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NBS MONOGRAPH 46

Analysis of Coaxial

Two-Terminal Conical Capacitor

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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Government Printing Ofl&ce, Washington, D.C.
UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Analysis of Coaxial

Two-Terminal Conical Capacitor

M. C. Selby

National Bureau of Standards]Monograph 46

Issued April 6, 1962

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. - Price 20 cents

Contents

Page

1. Introduction 1

2. Conical capacitor 1

3. Computed and measured values 5

4. Multiple cone and modified shape electrodes 6

5. Appendix I. Derivation of equation (13) from equation (12) 7

6. Appendix II. Derivation of equation (16) 7

7. Appendix III. Graphical field mapping 8

8. Appendix IV. Justifications of basic assumption for approximate

solution 10

9. Appendix V. Precautions for measurement and application 11

10. Appendix VI. Rigorous solution of problem 11

11. References 14

III

Analysis of Coaxial Two-Terminal Conical Capacitor

M. C. Selby

Adjustable capacitors having electrodes in the form of coaxial cones or frustums have

been used on rare occasions in the past; but their potential superiority to other types of capacitors for some important applications have been overlooked. The advantage of this

geometry over cylindrical or disk forms is that the practical capacitance range is several times

larger. An example cites the capacitance ranges of a disk, cylindrical, and conical type to be

10, 40, and 168 to one, respectively. An approximate equation was derived for this conical

capacitor and close agreement is shown between computed and measured values of capacitance versus electrode displacement. Multiple cone and different shape electrodes are suggested to obtain large values of capacitance with an appreciable saving of space and further

increased range of capacitance. The electric field is plotted and its construction steps for

axial symmetry are given.

1. Introduction

Adjustable two-terminal capacitors of coaxial form are frequently more suitable for a given application than the conventional rotary or compression types [iV. Variation of capacitance in the former is obtained by relative linear displacement of one electrode, usually a cylinder or a disk, with respect to another similar stationary electrode. Unfortunately, cylindrical

and disk electrodes have a relatively limited capacity range.

Let us assume a certain given cylindrical space within which an adjustable capacitor with

a maximum practical capacitance range is to be placed. End and shielding effects are assumed

negligible. Capacitance values with a reproducibility of the order of 0.2 of a percent or better

are to be available throughout the entire range.

Conical electrodes are intuitively attractive because they represent an intermediate case between cylindrical and disk electrodes. For coaxial cylinders, capacitance is directly proportional to the length of the meshed sections if end effects are neglected. Thus if the available travel is 2 in. and 0.05 in. is the minimum displacement that can be reproduced to 0.2

percent, the available capacitance range is about 40 to 1. On the other hand, for practical

purposes, two parallel, equal-diameter disk electrodes, placed inside a tube of approximately

the same diameter, cease to respond to the capacitive law alone when their separation exceeds

approxunately one radius of the disks; at these distances the effects of the TMoi mode induced

by the high-voltage electrode inside the tube become increasingly noticeable. Since we are

interested in the purely capacitive range we see that, for the example cited, and for 1-in. disks,

the range would be approximately 10:1. It is shown below that with conical electrodes one

can realize a range many times that of a cylindrical or disk capacitor.

2. Conical Capacitor

Consider a capacitor formed hy two right conductive frustums, as shown in figures 1 and 2. The cross section in figure 2 shows the essential dimensional elements of this capacitor. Let

the sections of the conical-electrode surfaces, indicated by the length I in figure 2, be referred

to as the "meshed" surfaces. Let also:

2^= angle of the cones Z)=adjustable distance between the apexes of the cones. As D increases, the capacity

decreases because the distances between the conical surfaces increase and the length

of meshed surfaces is reduced.

A. = the height of the inside cone (A) from its apex to the base of its effective meshed

surface.

' Figures in brackets indicate the literature references at the end of this Monograph.

1
The electrostatic shield, shown in figure 1, is normally grounded and either electrode,

male or female, may be connected to it. Connection of the male to the shield is particularly

advisable when measurement of capacitance increments of this capacitor is to be done: this

eliminates interfering varying capacitance of the male electrode to ground. This is discussed

further in appendix V.

The problem is to determine the capacitance of this type of construction. A rigorous

analytical solution seems unfortunately unobtainable at this time. The application of

standard mathematical techniques did not yield a rigorous solution (see appendix VI).

In arriving at any mathematical expression of capacitance, the following steps are generally involved:

(a) Determination of the potential distribution within the spaces occupied by the

electrodes for given sm-face potentials of the electrodes,

(b) determination of potential gradients at the desired electrode surfaces,

(c) determination of charge distribution and of the total charge (by integration) on the

electrode in question, and

(d) determination of the capacitance between the given electrode and its surroundings

from the known charge and the surface potentials.

The approximate approach given below jdelds satisfactory agreement with experimental

results for the application at hand. In order to develop more confidence in the approximate

solution, one of the field construction methods, the graphical, was partly employed to justify

introductory assumptions; the steps are given in appendixes III and IV. Appendix III dis- cusses the field construction and principles involved, and IV discusses the justification of the

basic assumptions used for the approximate solution. Experimental results obtained from

construction and measurements are given further in the text. Figure 3 ^ shows the electrostatic field and potential distribution in a plane passing

through the axis and any base diameter of the cones; the lines of force between the meshed

surfaces are everywhere essentially straight except for a relatively small fringing flux region at

' See appendix III for method of construction and meaning of symbols.

2
Figure 3. Electric field map in a homogeneous isotropic dielectric of a coaxial parallel-cone capacitor.

the ends of the frustums; the latter region becomes more negligible as the spacing between

the electrodes decreases and as the ratio of the height of the frustums to this spacing in- creases. It should be clearly understood that the actual limit on the maximum spacing for

which the approximation holds will have to be determined by measurements and agreement

with an analytical expression to the accuracy desired. One may notice in figure 3 a slight

departure of the equipotential surfaces from parallelism with the electrodes and with each

other; that this may be neglected in the present problem is shown in appendix IV. The

potential gradient is therefore perpendicular to the two metal surfaces nearly everywhere in

the space between them. Since the conical conducting surfaces are parallel and coaxial, it follows that the lines of force in the dielectric space (assumed to be air) are everywhere nor- mal to both surfaces. Moreover, since the electrodes are equipotential surfaces, it follows

also that everywhere over these surfaces and over any other surface parallel to and located

between these conducting surfaces

where E is the potential gradient or the electric field intensity and V is unit vector normal to

the siirfaces. One may show the above to be true as follows [2]. Any surface parallel to and located between the two parallel conical conducting surfaces

is an equipotential surface. E is constant over any one of these surfaces because for any given

increment, An, along the normal from a given equipotential surface there will be the same

increment, Av, at all points of this surface to another equipotential surface. Thus equation

(1) holds as long as the electrostatic lines are essentially straight. For the same reason E is

constant over either one of the two conducting surfaces as well. But

where a is the surface charge density and is a constant. Therefore, a is constant over the

surface of either conductor, and the flux-line density which is proportional to o- is also constant over any of the equipotential surfaces. In reality cr is never precisely constant over the con- ductor surfaces; it is a maximum at the apex of the inside cone and a minimum at the apex of

the inside conical surface of the outer cone; the nonuniformity of a is more pronounced for

wide spacings between the conductors.

Referring to figure 2 and taking the surface, Sf, of any other coaxial frustum of half-angle,

e, located between the two conical electrodes and applying Gauss law to this surface, we have

^ dv , . K ' •»7=zi = —-^=constant

an

(1)

(2)

(3)

3
The dielectric constant of the interelectrode space is e and the integral is applied only to

the conical sm'face above because the surfaces of the bases of the frustum by assumption do

not contribute any electric flux.

The surface of the frustum of altitude hi and base radii Ri and B2 is

(4)

In order to express capacitance values in terms of ID and h, we have from the geometry of

the figure:

R^=r cos e (5)

hi=l cos 6

R2=r cos 9—1 sin 9 and

Sf=Tr(2r cos 9—1 sin 9)^!l^ cos^ 9+1^ sin^ 9

=7rZ(2r cos 9—1 sin 9)

(6)

(7)

(8)

where r is the length of the normal from the axis to this surface at its larger base and / is the

length of the meshed conical surfaces.

Substituting into equation (3)

and

Q =eTlE(2r cos 9—1 sin 9)

E=Q/[eTrl{2r COS 9— I sin 9)]. (9)

Because of conical symmetry, the potential at the surface of this frustum, as well as of all others, depends only on r; therefore, the potential difference, V, between frustum A and B is

dr Q

(2r cos 9— I sin 9) 2eTl cos 9 In (2r cos 9—1 sin 9)

V-- Q In

{2rt, cos 9—1 sin 9)

2eirZ cos 9 (2/'a cos 9—1 sin 9)

Rearranging to obtain the capacitance we have

Q 2irel cos 9

(10)

(11)

V 2^6 cos 9—1 sin 9

2r„ cos 9—1 sin 9

(12)

Expressing and in terms of h and D (see appendix I), we have

2-Kd cos 9

ln[l +

2D cos^ 9 2h—l cos 6*J

(13)

When the internal electrode is a cone, it may be considered a frustum having a negligibly small

top section and an altitude h=l cos 9. Then

C= 2Treh

In [l+2{D/h) cos^ 9] (14)

Using the rationalized AlKS units, h is in meters (both D and h in the logarithmic term may be

expressed in anj- identical units), and es8.854X 10"^^ farads per meter (in vacuum and approximately in air) and

C=55.6/ti

ri I o^7->/;.N picofarads. (1.5) In [1+2(Z)//!,) cos^ 9]

^

4
3. Computed and Measured Values

Figure 4 compares experimental data with calculations based on eq (15). 0=1O°34',

(1)+^.) =2. 845in., height of enclosing cone=2.845 in., and the maximmn travel of the inner cone was 2 in. The capacitance between the cones was measured for various values of B (see fig.

2) and is given in figure 4. As D was increased by a given amount, h was, of course, decreased

lO' C 1 1 1 1 1 1 1 1

P

1 1 1 1 1

r

1 T

1 H

O COMPUTED VALUE X MEASURED VALUE

Fig URE 4. Capacitance values of right-cone

continuously adjustable capacitor versus dis- placement.

0.2 0.4 0,6 0.8 LO 1.2 L4 2.0

D, INCHES by the same amount. The maximum value of D used was 2 in. ; for tliis value of D there were

still more than % in. of the cones meshed ; this helped reduce the end effects of the cones. Other

steps to reduce this effect are described in the section below on precautions for measurement

and application.

In order to compare the range of the conical capacitor with others, one must do so within

the same mechanical precision limits, e.g., one nmst find the minimum value of D consistent

with a reproducibility of say 0.2 percent in capacitance which is the precision considered above

in the hypothetical cylindrical example. The derivative of C with respect to D or h will yield

the necessary information.

Tlae derivative is given in appendix II and results in

dC

where

Fill):

C h

2iD+h) cos' e

{Ji+2DcobH) l^ln (l-\-2 j cos- dj^

(16)

The error in the capacitance is thus larger than that in h by the factor [l-{-F(h)]. It is readily seen that F(h) is a pure numeric, so that h and D may be stated in any consistent units. The table below gives some computed values of F{h) and expected errors for the

capacitor of figure 4 for

This value of ^h/h follows from the fact that at minimum spacing (Z) =0.050 in.) wliere /;. =2.795

a reproducibility of setting of 0.0001 in. is desired.

D h Fih) ac/c C max. C mill. C max. C min.

in.0. 050

. 100

. 500

in.2. 795

2. 745

2. 345

56

28

5

%

0. 2

. 1

. 03

116. 1 0. 69 168

5
It appears, therefore, that for a resetabihty of h to 0.1 percent, reproducibiht}* of capacitance vahies to 0.2 percent, and a maximum displacement of 2 in., the range of tlie conical

capacitor of the above dimensions would be about 168 to 1, as against 40 to 1 for the cylindrical and 10 to 1 for the disk type. The conical capacitor recently found practical apphcation at NBS in an attenuator-thermoelectric (AT) rf voltmeter for voltages from an average

of 5 to 1,000 V at frequencies of 1 to 10 Mc/s [3]. As was observed above the heights of the cones were chosen to be close to 3 in. as compared to a maximum displacement of the 2 in. in order to reduce distortion of the electric

field between the cones. A similar precaution for cylindrical electrodes would further reduce

the capacitance range by a considerable amount depending on the diameter ratios of the

two cylinders.

Figure 4 shows satisfactory agreement between computed and measured capacitance

values of an experimental conical capacitor. The agreement was within ±0.5 percent for

all values of D to 1.6 in. For lower displacements the measured values are getting increasingly lower; the discrepancy is about 10 percent, for Z>=0.05 in., apparently as a result of

limited accuracy in measm-ing true cone heights as well as limited mechanical perfection.

Vahdity of formula (15) was also established for 5=20° and 45° with good agreement

between computed and measured values.

4. Multiple Cone and Modified Shape Electrodes

Several female and male conical conductors may be combined to form respective electrodes of a single adjustable capacitor. Such a capacitor having three pairs of conductors

is shown in figure 5. The advantage of tlris structure is that it renders capacitance values

Figure 5. Coaxial conical capacitor employing

three pairs of cones.

essentially equal to the values of a single pair times the number of pairs for the same available cylindrical space. In the example cited above the value of the conical angle may be

doubled. This would require a shielding tube of about twice the original diameter. Yet

the values of the new capacitance, as may be computed using eq (15), will increase by a

negligible amount over the previous. However, one may place three pairs of cones of the

original dimensions in the new shielding tube with a resultant capacitance three times the

original values for each setting of the traveling electrode. The conclusion is that by means

of multiple-cone electrodes one may obtain continuous coaxial capacitance ranges of say

1 to 150, or 2 to 300, or 3 to 450, etc., pf within practical space limitations. Moreover, longitudinal displacements of high accuracy at large values of D are relatively easy to obtain;

this enables the realization of still larger capacitance ranges by reducing, say, the 3 pf capacity to 1 pf. Extrapolation of the curve of figure 4 shows that for dimensions equivalent to

the constructed model a three-cone assembly plus an additional }2-in. displacement would

increase the range by about two to one and would thus furnish an overall range of capacitance

of about 300 to 1 without sacrifice in accuracy.

It is evident that one can obtain different curves of capacitance versus displacement by

changing the cross-sectional contour of one or both surfaces of the electrodes from conical

to other suitable shapes. This may be accomplished without affecting the overall range or

ratio of maximum to minimum capacitances obtained with conical shapes. One should even

6
be able to increase the range further by making use of additional ring-flanges at the bases

of the cones; the cross-sectional contours of these rings may in turn be shaped to produce a

certain desired function of capacitance versus displacement. The idea is akin to the wellknown shaping of the plates of variable air capacitors to obtain various responses, e.g., linear

capacitance, linear frequency, linear wavelength, etc. The graphical field mapping method

described in appendix III may be of assistance in the design of such capacitors.

5. Appendix I. Derivation of Equation (13) From Equation (12)

Referring to figure 2

ra=(h-\-ra sin 6) sin d=h sin d-'ri'a sin^ 6 (17)

rb=(D +h +ra sin 6) sin 6 (18) or

" cos'e

^^^^ and

r,=(D+h+h sin e=[D cos2e+/t(cos2e+sin^e)]

^^= (Z> cos^ e+h) (20) \ COS'' 6/ cos- 6 cos^ 6

Substituting into the denominator of eq (12) the values of and expressed in terms of

D and h, we have

o n J n 2{I)cos^e+h) -^^^j^ COS i9— / sin (9 , n i in ?

, 2r„ cos d—l sn\ d cos- d , 2(D cos^ d+K) — l cos d In =ln =ln —^ 2ra COS ^— Z sin ^ ^ 4 sin ^ „ , . 2h—lcosd

2 COS e —Z sm i9 cos^ B

, 2D cos' d+i2h-l cosd) , T, 2D cos'

6

=ln ' , =ln 1

' 2h—lcosd

, n I 2D cos'

6

-\ ^^^L^+2A-/cos J

^-1)

Therefore ^ 2Trel cos d

, 2D cos' e I

^^^^ ^^L^+2/.-/cos^J

which is the same as eq (13) above.

6. Appendix II. Derivation of Equation (16)

Starting with eq (14)

2weh

In [l+2{D/h) cos' 6]

Let Z)-t-/i= height of outside cone =a constant =H.

Let 2we=Ki and 2 cos- d=K2. Then, D=H-~h and

h

(23)

(24)
(25)

h , < 1 (26)

(27)

but

~Kih"K,dh Kih

KH

[h+K-AH-h)] [in (l+i^. ^^^^^

X (28)

and

dC_dh

C h ^ 1 +

ih+K,D) [ln(^l+K,

(29)

or

dC^rJh

C h

2H cos^ d

{h+2D cos' S) [in

(

1+2 ^cos^

(30)

The interpretation of eq (30) is that the error in Cis equal to the error in h plus another

multiple of that error in This multiple is expressed by the second term of (30); namely, by

2{D+h) cos' e (31)

Qi+2D cos' 6) [in (l+2 j cos' 6^

which is also given in eq (16) above.

7. Appendix III. Graphical Field Mapping

Maps showing the distribution of the equipotential surfaces and electric flux lines provide

a tool to compute the capacitance between electrodes of any shape. The principle is based on

subdividing the space between the electrodes into local-equal-capacitance cells [4, 5, 6]. These

cells are counted and are combined in series-parallel arrangements yielding the total capacitance

between the electrodes. The simplest case is that of "two-dimensional fields" to which the

method has been apphed widely for many years. The field in this case is represented by

means of "curvilinear squares" in one cross-sectional plane and is identical to the field in all other planes parallel to it. The lines of force lie within these planes. Figure 6 shows such a

field confined between two wedge-shaped electrodes in a homogeneous isotropic dielectric.

One might mistakenly assume that such a field distribution holds for the case of a conical

capacitor. This is not so because here the field is not the same in any system of planes intersecting the cones parallel to their axis. Nor does the case of coaxial cylindrical electrodes

8
Figure 6. Electric field map in a homogeneous

isotropic dielectric of a parallel-side, wedgeshape capacitor.

apply to cones because the system of planes perpendicular to the axis of the cylinders does

not furnish the necessary conditions to describe a field ; the lines of force intersect these planes

at some angles, and the fields are not identical in all of these planes.

For coaxial conical surfaces the "axial-symmetry field" is applicable. These fields are more complicated than the "curvilinear square" fields, but they still have a major advantage

over the case in which no type of symmetry exists, in that the fields may be represented in one

plane. The "cells" are shown as "curvilinear rectangles." For the present purpose it will

not be necessary to count and sum up the cells. It will be sufficient to show that the potential

gradient is essentially the same all along the surface of the inner cone. This is shown graphically in figure 3. As discussed in appendix IV, the assumption of uniform charge density

and the integration leading to eq (13) are therefore justified. The case of relatively large

displacement of the adjustable electrode was intentionally chosen for this figure to show up

field distortions better.

It is generally immaterial what procedure one follows in constructing a field map, as long- as the final map meets certain tests. The tests for this case are stated below. One may use

a trial and error procedure from start to finish. Or one may use short cuts by means of approximate computations; the approximate positions of the curves are then readjusted by trial

and error to satisfy the test requirements as best as possible. This latter short-cut approach

was applicable here.

The field construction steps were briefly as follows. A field in a space having axial symmetry ma}' be subcUvided into cells having equal "local" capacitance values bC where

p is the effective distance of the cell from the axis, i.e., its radius of rotation, bs and U are or- thogonal curvilinear line elements forming the "rectangle," U is the distance between the side

representing two equipotential sm-faces, bs is the distance representing the width of a "tube

of flux" and e is the dielectric constant. The cell is thus a ring having an effective radius p and a cross section of bs bl. It follows that in order for the cells to have the same capacitance either bs or bl must

be varied as p varies. In case of a coaxial fine, the distance from the axis of rotation increases

at a given point along the axis, p increases, bs is kept constant, and bl is made to be directly

proportional to p. From the geometry of a coaxial right circular conical capacitor with relatively small apex angles the location of the equipotential surface whose potential is midway

between the potential of the two cones is given approximately by the four equations given

below It follows from the above that

bC=2Te

pbS.

(32)

+ 5^2 =r=constant dimension of a ring along the radius

9
For small values of d,

Pi Pa l/2r+pi 1/2T+P1

4pf —4paPi— rpa =0. Therefore

'a (33)

P2^i/2r+pi (34)

5^1 ^2 (Pl — Pa) (35)

(36)

Here pa is the radius of rotation of the inner cone at the chosen point along the axis and T

is the constant distance between the electrodes measured along that radius; only the positive

value of p is used, pi is the radius of the center of the cell closer to the axis. p2 is the approximate radius of the center of the second cell, farther from the axis, Ui is the approximate distance

between the inner cone and the new equipotential plane, is the distance between the outer cone and the new equipotential plane.

One must check the results hj seeing that

Any other approximate equipotential-line position corresponding to the above given

location along the axis may be found in a similar manner; one may consider the new equipotential line determined above as the surface of the inner (or outer) cone and split the rest

of the upper (or lower) space into two effective cell layers, thus rendering positions for surfaces

having (or %) of the potential-difference given. Choosing several positions along the

axis, one may plot the sections of equipotential planes with the plane of the paper.

Having approximately located the equipotential lines one must next locate the "flux

tube" boundaries for equal cells. These are fixed by the values of 6's which should in our

case vary inversely proportional to p as one travels along the axis of symmetry.

The test of the accuracy of the finished plot is in its satisfying the following conditions:

(1) The field lines must be orthogonal, (2) the flux lines as well as the flux-tube boundaiies

must be normal to the conductor surfaces, and (3) the "rectangles" having the same value

of p must be similar. The position of the field lines are, therefore, adjusted by trial and error

to satisfy this test. The above conditions seem to be met in the plot of figure 3 to a sufficient

acc\u-acy.

8. Appendix IV. Justifications of Basic Assumption for Approximate

Solution

The approximate expression (13) was derived on the assumption that the equipotential

surfaces are everywhere parallel. Figure 3 shows these surfaces departing from parallelism

by a noticeable amount; as a result, one would at a first glance expect a considerable disagreement between experimental and computed data. The following reasons may explain wh}' no

appreciable discrepancy was observed at the measurement accuracies quoted.

The basis of the assumption that the charge distribution over the conical surfaces is essentially uniform is a physical visualization of two closely spaced parallel conducting closed

surfaces of any shape. If one assumes a certain potential of the inner conductor and zero

potential on the outer (shield), the charge distribution over the inner will be a normal function

of the contour when the spacing between these conductors is infinit}^. As this spacing is reduced to zero the surface charges will approach imiformity.

10
At relatively small spacings, uniform charge distribution may be assumed with fairly

good accuracy.

Maxwell, Howe, Schelkunoff, and Friis [7, 8, 9], and others have shown that fairly accurate

capacitance evaluations result from assuming uniform charge distribution even for simple,

unshielded conductors. If one assumes uniform charge distribution over a particular con- ductor of a system, its average potential will be closely equal to the uniform potential it attains

when the same total charge has its natural distribution. Howe has shown that an error less

than 1 percent in capacitance results from assuming uniform charge distribution on a long

straight wire in free space. Maxwell and others have shown that first-order errors in the

assumed charge distribution lead only to second-order errors in capacitance.

An additional justification may be found in the fact that the relative potential distribution

in our case is changing with displacement; the departure from parallelism, and hence the error, will increase with the value of {r„—ra). It will also increase for increased conical angles and

with increased ratios of {ri,—ra)/h. Since the conical angles as well as the ratios of {rD—ra)/h over most of the range in the present case were relatively small, this may account for a further

reduction in the expected discrepancy.

9. Appendix V. Precautions for Measurement and Application

Though the precautions to be taken during measurement and applications of conical capacitors are obvious, it may be worthwhile to mention some of these here.

Since either the male or the female cones may be chosen as the traveliag electrode, one

must watch out for the effect of the stray capacitance from the male to the surrounding objects

and ground.

For measurement purposes it is best to ground the male and to do it in such a way that the

grounding leads are always kept as far as possible away from the female (outer) cone. The

male cone should be mounted on an insulating base placed at the largest diameter of the male.

The female-cone rim (or wall) thickness should be kept at a minimum in order to reduce fringing

flux. To still further reduce this fringing flux it is advisable to place a guard band closely

around the opening of the female cone; this band should be insulated from the female and

should be grounded. To obtain true increments of individual cones of multiple cone units it ma}^ be best to start with the well assembled complete unit first and then remove one male at a time.

In applications one must keep in mind the input impedance variation of the male to

ground. The male cone is usually the grounded traveling electrode. In case of the AT voltmeter [3], for example, the use of the male as the high-potential constant input impedance

electrode is necessary. Where application requires it, one may add flanges or rings at the bases

of the conical electrodes to increase the maximum capacity and range of this type of capacitor

still further. One may also design contour shapes other than conical in order to obtain various

functions of capacitance increments versus displacement similar to the various shapes (linear

frequency, linear capacitance, linear wavelength, etc.) used with variable parallel-plate air

capacitors.

10. Appendix VI. Rigorous Solution of Problem

A solution is understood to be "rigorous" when no approximations are made in any of

the steps of its derivation from a basic differential equation and when this solution is expressed

in a finite number of terms or in a convergent series of terms with the general term precisely

known. A rigorous solution must satisfy the differential equation and boundary condition

and must also be the only solution possible. The rigorous solution of a boundary value problem in electrostatic field distribution leading to the determination of capacitance between

electrodes, requires the following steps :

1. Selection of coordinate system,

2. expression of Laplace's equation in this system,

3. general solution of this equation,

11
4. application of boundary values and determination of constants to obtain a solution for the

potential distribution of the particular problem at hand.

5. the steps mentioned previously leading from a known potential distribution to capacitance.

The criteria for choosing a coordinate system are the difficulties anticipated in steps 3

and 4. Stratton [10] concluded that the only practical general procedure to solve Laplace's

equation is to employ a set of orthogonal curvilinear coordinates and the method of separation

of variables or "product solution".

As was discussed above under field construction, a three-dimensional coordinate system

must be used in this case despite the symmetry about the common axis of the cones [11].

The potential at any point in space is in this approach equal to the product of independent

functions, say, Rd^, where is a function of the radius vector alone, 6 of one of the coordinate

angles alone, and ^ of the other coordinate angles alone. Prescribed boundaries should coincide

with surfaces of the families of the coordinates. Oblique systems of coordinates, though of

greatest practical importance, unfortunately lead to partial differential equations which cannot

be solved as yet. Material on this subject, written at later dates [12, 13, 14, 15] does not seem

to shed further light on the subject nor does it offer a practical way of handling a problem like

the present where an oblique system seems to be the most suitable. This later system should

preferably consist of right conical surfaces, spherical surfaces and planes; the apexes of the cones should^move along the z axis. Figure 7 shows the coordinates of this system.

Figure 7. Oblique three-dimensional conical

coordinate system.

a is a constant solid angle. Coordinates r, <A, 2 fix location

of point P; z=2i renders a family of parallel coaxial conical

surfaces; r=ri, a family of concentric spherical surfaces; and

<t>H4>i, s family of planes through axis z.

N

N

Several texts [16, 17, 18] have treated the conical case and, unfortunately, limited the

analy^sis to systems of cones having a single apex. The method of images is widely used in cases of point and line charges; however it is impractical for more complex cases like the present.

The method of superposition of fields produced individually inside one cone and outside another

cannot be used because of the inevitable interaction effect which cannot possibly be neglected

when the cones are brought closer to each other.

A promising approach to the present case in orthogonal coordinates seems to be the application of the series expansion solution [19, 20, 21, 22, 23, 24]. This approach requires that at a fixed, finite value of one of the functions of, say, Rd (e.g., for R—Ri9^0 and Ri^^)

the value of the other should result in an expandable series, say, Vi=Ri6 where 9 is an expandable function in terms of Fom-ier trigonometric or Legendre series. The constants of the terms

12
can then be found; Ri is a factor incorporated in these constants and may be treated as a variable parameter of the final solution. The value of Ri is arbitrary; the only requirement is that

it be much larger than D. The particular value of Ri chosen will affect the accuracy of the

solution; to this extent the solution ceases to be "rigorous". Once determined for the boundary

siu-face where V=Vi, these constants are put into the general solution which thus renders the

value of V for all values of r. These steps may be specifically outlined as follows.

The general solution of Laplace's equation in polar spherical coordinates for the case of

axial symmetry is [25, 26]

V=f: (^nr"+5„/-"-0[CiP„(cos e)KlQn{cos e)] (38)

where An, Bn, C\, and K\ are constants, P„(cos 6) and Q„(cos 6) are Legendre coefficients of

first and second kind, respectively. For 0=0, Q«=°°. Therefore, the terms containing Q„

will remain in the solution for all values of r>7), where the space excludes the 0=0 region

from the solution (see fig. 8a), and will drop out for r <C. D because V cannot reach infinity

anyplace. In general, terms containing the Legendre coefficients of the second kind (second

kind zonal harmonics), Qn, drop out from solutions where the space boundaries of the voltage

distribution to be determined, include the axis of symmetry. On the other hand if this space

is everywhere away from the axis (i.e., for 05^0), these terms remain in the solution. It would

thus normally be necessary to apply two solutions to the case in question, one for r>D and

the other for r<CD; for r=D either solution should apply.

Because V cannot be infinite An must equal zero, otherwise for r= , V would be= co. One can also combine the constants and make BnC\=Cn and BnK\=KnThe solution for r>Z) thus reduces to

V=iz ^-«-irc„p„(cos 0)+i^„(?„(cos e)]. (39)

71 =0 The solution for r<Z) may be in this case neglected as it is of minor importance and

amounts only to a correction caused by fringing flux. It could, however, be treated in a similar

manner as the rest of the field.

A glance at figure 8a will reveal that for r>>D the shape of the function of V versus e

0 r--0, B--0

Figure 8. (a) Polar spherical coordinate systern t, 6, (j); 6 =0 is an axis of symmetry; (b)

Potential distribution function for rJ5>D. V

Vir—i—

I

-TT -a 0 +a +7r

(b)

13
will be similar to a rectangular pulse. One can assign a value r=ri (say, Vy^lOOD or lOOOZ))

for which the pulse is rectangular to any accuracy desired. The width of the pulse is 2a, the

period is 2Tr and its amplitude is Vi. Figure 8b shows one cycle of this function, between —tt and +7r. Such a restricted piecewise continuous function can be expanded in the interval

(—IT, ir) in a series of Legendre polynomials in the form of [20, 21, 24, 27, 28, 29]

F(cos =2 C„P„ (cos e) (40)

7!=0

where the coefficients Cn are given by

a-=^^J_ Fi9)P,icosd)sindde. (41)

The solution is expressed in (39) in terms of Legendre functions of both first and second

kind. In order to attempt a series expansion it seems best to express this solution in terms of

the first kind only, using the interrelation between the two shown, for example, by Smythe

[30] in an alternate general solution as

d=AiPM+B^ [i/2P„(m) In P.-i(m)- • •] (42)

or dJ^Ai+B,

I

In ^J] PM-B P(„-u(m)- . . . here /x=cos 6. (43)

The general solution is then

V=±r---'[(Ai+B.l\n'^)pM-B^^P,.-M- . . .]. (44)

The solution of the problem depends thus on the prospects of having (44) developed into a Fourier-Legendre-series. Once such a series could be obtained the next steps, as indicated

above, would be to (1) find the constants of this series from figure 8b, (2) substitute r=ri,

and V—Vi, into (44), and (3) equate the two series term by term to find the coefficients for

the solution (44).

No development of (44) into a Fourier-Legendre [31] series seems to be available at

present. The presence of the term In ^"^^ and of an additional constant factor for the term

P„ (fi) makes such an approach doubtfxil, despite the fact that figure 8b strongly supports it.

11. References

[1] J. C. Balsbaugh and P. H. Moon, A bridge for precision power-factor—measurements on small oil samples, Trans. AIEE 52, 528-536 (June 1933).

[2] Leigh Page, Introduction to Theroretical Physics, chapter 10, (D. Van Nostrand Co., Inc., Princeton, N.J.,

1935). F. W. Sears, Electricity and Magnetism, chapters 1, 2, and 3 (Addison-Wesley Publishing

Co., Inc., Reading, Mass., 1954).

[3] These types of voltmeters were announced and briefly described in NBS Tech. News Bui. 40, 29-30

(1956).

[4] A. D. Moore, Fundamentals of Electrical Design, p. 61 (McGraw-Hill Book Co., Inc., New York, N.Y.,

1927).

[5] S. S. Attwood, Electric and Magnetic Fields, pp. 62-69, 74-75, 174-175 (John Wiley and Sons, Inc., New York, N.Y., 2d ed., 1941).

[6] E. Weber, Electro-Magnetic Fields, vol. 1, pp. 197-204 (John Wiley and Sons, Inc., New York, N.Y., 1950).

Additional references are listed on pp. 204 and 205.

[7] J. C. Maxwell, A Treatise of Electricity and Magnetism, vol. 1, pp. 148-154 (Oxford University Press,

New York, N.Y., 1904).

(8] S. A. Schelkunoff and H. T. Friis, Antennas Theory and Practice, pp. 313-317 (John Wiley and Sons,

Inc., New York, N.Y., 1952).

14
[9] G. W. 0. Howe, The calculation of aerial capacitances, Wireless Eng. 20, 157-158 (1943). The capacity

of an inverted cone and the distribution of its charge, Proc. Phys. Soc. (London, England), 39, 339-344

(1917).

[10] J. M. Stratton, Electro-Magnetic Theory, pp. 47-51, 194-197 (McGraw-Hill Book Co., Inc., New York,

N.Y., 1941).

[11] W. R. Smythe, Static and Dynamic Electricity, p. Ill (McGraw-Hill Book Co., Inc., New York, N.Y.,

1950).

[12] Smythe, ibid., pp. 50-95, 133-155.

[13] S. Ramo and J. R. Whinnery, Fields and Waves in Modern Radio, pp. 145, 173-174 (John Wiley and

Sons, Inc., New York, N.Y., 1953).

[14] Stratton, op. cit., pp. 47, 197.

[15] S. A. Schelkunoff, Applied Mathematics for Engineers and Scientists, pp. 138-160, 417-427 (D. Van

Nostrand Co., Inc., Princeton, N.J., 1948).

[16] Smythe, op. cit., p. 146.

[17] Weber, op. cit., pp. 493-495.

[18] W. E. Beyerly, An Elementary Treatise on Fourier Series and Spherical, Cylindrical, and Ellipsoidal

Harmonics, pp. 191-194 (Ginn and Co., Boston, Mass., 1893).

[19] Ramo and Whinnery, op. cit., pp. 171-176.

[20] I. S. SokolnikofP, Higher Mathematics for Engineers and Physicists, pp. 342-386 (McGraw-Hill Book

Co., Inc., New York, N.Y., 1941).

[21] W. Kaplan, Advanced Calculus, pp. 425-427 (Addison- Wesley PubUshing Co., Inc., Reading, Mass.,

1953).

[22] Smythe, loc. cit.

[23] R. V. Churchill, Fourier Series and Boundary Value Problems, pp. 175-178 (McGraw-Hill Book Co.,

Inc., New York, N.Y., 1941).

[24] W. C. Johnson, Mathematical and Physical Principles of Engineering Analysis, pp. 242-245 (McGrawHill Book Co., Inc., New York, N.Y., 1944).

[25] Smythe, op. cit., pp. 129, 143.

[26] J. C. Slater and N. H. Frank, Introduction to Theoretical Physics, pp. 202-203 (McGraw-Hill Book Co.,

Inc., New York, N.Y., 1933).

[27] J. H. Jeans, Mathematical Theory of Electricity and Magnetism, p. 223 (Cambridge University Press,

New York, N.Y., 1925).

[28] T. M. MacRobert, Spherical Harmonics, pp. 95-96, 102-103 (Dover Pubhcations, Inc., New York,

N.Y., 2d edition, 1948).

[29] Beyerly, op. cit., p. 170.

[30] Smythe, op. cit., p. 145.

[31] Among the references consulted for such a development were also: E. W. Hobson, Spherical and

Ellipsoidal Harmonics (Chelsea Publishing Co., New York, N.Y., 1931), according to Smythe (op. cit.,

p. 217, 1939 and 1950) "includes almost everything known about the subject."

P. Franklin, Methods of Advanced Calculus, chapter XI (McGraw-Hill Book Co., Inc., New York, N.Y.,

1944).

J. D. Kraus, Electromagnetics, pp. 519-541 (McGraw-Hill Book Co., Inc., New York, N.Y., 1953).

F. D. Murnaghan, Introduction to Applied Mathematics, pp. 166-174 (John Wiley and Sons, Inc., New

York, N.Y., 1948). E. T. Whittaker and G. N. Watson, A Course of Modern Analysis, pp. 303-323 (Cambridge University

Press, New York, N.Y., 1952).

15

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