US3748708A

US3748708A - Process for controlling cathode ray tube cutoff voltage by cathode insertion with accelerating grid compensation

Info

Publication number: US3748708A
Application number: US00198834A
Authority: US
Inventors: R Schweitzer; H Drury
Original assignee: Philco Ford Corp
Current assignee: Space Systems Loral LLC
Priority date: 1971-11-15
Filing date: 1971-11-15
Publication date: 1973-07-31
Anticipated expiration: 1990-07-31

Abstract

A process for positioning the electrodes of an electron gun in a cathode-ray tube with respect to one another to cause the electron gun to have a cutoff voltage closely conforming to a predetermined value. The cathode to control grid interelectrode capacitance of the gun is measured on a capacitance bridge during the cathode insertion operation. An initial bridge balance is obtained with the cathode just barely inserted into its support structure. A reference capacitor equal to the change in capacitance required to bring the cathode to its desired position is added to the standard arm of the bridge, and the bridge rebalanced during insertion by moving the cathode toward the control grid. The effect of variations in control grid to accelerating grid spacing is compensated by connecting the control grid to accelerating grid interelectrode capacitance in tandem with a series capacitor across the control grid to cathode interelectrode capacitance when rebalancing the bridge.

Description

United States Patent [191 Schweitzer et al.

11 3,748,708 [451 July 31, 1973 'PROCESS FOR CONTROLLING CATHODE RAY TUBE CUTOFF VOLTAGE BY CATHODE INSERTION WITH ACCELERATING GRID COMPENSATION [75] inventors: Robert W. Schweitzer, Kokomo,

lnd.; Harvey J. Drury, Sellersville, Pa.

[73] Assignee: Philco-Ford Corporation,

Philadelphia, Pa.

[22] Filed: Nov. 15, 1971' [2]] Appl. No.: 198,834

[5 6] References Cited UNITED STATES PATENTS 6/1946 Beggs 29/2515 10/1970 Baur et a1. 29/25.l5

Primary Examiner-LowellA. Larson Attorney-Robert D. Sanborn [57] ABSTRACT A process for positioning the electrodes of an electron gun in a cathode-ray tube with respect to one another to cause the electron gun to have a cutoff voltage closely conforming to a predetermined value. The cathode to control grid interelectrode capacitance of the gun is measured on a capacitance bridge during the cathode insertion operation. An initial bridge balance is obtained with the cathode just barely inserted into its support structure. A reference capacitor equal to the change in capacitance required to bring the cathode to its desired position is added to the standard arm of the bridge, and the bridge rebalanced during insertion by moving the cathode toward the control grid. The effect of variations in control grid to accelerating grid spacing is compensated by connecting the control'grid to accelerating grid interelectrode capacitance in tandem with a series capacitor across the control grid to cathode interelectrode capacitance when rebalancing the bridge.

13 Claims, 4 Drawing Figures [4 1 July 31, 1973 United States Patent [1 1 Schweitzer et al.

H m u v 7 if? a MM v n u w v w a FIIIIW IVMw IIML m r M 2 4 0 mm M ma n m J H a \X\fl\\\\\\\ \\\\\\\\\\\\\\k\ 0////////. r E a 4 I 7 i1 .11.; 0 ill. 1 i K a n H c I BACKGROUND OF THE INVENTION This invention relates to the precision insertion of the cathode in a cathode ray tube (CRT) electron gun. The

spacing being the control grid and the cathode and the spacing between the control grid and the accelerating grid act directly to control grid bias voltage at which zero beam current occurs, i.e., the cut-off voltage. ln monochrome tubes variations in cut-off voltage are of secondary importance and a wide range can be toleratedQI-Iowever, in order to achieve good color balance in a color CRT, it is necessary that the cut-off voltage of the three color guns be the same. Because of tolerances in gun parts, tolerances in the manufacturing jigs and fixtures, and variations in handling, slight variations exist in manufactured guns. Such variations are slight but their cumulative effects canproduce substantial variations in the electrical performance of the guns. While adjustable operating voltages are provided in color television receivers to accommdate slight differences in cut-off voltages, if cut-off voltages differ too greatly from one gun to the other, color balance over the entire grey scale of the CRT is impossible to achieve.

Desirably such variations could be reduced by making the CRT guns in two steps. The gun trio is manufactured completely except'for the cathodes. Then the cathodes are pushed or'inserted into supports in the gun and welded in place. If the cathode is correctly spaced within the gun assembly, cut-off will be at the desired value. One technique of the prior art is to control insertion to a fixed spacing from the control grid as indicated by a gaging device. Several such devices are available for production use, but they are necessarily complex and difficult to use and maintain. Furthermore, their accuracy leaves much to be desired and they cannot compensate for variation in control grid to accelerating grid spacing.

Some prior manufacturing processes have attempted to control cut-off voltage by positioning electrodes to achieve precise interelectrode capacitance. Since the capacitance, between any two conductorsvaries inversely with the spacing between them, cathode to control grid spacing can be controlled by measuring the cathode to control grid interelectrode capacitance on a capacitance bridge and moving the cathode until the desired reading is obtained. A similar process can then be employed to adjust the capacitance and hence the spacing between the accelerating grid and the control grid. If these two parameters along with control grid aperture size are carefully controlled, excellent cut-off voltage control can be achieved. The above procedure requires two complete measurement and electrode adjustment steps, and requires suitable apparatus to position the electrodes after gun assembly. It is, therefore, disadvantageous because it is of limited range capability, time consuming, and expensive.

SUMMARY OF THEINVENTION An object of this invention is to provide an improved method for manufacturing color CRTss 2 Another object isto'provide an improved method for controlling CRT gun cut-offvoltage while adjusting only the spacing between cathode and controlgrid.

Another object is to compensate for variations in the spacing between thecontrol grid and the accelerating grid, that effect cut-off voltage, by adjusting only the cathode to control grid spacing.

Another object is to provide a method forcontrolling cut-off voltage which is especially adaptable to automatic manufacturing techniques.

These'and other objects are achieved by connecting a capacitancebridge between the control grid and cathode electrodes of the CRT gun while it is located in a cathode insertion machine. The cathode is inserted until it just touches its mount and the bridge zeroed or balanced. Then a predetermined degree of unbalance is established. This unbalance is controlled so that as the cathode is inserted, a new balance will occur when the cathode is at its desired spacing from the control grid. At this point in the insertion cycle the cathode is welded into its support. Compensation for variations in control grid to accelerating grid variations on gun cutoff voltage are accomplished during insertion by connecting a predetermined fixed capacitance between the cathode and the accelerating grid. During initial bridge balance the accelerating grid is connected to the brige ground or neutral connection. After zero is established and the incremental bridge capacitance inserted, the ground on the accelerating grid is removed and cathode insertion to null performed. Since the fixed capacitance results in accelerating grid capacitance appearing in the bridge circuit, cathode insertion will be a function of accelerating grid spacing. For a particular gun design a fixed capacitance value can be selected so that final insertion will automatically compensate for variations in accelerating grid spacing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side view in partial section of a cathode insertion machine showingacapacitance bridge connected thereto for measuring control grid to cathode capacitance and including the added bridge circuit components necessary to practice the invention,

FIG. 2 is an enlarged view of a cathode ray tube gun, showing connection points and cathode insertion details,

FIG. 3 is a graph showing control grid to cathode capacitance as a function of spacing, and

FIG. 4 shows the critical cathode ray tube gun parts and the connections to the capacitance bridge required to practice the invention.

DESCRIPTION OF THE EQUIPMENT FIG. 1 shows the essential portions of a cathode insertion machine 1 and a capacitance bridge 2. A portion of the color CRT gun assembly is shown as 3 in the drawing. In a typical manufacturing sequence the cathode mount, control, focus, and accelerating electrodes are assembled by means of tabs that are pressed into elongated glass beads so as to achieve a unitary assembly. The cathodes are then inserted into their mounts to establish a controlled spacing and are spot welded into position. After the cathodes are welded in place the gun assembly is completed by heater insertion into the cathodes, stem mounting, electrode connection welding, and convergence cup mounting. These latter parts are not shown.

The beaded gun structure 3, detailed in FIG. 2, is impaled as shown on mandrel 4 which is shaped for a snug fit inside the electron gun. In operation, post 5 is retracted by handle 6 to its lower position, shown dotted. The gun 3 is located on the mandrel 4 and the post 5 raised to the operating position as shown by the solid lines. As post 5 is raisedinto position, contact finger 7 engages the cathode mount 8. Contact to control grid 9 and accelerating grid 10 is made by means of a pair of movable contacts 1 l and 12 operated by an air cylinder contact actuator 13.-Cathode 14, which is to be inserted as described in detail hereinafter, is pushed onto I mandrel 15 where it is held in place by means of a vacuum applied through a hollow center portion. The cathode mandrel 15 is carried on a spring loaded insertion press 16 and is insulated therefrom by insulator 17. Insertion press 16 is mounted on rack 18 which slides vertically on a set of guide bars, one of which is shown at 19. Rack 18 is operated by an air driven actuator 20 between its upper extreme as shown and a lower extreme which is adjusted by means of. stop 21 so that the cathode 14 just touches the shoulder of cathode mount 8. In the upper position of rack, 18, the cathode to be inserted into the gun is loaded onto mandrel 15 where it is held in position by virtue of the vacuum. Rack 18 is then lowered'to its bottom position. Insertion press tab 22 is then swung into position below lead screw 23 which, when rack 18 is in its lower position, will operate insertion press 16. As lead screw 23 is operated, either by hand or by an electric motor (not shown), the cathode 14 will be forced into its mount 8. When the correct insertion is achieved as indicated on bridge 2 (and to be described in detail hereinafter) the insertion is stopped and the cathode welded to its mount by means of a plurality of spot welding electrodes (not shown) that are associated with the machine 1.

After welding, lead screw 23 is retractedto its former position where it clears insertion press tab 22 and this action removes the mandrel 15 from cathode 14 which remains in the gun assembly. Insertion pres tab 22 is swung to the position shown and rack 18 raised to its upper position ready to receive a new cathode for the next insertion. Contact actuator 13 is operated to withdraw contacts 11 and 12, and rack 5 is lowered by handle'6 so that the next gun can be located in position for cathode insertion.

Capacitance bridge 2, as shown in FIG. 1, is a partial schematic of a commercial device such as a Boonton Model 77A manufactured by the Boonton Electronics Corporation. Only the capacitance measuring section is shown. The bridge measurement circuits are completely contained inside a shielded structure and the shield connection designated as ground connection 30. Such a bridge will indicate the capacitance connected between the Hi TEST terminal 31 and Lo TEST Terminal 32 while ignoring stray capacitance from either terminal to ground. This enables the connections to the cathode and control grid to be made using relatively long shielded leads without swamping the capacitance being measured. An r-f signal source 33 operating, for example at lMHz, supplies energy to the bridge through electrostatically shielded transformer 34, the

center tap of which is grounded to the shield.

The bridge is read out on differential capacitor 35 in units established by the setting on range capacitor 36. The bridge is zero adjusted by differential capacitor 37. R-f voltmeter 44 is used for indicating the bride balance or null condition. Such a bridge can be used in two ways. In the first, control 35 is set to zero and con-' trol 37 adjusted for a null with the bridge unconnected. An unknown capacitance can be connectedbetween the Hi and

Lo Test terminals

31 and 32 and the con trols 35 and 36 adjusted for a null on meter 38. The dials read out the unknown capacitance value. In the second way the bridge can be used simply as a null device to indicate the comparison between a capacitor connected between

STANDARD terminals

38 and 39 and the capacitance connected between

TEST terminals

31 and 32. V

Such a bridge is insensitive to any capacitance to ground because of the centertapped transformer 34. If any capacitance exists between the Hi terminal 31 and the ground terminal 30, the transformer reflects an equal capaciance to terminal 38 on the other side of the bridge thereby maintaining balance. Any capacitance appearing between the L0 terminal 32 and ground does not affect bridge balance because it appears across voltmeter 44 and at worst would merely reduce the maximum meter indication. I

To control cathode insertion from a capacitance standpoint, the bridge ground is connected to the frame of the insertion machine 1. Cathode contact 7 is connected to the bridge Hi terminal 31 and a flexible wire connects mandrel 15 to the same point. The L0 terminal 32 is connected to the gun control grid 9 by way of contact 11. Compensating capacitor 40 (the action of which will be explained more fully hereinafter) is connected between the cathode connection and the accelerating grid 10 thru contact 12. A switch 41 permits grounding the juncture of accelerating grid 10 and capacitor 40. Reference capacitor 42 can be connected by way of switch 43 between

STANDARD terminals

38 and 39 of the bridge. (The value of capacitor'42 will be discussed more fully hereinafter.)

FIG. 2 shows the details of gun 3 in FIG. 1. Cathode 14, located on mandrel 15, is shown .as just entering electrode 8. This is the position it will assume when rack 18 (FIG. 1) is in the lower position. The dotted portion shows the location of cathode 14 after insertion.

FIG. 3 shows how control grid to cathode capacitance varies with cathode insertion and represents a typical measurement series. Point A on the curve represents about l.6pf, the value obtained with the cathode face about 0.030 inch back from the control grid.

At point B the cathode is at about its desired location and the capacitance is about 2.25 pf. At point C the cathode touches the control grid. Since the curve angles sharply upward at point B, it can be seen that useful position information is available in terms of a capacitance measurement. It is this characteristic that makes the present technique feasible. The flat early portion of the curve between points A and B shows thatthere is little change in capacitance before the optimum cathode position because the cathode contributes very little to total capacitiance. Since the capacitance at point A is due mainly to stray capacitance between the control grid and the cathode mounting structure, and, since this parameter is subject to substantial variation due'to manufacturing tolerances, its effect must be avoided to obtain useful cathode position information. The actual cathode surface contribution is measured by the rise in capacitance between points A and B of about 0.65 pf. Therefore if the residual gun capacitance present at point A is nulled out at the start of cathode'insertion, and only the rise observed during insertion, very accurate cathode spacing can be achieved.

In practice standard capacitor 42 is made equal to the desired value for the tube type being manufactured (0.65 pf for the device of FIG. 3). Cathode insertion is commenced and when the insertion condition shown in solid lines in FIG. 2 is achieved switch 43 is opened (disconnecting capacitor 42) and the bridge balanced by operating control 35 for a null on meter 44. Switch 43 is then closed and the bridge unbalanced by the addition of capacitor 42. Insertion is continued by operating lead screw 23 until the bridge is rebalanced indicating arrival at point B of FIG. 3. The initial balance eliminates the unavoidable variations of basic stray capacitance due to parts and assembly tolerances. Thus when final balance is achieved on insertion, the final capacitance value will be due entirely to that contributed by the cathode as it approaches the control grid.

DESCRIPTION OF THE INVENTION FIG. 4 shows the details of the essential elements of FIG. 1 as they relate to the invention. Standard capacitor 42 is selected to be equal to the cathode to control grid capacitance that will be added to the structure by insertionof the cathode to its desired position. Compensating capacitor 40 and switch 41 are employed to compensate for the proximity of accelerating grid which introduces stray capcitance 45 between the two grids. It can be seen that as accelerating grid 10 approaches control grid 9 it will raise the gun cut-off voltage because its field will have more influence on electron flow from the cathode. This can be compensated by inserting the cathode to a lesser amount thereby reducingthe field effect.

In operation, cathode insertion is commenced as described above where cathode 14 just touches electrode 8. At this point switch 43 is opened and switch 41 .is closed. When the bridge is balanced with its control 35, the accelerating grid 10 is grounded thereby removing the effect of stray capacitance 45, and compensating capacitor 40 from the measurement circuit. They become stray capacitances to ground which the bridge ignores. After initial balance switch 43 is closed to apply capacitor 42 to the bridge and switch 41 is opened which places compensating capacitor 40 and stray capacitance 45 in series and their combination appears in parallel with control grid to cathode capacitance 48. Insertion to the new bridge null is accomplished as described above but accelerating grid compensation is automatically accomplished. For example if the accelerating grid is too close to the control grid, capacitance 45 will be larger. The final bridge null will then occur with a smaller value of capacitance 48 which means a greater cathode to'control grid spacing.

Capacitor 40 is selected to give approximate compensation for the gun being manufactured and its value is chosen on an empirical basis. To do this a number of standard gun mounts are made up and several different control grid to accelerating grid spacing values established, both above and below the desired value. Cathode insertions are then made as described for several values of capacitor 40. Cut-off values are then measured for the assembled guns and a plot is made of the value of capacitor 40 versus accelerating grid compensation. The capacitor value giving the best compensation is the one used in producing that type of gun.

pacitor having a value equal to the desired control grid to cathode capacitance to the REFERENCE arm of the bridge. The same effect could'be achieved by removing a capacitorhavitig the same value of capacitance from the TEST arh after the initial balance. In this case the operation-ofthe associated switch would be the opposite of that described. Furthermore to speed the pro-- cess the' brid g'e operation 'could bemade automatic by including"suitable servo mechanisms (not shown). Upon actuation such a bridge automatically adjusts to the null condition by means'of an'electrically operated servo mechanism. If desired, cathode insertiohcanbe automatically operated with a null seeking servor'n'e'cl'ianism operating lead screw 23.'Such a device would operate as follows: Cathode insertion is started as indicated in the solid lines of FIGQZ. The bridge servomechanism is actuated to zero with capacitor 42 disconnected and the accelerating grid grounded. Capacitor 42 is then connected, the accelerating grid ungrounded, and the insertion servomechanism actuated. The cathode is then automatically inserted to its compensated position whereupon it is welded into place.

It can be seen from FIG. 1 that connection of mandrel 4 to the machine frame grounds the CRT gun portions adjacent to theaccelerating grid 10. This avoids stray capacitance effects to the grounded gun parts. In

the fabrication of gun trio assemblies (employed in certain color CRTs) and to avoid the undesirable stray capacitance of guns adjacent to the one being processed it is also desirable to ground the parts of the two guns not being processed. At leasrtheir' control grids should be grounded. To this'end two more air actuator operated grounding contacts(.not shown) could be incorporated into the machine. v

Further modifications of the invention will occur to those skilled in the art. The scope of theinvention should therefore be limited only by the following claims.

We claim:

1. The method of manufacturing a cathode ray tube electron gun comprising a cathode, a control grid and an accelerating grid wherein said gun parts except for said cathode are assembled into a unitary structure and said cathode is subsequently inserted to a predetermined spacing relative to said control grid, said spacing being determined with the aid ofa capacitance bridge having measurement terminals and a ground terminal and having the property of "excluding from the measurement the effect of any capacitance connected to said ground terminal, said method including the steps: a. starting the insertion of said cathode into said gun, b. connecting a compensating capacitor between said cathode and said accelerating grid, c. connecting said bridge measurement terminals between said cathode and said control grid, d. connecting said accelerating grid to said ground terminal, e. balancing said bridge, f. unbalancing said bridge by a predetermined value, g. removing said ground connection from said accelerating grid, and I i h. inserting said cathode until said bridge is again balanced. 2. The method of claim 1 wherein each of the steps is applied to each of a plurality of cathode ray tube electron guns in a color cathode ray tube.

tube electron gun comprises a cathode, a control grid, and an accelerating grid, and said cathode is said first electrode, said control grid is said second electrode, and said accelerating grid is saidthird electrode.

, is applied to each of a plurality of cathode ray tube electron guns in a color cathode ray tube.

tube electron gun comprises a cathode, a control grid, and an accelerating grid, and said cathode is said first electrode, said control grid is said second electrode, and said accelerating grid is said third electrode.

is applied to each of a plurality of cathode ray tube electron guns in a color cathode ray tube.

3. In a method for manufacturing a cathode ray tube 4. The method of claim 3 wherein said cathode ray 5. The method of claim 4 wherein each of the steps 6. in a method of manufacturing a cathode ray tube electron gun having a plurality of electrodes; which method comprises the steps of using a reference capacitor to obtain a reading indicative of a desired interelectrode capacitance between a first and second electrode; removing said reference capacitor and substituting therefor the capacitance between said first and second electrodes; and adjusting the position of said first electrode until said reading is obtained again; the improvement which comprises:

8. The method of claim 7 wherein each of the steps 9. In a process for manufacturing a cathode ray tube electron gun comprising a first electrode, a second electrode and a third electrode, said process compris ing the steps of:

a. establishing a capacitance measuring bridge having a test arm and a standard arm,

b. connecting said test arm of said bridge between said first and second electrodes,

c. obtaining a reading dependent on the interelectrode capacitance between said first and second electrodes, with said electrodes initially spaced apart,

d. adding to said standard arm of said bridge, a reference capacitor indicative of the change in said interelectrode capacitance when the spacing between said first and second electrodes is adjusted to its desired value, and

' e. adjusting the position of said first electrode until said reading is again obtained, the improvement comprising:

compensating for the spacing between said second and said third electrode by:

f. including in said reference capacitor an amount of capacitance indicative of the series combination of a compensating capacitor and the desired interelectrode capacitance between said second electrode and said third electrode, and

g. prior to adjusting the position of said first electrode, adding to said measurement arm the interelectrode capacitance between said second and third electrodes connected in series with said compensating capacitor.

10. The process of claim 9 wherein said cathode ray tube electron gun comprises a cathode, a control grid, and an accelerating grid, and said cathode is said first electrode, said control grid is said second electrode, and said accelerating grid is said third electrode.

11. The process of claim 9 wherein said bridge includes a center tapped transformer balanced source having a ground terminal whereby the capacitance of any element connected between said ground terminal and the terminals of said test arm is not included in measurements made by said bridge.

12. The process of claim 11 wherein said accelerating grid is connected to said ground terminal when said reading is obtained in step (c) with said first and second electrodes widely spaced apart, and said accelerating grid is disconnected from said ground terminal when said reference capacitance is added to said standard arm in step (d).

13. The process of claim 12 wherein each of the steps is applied to each of a plurality of cathode ray tube electron guns in a color cathode ray tube.

Claims

1. The method of manufacturing a cathode ray tube electron gun comprising a cathode, a control grid and an accelerating grid wherein said gun parts except for said cathode are assembled into a unitary structure and said cathode is subsequently inserted to a predetermined spacing relative to said control grid, said spacing being determined with the aid of a capacitance bridge having measurement terminals and a ground terminal and having the property of excluding from the measurement the effect of any capacitance connected to said ground terminal, said method including the steps: a. starting the insertion of said cathode into said gun, b. connecting a compensating capacitor between said cathode and said accelerating grid, c. connecting said bridge measurement terminals between said cathode and said control grid, d. connecting said accelerating grid to said ground terminal, e. balancing said bridge, f. unbalancing said bridge by a predetermined value, g. removing said ground connection from said accelerating grid, and h. inserting said cathode until said bridge is again balanced.

2. The method of claim 1 wherein each of the steps is applied to each of a plurality of cathode ray tube electron guns in a color cathode ray tube.

3. In a method for manufacturing a cathode ray tube electron gun having a plurality of electrodes, which method comprises adjusting the spacing between a first electrode and a second electrode by moving said first electrode to a position where the capacitance measured between said first and second electrodes reaches a predetermined value and then securing said first electrode in place, the improvement which comprises: compensating the location of said first electrode for variations in the spacing between said second electrode and a third electrode by including in said capacitance measured a compensating capacitor connected between said first electrode and said third electrode.

4. The method of claim 3 wherein said cathode ray tube electron gun comprises a cathode, a control grid, and an accelerating grid, and said cathode is said first electrode, said control grid is said second electrode, and said accelerating grid is said third electrode.

5. The method of claim 4 wherein each of the steps is applied to each of a plurality of cathode ray tube electron guns in a color cathode ray tube.

6. In a method of manufacturing a cathode ray tube electron gun having a plurality of electrodes; which method comprises the steps of using a reference capacitor to obtain a reading indicative of a desired interelectrode capacitance between a first and second electrode; removing said reference capacitor and substituting therefor the capacitance between said first and second electrodes; and adjusting the position of said first electrode until said reading is obtained again; the improvement which comprises: compensating for variations in the spacing between said second electrode and a third electrode by including in said reference capacitor an amount of capacitance indicative of the series combination of a compensating capacitor and the inter-electrode capacitance between said second electrode and said third electrode, and substituting therefor, in parallel with the capacitance between said first and second electrodes, thE capacitance between said second and third electrodes connected in series with said compensating capacitor.

7. The method of claim 6 wherein said cathode ray tube electron gun comprises a cathode, a control grid, and an accelerating grid, and said cathode is said first electrode, said control grid is said second electrode, and said accelerating grid is said third electrode.

8. The method of claim 7 wherein each of the steps is applied to each of a plurality of cathode ray tube electron guns in a color cathode ray tube.

9. In a process for manufacturing a cathode ray tube electron gun comprising a first electrode, a second electrode and a third electrode, said process comprising the steps of: a. establishing a capacitance measuring bridge having a test arm and a standard arm, b. connecting said test arm of said bridge between said first and second electrodes, c. obtaining a reading dependent on the interelectrode capacitance between said first and second electrodes, with said electrodes initially spaced apart, d. adding to said standard arm of said bridge, a reference capacitor indicative of the change in said interelectrode capacitance when the spacing between said first and second electrodes is adjusted to its desired value, and e. adjusting the position of said first electrode until said reading is again obtained, the improvement comprising: compensating for the spacing between said second and said third electrode by: f. including in said reference capacitor an amount of capacitance indicative of the series combination of a compensating capacitor and the desired interelectrode capacitance between said second electrode and said third electrode, and g. prior to adjusting the position of said first electrode, adding to said measurement arm the interelectrode capacitance between said second and third electrodes connected in series with said compensating capacitor.