US2918421A

US2918421A - Measuring means for the bath resistance of aluminum reduction cells

Info

Publication number: US2918421A
Application number: US665777A
Authority: US
Inventors: Carl J Lundborg
Original assignee: Anaconda Aluminum Co
Current assignee: Atlantic Richfield Co
Priority date: 1957-06-14
Filing date: 1957-06-14
Publication date: 1959-12-22
Anticipated expiration: 1976-12-22

Description

Dec. 22, 1959 LUNDBORG 2,918,421

C. J. MEASURING MEANS FOR THE BATH RESISTANCE OF ALUMINUM REDUCTION CELLS Filed June 14. 1957 K24 INVENTOR CARL J. LUN DBORG ATTORNEYS (M. ma

United States Patent MEASURING MEANS FOR THE BATH RESIST- AN CE OF ALUMINUM REDUCTION CELLS Carl J. Lundborg, Butte, Mont, assignor to Anaconda Aluminum Company, New York, N.Y., a corporation of Montana This invention relates to electrical apparatus for measuring the resistance between the cathode and movable anode of an aluminum reduction electrolytic cell, to provide a guide for adjusting the movable anode to its optimum position with respect to the cathode. In a preferred embodiment, the invention provides means to measure the resistance, and hence to indicate the optimum anode adjustment, in any one of a series of aluininum reduction cells. In general these results are achieved by the provision of electrical circuits which compensate for those portions of the total voltage drop through the cell due to fixed resistances of the cell and its power supply circuit and due to the electrochemical reaction taking place in the cell, and which utilize the voltage after such compensations have been made to indicate the resistance of the electrolytic bath between the anode and cathode.

Aluminum metal is conventionally produced by the electrolytic decomposition of alumina in a molten bath of alumina and cryolitecontained in an electrolytic reduction cell (or pot, as it is usually called). In such a process, current is passed through the bath between a conductive carbon lining serving as the cathode of the cell and a conductive carbon anode partially immersed in the bath. Aluminum metal deposits at the cathode and oxygen is released at the anode. At the high temperature prevailing, the oxygen combines with the carbon of the anode. The anode therefore is mounted on jacks so that it may be lowered into the bath as it is consumed by oxidation, thus permitting a substantially constant distance to be maintained between the anode and cathode. In this manner the bath resistance to the flow of current through the cell, and the amount of current which flows through the cell, are both held at substantially constant .values by continuous adjustment of the jacks. It is the primary object of this invention to provide means for gauging the required adjustment of the anode by indicating the value of the bath resistance.

The total voltage applied across the terminals of an aluminum reduction cell must equal the decomposition potential of aluminua plus the sum of the voltage drops due to the fixed resistances of the cell and its power connections and to the somewhat variable resistance of the electrolytic bath between the anode and the cathode. The fixed resistances of a given cell are readily measurable, and the decomposition potential of alumina in a molten cryolite solution is known to be substantially equal to 1.7 volts. The bath resistance, however, tends to increase as the bottom face of the anode burns away and widens the spacing between anode and cathode, and in consequence the current through the cell tends to diminish. It is therefore necessary to adjust the position of the anode, either continuously or from time to time, to maintain the bath resistance substantially constant at an optimum predetermined value.

Heretofore it has been common practice to adjust the position of the anode with respect to the cathode to ice secure a predetermined over-all voltage drop through the cell. This voltage drop is a function of the current flowing through the cell, and such current-may (and frequently does) change during adjustment of the anode position. it is therefore preferable to make the anode adjustment to maintain a predetermined value of bath resistance. The present invention provides means for determining the bath resistance at any moment and gauging the anode adjustment accordingly.

The bath resistance measuring means of the invention comprises a proportional current circuit including a potentiometer resistance element, an adjustable resistance and a source of direct current connected in series. This circuit further includes means for maintaining the current flowing in it in fixed proportion to the current flowing through the electrolytic cell. Other circuit means are provided for applying a residual voltage, equal to the total electrolytic cell voltage minus the decomposition potential of alumina, across the adjustable resistance plus a variable portion of the potentiometer resistance in said proportional current circuit; and means are provided for varying the variable portion of the potentiometer resistance to a point where the voltage drop due to current flowing through said variable portion and said adjustable resistance equals the residual voltage appiied thereto. At the point of balance, the magnitude of the variable portion of the potentiometer resistance is proportional to the resistance of the electrolytic bath, provided the adjustable resistance has been set at a value such that the voltage drop across it due to current flowing through it is equal to the voltage drop due to current flowing through the fixed resistances of the electrolytic cell. v

The circuit means for applying the correct residual voltage to the adjustable resistance and the variable portion of the potentiometer resistance in. the proportional current circuit preferably comprises a circuit connecting the terminals of the electrolytic cell, in series with a source of back E.M.F. equal to the decomposition of voltage alumina, across said resistances. Advantageously a currentsensing means is connected in series with the source of back E.M.F., and an electric motor is provided to adjust the potentiometer to a position at which the current-sensing means gives a null indication. The current-sensing means in turn may serve to actuate the circuit through which the motor is energized, so as to drive the motor in the direction necessary to produce a null indication in the current-sensing means.

The means for maintaining the current in the proportional current circuit in fixed proportion to the current flowing through the electrolytic cell advantageously comprises a shunt resistor connected effectively in parallel with a shunt in the electrolytic cell circuit and therefore normally carrying a current proportional to the current flowing through the electrolytic cell. The proportional current circuit then includes, in series with the other components, a fixed resistance and a variable resistance; and the fixed resistance of this circuit is connected also to the shunt resistor through current-sensing means which give a null indication when the current through the fixed resistance of the proportional current circuit is in a fixed proportion to the current through the shunt resistor and hence to the current through the electrolytic cell. A motor is advantageously provided to actuate the variable resistance in the proportional current circuit. This motor preferably is energized through a circuit actuated in response to the indications of the current-sensing means in circuit with the fixed resistance and the shunt resistor, thereby automatically to vary the value of the variable resistance as required to maintain the current in the proportional current circuit at all times in fixed proportion to the current through the electrolytic cell.

Ordinarily a considerable number of aluminum reduction pots or cells are connected in series. it is advantageous to locate the bath resistance indicating means provided by this invention at a central control point, and to provide switching means by which the terminals of any one of the cells or pots may be selectively connected to the appropriate resistances of the proportional current circuit in order to obtain a measurement of the bath resistance of such cell. If desired, the motor means for varying the variab e portion of the potentiometer resistance in the-proportional current circuit may also actuate a visual indicator calibrated to provide a direct visual indication of the cell bath resistance.

A preferred embodiment of the measuring means of this invention is described herein with reference to the accompanying drawing, the single figure of which is a circuit diagram of the new measuring means.

A number of electrolytic cells 1, 1, .1 are connected in series (in what is commonly called a pot line) to a primary source 2 of direct current by a power line 3. Each cell contains a molten electrolytic bath 4 of alumina and cryolite in a carbon-lined pot 5, the carbon lining of which serves as the cathode of the cell. Immersed in the electrolytic bath 4 is a carbon anode 6 supported by a frame which is mounted on jacks 7. Current from the source 2 flows serially through the anode 6, the bath 4, and the cathode of each cell.

The total voltage across the terminals of any one of the cells is the sum of (a) the voltage drop across the fixed resistances of the cell (i.e. the anode resistance, the cathode resistance, and the resistance of the associated conductors and connections), (b) the voltage drop across the variable resistance of the bath between the anode and the cathode, and (c) the electrochemical decomposition potential of the alumina. In a given cell only the bath resistance varies, for the fixed resistances are determined by the construction and materials of the cell and the connections to it, and the decomposition potential of alumina is a constant quantity (1.7 volts). To maintain an optimum voltage drop across any one cell, and a substantially uniform flow of current through it, it is therefore necessary to control the bath resistance by keeping the anode at a substantially constant predetermined distance from the cathode. This is accomplished by lowering the anode into the bath by means of the jacks 7 as the lower end of the anode is consumed by oxidation.

An accurate indication of the distance through the bath between the anode and the cathode can be obtained by measuring the bath resistance. However, to do so it is first necessary to isolate a voltage drop equal to that due to bath resistance. According to the present invention this is accomplished by compensating for the voltage drop due to fixed resistances of the cell and for the decomposition potential of alumina, and determining from the residual voltage drop the resistance of the bath.

The current through each of the cells in the pot line circuit flows through an accurately calibrated shunt 8. A shunt resistor 9 of accurately known resistance is connected through a conventional magnetic amplifier 19 in parallel with the shunt. A current i which is directly proportional at all times to the current I through the pot line consequently flows through the shunt resistor.

A proportional current curcuit 11 is provided, includmg lnseries a DC. voltage source 12, the resistance element 13 of a potentiometer, an adjustable resistance 14, a variable resistance 15, and a fixed resistance 16. The fixed resistance 16 also is connected to the shunt resistor 9 through current-sensing means 17. So long as the voltage drop impressed on the shunt resistor 9 by the magnetic amplifier is equal to the voltage drop across the fixed resistance 16 due to current flowing in the circut 11, the current-sensing means 17 will yield a null indication. The variable resistance 15 is controlled by a motor 18 which is energized by a circuit 19 responsive to the fiow of current through the current-sensing means 17. Whenever current flows through the latter, the motor circuit is energized to actuate the motor 18 in the direction necessary to reduce the current flow through the sensing means to zero.

Thus, an increase (or decrease) in the pot line current will cause a proportional increase (or decrease) in current through the shunt resistor 9. This will cause the voltage drop across the shunt resistor 9 to exceed (or become less than) that across the fixed resistance 16. The resulting flow of current through the current-sensing means 17 will then energize the motor 18 in the direction necessary to decrease (or increase) the variable resistance 15, and thereby increase (or decrease) the current in the proportional current circuit 11 until a null response is again obtained from the current-sensing means. By this arrangement the current in the proportional current circuit 11 is always and automatically maintained in fixed proportion to the current flowing through the pot line circuit 3, regardless of variations that occur in the magnitude of such current.

Leads are connected from the terminals of each of the electrolytic cells 1, 1' 1 to the points of a twopole multi-throw selector switch 20. The two poles of the switch thus may be selectively connected to the terminals of any one of the electrolytic cells. In the circuit diagram, the switch poles are shown connected to the terminals of the first cell 1.

A circuit 21 is provided to connect any one of the electrolytic cells through the selector switch 20 and through a source of back E.M.F. 22 across the adjustable resistance 14 and (through a potentiometer slider 23) a variable portion of the potentiometer resistance element 13 of the proportional current circuit 11. The circuit 21 thus impresses across these resistances the full value of the electrolytic cell voltage E, minus the amount of the back E.M.F. of the source 22. This back E.M.F. is a fixed DC. voltage equal in value to the decomposition potential of alumina, E, (1.7 volts). The polarity of the back E.M.F. source 22 is arranged relative to the polarity of the electrolytic cell with which the circuit 21 is connected, so that the residual voltage E impressed across the resistances of the proportional current circuit is equal to the electrolytic cell voltage minus the decomposition potential of alumina, i.e. it is 1.7 volts less than the actual cell voltage. The residual voltage E thus is equal to the voltage drop in the electrolytic cell due to the flow of the electrolytic cell current I through the fixed resistances and the bath resistance of the cell.

As described above, the flow of current through the proportional current circuit, and hence through the adjustable resistance 14 and potentiometer resistance element 13, is automatically maintained in fixed proportion to the amount of current i flowing through the electrolytic cells. The adjustable resistance 14 can be preset to a value such that the voltage drop through it due to the flow Of the proportional current i is equal to that portion of the voltage drop in the electrolytic cell due to the flow of the cell current I through the fixed resistances of the cell. Then, the potentiometer slider 23 can be adjusted to a position such that the residual voltage E impressed across the sum of the adjustable resistance 14 and'the variable portion of the potentiometer resistance element 13 is just balanced by the voltage drop due to the flow of the proportional current i through these resistances. At this position of balance, and with the adjustable resistance 14 preset to the value stated above, the magnitude of the variable portion of the potentiometer resistance across which the residual voltage E is impressed will bear a definite proportional relation to the resistance of the electrolytic cell bath.

It is thus evident that by presetting the adjustable resistance 1 4 in the manner above outlined, and by then current through the current-sensing element.

moving the potentiometer slider 23 to balance the residual voltage E against the voltage drop due to flow of proportional current i through the adjustable resistances 14 and the potentiometer resistance 13, an indication of the cell bath resistance can be obtained. This resistance can be made visually observable and directly readable in magnitude by coupling the potentiometer slider 23 to the pointer of an indicator 24 which is calibrated to read directly the value of the cell bath resistance.

Preferably, in order to facilitate operation of the apparatus and render it substantially completely automatic, a current-sensing element 25 is included in the circuit 21 through which the residual voltage E is impressed across the resistances of the proportional current circuit. A motor 26 is then provided to operate the potentiometer slider 23 and the pointer of the indicator 24. This motor 26 is actuated through an energizing circuit 27 which is responsive to the flow, and to the direction of flow, of Through the circuit 27 the motor is energized, whenever there is a flow of current through the sensing means 25, in a direction to move the potentiometer s ider to the point where the residual voltage E is just balanced by the voltage drop due to flow of the proportional current i through the adjustable resistance 14 and the variable portion of the potentiometer resistance. At this po nt of balance, the current-sensing element 25 gives a null indication and deenergizes the motor 26.

With the circuits and apparatus described above, the actual value at any moment of the resistance of the electro'yte bath in any one of the cells in the pot line can be quickly and visually indicated by simp y moving the selector switch 20 to the position where its poles are connected to the terminals of that electrolytic cell. provided only that the setting of the adiusab e resistance 14 is such that the volta e drop due to flow of the proportional current i throu h that resistance is equal to the voltage drop due to the flow of cell current I through the tired resistances of the cell for which the indic tion is being ta en. Generally the fixed resistances of each of a series of identical electrolytic cells wi l be the same and when such is the case, no resetting of the adiustab e resistance 14 is necessary when the circuit 21 is switched from one cell to another. If, however, there are diflerences in the fi ed resistances of various of the cells then detents or o her means may be provided on the mechani m of the adjustable resistance to insure rapid setting of it at the correct value for the ce l to whi h connection is made through the se ector switch 20. Indeed, it is possible to interlock the selector switch and the adjustable resistance 14 so that the correct setting of the former is achieved automatically upon moving the selector switch. It is of course apparent that once the fixed resistances of any one of the cells has been determined, and the corresponding sett ng of the adjustable resistance for that cell h s been ascertained, the adjustable resistance will always be returned to that setting whenever the bath resistance of that particular cell is to be determined.

With the measuring means of this invention, an accurate and rapid determination of the bath resistance of any electrolytic cell is a pot line can be made from a convenient central location. By the provision of suitable remote contro-ls for the jacks 7, the anode of that cell can be adjusted from that same central location to maintain optimum length of the electrical path between anode and cathode.

I claim:

1. The combination with an aluminum reduction cell having a cathode and an anode which is movable relative to saidcathode immersed in an electrolytic bath, and circuit means connecting said cell to a source of direct current, of means for measuring the resistance of the electrolytic bath between the anode and cathode comprising a proportional current circuit including a potentiometer resistance element, an adjustable resistance and a 6 source of direct current connected in series, "means "for maintaining the current flowing in said proportional current circuit in fixed proportion to the current flowing through said electrolytic cell, circuit means for applying a residual voltage equal to the total electrolytic cell voltage minus the decomposition voltage of alumina across said adjustable resistance and a variable portion of the potentiometer resistance element of said proportional current circuit, and means for varying said variable portion of the potentiometer resistance to balance the voltage due to current flowing through said variable portion and said adjustable resistance against the residual voltage applied thereto, whereby the magnitude of the variable portion of the potentiometer resistance is proportional to the resistance of the electrolytic bath when the value of said adjustable resistance is such that the voltage drop due to current flowing therethrough equals the voltage drop due to current flowing through the fixed resistances of the electrolytic cell. p

2. The combination with an aluminum reduction cell having a cathode and an anode which is movable relative to said cathode immersed in an electrolytic bath, and circuit means connecting said cell to a source of direct current, of means for measuring the: resistance of the e ectrolytic bath between the anode and the cathode comprising a proportional current circuit including a potentiometer resistance element, an adjustable resistance and a source of direct current connected in series, means for maintaining the current, flowing in said proportional current circuit in fixed proportion to the current flowing through said elec rolytic cell, circuit means connecting the terminals of said cell in series with a source of back EMF. equal to the decomposition voltage of alumina across said adjustable resistance and a variable portion of the potentiometer resistance element of said proportional current circuit, and means for varying said variable portion of the potentiometer resistance to balance the voltage drop due to current flowing through said variable portion and said ad'ustable resistance against the residual voltage applied thereto from the terminals of the electrolytic cell, whereby the magnitude of the variable portion of the potentiometer resistance is proportional to the resistance of the electrolytic bath when the value of said adjustable resistance is such that the voltage drop due to current flowing therethrough equals the voltage drop due to current flowing through the fixed resistances of the electrolytic cell,

3. The combination with an aluminum reduction cell having a cathode and an anode which is movable relative to said cathode immersed in an electrolytic bath, and circuit means connecting said cell to a source of direct current, of means for measuring the resistance of the electrolytic bath between the anode and the cathode comprising a proportional current circuit including a potentiometer resistance element, an adjustable resistance and a source of direct current connected in series, means for maintaining the current flowing in said proportional current circuit in fixed proportion to the current flowing through said electrolytic cell, circuit means connecting the terminals of said cell in series with a source of back equal to the decomposition voltage of alumina through current-sensing means across said adjustable resistance and a variable portion of the potentiometer resistance elemerit of said proportional current circuit, motor means for varying said variable portion of the potentiometer resistance, and a motor energizing circuit responsive to the flow of current through said current-sensing means, whereby said motor means is energized in a direction to vary the variable portion of the potentiometer resistance to balance the voltage drop due to current flowing through said variable portion and said adjustable resistance against the voltage applied thereto through said current-sensing means, and whereby the magnitude of the variable portion of the potentiometer resistance is automatically maintained proportional to the resistance of the electrolytic trolytic cell having a cathode and an anode which is movable relative to said cathode immersed in an electrolytic bath, and circuit means connecting said cell to a source of direct current, of means for measuring the resistance of the electrolytic bath between the anode and cathode comprising a shunt resistor connected effectively in parallel with a shunt in said electrolytic cell circuit and normally carrying a current proportional to the current flowing through said electrolytic cell, a proportional current circuit including a potentiometer resistance element, an adjustable resistance, a variable resistance, and a fixed resistance connected in series with a source of direct current, the fixed resistance of said proportional current circuit being connected to said shunt resistor through current-sensing means, motor means for varying said variable resistance, a motor actuating circuit energized in response to the flow of current through said currentsensing means for energizing said motor means in a direction to vary said variable resistance as required to maintain the current flowing in the proportional current circuit and through the fixed resistance thereof in fixed proportion to the current flowing through said shunt resistor, circuit means for applying a residual voltage equal to the total electrolytic cell voltage minus the decomposition voltage of alumina across said adjustable resistance and a variable portion of the potentiometer resistance element of said proportional current circuit, and means for varying said variable portion of the potentiometer resistance to balance the voltage drop due to current flowing through said variable portion and said adjustable resistance against the residual voltage applied thereto, whereby the magnitude of the variable portion of the potentiometer resistance is proportional to the resistance of the electrolytic bath when the value of said adjustable resistance is such that the voltage drop due to current flowing therethrough equals the voltage drop due to current flowing through the fixed resistances of the electrolytic cell.

6. The combination with an aluminum reduction cell having a cathode and an anode which is movable relative to said cathode immersed in an electrolytic bath, and circuit means connecting said cell to a source of direct current,.of means for measuring the resistance of the electrolytic bath between the anode and cathode comprising a shunt resistor connected efiectively in parallel with a shunt in said electrolytic cell circuit and normally carrying a current proportional to the current flowing through said electrolytic cell, a proportional current circuit including a potentiometer resistance element, an adjustable resistance, a variable resistance, and a fixed resistance connected in series with a source of direct current, the fixed resistance of said proportional current circuit being connected to said shunt resistor through first current-sensing means, first motor means for varying said variable resistance, a motor actuating circuit energized in response to the flow of current through said current-sensing means for energizing said first motor means in a direction to vary said variable resistance as required to maintain the current flowing in the proportional current circuit and through the fixed resistance thereof in fixed proportion to the current flowing through said shunt resistor, circuit means connecting the terminals of said cell in series with a source of back E.M.F. equal to the decomposition voltage of alumina through second current-sensing means across said adjustable resistance and a variable portion of the potentiometer resistance element of said proportional current circuit, second motor means for varying said variable portion of the potentiometer resistance, and a motor energizing circuit responsive to current flow through said second current-sensing means for energizing said second motor means in a direction to vary the variable portion of the potentiometer resistance to balance the voltage drop due to current flowing through said variable portion and said adjustable resistance against the voltage applied thereto through said second currentsensing means, whereby the magnitude of the variable portion of the potentiometer resistance is automatically maintained proportional to the resistance of the electrolytic bath when said adjustable resistance has been preset to a value such that the voltage drop due to current flowing therethrough equals the voltage drop due to current flowing through the fixed resistances of the electrolytic cell.

7. The combination according to claim 3, in which a plurality of aluminum reduction cells are connected in series, including a selector switch for connecting the terminals of any one of said cells selectively through the source of back and the current-sensing means across the variable portion of the potentiometer resistance and the adjustable resistance.

8. The combination with a plurality of aluminum reduction cells each having a cathode and an anode which is movable relative to said cathode immersed in an electrolytic bath, and circuit means connecting said cells in series with a source of direct current, of means for measuring the resistance of the electrolytic bath between the anode and cathode of any one of said cells comprising a proportional current circuit including a potentiometer resistance element, an adjustable resistance and a source of direct current connected in series, means for maintaining the current flowing in said proportional current circuit in fixed proportion to the current flowing through said series of electrolytic cells, circuit means including a selector switch for connecting the terminals of any one of said cells through a source of back E.M.F. equal to the decomposition voltage of alumina across said adjustable resistance and a variable portion of the potentiometer resistance element of said proportional current circuit, and means for varying said variable portion of the potentiometer resistance to balance the voltage drop due to current flowing through said variable portion and said adjustable resistance against the voltage applied thereto from the terminals of an electrolytic cell, whereby the magnitude of the variable portion of the potentiometer resistance is proportional to the resistance of the electrolytic bath in that cell to which it is connected through said selector switch means when said adjustable resistance is preset to a value such that the voltage drop due to current flowing therethrough equals the voltage drop due to current flowing through the fixed resistances of said electrolytic cell.

References Cited in the file of this patent UNITED STATES PATENTS 2,545,411 Perret-Bit Mar. 13, 1951 2,545,412 Ferret-Bit Mar. 13, 1951 2,545,413 Ferret-Bit Mar. 13, 1951 2,731,412 Ferrand Jan. 17, 1956

Claims

1. THE COMBINATION WITH AN ALUMINUM REDUCTION CELL HAVING A CATHODE AND AN ANODE WHICH IS MOVABLE RELATIVE TO SAID CATHODE IMMERSED IN AN ELECTROLYTIC BATH, AND CIRCUIT MEANS CONNECTING SAID CELL TO A SOURCE OF DIRECT CURRENT, OF MEANS FOR MEASURING THE RESISTANCE OF THE ELECTROLYTIC BATH BETWEEN THE ANODE AND CATHODE COMPRISING A PROPORTIONAL CURRENT CIRCUIT INCLUDING A POTENTIOMETER RESISTANCE ELEMENT, AN ADJUSTABLE RESISTANCE AND A SOURCE OF DIRECT CURRENT CONNECTED IN SERIES, MEANS FOR MAINTAINING THE CURRENT FLOWING SAID PORPORTIONAL CURRENT CIRCUIT IN FIXED PROPORTION TO THE CURRENT FLOWING THROUGHT SAID ELECTROLYTIC CELL, CIRCUIT MEANS FOR APPLYING A RESIDUAL VOLTAGE EQUAL TO THE TOTAL ELECTROLYTIC CELL VOLTAGE MINUS THE DECOMPOSTION VOLTAGE OF ALUMINA ACROSS SAID ADJUSTABLE RESISTANCE AND VARIABLE PORTION OF THE POTENTIOMENER RESISTANCE ELEMENT OF SAID PROPORTIONAL CURRENT CIRCUIT, AND MEANS FOR VARYING SAID VARIABLE PORTION OF THE POTENTIOMENTER RESISTANCE TO BALANCE THE VOLTAGE DUE TO CURRENT FLOWING THROUGH SAID VARIABLE PORTION AND SAID ADJUSTABLE RESISTANCE AGAINST THE RESIDUAL VOLTAGE APPLIED THERETO, WHEREBY THE MAGNITUDE OF THE VARIABLE PORTION OF THE POTENTIOMETER RESISTANCE IS PROPORTIONAL TO THE RESISTANCE OF THE ELECTROLYTIC BATH WHEN THE VALUE OF SAID ADJUSTABLE RESISTANCE IS SUCH THAT THE VOLTAGE DROP DUE TO CURRENT FLOWING THERETHROUGH EQUALS THE VOLTAGE DROP DUE TO CURRENT FLOWING THROUGH THE FIXED RESISTANCE OF THE ELECTROLYTIC CELL.