US2986512A

US2986512A - Control system and method

Info

Publication number: US2986512A
Application number: US777377A
Authority: US
Inventors: Rolland C Sabins
Original assignee: Individual
Current assignee: Individual
Priority date: 1958-12-01
Filing date: 1958-12-01
Publication date: 1961-05-30
Anticipated expiration: 1978-05-30

Description

May 30, 1961 R. c. SABINS CONTROL SYSTEM AND METHOD Filed Dec. 1, 1958 E5 2 n 1N9 :22: 52; 3 8 k 5% 2 i; .Qk o w W I E: g mm K 25: E3: t n\ 3528 :2: 00. A 5:22 55E 5% 5E; g 5% 5% 2 3 2 2 L. N I. 3 4 4 m N A 4 4 N M E A o m NW mm mm ATTORNEYS United States Patent 9 F CONTROL SYSTEM AND METHGD Rolland C. Sabins, 522 Cataling Blvd, San Diego 61, Calif.

Filed Dec. 1, 1958, Ser. No. 777,377

10 Claims. (Cl. 204-196) This invention relates generally to a control system and method and more particularly to a control system and method for automatically providing cathodic protection for various types of structures, vessels and the like which are normally submerged in water or some solution which acts as an electrolyte.

Control systems heretofore provided for cathodic protection have often been of a type with manually controlled means for regulating the current impressed on the submerged structure such as the hull of a ship or a barge. Such means has been found to be unsatisfactory because of the varying current requirements necessary to provide adequate protection for the submerged structure. As is Well known, the current requirement to provide satisfactory protection is dependent upon many factors as for example, the speed of movement of the hull through the water, temperature, and ionic content of the water through which the hull is moving, etc. Attempts to provide automatic control to take care of these variations have heretofore not been completely satisfactory especially when subjected to shock and vibration, such as would be produced by gunfire aboard navy ships. This, in large part, has been due to the fact that apparatus of this type must function for long periods of time without maintenance because a ship carrying the system may not return to its home port for months and even years. Automatic control is also complicated by the fact that the system must operate over a wide range of impressed currents. It must respond to very small control currents and must provide magnification of a very high order. There is a need for an automatic control system which will be suitable for use on large vessels and which will require very little, if any maintenance.

In general, it is an object of the present invention to provide a control system and method which will automatically control the current impressed on the structure to be protected to maintain the structure at a predetermined polarization.

Another object of the invention is to provide a system and apparatus of the above character in which the current impressed on the structure is determined by an adjusted electrochemical reference potential.

Another object of the invention is to provide a system of the above character in which a reference circuit, which employs a cathode, an anode, and a bi-polar electrode, is used in a monitoring circuit for generating a reference potential.

Another object of the invention is to provide a system of the above character in which a reference circuit is always connected in the monitoring circuit.

Another object of the invention is to provide a system of the above character which has no moving parts necessary for the normal automatic functioning of the system.

Another object of the invention is to provide a system of the above character which requires very little, if any, maintenance.

,Another object of the invention is to provide a system Patented May so, 1961 of the above character which consists of static, solid state components.

Another object of the invention is to provide a system of the above character which is highly resistant to shock and vibration.

Another object of the invention is to provide a system of the above character having inert electrodes utilized as the anode and bi-polar electrode of the reference circuit.

Another object of the invention is to provide a system of the above character in which the demand is substantially satisfied.

A further object of the present invention is to provide a system of the above character in which, in the event of failure of the reference circuit potential, over-polarization of the structure is prevented and, in the instant embodiment, the means for impressing extraneous current on to the structure is rendered ineffective.

Further objects and advantages will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the invention is illustrated.

In the drawings:

Fig. l is a circuit diagram incorporating the present invention with certain parts illustrated schematically; and

Fig. 2 is .a circuit diagram of an alternate form of reference means.

The present application is a continuation-in-part of my co-pending application Serial No. 775,523, filed No: vember 21, 1958 and entitled Control System and Meth. 0d. This latter application teaches the employment of a reference electrode connected by a conductor to the hull of the structure to be protected. The current flow which passed through the electrolyte from the hull .of the structure to such reference electrode was passed through a meter having a movable element responsive to the flow of reference current. This movable element in turn initiated the operation of the vautomatic impressed current supply means described in that application. In contrast, the present application does not depend upon the constant flow of a reference current, but instead relies upon a predetermined potential established in areference circuit which includes an anode, a cathode, and a bi-polar electrode. In addition, no meter having a movable element is necessary to the operation of the present system, and for that reason'the present system is extremely resistant to shock and vibration. All portions of the system are made of solid state materials, and no movable elements are employed which might malfunction or fail under the stress of shock or vibration.

In general, the present invention comprises .a control system and method for automatically providing cathodic protection for various types of structures such as the hulls of vessels and barges, and the like, normally immersed or submerged in fresh or salt water or other electrolyte, and providing cathodic protection for various objects such as piping systems which are subjected to an electrolyte. A current is impressed on the structure to cause the structure or object to serve as a cathode. A monitoring circuit is provided in which a reference current is generated at a predetermined potential, and this reference current is utilized directly for controlling the amount of current impressed on the structure or object to maintain the structure or object at a relatively constant predetermined polarization to prevent corrosion of the structure or object.

As shown in the drawing, my control system comprises, generally, impressed current power supply means which preferably includes a signal amplifier 10, a control re: actor 12, a power reactor 14 and a power rectifierllfi; monitoring means which includes a reference current supply and control 18 in a reference circuit 28 which includes an anode 22 and a bi-polar electrode 24. The impressed current supply means is connected between an anode array or assembly 26 and a structure 28 to be protected. The anode assembly 26 and the structure 28 are shown submerged or immersed in an electrolyte 30 which, for example, can be sea water. For illustrative purposes, I have chosen the hull of a ship as the structure or object to be protected; and the anode assembly 26 can be of any suitable type such as that disclosed in my co-pending application entitled Electrolytic System, Serial No. 715,440 filed February 14, 1958.

As stated, the monitoring means includes a reference circuit which is utilized for generating a reference potential. As shown, the pilot anode 22 and drive bipolar electrode 24 of reference circuit 20 are also submerged in the electrolyte 38 and are generally mounted relatively close to the structure 28 to be protected. The anode 22 and the bi-polar electrode 24 can be of any suitable inert material, but are preferably made of platinum. In addition, anode 22 and the bi-polar electrode 24 are preferably mounted very close to each other in order to minimize the effect of variations in the environment or physical characteristics of the electrolyte forming the electrolytic path between them. Electrode 24 is termed bi-polar because it is anodic with respect to structure 28, but cathodic with respect to anode 22. Thus it will be seen that it serves in the overall system as both a cathode and an anode, depending upon whether it is considered in relation to structure 28 or to anode 22.

As shown and as hereinafter described, the anode 22 and the bi-polar electrode 24 are normally connected together. Any suitable means may be employed for impressing a source of D.C. current on the bi-polar electrode 24 to increase the potential thereof. In the specific embodiments of Fig. 1, the anode 22 and bi-polar electrode are connected by full wave bridge 32 comprising four reetifiers 34, the positive terminal of the output of the full wave bridge 32 being connected to anode 22 by a conductor 36, and the negative or ground terminal of the bridge being connected to the bi-polar electrode 24 by conductor 38. The negative terminal of the bridge is also connected by a conductor 40 to the structure 28 through signal amplifier 10.

A suitable power source for reference circuit 20 is provided by a variable transformer 42 of the auto transformer type. The winding of transformer 42 is connected to a suitable source of power 44 by

conductors

46 and 48, and one end of the winding of transformer 42 is connected to one end of an isolation transformer 50 by a conductor 52, and the other end of isolation transformer 50 is connected to a wiper element 54 which operates along the length of the winding of transformer 42 in the conventional manner.

The secondary winding of isolation transformer 50 is connected across the bridge 32 by a pair of

conductors

56 and 58. Thus, the power supply means 44 is effective, through the operation of

transformers

42 and 50, to provide a regulated or adjusted potential to bridge 32. The wiper or arm 54 may be operated to obtain a level of potential at bridge 32 which will be effective to establish a reference potential for monitoring the impressed current supply to structure 28, as will hereinafter be further described.

Bridge 32 operates to impress a current upon anode 22, and this current is caused to pass through the bridge to the bipolar electrode 24, and thence by electrolytic action through the water path from the bi-polar electrode 24 to anode 22. By adjusting the position of wiper 54 it will be seen that the potential at which anode 22 and the bi-polar electrode 24 are maintained can be adjusted to a predetermined level.

Assuming that the bi-polar electrode 24 and structure 28 are at the same level of potential there will be negligible current flow through conductor 40 to signal amplifier 10 and thence back through a conductor 60 to structure 28. Under these conditions we will assume that the level of potential for hull 28 is at the proper level to afford cathodic protection. However, now assuming that the potential level of the structure 28 falls below the desired level, it will be apparent that its potential with respect to the bi-polar electrode 24 will change, causing an increased flow of current through conductor 40, a coil 180 of amplifier 10, and then through conductor 60 to structure 28. This increased current provides the signal which initiates or actuates the automatic impressed current supply. This however is only responsive to saturation current in the forward direction. It is important to note, therefore, that the electrical potential of reference circuit 20 can be established at a level which, if it exists on structure 28 also, will be sufiicient to provide cathodic protection to structure 28, and that the monitoring circuit will increase the output of the impressed current supply means at any time that this established level of potential falls.

Going now to the impressed current system which is effective to raise the potential of structure 28 through utilization of the reference current flowing through the monitoring means at amplifier 10, it will be seen that amplifier 10 accepts the D.C. input at a pair of

terminals

62 and 64. Amplifier 10 is supplied with power from a five volt A.C. source 63. This A.C. input is controlled by impedance as a result of the A.C. windings on the reactor cores. The controlled A.C. is rectified and supplies the D.C. output. The D.C. current in coil 100 controls the saturation of the cores, resulting in control of the D.C. output which is fed from a pair of DC.

output terminals

66 and 68, through a pair of conductors '70 and 72 to a pair of

D.C. input terminals

74 and 76 of the amplifier or control reactor 12. In the present embodiment, at full output the incoming signal will fully saturate at 20 micro-amperes, in which case the amplifier 10 of the present system puts out a signal in excess of 300 micro-amperes. It is important to note that signal amplifier 18 is of that type which saturates only in a forward direction, that is, from terminal 62 to terminal 64. Current flowing in the opposite direction will thus be ineffective to saturate the core of amplifier 10 and, accordingly, no signal input will be made to amplifier 10 by reason of any such reverse flow. This is important, as will be seen, because if the potential of structure 28 should for some reason be greater than the potential of bi-polar electrode 24, and if a reverse flow of current through amplifier 10 would saturate the D.C. input core thereof, which as stated is not the case, the impressed current supply means would be actuated and tend to raise the potential of structure 28 to a still greater level. The reverse flow would then increase, and the system would be uncontrolled. For this reason amplifier 10 is made to saturate or be actuated only by a flow of current in the forward direction from terminal 62 to terminal 64.

Control reactor 12, which is provided with power from a suitable 110 volt A.C. source 78, then amplifies the input signal in excess of 300 milli-amperes which is fed through a pair of

output terminals

80 and 82, and through a pair of

conductors

84 and 86 to the

D.C. input terminals

88 and 90 of power reactor 14. Reactor 14, which is connected to a suitable 110 volt A.C. power source 92, further amplifies the signal, and this signal is fed to the AC.

input terminals

94 and 96 of power rectifier 16. The AC. input is then rectified to a D.C. output which is connected to anode array 26 and structure 28 by a pair of

conductors

98 and 100 which in turn are connected, respectively, to the positive and negative output terminals 102 and 164 of rectifier 16.

From the foregoing it will be seen that amplifier 10, reactor 12, reactor 14, and rectifier 16 are in effect various stages of an amplification system for accepting a small D.C. input signal and amplifying it to a rather large D.C. output signal. The D.C. output signal comprises the impressed current for raising the electrical potential of structure 28 to provide the necessary cathodic protection therefor.

Thus, assuming as before that structure 28 is below the necessary potential to provide cathodic protection, the small input signal from reference circuit 20 of the monitoring means will be amplified through hte amplification system just described, and the D.C. output of rectifier 16 will impress a current through conductor 100 to structure 28, and thence through the electrolytic or water path to anode array 26 to complete the impressed electrochemical circuit.

As the control signal continues to flow through the conductor 40 the impressed current supply will be applied through the conductor 100 until such time as the potential of structure 28 reaches a level corresponding substantially with the potential of the bi-polar electrode 24. At this time, since there is substantially no potential difference between the bi-polar electrode 24 and structure 28, there will be a diminished flow of current through conductor 40 until a balance is achieved between the impressed current supply means for anode array 26 and the reference circuit 20. The leveling of the potentials of the bi-polar electrode 24 and structure 28 brings the system into balance.

Although it was assumed that the potential of structure 28 dropped an appreciable extent to initiate the above operation, it will be apparent that no appreciable drop will normally occur because the system tends to maintain a continuous balance between the impressed current system and the reference circuit 20. That is, any slight unbalance between the potentials of the bi polar electrode 24 and structure 28 will be immediately corrected by the actuation of the impressed current system through the flowing of the control signal through conductor 40. Likewise, as soon as the impressed current means has brought the potential of structure 28 to its proper level the potential between 24 and '28 comes into balance and no appreciable current flow occurs through conductor 40. At all times during this balancing of the potentials of the bi-polar electrode 24 and structure 28 it will be seen that reference circuit 20 provides a constant reference potential. That is, the adjusted potential of anode 22 and the bi-polar electrode 24 always remains substantially constant and thereby provides a continuous, reliable and relatively unchanging reference base or reference potential.

The reference potential in reference circuit 20 is, as previously stated, that potential at which satisfactory cathodic protection is provided to structure 28 when structure 28 is also at that potential. The potential of reference circuit 20 may be established independently by utilizing the well known reference electrode or half cell (not shown). Such a reference electrode can be of any suitable type such as the silver-silver chloride reference electrode which has a potential in the electrolyte 30 which dilfers from the potential of the structure 28 when it is submerged in the electrolyte. The reference electrode can be utilized in combination with any suitable microvoltmeter or sensitive recording instrument to determine the achieved polarization level of the structure.

The level of the current flow necessary to cathodicaily protect the structure 28 may be determined by adjusting the position of wiper 54 until the independently connected reference electrode indicates that the potential of structure 28 is satisfactory to provide cathodic protection. Once this position of wiper 54 is determined, the polarization reference electrode and meter could be disconnected, but for practical purposes it is usually left installed to afford a constant reading or recording of the potential of structure 28.

The resistance oflered by the coil input and the signal amplifier and which is in series with the monitoring circuit provides a stabilizing effect of the monitoring circuit. It has been found that the sensitivity of the 6 monitoring circuit is materially enhanced by the addition of a second resistance 105 and this second resistance may be variable if so desired. In the instant embodiment the coil in the monitoring circuit and the signal amplifier has a resistance of approximately 1500 ohms and it has been found that the sensitivity of the monitoring circuit is enhanced to a material extent by the addition of a resistance such as resistance having a value of 500' ohms. Resistance 105 tends to reduce only slightly the magnitude of the signal current flowing through conductor 105, but it in conjunction with the resistance in the coil, in the series therewith in the amplifier 10, olfers a comparatively high resistance path as compared to the reference circuit of anode 22 and bi-polar electrode 24. Thus, any unusual surges or fluctuations in current =flow which may occur in the reference circuit will tend to be blocked from the monitoring circuit by resistance 105. That is, the fluctuations will tend to be maintained in the reference circuit and will not be passed on into the monitoring circuit. This stabilization is believed to occur by reason of the tendency of current surges to take the lower resistance path of the reference circuit rather than the somewhat higher resistance path of the monitoring circuit including resistance 105, conductor 40, signal amplifier 10, conductor 60 and the water path from the structure 28 to bi-polar electrode 24. The resistance of resistance 105 may be varied until it has been determined that undesirable fluctuations have been sufiiciently damped out.

The signal amplifier 10, the control reactor 12, the power reactor 14, and the power rectifier 16 may be of the type shown in my co-pending application Serial No. 734,322, filed May 9, 1958.

Referring now to Fig. 2, there is illustrated an alternative reference means 106 for establishing a reference potential. A similar reference current supply and control 18 is provided comprising the transformer 42, the wiper arm 54 and the isolation transformer 50. However, a modified form of bridge is employed in the reference circuit 107 which includes a pair of rectifiers 108 and 110. One side of the secondary winding of isolation transformer 50 is connected to anode .22 by a conductor 112, and the other side of the secondary winding is connected to bi-polar electrode 24 by a conductor 114. Rectifiers 108 and 110 are connected, as illustrated, across

conductors

112 and 114, and conductor 40 is connected intermediate the two rectifiers. In operation, alternating current from transformer 50 flows on the first half of its cycle to bi-polar electrode 24 through-conductor 114 and thence through the elec trolyte path to anode 22. In the other half of the AC. cycle current flows through conductor 112 to anode 22 and thence through the electrolyte path to bi-polar electrode 24. Thus first anode 22 and then bi-polar elec-. trode 24 become the more positive or the more negative with respect to the other, and in this regard it is important to note that the term anode as applied to element'22 and the term bi-polar electrode as applied to element 24, are used for convenience only with respect to the reference means of Fig. 2. More particularly, it will be seen that depending upon the direction of current flow to

elements

22 and 24, either may be the anode or the bi-polar electrode depending upon the current flow at that particular instant.

Thus a reference potential is established between the

elements

22 and 24, and this reference potential is, as will be described, used in the monitoring circuit. Current during one half of the AC. cycle will flow from element 22 through the electrolytic path to element 24 and thence back through conductor 114 to the power source. The disposition of rectifier 110 is such that if the potential of structure 28 is at this time less than the potential of the reference circuit 107 of

elements

22 and 24, current will flow through rectifier 110, will be blocked by rectifier 108, and will then flow through conductor 40.

From conductor 40 the reference current will pass to amplifier 1G, to structure 28 and thence back through the electrolytic path to reference circuit 107. On the other "half of the AC. cycle, current will flow from element 24 through the electrolytic path to element 22 and thence back through conductor 112 to the power source. Again, assuming the potential of structure 28 is less than that of reference circuit 107, current will flow through rectifier '108 to conductor 40, being blocked by rectifier 110. From conductor 40 the current path is to amplifier 10, to structure 28, and thence back through the electrolyte to reference circuit 107.

There are no moving parts in the system. Thus, in contrast to other systems, my system has no mechanical relays, contact points, servo mechanisms, motorized 'variacs or other moving parts which would require extensive and continued maintenance. By utilizing saturable core reactors, magnifications of a high order are obtained by devices which have a relatively long life and require no maintenance. A precise control is obtained which has an almost instantaneous response and which closely follows the demand on the system. Such performance coupled with the lack of maintenance is particularly important in systems of this type which are -often installed in large ships which may not return to their home ports for long periods of time. If this were not the case, a breakdown of the system while the ship was out of port could permit severe damage to occur to the hull of the ship before the system could again be placed in operation.

It is also apparent, from the foregoing, that I have provided a system in which the polarization level of the reference bi-polar electrode is maintained constant or substantially so. Thus providing means to establish a predetermined polarization balance between a structure or ship to be cathodically protected and means to provide the required impressed current supply to the structure, responsive to the polarization balance demand.

Also it is apparent that I have provided a system which falls safe" in the event of failure, for any reason whatsoever, of the reference circuit. In the event of failure of the reference circuit, there will be no flow of current through conductor 40 to actuate or energize signal amplifier so that the means for impressing current will be rendered ineffective or inoperative. In this manner, paint stripping and other damage to the structure 2-8 due to over polarization is prevented;

It is also apparent that in addition to being useful for the cathodic protection of the hulls of ships, barges and other floating vessels, my system can be used for cathodically protecting other structures such as underwater foundations, pipe lines, storage reservoirs and the like.

While the form of embodiment herein shown and described constitutes a preferred form, it is to be understood that other forms may be adopted falling within the scope of the claims that follow:

I claim:

1. In a control system for cathodically protecting a structure immersed in an electrolyte, an anode immersed in the electrolyte; means connected to said anode and said structure and operative to impress a current flow between said anode and the structure to raise the potential of said structure and cause it to serve as a cathode; and a monitoring circuit connected to said structure and including a reference electrode immersed in the electrolyte, a second electrode immersed in the electrolyte, a source of direct current connected with the last two mentioned electrodes for maintaining said reference electrode at an adjusted level of. potential whereby, when there is a change in the potential of said structure, a reference current is caused to flow in said monitoring circuit between the structure and said reference electrode, said first mentioned means being electrically coupled to said monitoring circuit and controlled thereby.

*2. A control system as in claim 1, wherein the refer once electrode includes an electrode of substantially insert material connected to said monitoring circuit.

3. In a controlled system for cathodically protecting a structure immersed in an electrolyte, an anode immersed in the electrolyte; means connected to said anode and said structure and operative to impress a current flow between said anode and the structure to raise the potential of said structure and cause it to serve as a cathode; a monitoring circuit connected to said structure and including a reference electrode immersed in the electrolyte, a second electrode immersed in the electrolyte, a source of direct current connected with the last two mentioned electrodes for maintaining said said reference electrode at an adjusted level of potential; and means responsive to the flow of current in the monitor circuit for controlling the first mentioned means. I

4. In a control system for cathodically protecting a structure immersed in an electrolyte, an anode immersed in the electrolyte; means connected to said anode and said structure and operative to impress a current flow between said anode and the structure to raise the potential of said structure and cause it to serve as a cathode; a monitoring circuit connected to said structure and including a reference electrode immersed in the electrolyte, a second electrode immersed in the electrolyte, a source of direct current connected with the last two mentioned electrodes for maintaining said reference electrode at an adjusted level of potential; and means responsive to the increase and decrease in flows of current in the monitor circuit for increasing and decreasing, respectively, the value of the impressed current.

5. In a control system for cathodically protecting a structure immersed in an electrolyte, an anode immersed in the electrolyte; means connected to said anode and said structure and operative to impress a current flow between said anode and the structure to raise the potential of said structure and cause it to serve as a cathode; and a monitoring circuit connectedto said structure and comprising reference means including a second anode and a bi-polar electrode immersed in the electrolyte, means connecting said second anode and said bi-polar electrode, and including a controlled extraneous power source for maintaining said bi-polar electrode at an adjusted level of potential whereby when there is a change in the potential of said structure, a reference current is caused to flow in said monitoring circuit between the structure and said electrode, said first mentioned means being electrically coupled to said monitoring circuit and controlled thereby.

6. A control system as claimed in claim 3, wherein said anode and said reference electrode of said reference means are composed of substantially inert material.

7. In a control system for cathodically protecting a structure immersed in an electrolyte, an anode immersed in the electrolyte; means connected to said anode and said structure and operative to impress a current flow between said anode and the structure to raise the potential of said structure and cause it to serve as a cathode; a monitoring circuit connected to said structure and comprising reference means including an anode and a bi-polar electrode immersed in the electrolyte, means connecting said anode and said bi-polar electrode, and including a controlled extraneous power source for maintaining said bi-polar electrode at an adjusted level of potential; and means responsive to the flow of current in the monitor circuit for controlling the first mentioned means.

8. In a control system for cathodically protecting a structure immersed in an electrolyte, an anode immersed in the electrolyte; means connected to said anode and said structure and operative to impress a current flow between said anode and the structure to raise the potential of said structure and cause it to serve as a cathode; a monitoring circuit connected to said structure and comprising reference means including an anode and a bipolar electrode immersed in the electrolyte, means connecting said anode and said bi-polar electrode, and ineluding a controlled extraneous power source for main- 10. A control system as in claim 9, wherein the resisttaining said bi-polar electrode at an adjusted level of ance means are variable. potential; and means responsive to the increase and decrease in flows of current in the monitor circuit for in- References Cited in the filfi of this Patel!t creasing and decreasing, respectively, the value of the 5 UNITED STATES PATENTS impressed current.

9. A control system as in claim 5, wherein the monitor- 2,759,387 Mllfls 1956 ing circuit includes resistance means which dampens cur- 2,903,405 Sablns P 1959 rent surges occurring in said reference means.

Claims

1. IN A CONTROL SYSTEM FOR CATHODICALLY PROTECTING A STRUCTURE IMMERSED IN AN ELECTROLYTE, AN ANODE IMMERSED IN THE ELECTROLYTE; MEANS CONNECTED TO SAID ANODE AND SAID STRUCTURE AND OPERATIVE TO IMPRESS A CURRENT FLOW BETWEEN SAID ANODE AND THE STRUCTURE TO RAISE THE POTENTIAL OF SAID STRUCTURE AND CAUSE IT TO SERVE AS A CATHODE; AND A MONITORING CIRCUIT CONNECTED TO SAID STRUCTURE AND INCLUDING A REFERENCE ELECTRODE IMMERSED IN THE ELECTROLYTE, A SECOND ELECTRODE IMMERSED IN THE ELECTROLYTE, A SOURCE OF DIRECT CURRENT CONNECTED WITH THE LAST TWO MENTIONED ELECTRODES FOR MAINTAINING SAID REFERENCE ELECTRODE AT AN ADJUSTED