Abstract:
The present invention&#39;s unique utilization of an electrolytic solution renders more efficient the conduction of electricity between two objects (such as two parts of electrical or electromechanical machinery) in relative motion. According to typical inventive practice, the electrolytic solution is a “strong” electrolytic solution of a “dual-valance” nature, including two compatible solute compounds containing electron donor ions and electron acceptor ions. When inventively implemented at the current collector interface, the electrolytic solution serves to improve current transfer across the interface. During machine operation in which a brush (e.g., fiber brush) slidingly contacts a first machine part while being affixed to a second machine part, the inventive additive represents an auxiliary vehicle for conducting electrical current from one machine part to the other machine part. Among the benefits of inventive practice are reduced conduction power losses, reduced friction power losses, and reduced wear rate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional application No. 60/616,843, filed 8 Oct. 2004, hereby incorporated herein by reference, entitled “Ion Conducting Electrolyte Brush Additives,” joint inventors William A. Lynch and Neal A. Sondergaard. 

   STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment of any royalties thereon or therefor. 

   This application is related to U.S. nonprovisional application Ser. No. 10/863,844, filed 3 Jun. 2004, hereby incorporated herein by reference, entitled “Electrical Current Transferring and Brush Pressure Exerting Interlocking Slip Ring Assembly,” joint inventors William A. Lynch, Wayne Marks, Jr. and Neal A. Sondergaard. 
   This application is related to U.S. nonprovisional application Ser. No. 10/985,074, filed 5 Nov. 2004, hereby incorporated herein by reference, entitled “Solid and Liquid Hybrid Current Transferring Brush,” joint inventors Neal A. Sondergaard and William A. Lynch. 
   This application is related to U.S. nonprovisional application Ser. No. 10/985,075, filed 5 Nov. 2004, hereby incorporated herein by reference, entitled “Folded Foil and Metal Fiber Braid Electrical Current Collector Brush,” joint inventors William A. Lynch, Neal A. Sondergaard and Wayne Marks, Jr. 
   This application is related to U.S. nonprovisional application Ser. No. 11/033,619, filed 13 Jan. 2005, hereby incorporated herein by reference, entitled “Quad Shaft Contrarotating Homopolar Motor,” joint inventors William A. Lynch and Neal A. Sondergaard. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to machinery involving the conduction of electrical current between parts moving relative to each other, more particularly to methods and devices for effecting or facilitating such electrical conduction. 
   Various kinds of motors, generators and other electrical apparatus require the conduction of electricity between two relatively moving parts. Such mechanical arrangements usually involve the conduction of current between a stationary part (stator) and a rotating part (rotor). A device known as a “brush” or “current collector” is normally used for making sliding contact between stationary and rotating parts so as to conduct electrical current therebetween. A conventional current collection assembly includes a brush and a “holder” (for the brush) as two separate components that are attached to each other. The holder is also attached to either the stationary part or the rotating part of the machinery. 
   Depending on the particular machinery, a brush can be used to conduct current in either direction (i.e., either from the stationary part to the rotating part, or vice versa), and can be fixed with respect to either the rotating part or the stationary part. Among the desirable qualities of a brush are high current-carrying capacity (e.g., in terms of capability of carrying a high amount of current per unit area of the interface between the brush and the surface contacted thereby), low resistance, low friction, and high wear resistance. Current collection brush technology has grown in interest with the advent and continued development of homopolar machine technology, particularly in the realm of homopolar motors (which operate on direct current) such as those that are currently envisioned for naval ship propulsion. 
   Conventional brushes include solid (e.g., carbon) brushes and metal fiber (e.g., copper) brushes. The majority of brushes currently used are of the solid carbon variety. Solid carbon brushes provide limited power densities due to their characteristically small number of contact spots. In addition, solid carbon brushes tend to have a short life and to produce conductive wear debris, resulting in frequent brush replacement and frequent machinery cleaning and associated high maintenance costs. Generally speaking, as compared with solid carbon brushes, copper fiber brushes are considered to afford superior performance. However, copper fiber brushes are currently expensive to produce and can support only moderate current densities. 
   Furthermore, fiber brushes are prone to wear, frequently manifested as a fiber brush “wear-in” contour that matches the curvature of the rotor that is in constant running contact with the fiber brush. The fiber brush wear, sometimes resembling welding-like damage, may result from friction and/or from electrical sparking associated with the fiber brush&#39;s sliding contact with the relatively moving machine part. Moreover, the electrical conductivity of fiber brushes is characterized by discontinuities concomitant with the intermittencies of contact by the fibers with the relatively moving machine part. Such contact intermittencies are occasioned by the running nature of the contact in conjunction with the roughness, often microscopic, of the surface of the relatively moving machine part. It has been estimated that a typical brush fiber is in actual physical contact with the relatively moving machine part only about one-third of the time during which the machinery is in operation; hence, each fiber represents a non-contributor to the overall electrical conduction during about two-thirds of the period of machine operation. 
   Liquid metal additives have been investigated for use in conjunction with fiber brushes in order to alleviate the above-noted conductive intermittency. Although a liquid metal material can succeed in lending constancy or smoothness to the electrical conduction between the brush and the slidingly contacted machine part, the high electrical conductivity of the liquid metal material invites electrical problems in the machinery such as involving short circuiting. Liquid metal brushes are capable of supporting very high current densities, but more research is needed in this area because of problems concerning stability and reactivity. Liquid metals have been tested, with some success, in association with small brushes for high performance current collection in homopolar motors. Generally, liquid metals require very clean operating environments absent oxygen and water. Since liquid metals are highly conductive, unwanted electrical connection of adjacent slip rings can occur (especially at locations where the liquid metal may drip or migrate, such as the bottom of the machine), resulting in short circuits in the machine. 
   Notable liquid metals that have been tested in current collection context are sodium-potassium (NaK) alloy and various gallium alloys. NaK (an alloy of sodium with potassium, approximately 22% Na and 78% K) has performed well in a range of applications, including both generators and motors. NaK does not react with most materials normally used in electrical machines; however, NaK is a water-reactive, caustic alkali metal, so spillage of this hazardous material is to be avoided. Gallium alloys have similar conductivity attributes, but because of their higher densities and viscosities these metallic materials are best suited for use with low speed propulsion motors. Because gallium alloys slowly react with copper (possibly by forming an alloy with the copper), all copper surfaces must be plated with a suitable non-reactive metal; nevertheless, gallium alloys are non-hazardous and should be further evaluated for current collector applications. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to improve the conduction of electrical current between a fiber brush and a machine part that the fiber brush slidably contacts. 
   Another object of the present invention is to reduce the wear of a fiber brush that slidably contacts a machine part. 
   The present invention provides a liquid additive—a “strong” electrolytic solution—that is designed to enhance the performance of brushes by providing a uniform electrically conductive interface. The inventive electrolytic solution promotes constancy of electrical conduction at the interface by compensating, via ionic electrical conduction, for intermittencies of electronic electrical conduction that are associated with the “fiber bouncing” resulting from microscopic surface roughness characterizing the running surface of the object that contacts the brush fibers while moving relative thereto. A typical inventive electrolytic solution contains both electron acceptor ions and electron donor ions, which can conduct electrical current without undergoing any net change in composition. Such a dual electrolytic solution, containing both an oxidizing agent and a reducing agent, serves not only to provide a uniform electrically conductive interface, but also serves to protect the electrodes in applications such as involving high current density slip ring current collector devices. 
   In accordance with typical embodiments of the present invention, a combination suitable for current collection applications comprises electronic conduction means and ionic conduction means. The electronic conduction means includes plural (e.g., multiple) fibers for effecting electronic conduction during relatively moving contact of the fibers with a structure. The contact of at least some of the fibers is characterized by intermittency associated with the relatively moving nature of the contact along with surface roughness characterizing the structure. The electronic conduction is characterized by discontinuity associated with the intermittency. The ionic conduction means includes a strong electrolytic solution for effecting ionic conduction when placed in the vicinity of the contact. The ionic conduction supplements the electronic conduction so as to promote the constancy of the overall conduction of electricity, the overall conduction representing the sum of the ionic conduction and the electronic conduction. According to frequent inventive practice, the electrolytic solution is an inorganic aqueous solution including an oxidizing agent and a reducing agent; however, non-aqueous solutions and/or organic solutes may also be utilized. An at least substantially neutral electrochemical cell reaction occurs in which the fibers constitute a first electrode and the structure constitutes a second electrode. 
   The conventional scientific (electrical, chemical and electrochemical) concepts noted in the instant paragraph are germane to the present invention. A chemical solution includes a solvent and a solute; the solute is dissolved in the solvent. An aqueous solution is a chemical solution in which the solvent is water. An electrolyte is a chemical substance that, when dissolved, ionizes to produce an electrically conductive medium. An ion is an electrically charged chemical particle, such as an atom, a molecule or a molecular fragment. An electrolytic solution is a chemical solution that can conduct electricity because its solute is ionically dissociated. Ions can be separated from an electrolytic solution using electrically charged electrodes. Many aqueous solutions are electrolytic solutions, as many substances (e.g., acids, bases, and salts) dissociate in water to form positively charged ions (cations) and/or negatively charged ions (anions) that enable the solution to conduct electricity. Anions are characterized by an excess of one or more electrons. Cations are characterized by a deficiency of one or more electrons. Electrical current is the movement of electrical charges in a conductor. There are two basic kinds of conductors, namely, an electronic conductor (a material that conducts electricity by electron motion) and an ionic conductor (a material that conducts electricity by ion motion). An electrolytic solution is a notable example of an ionic conductor. According to electronic conduction, electrical charges are carried by electrons. According to ionic conduction, electrical charges are carried by ions. 
   The present invention&#39;s liquid additive represents an attractive alternative to liquid metals for use in association with current collector brushes. The inventive liquid material typically includes moderately strongly conductive electrolytes in solution. Although the inventive electrolytic solution has sufficient conductivity to enhance brush operation, the inventive electrolytic solution is significantly less conductive than liquid metals, thereby significantly reducing the risks of shunt leakage and short circuiting. In addition, the present invention&#39;s electrolytic additives should be sufficiently conductive to allow for brush operation with a significantly thicker film than the water (H 2 O) and carbon dioxide (CO 2 ) film that is currently used in a vaporous manner for lubrication and/or anti-oxygenation purposes in association with fiber brushes. 
   The water-plus-CO2 solution that is currently known in fiber brush current collection context is one of many known formulations of weak electrolytic solution. A weak electrolytic solution is one in which there is partial (typically, slight or minimal) ionization (dissociation of ions) of the electrolytes in the solution; that is, the electrolyte solute(s) behave(s) weakly in that solvent. A strong electrolytic solution is one in which there is complete or nearly complete ionization (dissociation of ions) of the electrolytes in the solution; that is, the electrolyte solute(s) behave(s) strongly in that solvent. Data on solution and other material characteristics can be found in reference text books such as  CRC Handbook of Chemistry and Physics,  82 nd  Edition, edited by David R. Lide, QD65.C57, 2001, as well as in manufacturer&#39;s data. For purposes of quantitatively demarcating herein between a weak electrolytic solution and a strong electrolytic solution, hydrofluoric acid (HF), which is partially dissociated in water, is availed of herein (with some approximation) to establish the lower limit of a strong electrolytic solution. Hydrofluoric acid serves as baseline for a moderately strong electrolyte, having a conductivity at 1 mole/liter of 24.3 mS/cm and a pH of 3.2. 
   The term “weak electrolytic solution,” as used herein, is an electrolytic solution characterized by an electrical conductivity of less than 10 mS/cm at 1 molar concentration. Hence, hydrofluoric acid is proximate the lower end of the range corresponding to the strong electrolytic solution category. Most weak electrolytic solutions, such as carbonic acid (CO2 solution), acetic acid solution (vinegar) and ammonia solution, are orders of magnitude weaker than this and are nearly neutral in pH. The term “strong electrolytic solution,” as used herein, is an electrolytic solution characterized by an electrical conductivity of 10 to 1000 mS/cm at 1 molar concentration. More generally expressed, a weak electrolytic solution is less conductive than hydrofluoric acid, and a strong electrolytic solution is at least as conductive as hydrofluoric acid. Generally, a strong electrolytic solution is very much more electrically conductive than is a weak electrolytic solution. A typical electrolytic solution in accordance with the present invention is a strong electrolytic solution; nevertheless, some weak electrolytic solutions may also lend themselves to inventive practice. Generally, a weak electrolytic solution (such as CO2 aqueous solution) cannot be applied as thickly to a current collection interface as can a strong electrolytic solution, as a weak electrolytic solution that is laid on too thickly will inhibit the electrical conduction at the interface. A thick film of inventive electrolytic solution can permit a brush to operate at a very light load, thereby significantly reducing both the wear and the power loss. 
   A typical formulation of the present invention&#39;s electrolytic additive solution is significantly more electrically conductive than is the CO2 water solution conventionally used for non-conductive purposes with fiber brushes. For instance, at 25° C.: A 10 molar HCl solution has a conductivity of 708 ms/cm; a 1 molar KCl solution has a conductivity of 109 mS/cm; a CO2 saturated water solution has a conductivity of 0.0011 mS/cm; seawater has a conductivity of 53 mS/cm. Accordingly, a thicker liquid layer at the interface is practical with those among the present invention&#39;s ionic conductive additives that are strongly electrolytic. Inventive practice is very much preferred with strong electrolytic solutions rather than weak electrolytic solutions, as the latter only permits application in the interface as a thin layer. In fact, a weak electrolytic solution may be insignificantly capable or thoroughly incapable of providing, in accordance with inventive principles, an ionically conductive vehicle that serves to supplement an electronically conductive vehicle for conducting electricity. Weak electrolytic solutions cannot significantly advance electrical conduction uniformity, due to the insufficiency of the ionic electrical conduction that would be associated with the relatively low number of ions of the weak electrolytes. Nor is a weak electrolytic solution capable of counteracting the physical tendencies toward asperities in electronic electrical conductivities by brush fibers. Furthermore, a thin layer of a weak electrolytic solution, even in dual-species oxidant-reductant form, is not strong enough to reduce brush wear through an electrochemical neutralization mechanism. 
   Many embodiments of the inventive additive eliminate wear caused not only by fiber bouncing mechanisms but also by electroplating mechanisms. The present invention&#39;s electrolytic solution, as typically embodied, is an electrolytic solution similar to the kind that has been used (and known to work) in flow batteries. A typical inventive electrolytic solution includes both electron donor ions and electron acceptor ions within an aqueous solution. 
   According to frequent inventive practice, the inventive electrolytic solution is situated so as to form a film in the interface between a brush and a relatively moving machine part. During operation of the machinery, the inventive electrolytic solution provides, through the inventive electrolytic solution, an electrochemical path for current flow. For inventive embodiments in which the electrolytic solution contains both an oxidant and a reductant, current is carried by the present invention&#39;s electrolytic solution so that the ensuing net reaction within it is neutral (zero). The total reaction of the inventive electrolytic solution is an electrochemical nullity; hence, the inventive electrolytic solution should last indefinitely. 
   A dual-species oxidant-reductant kind of inventive electrolytic solution (i.e., one which includes both proton donor ions and proton acceptor ions) can minimize brush wear associated with anodal electrochemical etching mechanisms, thereby eliminating the anode-cathode effect previously observed in testing of many types of metal fiber brushes. The inventive additive provides an alternate low impedance current conduction path, thereby eliminating accelerated positive brush wear that may be associated with the formation of metal oxide or metal cations and the accompanying anodic etching. According to typical inventive practice, during machine operation a brush experiences ultra low friction and light loading; the inventive liquid interface layer affords minimal frictional loss and minimal brush wear. A moderately strong conductivity electrolytic additive material can provide optimal trade-off between transport current conduction-related power loss and magnetically induced circulating current-related power loss. No net change will occur in the solution or at the electrodes, which are thus inert electrodes. Slip ring motion can result in mixing of the electrolytic solution, thereby preventing polarization or stratification in the electrolyte. 
   Generally, the present invention&#39;s additive materials are corrosive. Therefore, for many inventive embodiments, gold-plated silver brushes may be required, and other parts of the machinery may require plating with a hard noble metal such as iridium. The prospects for successful practice of the present invention are great so long as a given inventive additive is teamed with suitable metal materials for the two relatively moving machine parts (e.g., fiber brush and rotor) that define the interface at which the inventive additive is to be situated. For instance, depending on the inventive embodiment, copper may or may not be a suitable electrically conductive material for the brush fibers. Copper-based Cu(I) and Cu(II) solutions can also provide acceptor and donor ions, but stable cuprous Cu(I) cations only exist in solutions with complex ions such as halide salts and acids. These copper-based electrolytic solutions should not be used with copper brushes, but can be used with gold brushes and possibly with gold-plated silver brushes. vanadium, titanium and polysulfide solutions may be suitable. Chromium, vanadium, titanium and polysulfide solutions may be suitable for use with copper brushes. Copper brushes may corrode or become coated with a thin passive coating. Silver brushes would represent a higher risk than gold brushes but a lower risk than copper brushes, and may be suitable for most conductive solutions. Gold brushes are almost certainly compatible with most solutions, but have higher initial costs. 
   There are at least two preferred modes for applying an inventive electrolytic solution in a current collector context in accordance with the present invention. According to a first inventive mode, the electrolytic solution is dripped onto the brush (or brushes). According to a second inventive mode, the electrolytic solution is contained in a sealed environment for the current collector apparatus such as including slip rings. According to a third mode for applying an inventive electrolytic solution in a current collector context in accordance with the present invention, the electrolytic solution is caused to be transmitted by the brush fibers themselves in a manner pursuant to principles disclosed by the aforementioned U.S. patent application entitled “Solid and Liquid Hybrid Current Transferring Brush,” joint inventors Neal A. Sondergaard and William A. Lynch. 
   Inventive practice of liquid additives should be suitable in association with solid brushes. Nevertheless, fiber brushes will generally provide greater current densities because of their larger surface areas in contact with the inventive electrolytic fluid. The present invention admits of diverse application. For instance, a slip ring can use two solid non-contacting surfaces, or a non-contacting fiber brush, or a lightly loaded fiber brush, or two non-contacting fiber brushes. If the electrolytic solution is corrosive to low temperature solders, the fiber brushes will likely require attachment to their holders using an alternative material such as eutectic 72% silver 28% copper braze, to avoid solder corrosion. Alternatively, the present invention can be practiced so as to implement a novel unitary device conceived by the present inventors, which combines the brush and holder components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: 
       FIG. 1  is an elevation view of a portion of machinery in which a fiber brush is mounted (via a holder) onto a stator and is in sliding contact with a rotor, wherein, in accordance with an embodiment of the present invention, an electrolytic solution is dripped upon the interface between the fiber brush and the rotor. 
       FIG. 2  is an elevation view, similar to the view of  FIG. 1 , of a portion of machinery in which a fiber brush is mounted (via a holder) onto a stator and is in sliding contact with a rotor, wherein, in accordance with an embodiment of the present invention, an electrolytic solution is contained surrounding the interface between the fiber brush and the rotor. 
       FIG. 3  is a partial and enlarged version of the view of  FIG. 1 , illustrating “jumping” of the brush fibers (and concomitant intermittency of contact of the brush fibers with the rotor&#39;s running surface) due to microscopic roughness of the rotor&#39;s running surface. 
       FIG. 4  is a perspective view of a single brush fiber forming an electrical contact with a relatively moving object (such as a rotor) in the absence of the present invention&#39;s additive.  FIG. 4  illustrates asperity characterizing the electrical conduction between the brush fiber and the relatively moving object. 
       FIG. 5  is a perspective view, similar to the view shown in  FIG. 4 , of a single brush fiber forming an electrical contact with a relatively moving object (such as a rotor) in the presence of the present invention&#39;s additive.  FIG. 5  illustrates an amelioration of the asperity that is shown in  FIG. 4 , resulting in an enlargement of the area of electrical conductivity between the brush fiber tip and the relatively moving object. 
       FIG. 6  is a diagram of a hypothetical electrochemical brush wear mechanism in the absence of the present invention&#39;s additive. 
       FIG. 7  is a diagram, similar to the diagram shown in  FIG. 6 , of a hypothetical electron shuttling mechanism in accordance with the present invention. The present invention&#39;s electron shuttling mechanism is a brush wear-preventive mechanism that occurs in the presence of an inventive additive such as the Vanadium-based additive shown in  FIG. 7 , which is an aqueous electrolyte solution that includes dual valence cations consisting of electron acceptors and electron donors. 
       FIG. 8  is a table listing dual valance electron donor and electron acceptor conductive electrolytes that may be suitable for inventive practice, and also indicating metals that may be compatible therewith. 
       FIG. 9  is a table listing electrical conductivities of various materials. 
       FIG. 10  is a table indicating solubilities of various combinations of cations and anions in water. 
       FIG. 11  is a table indicating various combinations of metal brush materials, electrolytic solutions, and metal plating materials that may be appropriate for further investigation, or practice, in accordance with the present invention. 
       FIG. 12  is a table indicating some inventive test results of compatibilities of various combinations of metal electrode materials and electrolytic solutions. 
       FIG. 13  is a table indicating inventive solution characteristics and test data for four Iron-based single-valence electrolytes and two Iron-based dual-valence electrolytes. 
       FIG. 14  is a graphical representation, known as a “Pourbaix Diagram,” that indicates metal stability regions and conditions for selected additive solutions in accordance with the present invention. 
       FIG. 15  is a graph indicating conductivity (high conductivity proportional to low millivolts) as a function of temperature (degrees Celsius) for three different additive solutions in accordance with the present invention. 
       FIG. 16  is a table indicating, for each of three additive solutions in accordance with the present invention, conductivity (high conductivity proportional to low millivolts) of the inventive additive solution as a function of temperature (degrees C.). 
       FIG. 17  is a table indicating characteristics and test data for each of three iron-based dual valance aqueous solutions and four copper-based dual valance aqueous solutions in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 1  and  FIG. 2 , a holder  106  is mounted upon a stator  102 . Brush  108 , which includes multiple fibers  109  and a bonded section  107  of fibers  109 , is fixedly held at bonded section  107  by holder  106 . The exposed portions of brush fibers  109  contact rotor  104  at interface  122 . Rotor  104  rotates (about a rotational axis) in a rotational direction r and is hence moving relative to the stationary brush  108 . Interface  122 , the interface between the exposed brush fibers  109  and the rotor  104  surface, is constantly moving along the rotor  104  surface in accordance with the rotation of rotor  104 . Outside of inventive practice, a holder is used not only to hold a brush but also to exert a bias (e.g., in spring-like fashion) against the brush in the direction of the relatively moving object that is slidingly contacted by the brush. In inventive practice, depending on the inventive embodiment, holder  106  may be devised to exert a less forceful bias (e.g., a slight bias) against brush  108  than is usually seen by a holder with respect to a brush in non-inventive practice; in fact, in inventive practice, a holder  106  may even be designed so that exertion of bias against the brush  108  is obviated completely. 
     FIG. 1  and  FIG. 2  can each be conceived as portraying machinery  100  either of a motor variety (wherein current is conducted from power supply  200 M, to stator  102 , then through holder  106 , then through brush  108  and electrolytic solution film  22 , to rotor  104 ) or a generator variety (wherein current is conducted from power supply  200 G, to rotor  104 , then through electrolytic solution film  22 , then through brush  108 , then through holder  106 , to stator  102 ). In the case of motor-type machinery  100 , an electrical power supply (e.g., battery)  200 M is electrically connected, either directly or indirectly, to stator  102 ; stator  102  receives the current (e.g., direct current) from power supply  200 M and conducts the current to the current collection apparatus (which includes holder  106 , brush  108  and electrolytic solution film  22 ), which conducts the current to rotor  104 . In the case of generator-type machinery  100 , an electrical power supply (e.g., battery)  200 G is electrically connected (usually indirectly through another current collector slip ring) to rotor  104 ; rotor  104  receives the current (e.g., direct current) from power supply  200 G and conducts the current to the current collection apparatus (which includes holder  106 , brush  108  and electrolytic solution film  22 ), which conducts the current to stator  102 . Although  FIG. 1  and  FIG. 2  portray a brush  108  as affixed to a stator  102  for sliding contact of brush  108  with a rotor  104 , the present invention can also be practiced in arrangements in which a brush  108  is affixed to a rotor  104  for sliding contact of brush  108  with a stator  102 . 
   As illustrated in  FIG. 1 , a liquid electrolytic solution  20  is contained in a tank or receptacle such as a reservoir  10 . Electrolytic solution  20  is pumped from reservoir  10  via pump  12  and along pipe  14 , and is dispensed via nozzle  16  at the sliding contact interface  122  between the fibers  109  of brush  108  and the surface of rotor  104 . Drops  21  of electrolytic solution  20  are continually deposited (e.g., dripped) onto interface  122  so as to generate an ongoing film  22  of electrolytic solution  20 , film  22  extending at least substantially throughout interface  122 . By force of gravity, droplets  23  of electrolytic solution  20  fall beneath interface  122  (e.g., toward the ground or floor, or onto lower parts of the machinery). The solution may be collected and re-circulated to the reservoir  10 . 
   It is noteworthy that, as distinguished from liquid metal materials that have been tested in current collection context, the electrolytic solutions  20  in accordance with the present invention are well suited for an inventive “drip” configuration such as shown in  FIG. 1 . A liquid metal is much more highly electrically conductive than is a typical inventive electrolytic solution  20 . It would be less practical or more complicated to drip the liquid metal in an analogous manner because of the risk posed by falling liquid metal droplets (which are characterized by high electrical conductivity) of deleterious electrical events such as involving short-circuiting. 
   As illustrated in  FIG. 2 , a liquid electrolytic solution  20  is contained in a vessel such as an immersion chamber  24 . Due to the immersion of at least a portion of brush  108  in electrolytic solution  20  inside immersion chamber  24 , a film  22  of electrolytic solution  20  is continuously present at interface  122 . According to some inventive embodiments, apparatus such as including a reservoir  10 , a pump  12  and a pipe  14  is implemented to periodically (e.g., timed) or selectively replenish the electrolytic solution  20  contained within immersion chamber  24 . Similarly as provided by the inventive configuration shown in  FIG. 1 , a film  22  of electrolytic solution  20  is present at interface  122  on an ongoing basis and extends at least substantially throughout interface  122 . 
   Reference is now made to  FIG. 3 , which depicts the fiber  109  bouncing phenomenon that takes place at interface  122 . The distances of fibers  109  from the rotor  104  surface are exaggerated in  FIG. 3  for illustrative purposes. No mechanical surface is perfectly smooth. Even a macroscopically even rotor  104  surface is characterized, on a microscopic scale, by surface roughness. As rotor  104  rotates, the surface roughness of rotor  104  causes the fibers  109  to “bounce” (e.g., rebound, jump or ricochet) microscopic distances from the rotor  104  surface. According to L. Boyer, S. Noel and J. P. Chabrerie, Wear, vol. 116, pages 43-57 (1987), incorporated herein by reference, at any one moment in time only approximately one-third of the brush fibers at a current collector interface are in actual physical contact with the relatively moving surface. Based on this teaching by Boyer et al., perhaps it is fair to assume for many inventive applications that, at any particular moment in time during machine operation, less than one-half of fibers  109  will actually be contacting the running surface of rotor  104 . Because the fibers  109  bounce around, at least half and perhaps two-thirds of fibers  109  being separated from the rotor  104  surface from moment to moment, the electronic electrical conduction by the fibers  109  to (or from) rotor  104  is commensurately discontinuous (intermittent), and its average value is proportionally lower. 
   The film  22  of electrolytic solution  20  promotes constancy (uniformity) of the overall electrical conduction at interface  122 . The overall electrical conduction equals the sum of the electronic electrical conduction that occurs via the brush fibers  109  and the ionic electrical conduction that occurs via the electrolytic solution film  22 . The electrolytic solution film  22  provides an auxiliary electrically conductive mechanism (i.e., an ionically electrically conductive mechanism) that supplements the electronically electrically conductive mechanism provided by the brush fibers  109 . The ionic conduction by the electrolytic solution film  22  not only supplements, but also complements, the electronic conduction by the brush fibers  109 , by exerting a smoothing influence on the overall electrical conduction at interface  122 . Otherwise expressed, the electronically conductive discontinuity is tempered by the ionically conductive continuity, thereby promoting the evenness of the overall electrical conduction. 
   With reference to  FIG. 4  and  FIG. 5 , asperities are microscopic roughness elevations of a surface of a material. An asperity spot (abbreviated “a-spot”) is the point of physical contact between an asperity on one surface and an asperity on another surface. When the respective surfaces of electrical contacts are mated, physical contact occurs only at one or more asperity spots (a-spots). Each a-spot is elastically/plastically flattened to form a small contact area through which the electronic conduction of electrical current is constricted. The number of a-spots and their sizes and arrangement can affect the performance and longevity of the electrical contact. Most of the electrical resistance of brush  108  is associated with the voltage drop across interface  122 . As depicted in  FIG. 4  (which shows one asperity spot  130 ), the current in each fiber  109  is constricted in one or a few small asperity spots  130  on the tip  111  of each fiber  109 . The fiber tip  111  area that carries current is only a small portion of the total fiber tip  111  area. The area of the a-spot is known to the electrical contact industry to be small (e.g., on the order of 1% of the area of a typical fiber in a metal fiber brush), as disclosed in references such as Ragnar Holm,  Electric Contacts Theory and Application,  4 th  Edition, TK2821H74, 1967, and  Electrical Contacts Principles and Applications , Edited by Paul G. Slade, TK2821.E34, 1999. Each a-spot  130  is likely to occur at an asperity (high point) on the fiber tip  111  micro-roughness, and is likely to move around on the fiber tip  111  because of the micro-roughness of the running surface of rotor  104 . Moreover, fibers  109  are likely to jump over the micro-roughness characterizing rotor  104 &#39;s running surface, so each fiber  109  is not conducting electricity some of the time. The present invention&#39;s liquid additive  20  can serve to facilitate electrical conduction over the entire (or nearly the entire) fiber tip  111 , such as illustrated in  FIG. 5 . The constantly present film  22  of electrolytic solution  20  serves to increase the areas of conductance (a-spots  130 ) in fibers  109 . Depending on characteristics of the electrolytic film  22  such as thickness, the electrical current conduction may be described by the mechanism depicted in  FIG. 5 , or by the simultaneous combination (acting in parallel) of the mechanisms depicted in  FIG. 4  and  FIG. 5 . 
   Reference is now made to  FIG. 6 , which illustrates a hypothetical electrochemical wear mechanism in non-inventive practice. Under normal conditions, electrons are conducted through a small gap (greatly exaggerated in the illustration) consisting of a few molecular layers of water. Copper metal in the brush fiber can act as an electron donor, thereby becoming a copper ion Cu+ or Cu2+ and dissolving into solution. The copper ion can then act as an electron acceptor, thereby becoming metallic copper and plating on the rotor. This situation would cause increased fiber wear and could also increase rotor surface roughness if the plating process is not uniform. 
   With reference to  FIG. 7 , in accordance with the present invention, an ionic current shunt mechanism is provided by using an additive based on a dual-valance vanadium solution. Since the vanadium half-cell is less electro-positive than copper, V2+ preferentially acts as an electron donor rather than copper. This current shuttle mechanism can both prevent the ion current induced wear mechanism, and increase the electrical conductivity of the interface. 
   Among the various electrolytic solutions that are suitable for use in accordance with the present invention are certain electrolytic solutions that are known to be useful to store energy using redox-type reactions in flow batteries. Two known types of flow batteries are the Vanadium V(II,III)/V(IV,V) based battery system and the Sodium Bromide/Sodium Polysulfide based Innogy Regenesys battery system. These types of batteries store energy in two solutions respectively contained inside anolyte and catholyte tanks. Each of these solutions contains both electron acceptors and electron donors. As distinguished from flow battery applications, the present invention utilizes a single electrolytic solution rather than a plurality of electrolytic solutions. Not only compounds utilized in batteries, but other compounds, may be suitable for use in current collector applications in accordance with the present invention. Furthermore, the present invention utilizes the single electrolytic solution for ionic conduction purposes rather than for energy storage purposes. 
     FIG. 8  indicates the half-cell potentials and pH values expected for various ion-conducting solutions. For the anolyte, the Vanadium redox battery system uses a V(II) and V(III) electrolytic solution, which has a very low half-cell potential and should therefore be compatible with copper brushes, but it could therefore possibly evolve hydrogen gas. For the catholyte, the Vanadium redox battery system uses V(IV) and V(V) electrolytic solution, which has a high half cell potential and would require gold brushes in inventive practice. A V(III) and V(IV) electrolytic solution is also suitable for some inventive embodiments, albeit it is not used in the Vanadium redox battery because it would result in a low cell voltage. A V(III) and V(IV) solution is suitable for inventive use with silver brushes. Vanadium-based electrolytic solutions are made by dissolving vanadium oxide in sulfuric acid. The vanadium compounds are toxic. 
   The Regenesys system uses a catholyte containing NaBr and NaBr 3  solution, which has a very high half-cell potential and would require gold brushes in inventive practice. This solution also can liberate highly toxic bromine fumes. A solution containing KI and KI 3  should have similar characteristics; however, it would be less toxic and would have a lower half-cell potential. compatible with silver or palladium brushes. Iodine solution can be used as a precious metal etchant because of the formation of complex ions, and therefore may cause excessive corrosion in inventive practice; however, suitable mixtures of iodine electrolytic solution may be possible combined with metals that do not form complex ions. The Regenesys anolyte uses polysulfides Na 2 S 4  and Na 2 S 2 . In inventive practice, this alkali solution should be compatible with all brush types; however, the formation of low solubility sulfides may require maintenance in terms of cleaning and solution replenishment. 
   Iron and chromium solutions have also been tested in flow batteries. These are less toxic than those used in the Vanadium and Regenesys systems. Cr(II) and Cr(III) or titanium salts could be used in the anolyte solution; however; the solution&#39;s low half cell potential indicates a possibility of hydrogen gas evolution and poor stability of the Cr(II) ions. Fe(II) and Fe(III) could be used in the catholyte solution. The solution would include ferric chloride etching solution containing FeCl 3  and HCl, with ferrous chloride FeCl 2  added. Fe 2 (SO 4 ) 3 , H 2 SO 4  and FeSO 4  could also be used. The iron solutions should be compatible with gold brushes, and the iron sulfate solutions should also be compatible with gold and possibly palladium brushes. 
     FIG. 9  is a table of the conductivity of selected substances. According to typical inventive practice, the brush additives are strong electrolytes.  FIG. 9  indicates that the conductivity of strong electrolytes is intermediate between that of weak electrolytes and that of metals including liquid types. The present invention&#39;s additive is orders of magnitude more conductive than that of the CO2 solution often used with brushes, serves to enhance the conductivity of the interface, and may be applicable with a significantly thicker film layer. An inventive additive provides a lower conduction power loss and/or a lower frictional power loss by allowing the brush to operate at lighter load pressure. Further, an inventive additive facilitates lower wear rates, by minimizing electrochemical, mechanical and thermal wear mechanisms. Since the inventive additive is orders of magnitude less conductive than metals, it should not contribute significantly to electrical shorts or leakage currents, provided that the slip ring assemblies include suitable separation and creepage paths between adjacent turns. 
     FIG. 10  is a water solubility table that can be used by an inventive practitioner for selecting materials in inventive practice. The materials shown include possible brush metals, additive ions and spectator ions. Metals used in fiber brushes are soluble when combined with most anions, especially halides; therefore, electrochemical dissolving of metals can occur if they are present in an aqueous solution within the brush interface and less electropositive electron donors are not present in the solution. 
     FIG. 11  is a table of some possible combinations of materials that can be used by an inventive practitioner such as in a slip ring assembly, wherein an inventive additive electrolyte contains electron donor and electron acceptor ions. Other materials may be considered, and not all permutations of the materials are compatible. A variety of alloys may be considered in addition to the pure materials, and their characteristics such as hardness, corrosion resistance, electrical conductivity, and cost may be optimized for various parts of the machine. The characteristics of alloys are generally not the same as the average of the characteristics of the component materials, and hence some alloys may offer significant advantages. 
     FIG. 12  is a result of a preliminary compatibility test. Small samples of the metals were placed in small vials of 1 molar solution, and any changes were observed. Since no electrical current is transferred, this test result does not guarantee a practical combination; however, it is an indicator of likely material compatibilities and generally correlates with expected results. 
     FIG. 13  is a table of characteristics of Iron-based additive solutions, and includes experimental results. The pH, half-cell voltage E, and conductivity values C were measured using commercially available laboratory instruments. The electrode drop was measured using a simplified model of the inventive device. This included a pair of electrodes with one square centimeter area each separated by 2 centimeters, which were immersed in the test electrolyte solution. A small current source supplied the 10 milliamp test current to the apparatus. In this proof of concept experiment, the separation is much larger, and the current is much smaller, than would be expected in a practical current collector device. This set of experiments validates the present invention&#39;s concept of the dual-valance additive, because the conductivity of the two-component solution (which is made by mixing 50% each of the constituent solutions) is significantly lower than that of either the electron donor solution or the electron acceptor solution alone. 
     FIG. 14  is a compilation of data related to Pourbaix diagrams that are shown in greater detail in Marcel Pourbaix,  Atlas of Electrochemical Equilibria in Aqueous Solutions,  2 nd  Edition, QD553.P6613, 1974. Relative stabilities of metals in aqueous solution can be indicated in Pourbaix diagrams such as illustrated in  FIG. 14 . Comparative inspection of the Pourbaix diagram reveals relative stability of metals (indicated by contrasting shaded areas in the Pourbaix diagram) and possible characteristics of additive solutions (indicated by points in the Pourbaix diagram). Each metal is stable in the shaded area indicated as well as lower areas. The metal would be likely to dissolve forming metal cations in a solution with characteristics above its stability region. The Porbaix diagram is based on the stability of metals in chloride solutions. In inventive practice, the stability of brush fiber metals will be higher for acids with other anions such as SO4, which do not form complex ions with the metals. 
     FIG. 15  is a plot of the voltage drop across the electrodes as a function of temperature, which is inversely proportional to conductivity. This includes data on the original two iron-based additives as well as a copper-based additive. The copper-based additive provides a significantly lower voltage drop. 
     FIG. 16  is the tabular data presented in  FIG. 15 . The experiments were performed using 1 molar solutions with a pH of 1.00. In some cases, dilute acid and salt solutions were used to set the pH and to facilitate additive compound dissolving into solution. 
     FIG. 17  is similar to  FIG. 13 ; however,  FIG. 17  includes data on copper-based additives as well as variation in solution concentration. The copper-based additives provided significantly higher conductivity. Chloride anions provided higher conductivity than sulfate ions, and solutions with higher concentration provided higher conductivity. The iron-based additives are relatively simple to prepare by dissolving compounds in water and mixing 50% of each component solution, thereby making the present invention&#39;s dual-valance additive solution. Small amounts of dilute acid may be added to reduce the pH and to ensure no precipitate formation. 
   The copper-based additives are somewhat more difficult to prepare because of the low aqueous solubility of the Cu(I) (cuprous) ion. It complexes with chloride ions; therefore, addition of HCl (hydrochloric acid) is a possible technique for dissolving the Cu(I) chloride. Very concentrated acid was required to dissolve a 1 molar solution. This solution provided very high conductivity, but this highly acidic solution had a negative pH. Salt solution made with saturated NaCl was also used to produce a copper-based additive solution that was much less acidic and provided reasonably high conductivity. Further study of the copper-based additives as a function of salt concentration and pH could lead to further optimization of characteristics of the copper-based additive. A mildly alkali form of the copper-based additive can be made using a mixture of ammonium chloride and ammonium hydroxide in the solution. Other procedures would be required for other types of additives included in  FIG. 8  and  FIG. 14 . 
   The present invention, which is disclosed herein, is not to be limited by the embodiments described or illustrated herein, which are given by way of example and not of limitation. Other embodiments of the present invention will be apparent to those skilled in the art from a consideration of the instant disclosure or from practice of the present invention. Various omissions, modifications and changes to the principles disclosed herein may be made by one skilled in the art without departing from the true scope and spirit of the present invention, which is indicated by the following claims.