriation column opposite
the port.
9. The apparatus of any preceding claim, including a U-shaped bend at the top of said
elutriation column for impeding the fine particulate drawn off by said suction means
from falling back onto the fluidized bed and re-combining with the heavier granules.
10. The apparatus of claim 9, including an ambient air inlet intermediate said U-shaped
bend and said suction means to facilitate balancing the height of the fluidized bed
in said elutriation column.
11. Method for cleaning granules having lighter weight particulate material entrained
therewith comprising the steps of:
(a) feeding dirty granules on a stream of bipolar ionized air into a vertically disposed
elutriation column to effect neutralization of the granules and the entrained materials
so as to produce multi-level fluidization thereof as a floating suspension within
the elutriation column;
(b) drawing off the lighter weight particulate material at the upper portion of the
floating suspension by suction;
(c) allowing the heavier weight cleaned granules to separate from the floating suspension
by falling by gravity to the bottom of the elutriation column; and
(d) collecting the cleaned granules at the bottom of the elutriation column for re-use.
12. The method of claim 11, wherein the stream of bipolar ionized air is produced by exposing
the dirty granules to dual phase emission from pairs of facing static bars coupled
to opposite sides of a high voltage AC transformer so that the emission from one static
bar is 180° out of phase with that of the other.
13. The method of claim 12, wherein the static bars are disposed within a conductive tube
constituting a reference electrode therefor, said reference electrode being isolated
from ground whereby it floats with respect to ground to produce a balanced ion emission.
14. The method of claim 13, wherein the cleaned granules falling through the elutriation
column are exposed to a second set of opposed static bars coupled to opposite sides
of a high voltage AC transformer, said second set of opposed static bars having a
reference electrode isolated from ground.
15. Apparatus for neutralizing granules comprising:
(a) a conduit;
(b) means for propelling the granules through said conduit;
(c) at least one pair of electro-static ionizers mounted at diametrically opposing
portions of said conduit and having pointed electrodes facing one another within said
conduit, said conduit being conductive and constituting a reference electrode adjacently
spaced from said pointed electrodes;
(d) means for connecting the pointed electrodes of one of said ionizers to one side
of a centre-tapped-to-ground high voltage AC transformer and the pointed electrodes
of the other of said ionizers to the other side of said high voltage AC transformer
whereby a dual phase emission is effected 180° out of phase with each other to expose
the granules to bipolar ion emission.
16. The apparatus of claim 15, wherein the reference electrodes of said ionizers are isolated
from ground so that they float with respect to ground to provide balanced ion emission.
17. Apparatus for neutralizing articles, comprising:
(a) a pair of electro-static ionizers having pointed electrodes facing one another
and a reference electrode adjacently spaced with respect to said pointed electrodes;
(b) a high voltage AC power supply having a transformer including a secondary coil
being centre-tapped-to-ground;
(c) and means for connecting the pointed electrodes of one of the ionizers of the
pair to one side of the secondary coil and the pointed electrodes of the other of
said ionizers of said pair to the other side of the secondary coil whereby a dual
phase emission is provided 180° out of phase with each other to expose any articles
positioned between the pair of ionizers so as to effect balanced bipolar ion emission.
18. The apparatus of claim 17, wherein said reference electrode comprises a conductive
conduit to the opposite portions of which the ionizers are supported with the pointed
electrodes directed toward the interior portion thereof.
19. The apparatus of claim 17, wherein said reference electrode comprises a conductive
tube, and said pair of ionizers are mounted to diametrically opposing portions thereof
with the pointed electrodes directed interiorly.