US 3657647 A
An electrochemical coulometer having a bore of varying cross section is disclosed. The resistance of the coulometer varies with time in a manner corresponding to the bore cross section as electric current is passed through the device causing the electrolyte in the coulometer to take on varying configurations within the bore.
Description (OCR text may contain errors)
United States Patent Beusman et al.
[151 I 3,657,647 1451 Apr. 18, 1972 VARIABLE BORE MERCURY MICROCOULOMETER Inventors: I
Finger, Brewster, both of NY.
Curtis Instruments, Inc.
Filed: Feb. 10, 1970 Appl. No.: 10,179
Related US. Application Data 9 Continuation-impart of Ser. No. 683,903, Nov. 17, 1967, abandoned.
.....324/94, 200/152 L, 235/197 .0011 27/22 00111 21/42 ..324/93, 94, 68 ET; 204/195;
References Cited UNITED STATES PATENTS Curtis C. Beusman, Mount Kisco; Eugene P.
Primary Examiner-Gerard R. Strecker Attorney-Pennie, Edmonds, Morton, Taylor and Adams [5 7] ABSTRACT An electrochemical coulometer having a bore of varying cross section is disclosed. The resistance of the coulometer varies with time in a manner corresponding to the bore cross section as electric current is passed through the device causing the electrolyte in the coulometer to take on varying configurations within the bore.
9 Claims, 8 Drawing Figures VARIABLE BORE MERCURY MICROCOULOME'IIER CROSS-REFERENCE To RELATED APPLICATIONS BACKGROUND OF THE INVENTION The present invention relates toelectrochemical devices known as coulometers and, more particularly, to a coulometer type instrument whose resistance varies as a function of time.
The U. S. Pat. No. 3,045,178 issued to Lester .Corrsin on July 17, 1962 and entitled, Operating Time Indicator described the theory of operation and construction of a coulometric device of which the present invention is an improvement. The instrument described in that patent includes a body of electrically non-conductive material having a uniform bore .therethrough which supports two columns of liquid metal (e.g., mercury), the adjacent inmost ends of which are separated by, but maintained in conductive contact with a small volume of liquid electrolyte. The bore diameter is suffciently small that the capillary forces on the mercury and the liquidelectrolyte are greaterthan gravitational and inertial ef fects.The outermost ends of the two metal columns are maintained in contact with suitable conductive leads provided to connect the instrument to a source of electrical current. The flow of electrical current from one metal column to the other through the electrolyte causes metallic ions to migrate from the positive column (anode) to the negative column (cathode). In accordance with Faradays Law, liquid metal is electroplated from the anode column to the cathode column causing the anode column to decrease in length and the cathode column to elongate correspondingly, the change in column length being directly proportional to the total e1ectrical charge passed through the device. When such a device is connected to a source of constant current readout of the measured time (or time-current product) inay be obtained, for example, by a direct visual comparison of one column length against a calibrated scale.
SUMMARY OF THE INVENTION According to the present invention, a coulometric instrument of the type generally described above is provided wherein the cross-sectional area of the bore containing the two liquid metal columns and electrolyte varies along the length of the bore. The variation in bore cross section causes the electrolyte separating the two liquidmetal columns to assume varying configurations, resulting in a varying=electrolyte resistance as current is passed through the deviceLIn general, the cross section of the bore may be varied in any continuous or discontinuous fashion to permit the generation of a wide range of functions of coulometer resistance versus time.
In one embodiment of the invention the bore of the coulometer is constricted at one region along its length so that when a constant current source applied to the coulo'metric device has caused the electrolyte separating the metal columns to reach that region the electrical resistance of the device is increased, thus providing a readily detectable indication ofthat event. I
In a second embodiment of the invention, the bore of the device is constricted at a plurality of evenly spaced regions along its length so that the cross section of the bore varies in any desired manner suitable to the intended use of the device, e.g., in a regular periodic fashion. The resistance of this embodiment will vary accordingly in an aperiodic or in a periodic fashionas electrical charge is passed through the device.
In a third embodiment of the invention described herein, the effective cross section of the bore is varied by positioning a non-conductive body within the bore itself. The body itself may vary in cross-section to form the desired cross section of the bore.
When connected to a suitable source of electric current, the present invention provides a time-varying resistance element for general use in electronic circuitry with particular application in the-generation of periodic or aperiodic timing signals.
. These and further objects and advantages of the present invention are described in greater detail in the following description of specific embodiments.
BRIEF DESCRIPTION OF THE DRAWING FIG. Ia, lb and 1c are three sectional views of a first embodiment of the invention respectively showingthe electrolyte separating the metal columns at three different locations;
FIG. 2is a diagram depicting the variation of the electrical resistance of the first embodiment as a function of position of the electrolyte along the bore;
FIG. 3 is a sectional view of a portion of a second embodi-- DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS In the first embodiment, illustrated in FIG. in, tube 10 of nonconductive material such as glass, ceramic, epoxy resin or the like, has a bore 11 extending therethrough. The bore has a circular cross section throughout, but the cross-sectional area of the bore is substantially diminished at a region 12 midway along the length of the bore.
Two liquid metal columns 13 and 14 (e.g., mercury) are supported in the bore, extending inwardly from opposite ends of the tube and separated at the inmost ends by a space filled by a small volume of electrolyte 1.5 which is maintained in conductive contact with both columns. A suitable electrolyte may comprise a water solution of potassium iodide and mercuric iodide as described in the above-identified patent to Corrsin.
The bore 11 is sealed at both ends by epoxy resin seals 16 as shown. Two conductive leads 17 and 18, the innermost ends of which are immersed in the mercury columns provide electrical contact with the columns. The conductive leads are preferably made of a metal such as nickel which does not chemically combine with mercury.
When an external source of constant electrical current (not lead 18 is positive (i.e., the anode) the flow of current through the device causes mercury to be electroplated from column 14 to column 13. The metal column 131 grows at the expense of column 14 and the electrolyte 15 moves toward the anode lead 18. The process is completely reversible, and when the flow of current through the device is reversed the movement of the electrolyte also changes direction.
With the continued passage of current through the device the electrolyte 15 comes to occupy the position as shown in FIG. 1b, and as the current flow continues further, eventually it reaches the location shown in FIG. 10.
Since the resistance of the metal columns and conductive leads is extremely low, the resistance of the coulometer device shown in FIG. 1 is almost entirely due to the resistance of the electrolyte 15. The resistance through the electrolyte, in turn, is proportional to the distance across the space separating the metal columns and inversely proportional to the cross-sectional area of the space. In the positions shown in FIG. 1a and 1c the resistance across the electrolyte 15 is seen to be minimized since the distance separating the columns 13 and 14 in those positions is minimal and the area of the opposing ends of 13 and 14 is maximized. However, when the electrolyte 15 moves into the region 12 of decreased bore cross section as shown in FIG. lb it assumes a masimum resistance since in that configuration the distance between columns 13 and 14 is greatest and the area of their opposing column faces is least.
FIG. 2 shows this variation in the electrical resistance of the device of FIG. 1 plotted as resistance versus position of the electrolyte in the bore. The portions of the curve indicated by a, b and c in FIG. 2 correspond to the resistance values occuring when the electrolyte 15 is in the configurations of FIG. la, lb and 1c, respectively.
When energized by a source of electric current the embodiment illustrated in FIG. 1 provides an electrical element whose resistance varies with time in a predetermined manner.
The constriction of the bore in the region 12 may be made sharper or more gradual than that shown in FIG. 1 to achieve respectively a corresponding sharper or more gradual transition between low and high resistance values in FIG. 2. In general, the shape of the resistance vs. time characteristics of the coulometer of this invention correspond to the plot of bore cross-sectional area vs. bore length when a constant current is passed through the device. By varying the sectional area of the bore in a predetermined fashion along the bore length a corresponding predetermined coulometer resistance variation with time is obtained. A wide range of coulometer resistance vs. time characteristics may be achieved by the simple expedient of providing a bore with suitable cross-sectional area variation.
FIG. 3 shows a portion of a second embodiment of the invention having a tube 30 of non-conductive material such as glass, ceramic or the like and a bore 31 of circular cross section therethrough. The diameter of the bore is sharply diminished at a plurality of evenly spaced regions 32 along the length of the tube. Two liquid metal columns 33 and 34 (e.g., mercury), are within the bore separated by a volume of liquid electrolyte 35 as in the first embodiment described above. Although not shown in FIG. 3, the bore is sealed at both ends by epoxy-resin seals and conductive metal leads make contact with the metal columns in the same manner as in the embodiment ofFIG. 1.
As current from an external source is passed through the device of FIG. 3 the column electroplating operation takes place, causing the electrolyte 35 to move along the length of the bore 31. When the electrolyte is in a position of maximum bore diameter as shown in FIG. 3 it has a configuration for a minimum electrical resistance as previously described. When, however, the electrolyte moves into a region 32 of the bore having diminished diameter its configuration causes it to have a maximum electrical resistance. This variation in electrolyte resistance (and hence coulometer resistance) with the movement of the electrolyte 35 along the bore length is depicted in FIG. 4 which plots resistance versus electrolyte position.
The cross section of the second embodiment is shown in FIG. 3 to vary in a regularly repeating rectangular fashion and the corresponding resistance characteristic of FIG. 4 also varies in a periodic, square-wave manner. The bore section variation need not, of course, be rectangular, regular or periodic.
A third embodiment of this invention, shown in FIG. has a cylindrical tube 50 of non-conductive material with a uniform cylindrical bore 51 therethrough. Two liquid metal columns 52 and 53 within the bore are separated by a volume of liquid electrolyte 54 as in the two previously described embodiments. Both ends of the bore are sealed by epoxy resin seals 55 and conductive metal leads 56 and 57 make contact with the respective metal columns. Additionally, an inserted body 58 of non-conductive material such as glass, ceramic or the like is positioned within the bore and fixed to the end of conductive lead 57. The inserted body 58 is cylindrical in form of dimensions less than the dimensions of the bore.
When current from an external source is passed through the device of FIG. 5 the electrolyte 54 moves along the length of the bore in the manner already described. When the electrolyte 54 occupies a position in a region of the bore not also occupied by the inserted body 58, such as that shown in FIG.
5, the electrolyte has a configuration for minimum resistance.
When, however, the electrolyte moves into a region of the bore also occupied by the inserted body 58 its configuration alters since the same volume of electrolyte is distributed along a greater length of bore and has a correspondingly decreased cross section. In this altered configuration the resistance of the electrolyte is increased causing the resistance of the device of FIG. 5 to'have a characteristic curve similar to that depicted in FIG. 2.
The non-conductive inserted body 58 is a means for varying the effective cross-sectional area of the bore as seen by the liquid electrolyte 53. The presence of the inserted body forces the electrolyte to take on a different configuration just as the bore constriction by the material of the coulometer tube did in the first two embodiments. The inserted body need not have a regular cross section itself, but variations in its cross section will be reflected by corresponding variations in the resistance characteristic of the device. Furthermore, although the tube bore 51 has a uniform section in FIG. 5, it should be understood that an inserted body may be used in conjunction with a coulometer tube having a varying bore section so long as the effective cross-sectional area of the bore seen by the electrolyte varies along the length of the bore.
FIG. 6 is a block diagram of a typical circuit arrangement for utilizing the above-described coulometers as timing devices comprising, in essence, a variable bore coulometer 60, a pair of terminals 61A and 61B for applying DC electrical power to the coulometer, and means for detecting changes in the resistance of the coulometer. In the specific circuit shown, the resistance detecting means comprises a resistor 62 connected in series with the coulometer and a voltage detector 63 connected in parallel with the coulometer. Preferably, the voltage detector 63 is a digital detector such as an amplifier, a
neon glow tube or a voltage controlled switch (such as a silicon controlled rectifier) adapted to operate only when the variable bore coulometer is in its high resistance state. In operation, a DC voltage source 64 is applied to the device through terminals 61A and 618 for connecting the source in series with the resistor and coulometer. The value of resistor 62 can be chosen to limit current through the coulometer at its low resistance state. The resistor and coulometer together act as a voltage divider network. When the resistance of the coulometer increases, the voltage drop across it increases. The area of the one or more constrictions in the tube is chosen so that the voltage increase is sufficient to activate the voltage detector, and the length of each constriction is chosen to activate the detector for a desired length of operating time. The area of the large area sections between constrictions can be chosen to deactivate the detector, and the length of these sections is chosen to control the operating time between successive activations.
A significant advantage of this device is that the detector can be sequentially activated and deactivated many times before the electrolyte reaches the cathode contact and the device stops working.
1. In a coulometer having a body of non-conductive material with, a capillary bore therein, two columns of liquid metal within said bore, each column extending from an end of the bore toward the other column such that a space not occupied by column metal exists between the inmost ends of the columns, a liquid electrolyte in said bore filling said space and being in contact with the inmost ends of the columns, and conductive means for connecting the columns to an external source of current, the improvement wherein the effective cross-sectional area of said bore varies along the length of said bore to provide a coulometer whose resistance varies in a predetermined manner with respect to electrolyte position.
2. A device according to claim 1 wherein the cross-sectional area of said capillary bore is reduced in one or more portions between its ends.
3. The improvement according to claim 1 wherein the crosssectional area of said bore varies in a regular periodic fashion.
4. The improvement according to claim 1 wherein an inserted body of non-conductive material is positioned within said bore to vary the effective cross-sectional area of said 5'. The improvement according to claim 1 wherein said bore has a uniform cross section and an inserted body of non-conductive material is positioned within said bore to vary the cross section area of said space along the length of said bore.
6. A timing device comprising:
a coulometer according to claim 1;
means for applying a DC voltage across said coulometer;
means for detecting variations in the resistance of said coulometer.
7. A timing device according to claim 6 wherein said means for detecting variations in the resistance of said coulometer comprises a resistor seriallyconnected with said coulometer and a voltage detector connected in parallel with the coulometer.
8. A device according to claim 7 wherein said voltage detector is adapted to be in the off state when the electrolyte in said coulometer is in the portion of said coulometer having the largest cross-sectional area and to be in the on state when the electrolyte is in the narrowest portion of said coulometer.
9. A device according to claim 8 wherein said voltage detector is a voltage controlled switch.