AlllDE IMESm

NBS

Publications

NATL INST OF STANDARDS *( jECH RjC.

A11 1021 4221

\

*<'*fAU 0* *^ NBS TECHNICAL NOTE 1191

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Electrical Parameters

in 60-Hz Biological

Exposure Systems and

Their Measurement:

A Primer
TECHNOLOGY

Research Momiation Center

Gaithersburg, MD 20899

NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities^ — Radiation Research — Chemical Physics —

Analytical Chemistry — Materials Science

THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering^ — Manufacturing

Engineering — Building Technology — Fire Research — Chemical Engineering^

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center arc located at Boulder, CO 80303.
'BATIOl'TAL BUREAU OF STAliD/i-TlDS LIBRARY Electrical Parameters in 60-Hz Biological /\/g5^

Exposure Systems and Their Measurement: ^^^

A Primer ^575

^ _ N^./i^f

M. Misakian

Electrosystems Division

Center for Electronics and Electrical Engineering

National Bureau of Standards

Washington, DC 20234

Prepared for:

Division of Electric Energy Systems

Department of Energy

1000 Independence Avenue, SW

Washington, DC 20585

o^ y iP

CO Q*

U.S. DEPARTMENT OF COMMERCE, Malcolm Bal^rige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1984
National Bureau of Standards Technical Note 1191

Natl. Bur. Stand. (U.S.), Tech. Note 1191, 43 pages (Apr. 1984)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1984

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
TABLE OF CONTENTS

Page

LIST OF FIGURES V

LIST OF TABLES vi

Abstract 1

1. INTRODUCTION 1

2. AC TRANSMISSION LINE FIELDS 1

3. LABORATORY SIMULATION OF TRANSMISSION LINE FIELDS 3

3.1 Electric Field Between Parallel Plates 4

3.1.1 Stray Electric Fields and Shielding 4

3.2 Energizing Parallel Plates 4

3.3 Perturbations of Electric Field 6

3.3.1 Parallel Plate Spacing 6

3.3.2 Perturbations by Nearby Ground Planes 7

3.3.3 Perturbations by Objects Between Plates 7

3.3.4 Mutual Shielding of Test Animals 9

3.4 Magnetic Field of Rectangular Loops 9

3.5 Magnetic Field of Helmholtz Coils 11

3.5.1 Stray Magnetic Fields and Shielding 14

3.6 Energizing Magnetic Field Coils 14

3.7 Perturbations of Magnetic Field 15

4. MEASUREMENT OF FIELD PARAMETERS 17

4.1 Electric Field Measurements 17

4.2 Magnetic Field Measurements 19

4.3 Fieldmeter Calibration 20

4.3.1 Electric Fieldmeters 20

4.3.2 Magnetic Fieldmeters 20

4.4 Harmonic Content Measurements . 22

4.5 Phase Measurements 22

m
TABLE OF CONTENTS (continued)

Page

5. CORONA 28

6. ACKNOWLEDGMENTS 28

7. REFERENCES AND NOTES 29

8. APPENDIX 33

IV
LIST OF FIGURES

Page

Figure 1. Electric and magnetic field lines of single conductor

above ground plane 2

Figure 2. Electric field ellipses in vicinity of 3-phase

transmission line 2

Figure 3. Normalized electric field at the surfaces of and midway

between semi -infinite parallel plates 5

Figure 4. Perturbations of electric field in plastic enclosure due

to presence of water bottle, food, and plastic. The

relative field strength values prior to the introduction

of the enclosure and water bottle are shown in

parenthesis 8

Figure 5. Coordinate system and configuration of rectangular loops

for generating magnetic field 10

Figure 6. Normalized flux density profiles along z-axis and along

vertical line through point x = 0.25 m, y = 0.25 m. The

magnetic field is produced with two square coils of 1 m side dimension and separated by a distance of 0.5 m 12

Figure 7. Configuration of coils for generating magnetic field and

coordinate system 13

Figure 8. Schematic view of apparatus for simultaneously

generating an electric field and a rotating magnetic

field 16

Figure 9. Schematic view of flat probe-type electric fieldmeter 18

Figure 10. Schematic view of coil-type magnetic fieldmeter 18

Figure 11. Percentage departure of B^ from 62(0,0,0) for

positions in the plane of a square loop 1 m x 1 m, and

3 cm above or below the plane (parentheses). A scale

drawing of a coil -type probe is outlined 21

Figure 12. Effect of 3rd harmonic on sine wave: (a) sine wave (top)

and sine wave combined with 6% 3rd harmonic, in phase

(bottom); (b) sine wave and sine wave combined with 6%

3rd harmonic, 180° out of phase; (c) sine wave and sine wave combined with 6% 3rd harmonic, 90° out of phase.

Using the probes described in the text, the waveforms

with 6% 3rd harmonic would correspond to what would be

observed if there was 2% 3rd harmonic in the field 23
LIST OF FIGURES (continued)

Page

Figure 13. Effect of 4th harmonic on sine wave: (a) sine wave (top)

combined with 4% 4th harmonic, in phase

sine wave and sine wave combined with 4%

180° out of phase; (c) sine wave and sine

with 4% 4th harmonic, 90° out of phase.

Using the probes described in the text, the waveforms

with 4% 4th harmonic would correspond to what would be

observed if there was 1% 4th harmonic in the field . . .

Effect of 4th

and sine wave

(bottom); (b)

4th harmonic,

wave combined

,24

Figure 14. Effect of 5th harmonic on sine wave: (a) sine wave (top)

combined with 3% 5th harmonic, in phase

sine wave and sine wave combined with 3%

180° out of phase, (c) sine wave and sine

with 3% 5th harmonic, 90° out of phase.

Using the probes described in the text, the waveforms

with 3% 5th harmonic would correspond to what would be

observed if there was 0.6% 5th harmonic in the field. .

Effect of 5th

and sine wave

(bottom); (b)

5th harmonic, wave combined

25

Figure 15. Determining phase relation between waveforms with

oscilloscope, (a) Waveforms 90° out of phase,

(b) waveforms 90° out of phase, (c) displacement

produced by 4.5° shift in phase is readily observed 27

LIST OF TABLES

Table 1.

Table 2.

Normalized electric field strength midway between plates

and at plate surfaces ,

Normalized flux density profiles,

5

12

VI
Electrical Parameters in 60-Hz Biological Exposure

Systems and Their Measurement: A Primer

ABSTRACT

This report presents material which is intended to provide assistance

in the measurement of a number of electrical parameters that are of

importance during bioeffects research involving 60-Hz electric and

magnetic fields. The parameters that are considered are the electric

field strength E, the magnetic induction or flux density B, field

uniformity, harmonfc content, phase relations between field

components, and corona. Descriptions of the fields and methods for

their laboratory generation are surveyed. The text is purposely

elementary with references provided to aid the interested reader in

obtaining a fuller understanding of many of the topics. It is shown

that using relatively simple instrumentation, it is possible to

characterize reasonably well the electric and magnetic fields used in

animal exposure studies.

Key words: biological effects; corona; electric field; ELF; magnetic

field; power frequency; 60-Hz; transmission lines.

1. INTRODUCTION

Because of the interest in recent years of possible biological effects of

electric and magnetic fields from ac high voltage transmission lines, many

laboratory studies have been conducted, or are under way, to determine what

effects can be associated with these fields [1-8]. This report identifies a number of electrical parameters which must be controlled during such laboratory

studies and describes procedures for their measurement. The report is intended

to serve as a measurement primer for those planning biological experiments with

60-Hz electric and magnetic fields, perhaps for the first time. The electrical

parameters which are considered are the electric field strength, E, the

magnetic induction, B, field uniformity, harmonic content, phase relations

between field components, and corona. Methods for generating 60-Hz electric

and magnetic fields are also briefly described.

It is noted that the parameters cited are a small subset of a larger list of

physical parameters with which an investigator must be concerned. For example,

such things as temperature, humidity, light and dark cycles (and the timing of

exposure), noise, vibrations, and short circuit currents during drinking of

water may also be of importance. The reader is referred to the literature for a discussion of these and other parameters [9-12]. This report, while tutorial in

nature, assumes that the reader is familiar with the subjects of electricity and

magnetism as presented in an introductory college physics course.

2. AC TRANSMISSION LINE FIELDS

The electric and magnetic field lines of a single conductor energized with GO-Hz

voltage and located above a ground plane are shown schematically at some instant

in figure 1. The magnetic induction or flux density B at a point can be

represented as a vector that oscillates in magnitude at a frequency of 60-Hz

along a line tangent to a circular magnetic field line. Similarly, the electric

field strength, E, can be represented as an oscillating vector tangent to an electric field line. These fields are sometimes referred to as single-phase ac fields [13].
Figure 1. Electric and magnetic field lines of single conductor

above ground plane.

/

\ S oQo ^ /

Conductors

\

Figure 2. Electric field ellipses in vicinity of 3-phase

transmission line.
Transmission lines normally have three or more conductors (or conductor bundles)

with multiple voltage phases. The electric and magnetic fields of a three-phase

transmission line have been calculated and illustrated by Deno [14] and a somewhat simplified sketch of the electric fields in the vicinity of a three-phase transmission line is given in figure 2. Three conductors are shown

above the ground plane in figure 2 and the phase of the 60-Hz voltage applied to

each conductor differs by 120° with respect to the conductor adjacent to it

(e.g., sinusoidal voltages Vsin(wt), Vsin(a)t + 120°), and Vsin(wt +240°) applied

to adjacent conductors where V is the peak voltage).

In general, the electric field strength E at a point in space can be represented

as a rotating vector which traces an ellipse in a plane perpendicular to the

conductors as shown in figure 2. Near ground level, the field ellipse

degenerates to a nearly vertical line. In the absence of nearby objects or irregular terrain, the field strength changes slowly from ground level to a height of about 1 or 2 meters, i.e., the field is approximately uniform. At

ground level the field vector oscillates along a line perpendicular to the

ground. Because of this spatial dependence of the transmission line electric

field, laboratory simulations of the field at or near ground level can be made

with a parallel plate apparatus (Section 3.1).

Although not shown in figure 2, the magnetic field at a point near a three-phase

transmission line can also be represented by a rotating vector in space but in

contrast to the electric field, the vector B is a rotating vector even at ground

level [14]. Thus, laboratory simulation of a three-phase transmission line

magnetic field near ground level requires an apparatus which can generate a rotating field (Section 3.6).

It can be shown that the elliptical representation of the fields for the

three-phase, single-circuit transmission line shown in figure 2 is also relevant

to lines with multiple circuits. That is, the fields from transmission lines,

in general, can be represented by rotating vectors [15]. The maximum electric

field strength near ground level within the right of way ranges in value from a few kV/m to about 9 kV/m, depending on the line voltage and conductor geometry.

Similarly, the magnetic field can be as great as 0.3 x lO"** tesla (0.3 gauss),

depending on the current (load) and conductor geometry.

3. LABORATORY SIMULATION OF TRANSMISSION LINE FIELDS

As noted above, a parallel -pi ate apparatus can be used to generate the

approximately uniform electric field that exists at or near the ground in the

vicinity of a transmission line. Test animals have typically been placed in

contact with the bottom plate and housed in plastic enclosures during exposure

to the electric field [16].

Simulations of rotating transmission line magnetic fields can be obtained with

two pairs of orthogonally oriented rectangular or square loops of many turns of

wire or circular loops known as Helmholtz coils. Such systems can be used in

conjunction with a parallel-plate, electric field apparatus to generate

simultaneously 60-Hz electric and magnetic fields. The electric fields produced

with parallel plates and the magnetic fields produced with orthogonally oriented

coils of wire are described in further detail in the next two sections. While

fields that are highly uniform can be genei^ated with the systems that are described, ambient electric and magnetic fields with variations of about ±5%

with respect to the fields at some central point have frequently been used in
experiments. What is considered acceptable field uniformity in a given

experiment must be determined by the researchers.

It should be noted that throughout this report the sinusoidal time dependence of

E and B and associated voltages and currents will be suppressed; use of

root-mean-square (r.m.s.) values for these quantitities will be assumed. It is

recalled that the r.m.s. or "effective value" of a sinusoidal current is equal

to the steady (dc) current that is required to develop the same amount of heat

as the ac current passing through the same resistance in the same time. The

r.m.s. current is equal to the peak sinusoidal current divided by fT. Similarly

the r.m.s. voltage, electric field strength, etc. are equal to their peak values

divided by fl.

3.1 Electric Fields Between Parallel Plates

Regions of nearly uniform electric field strength of known magnitude can be

produced with a parallel plate system provided that the spacing of the plates,

relative to the plate dimensions, is sufficiently small. The uniform field

value Eq is given by V/t where V is the applied potential difference between

the plates and t is the plate spacing. The magnitudes of the normalized field,

E/Eq, at the plate surfaces and in the midplane of semi -infinite parallel

plates can be theoretically determined [17] and are plotted as a function of

normalized distance x/t from the plate edge in figure 3; numerical values are given in Table 1.

The numerical values in Table 1 show that the departure from field uniformity

due to fringing field effects is 0.1% at a distance of one plate spacing from

the edge. For finite square plates, the fringing field effects from four edges

can be estimated by superposition when the effect from one edge is less than

0.2%. For example, when the side dimension of a square parallel plate system is

twice the plate spacing, the field strength at the center of the plate surfaces

is approximately 0.4% greater than Eq. These results are valid in the absence

of perturbations due to nearby grounded objects such as walls, floors,

equipment, etc. (The perturbations due to non-grounded objects is, in general,

less.)

A numerical calculation of the electric field between finite square parallel

plates has been reported in the literature [18]. Profiles of the electric field

between the plates are provided for different ratios of plate dimension to plate

spacing. This calculation also assumes the absence of perturbations due to

nearby ground planes.

3.1.1 Stray Electric Fields and Shielding

While attention in Section 3.1 focuses on the field between the parallel plates,

it should be remembered that the electric field also extends to regions beyond

the plates [17]. Thus, if sham exposed (control) animals are located nearby,

electrical shielding may be necessary for these animals. Use of grounded metal screen or hardware cloth readily reduces the field strength to ambient levels.

3.2 Energizing Parallel Plates

The parallel plates can be energized with a commercially available high voltage

power supply or with a variabte autotransformer-transformer combination. A

voltage regulator or line conditioner is frequently used to prevent fluctuations

of the line voltage to the autotransformer. An oscillator and power amplifier
(V) Semi-infinite plates

/

Plate edges

\

E 1 ; Eo = V/t

1

J2.6

2.2

1.8

X /t

o

UJ

liJ Q

-1HI

UL \

1 1 ' 1

' 1'

-

a

UlN

<

1.4

1.0

-\

Vc

Plate surface

; S —

o

z

0.6

«

1 1

x_ Midway between

plates

I.I.I. -

0.2 0.4 0.6 0.8 1.0

NORMALIZED DISTANCE

FROM EDGE (x/t)

Figure 3. Normalized electric field at the surfaces of and midway

between semi -infinite parallel plates.

Table 1. Normalized electric field strength midway

between plates and at plate surfaces

Midway Between Plates

x/t E/Eo

0.0698 0.837

0.1621 0.894

0.2965 0.949

0.4177 0.975

0.6821 0.995

0.7934 0.997

1.0000 0.999

Plate Surface

x/t E/Eo

0.0185 2.449

0.0829 1.414

0.1230 1.265

0.1624 1.183

0.2431 1.095

0.4376 1.025

0.6861 1.005

0.7954 1.002

1.0000 1.001
can also be used in place of the autotransformer. Systems for energizing the

plates should have provisions made for continuous metering of the voltage to the

plates, e.g., a panel meter on the power supply (during "double-blind" field

exposure studies, it may be necessary to make arrangements for recording the

"field-on" condition without visual cues for the investigators). While the

voltage across the plates can be measured with a calibrated electrostatic

voltmeter or high voltage divider and correlated with the voltage setting on the

power supply, it is perhaps more useful to establish a correspondence between

the electric field, which is readily measured (Section 4.1), and the power

supply setting. This correspondence should be checked regularly to insure

repeatable field exposure conditions.

Center-tap transformers which produce two high voltages of equal magnitude but

of opposite phase and "single-ended" transformers which produce a single high

voltage relative to ground potential have been used to energize the parallel

plates. The former approach minimizes perturbations of the electric field

between the plates by nearby grounded objects or planes (see below). With

certain precautions taken to minimize corona (see Section 5), electric field

strengths as large as 100 kV/m have been produced with parallel plate systems

for biological studies [19].

Because lethal voltages and currents are usually generated by the power supply,

precautions must be taken to prevent contact between personnel and the high

voltage leads and parallel plates during field exposure periods. The parallel

plates can be located in rooms with door interlocks which, when opened,

interrupt the voltage to the plates. Current limiting resistors in the high

voltage leads to the parallel plates can also be used as an added precaution.

Isolation of the power supply in a room separate from the parallel plates may be

required in some cases to prevent audible noise from the power supply reaching

the test animals. Vibrations of the parallel plates because of sinusoidal

electrical forces may also be of concern [9]. Additional remarks regarding the

energizing of parallel plates during experiments employing both electric and

magnetic fields are given in Section 3.6.

3.3 Perturbations of Electric Field

Two types of field perturbations are distinguished in this report. The first, over which control is not normally attempted, is the perturbation of the field

near and at the surface of a test animal. This perturbation depends on the

animal's geometry and electric potential. The second type of field

perturbation, over which some control can and should be exercised, is produced

by factors that influence the ambient field strength (i.e., as shown in fig. 3)

prior to the introduction of the test animal between the parallel plates or influence the "normal" geometric field perturbation at or near the surface of

the test animal after it is placed into the field. Field perturbations of the

second type are considered next.

3.3.1 Parallel Plate Spacing

While the information in Table 1 is useful for estimating the dimensions of a parallel plate system which has extended regions of field uniformity, the size

of the test animal can influence the choice of spacing between the plates. The

parallel plate spacing should be sufficiently large in order to avoid

significant perturbation of the surface charge distribution on the top plate by

the presence of the animal [19]. The measurements of Kaune [20] indicate that a
model animal in the shape of a hemi -ellipsoid on the bottom plate does not

significantly interact with the top plate charge distribution when the height of

the hemi -ellipsoid is as much as 28% of the plate spacing. Failure to provide

adequate spacing between the plates leads to a field-induced current in the

animal that is larger than that produced by an ambient field of the same magnitude but with greater parallel plate spacing. Alternatively, it can be

said that inadequate plate spacing can increase the field strength at the

surface of the animal beyond that caused solely by the animal's shape.

3.3.2 Perturbations by Nearby Ground Planes

The electric field values shown in figure 3 and Table 1 have been determined

assuming that there are no objects or ground planes nearby. In the presence of

ground planes, which can be approximated by a wall, floor, or ceiling, the

surface charge distributions on the parallel plates are perturbed and lead to

changes in the electric field. If one of the plates is grounded, the

perturbations can be quite large, particularly near the plate edge [21,22]. For

example, measurements have shown that the surface field strength near the edge

of a grounded plate can be reduced by more than 10% when a vertical ground plane

is two plate-spacings away from the edge of the parallel plates [21]. In

general, the field strength is reduced- at the grounded plate and increased at the energized plate; corresponding changes occur at points between the plates.

Since the unperturbed field on the plate surfaces increases as the edges are approached (fig. 3), some reduction in field strength near the edge of the

grounded plate may be desirable in order to obtain a greater region of

approximately uniform field where the test animals are located. The

perturbations due to nearby ground planes can be reduced by using a center-tap

transformer to energize the parallel plates. In any event, the variation of the

field strength where test animals will be placed for exposure should be

characterized and this is readily accomplished with the instrumentation

described in Section 4.1.

3.3.3 Perturbations by Objects Between Plates

Introduction of objects with large dielectric constants such as food or a plastic bottle filled with water between the parallel plates can cause significant perturbations of the field. Figure 4 shows a schematic view of a plastic animal enclosure containing food and water, and which has been used for

exposing animals to 60-Hz electric fields. The water and metal mesh floor are electrically connected to reduce the possibility of spark discharges between the

animal and water when the animal drinks. The field strength values with and

without parentheses in figure 4 represent measurements performed with the

enclosure removed and present, respectively. The measurements in the enclosure

reveal a low field region below the bottle that is approximately a third of the

uniform field value between the plates [23].

The influence of plastic enclosures on the field distribution at the surface of

a test animal has been studied by Kaune [20]. By measuring induced currents on isolated conductive patches on a hemi-ell ipsoid animal model, he has

demonstrated that the surface fields can be increased or decreased depending on the position of the animal relative to the plastic enclosure and the location on the surface of the animal. Contamination of the plastic surface from animal

contacts can also have significant effects on the field. Decreases in field

strength of 49% within an enclosure have been observed after rats have occupied

the enclosure for 16 days [24].
GLASS BOHLE

PLASTIC

ENCLOSURE

METAL SCREEN

FLOOR

METAL PLATE

SIDE VIEW

>

>.

TOP VIEW

9FIELD PROBE

WALL REMOVED FOR

E-FIELD MEASUREMENTS

Figure 4. Perturbations of electric field in plastic enclosure due

to presence of water bottle, food, and plastic. The

relative field strength values prior to the introduction

of the enclosure and water bottle are shown in

parenthesis

.
3.3.4 Mutual Shielding of Test Animals

The last source of field perturbation which is considered is caused by the

placement of more than one animal into the exposure system. While actually

another example of the type of perturbation considered in 3.3.3, it is treated

separately here to emphasize its importance.

Some animals, such as mice, if placed i up" and thereby change the field distri

separating the animals with a partition

between the animals is reduced. Howeve

proximity of adjacent rats, as determin

(or short-circuit) current in an animal

35% under certain conditions [20]. Mod

ellipsoids with conductive coatings can along the perimeter of a partitioned en exposure conditions for all the animals

nto the same plastic enclosure may "pile

bution at the surface of each animal. By

ed plastic enclosure, the interactions

r, the mutual shielding caused by the

ed by measurement of the total induced

, has been found to be as large as about

el animals in the shape of hemibe placed adjacent to animals that are closure to provide comparable field

[25].

3.4 Magnetic Field of Rectangular Loops of Many Turns

Figure 5 shows two rectangular loops of many turns of wire of dimensions 2a x 2b

separated by a distance s. The z-component of the magnetic flux density at a point P(x,y,z) due to a current in the lower loop is given by the

expression [21].

B zl

y 4

a=l

Tir

(-l)"d.

r [r + (-1)

a a IdT

C ] ar (r + d ) a a a

(1)

where

Ci = -Cit = a+x

C2 = -C3 = a-x

ri = / (a+x)^ + (b+y)^ + z'

r2 = / (a-x)^ + (b+y)^ + z'

di = d2 = y+b

dg = di, = y-b

r3 = / (a-x)2 + (b-y)2 + z^

r^ = / (a+x)^ + (b-y)2 + z^

N is the number of turns, I is the current and ]Xq is the magnetic permeability

of air. The conductors in the loop are assumed to be of negligible cross section. By superposition of fields, the flux density at P(x,y,z) due to a current I in both loops assuming the same number of turns in each loop, is

B_ = ^zl^ - IN I a=l

Tit

(-l)°'d,

r' [r' + (-1)°^^C ] r' (r' + d ) a a a

(2)

where
P(x,y,z)

^y

Figure 5. Coordinate system and configuration of rectangular loops

for generating magnetic field.

10
ri' = / (d+x)2 + (b+y)^ + (s-z)'

ra' = / (a-x)^ + (b+y)^ + (s-z)'^

ra' = / (a-x)2 + (b-y)^ + (s-z)^

r^' = / (a+x)2 + (b-y)2 + (s-z)^

It can also be shown that the x and y-components of the flux density for the two

loops are given by [26]

X Air ^,

a=l

Tlx + d ) a a a

^ (-l)"(s-z)

r'(r' + d )

M 4 r

•^ a=l

(-1)°^^ z

r [ r + (-1)°'^^C ]

(-l)^(s-z)

r' [r' +

(-1)°'"'^ C ]J

(3)

(4)

Figure 6 shows the variation along the z-axis of the normalized flux density,

B/B(0,0,0.25), for square coils of 1 m side dimension and 0.5 m spacing (because

of symmetry considerations, Bx and By vanish along the z-axis and B2 is

equal to the total flux density B). Also shown are the variations of the

normalized x, y, and z-components of the flux density along a vertical line

passing through the point (0.25, 0.25, 0). The corresponding numerical values

are given in Table 2. It is evident that for this sample calculation, the flux

density is within 4% of B(0, 0, 0.25) in a central volume of 20 cm x 20 cm x 20 cm.

Beginning with eq. (1), the principle of superposition can be used to develop

expressions for the magnetic induction due to two or more rectangular loops of

many turns with different values of a, b, N and spacing provided that their z-axes coincide.

It should be noted that the magnitude and time dependence of the magnetic flux

density at a point is determined by the sinusoidal current I once the

dimensions, a and b, and the number of turns, N, of the field windings have been

fixed.

3.5 Helmholtz Coil Magnetic Field

The z-component of the magnetic flux density at a point P(r,e), between two

coils of radius b with N turns of wire separated by a distance of 2d as shown in

figure 7 is given by [27]

B,(r,9) =_ y Ib2N

where

[2+A2P2(cos9)r2+A^P^(cose)r'*+...]

2(b2+d2)^5

(5)

Ao =

A, =

- 15d2-3(b2+d2)

(b2+d2)2

^ 15(b2+d2)2-210d2(b2+d2)+315d'^

4(b2+d2)'^

11
(0

z

O

X

D

O

LUN

_J

<

IEo

z

1.2

1.0

0.8

1 1 1 1

_ rBz(0.26,0.25,z)/B(0,0,0.25)

Bz(0,0,z)/B(0,0,0.25)

20 cm —»|

0.02 -

-0.02 - Bx (0.25,0.25, z)/B(0, 0,0. 25)

By (0.25,0. 25, z)/B(0,0,0. 25)

10 20 30 40 50

HEIGHT ABOVE BOTTOM COIL (z.cm)

Figure 6. Normalized flux density profiles along z-axis and along

vertical line through point x = 0.25 m, y = 0.25 m. The

magnetic field is produced with two square coils of 1 m

side dimension and separated by a distance of 0.5 m.

Table 2. Normalized flux density profiles

z(cm) B (0,0,z)/B(0,0,0.25) 8 (0.25,0.25,z )/B(0,0,0.25) B* ^(0.25,0.25,z)/B{0,0,0.25)

z z X ,y

1.1476

1.1471

1.0994

1.0377

0.9903

0.9730

0.9903

1.0377

1.0994

1.1471

1.1476

Negative sign indicates reversal of field direction.

0.9335

5 0.9637

10 0.9830

15 0.9937

20 0.9986

25 1.0000

30 0.9986

35 0.9937

40 0.9830

45 0.9637

50 0.9335

-0.0807

-0.0247

0.0128

0.0254

0.0179

0.0000

-0.0179

-0.0254

-0.0128

0.0247

0.0807

12
Figure 7. Configuration of coils for generating magnetic field and

coordinate system.

13
]1q is the permeability, I is the current, r and 9 are spherical coordinates,

and Pm(cose) are Legendre polynomials of order m; the conductor cross sections

are again assumed to be negligible.

For the condition 2d = b, A2 vanishes and the field becomes highly uniform in a small volume near the origin. For example, in a region jz-(b/2) <(b/10) along

the z-axis (i.e., 6 = 0), 82(2) varies from the central value by less than

0.012%. Coils that satisfy the condition 2d = b are known as Helmholtz coils.

The horizontal field component can be calculated but only with some difficulty

[28,29] and is not considered here.

3.5.1 Stray Magnetic Fields and Shielding

Equations (2) to (5) show that the magnetic field decreases as a function of

distance from the field generating loops and coils. For example, the magnetic

induction along the z-axis for a set of square loops 1 m x 1 m and 0.5 m spacing

decreases to 0.44% of the central value (between the loops) at a distance of

3.75 m from the center of the loop. For a central flux density of 2 x

10"'*

tesla (2 gauss), the stray field 3.75 m away is 8.8 x 10"^ tesla

(8.8 milligauss) which is more than three times the ambient magnetic field

levels reportedly found in homes [30].

Preventing stray ac magnetic fields produced by an exposure system from reaching

a sham exposed group of animals is, in general, more difficult and costly than

shielding 60-Hz electric fields. Metal sheets with large permeability or high

conductivity can be used for attenuating the stray fields to an "acceptable" (to

be set by the investigators) level if the exposed and sham exposed animals

cannot be separated with sufficient distance. Metals with large permeabilities

concentrate and confine the magnetic field lines to a degree that depends on the

angle of incidence of the flux lines and the permeability [31]. Limited

attenuation also occurs in metals of low permeability, but good conductivity,

such as copper because the varying magnetic field induces eddy currents in the

metal which reduces the flux density. The distance into the metal at which the

magnetic field is attenuated to e"^ of its value at the metal surface is known

as the skin depth and is given for copper by the expression [32]

6 = 0.066/f°-^ (6)

where f is the frequency of the field and 6 is given in meters. For 60-Hz

magnetic fields, the skin depth is nearly 1 cm. If shielding is required, a trial and error approach using metal sheets of high permeability and good

conductivity followed by measurements of the residual field may be the most

expeditious way to develop adequate shielding for the sham exposed animals.

3.6 Energizing Magnetic Field Coils

Current to the magnetic field coils can be provided with a variable autotransformer and current limiting resistor. As in the case of energizing the

parallel plates, use of a voltage regulator or line conditioner is desirable to

reduce fluctuations of the line voltage to the autotransformer. Also, as before, an oscillator-power amplifier combination may be used in the place of

the variable autotransformer. Knowledge of the current limiting resistance and

measurement of the voltage across the resistor allows one to determine the

current, I, using Ohm's law. The flux density can then be calculated from

14
equations (2) to (5). It is assumed here that the power rating of the current

limiting resistor is large enough to prevent significant change in resistance

value due to heating.

As noted in Section 2, the magnetic field from a three-phase transmission line

is a rotating magnetic field in a vertical plane at and near ground level.

Circularly polarized magnetic fields rotating in a vertical plane have recently

been adopted for use in the animal field exposure studies sponsored by the State

of New York Department of Health [8] and the production of such fields is

briefly considered here.

Circularly polarized magnetic fields can be produced at a point by two sets of

orthogonally oriented magnetic field loops or coils provided that the field

components there are equal in magnitude and differ in phase by 90°. If the

magnetic field components can be made nearly uniform in an exposure system where

test animals will be located, the animals will experience an approximately

circularly polarized magnetic field. Because the time dependence of the field

components is a function of the sinusoidal currents to the coils, the 90°

phase difference between the field components is obtained by phase-shifting the

current to one of the coils by 90° with respect to the other coil.

Figure 8 is a schematic view of an apparatus which permits simultaneous

excitation of two sets of orthogonally oriented magnetic field coils and a parallel plate electric field apparatus. If the impedance in the circuits to

the vertical field coil is dominated by the current limiting resistor and wire

resistance (and not the inductance of the coils), the vertical component of the

magnetic field and the electric field will be nearly in phase because the

current to the coil and the voltage applied to the plates will be in near phase

coincidence. The phase of the horizontal magnetic field component can be made

to differ from the vertical magnetic field by 90° by using the phase shifter.

The phase relationships between the electric and magnetic fields described above

are being used in the New York State-sponsored small animal studies. While

fixing the phase relationships between the fields is considered arbitrary at

this time because of the state of knowledge of power frequency field effects,

the practice insures a degree of consistency in exposure conditions for the

various investigations.

The schematic view of an exposure system in figure 8 is shown for illustrative

purposes and is not intended as a recommended design for an exposure apparatus.

Other arrangements such as employing more than two loops or coils along a given

axis (with unequal turns) or energizing the coils and plates with three-phase

line voltage can be used [33].

A final engineering note is that the current carrying capacity of insulated wire

used in the magnetic field windings may have to be reduced from the value

specified by the manufacturer in air because of excessive heating effects that

can occur in the "bundled" configuration.

3.7 Perturbations of Magnetic Field

The number of mechanisms for perturbing the magnetic field are fewer than for

the electric field case. Ferromagnetic materials such as iron and nickel can

15
Phase shifter

Power

amplifiers

Figure 8. Schematic view of apparatus for simultaneously

generating an electric field and a rotating magnetic

field.

16
cause large perturbations when introduced into a magnetic field but their use is

readily avoided in field exposure studies. Most other (non-ferromagnetic)

materials exhibit the weaker magnetic effects of paramagnetism and

diamagnetism [34] and perturb the field to a degree that can normally be

neglected. Non-ferromagnetic conductors such as copper and aluminum are "exceptions to the rule" because the ambient magnetic field varies with time and

induces eddy currents which can lead to perturbation of the magnitude and phase

of the magnetic field. The magnitude of the perturbation increases with eddy

current and decreases with distance from the conductor. Because parallel plate

conductors are used to generate the electric field in exposure systems that

employ electric and magnetic fields simultaneously, and test animals are usually

located on the surface of the bottom plate, the eddy current perturbations must

be characterized (measured) and, if necessary, reduced to an acceptable level.

Perhaps the most direct approach for reducing the perturbation is to reduce the

eddy currents by decreasing the conductivity of the parallel plate electrodes.

Techniques that have been employed include using thin metal foil mounted on dielectric surfaces for the plates, partitioning the plate into smaller but

connected sections, and using wide mesh metal screen for the bottom plate.

Methods for characterizing the perturbation of the magnetic field are discussed

in Section 4.5.

4. MEASUREMENT OF FIELD PARAMETERS

4.1 Electric Field Measurements

The measurement instrumentation which is considered here is intended for use in

exposure systems that are typically employed for small animal studies. The

electric fields in larger exposure systems, such as might be used for human

experiments, may be characterized with commercially available electric field

strength meters that are used for measurments in the vicinity of transmission

lines [13]. These probes are, in general, too large for use between parallel

plates employed for small animal studies (with spacings of the order of 0.5 m)

because of excessive interaction between the fieldmeter and the surface charge

distributions on the parallel plates.

Electric field strength measurements at the surface of a grounded bottom plate

of a parallel plate system are readily performed with the simple instrumentation

shown schematically in figure 9. A circular or square electrode fabricated from

double copper-clad printed circuit board serves as the sensing probe, and a current-to-voltage operational amplifier circuit and digital voltmeter act as the detector of an electric field strength meter [35]. The fieldmeter shown in

figure 9 determines the field strength, averaged over the area of the sensing

electrode, by measuring the current induced into the sensing electrode. Using

Gauss' law, it is readily shown that the relation between the electric field

strength at the surface of the electrode, E, and the induced current, I, is

I = ZirfeoAE (6)

where f is the frequency of the sinusoidal field, cq is the permittivity of

free space and A is the area of the sensing electrode. For example, the current

induced in a probe with a sensing area of 100 cm^ placed in a 10 kV/m electric

field is about 0.33 x 10"^ amperes (due to the finite thickness of the probe and

17
Copper

cladding

\ \J?lJUi77mi^))))))n))))»)rm,

Conducting

painty:?-

zznwr

Field sensing

electrode

Guard

band

e

r

DVM Shielded cable

to operational

amplifier

Figure 9. Schematic view of flat probe-type electric fieldmeter,

B

Shielding

Figure 10. Schematic view of coil -type magnetic fieldmeter,

18
the resultant field enhancement, the current is slightly higher). The output

voltage of the operational amplifier circuit (fig. 9) with a 1 megohm feedback

resistor would therefore be 0.33 volts. The detector circuit is shielded to

avoid electrical pick-up, and is usually located outside the parallel plate

system for remote observations. Microminiature coaxial cable with aluminized

plastic shielding is commercially available and is wery effective in reducing

electrical pick-up. The calibration of the fieldmeter is performed with a parallel plate system and is discussed further in Section 4.3.1. A fieldmeter

similar to that shown in figure 9 has been used for mapping electric field

profiles on the bottom plates of parallel plate exposure systems [22].

If the parallel plates are energized with a center-tapped transformer, the

fieldmeter described above can still be used for field strength measurements

provided that the detector (i.e., operational amplifier circuit and digital

voltmeter) is no longer referenced to ground but to the potential of the bottom

plate. For large parallel plates, this is readily accomplished by using a battery-operated detector housed in a metal box and placed on the bottom plate.

A small screened window permits viewing of the digital voltmeter display from a distance. The perturbing influence of the box on the field measurement can be

checked by introducing a second identical box at a location equal in distance

from the probe as the detector and observing the difference in field strength

value. Tests have shown that a detector housed in a metal box with dimensions

(length x width x height) of 21 cm x 16 cm x 7 cm does not measurably affect the

electric field near a disk probe, 10 cm in diameter, located 50 cm away. The

parallel plate spacing was 50 cm during these tests. Flat, electrically

floating, probes have been successfully used to characterize the electric field

strength at the surface of energized plates in biological exposure systems [36].

Electric field strength measurements at points between the parallel plates can

be made with several types of fieldmeters, but the complexity and cost of the

instrumentation is much greater than devices described above. Miniaturized

free-body type probes connected to a portion of the detector with a fiber-optic

link have been developed in recent years [37]. Fieldmeters which utilize the

Pockel's effect have also been developed [38]. A miniature fieldmeter with wire

connections between the probe and detector [39] is described in the Appendix.

4.2 iMagnetic Field Measurements

Power frequency magnetic fields can be measured with a fieldmeter consisting of

an electrically shielded coil of wire (probe) and a portable voltmeter detector

shown schematically in figure 10. The principle of operation of the fieldmeter

makes use of Faraday's Law which predicts that an electromotive force (emf) is

produced at the ends of an open loop of wire placed in a changing magnetic

field. For a loop of N turns placed in a sinusoidal ly varying field with

frequency f, the emf is

emf = 2TTfNAB (7)

where B is the flux density perpendicular to the area. A, of the loop. It is

assumed that the magnetic field due to the induced current in the probe (after

the probe is connected to a voltmeter) is small enough to be ignored. The emf

produced by a coil of 500 turns and an area of 100 cm^ when placed in a field of

lO*"* tesla (1 gauss) is 0.189 volts. The coil probe and voltmeter detector

19
should be well shielded to prevent electrical pick-up, particularly when

electric fields from an exposure system are present. Calibration of magnetic

fieldmeters is discussed in Section 4.3.2.

Coil -type magnetic fieldmeters can be readily fabricated and are also

commercially available. Hall effect magnetic fieldmeters which are commercially

available can also be used for 60-Hz magnetic field measurements. However, such

fieldmeters tend to have insufficient sensitivity for measurement near ambient

(10"^ tesla) flux densities.

4.3 Fieldmeter Calibration

The systems described earlier in Section 3 for generating electric and magnetic

fields can also be used for calibation purposes. Examples of calibration

apparatus for electric and magnetic fieldmeters are presented below.

4.3.1 Electric Fieldmeters

Calibration of the flat electric field probes described in Section 4.1 is

readily accomplished using a parallel plate system with a grounded bottom

plate. The probe is placed on the bottom plate which has dimensions

sufficiently large so that the edges of probe are no closer than two plate

spacings to any edge of the bottom plate. The spacing between the plates should

be at least as large as one and one-half times the side dimension of the probe,

for probes with thicknesses of 3 percent or less of the (probe) side dimension,

to insure that there is no significant perturbation of the surface charge

distributions on the top plate due to the presence of the probe. The guard band

should be at least as wide as 6% of the side dimension of the probe. Nearby

ground planes (e.g., floor, walls) should be no closer than two plate spacings

away from the parallel plates. With the above restrictions, the calibration

electric field will be within 0.5% of the uniform field value, V/t

(uncertainties in the value of V and t will add to the 0.5%).

Procedures for calibration of electric fieldmeters which can measure the field

strength at points above the bottom plate are described elsewhere [13, 40] and are not discussed here.

4.3.2 Magnetic Fieldmeters

Calibration of a magnetic fieldmeter is typically performed by introducing the

probe into a nearly uniform magnetic field of known magnitude and direction.

Helmholtz coils (Section 3.5) have frequently been employed to generate such

fields but the more simply constructed single loop of many turns with

rectangular geometry can also be used. The simplicity in construction is at the

expense of reduced uniformity, but sufficient accuracy is readily obtained. The

z-component of the magnetic flux density is given by Eq. (1). It is noted for

purposes of reference that 62(0,0,0) = MQlU/Tf-na for a square loop of side

dimension 2a. Equation 1 has been used to calculate the field values at and near the center of a square loop of dimensions 1 m x 1 m. Figure 11 shows

percentage departure from the central magnetic field value at nearby points in

the plane of the loop and 3 cm above and below the plane of the loop (indicated

in parenthesis). Also shown in figure 11 is an approximate outline (scale

drawing) of a commercially available magnetic field probe. A 1% change in the

loop dimension changes the values over the probe by about 1%. For the probe

20
2= 3 cm(1.6)

= 2.1

(1.8)o

2.3

8.0 cm

(2.6)o

3.0

B-field

probe

(3.6)o

4.2

(1.3), (2.5)^

1.8 3.0

(1.8)o

2.3

Figure 11. Percentage departure of B^ from Bz(0,0,0) for

positions in the plane of a square loop 1 m x 1 m, and

3 cm above or below the plane (parentheses). A scale

drawing of a coil -type probe is outlined.

21
shown in figure 11, the calibration field is within 2% of the central value,

Bz(0,0,0).

4.4 Harmonic Content Measurements

To reduce the possibility of attributing biological effects that may occur at frequencies [41] other than 60 Hz to the power frequency field, the harmonic

content in exposure fields should be minimized. When determining what is an acceptable level of harmonic content, it should be noted that the current

induced in a biological system by a particular harmonic field component will be

weighted by its harmonic number [42]. This result is suggested by the frequency

dependence in the expressions for induced current, eq. (6), and emf, eq. (7) in

conductors produced by electric and magnetic fields, respectively. If, for

example, the exposure field contains 3% of the third harmonic, the induced

current due to the third harmonic field component will be about 8.3% of the

total induced current.

Possible sources of harmonics in the exposure field include harmonics in the

line voltage, excessive loading of the transformer, or use of a transformer

designed for applications in which the presence of harmonics is of no consequence. The use of a line conditioner in the power supply, suggested

previously, can reduce the total harmonic content in the line voltage to less

than 0.5%.

If the power frequency harmonic content in the exposure fields are the order of

2 to 3%, the simple fieldmeters described earlier for electric and magnetic

field measurements (Sections 4.1 and 4.2) can be used with an oscilloscope to

verify the presence of the harmonics in a semiquantitative fashion. This is

possible because the distortion of a 60-Hz waveform is discernable visually by

the presence of 6% of the third harmonic and lesser amounts of the higher order

harmonics. Consequently, because the fieldmeters described previously enhance

the harmonics (eq. 6 and 7), the signal from, for example, the operational

amplifier of the electric field probe will reveal the presence of 2% of the 3rd

harmonic and lesser amounts of the higher order harmonics in the field.

Figures 12 through 14 compare pure 60-Hz waveforms with waveforms containing

various amounts of the 3rd, 4th, and 5th harmonic. The distortion produced by

the harmonics is seen to be a function of phase relation of the harmonic

relative to the fundamental.

The waveform of magnetic fields of order lO"** tesla (1 gauss), with enhanced

harmonics, can usually be observed by directly connecting the coil probe to an oscilloscope.

If a quantitative measure of the harmonic content of an electric or magnetic

field waveform is desired, the electrical signals can be decomposed into their

harmonic components with a spectrum or waveform analyzer. However, proper

accounting should be made of the weighting of each harmonic component if the

field probes described above are used for such measurements.

4.5 Phase Measurements

As noted earlier, the phase difference between the two magnetic field components

must be 90° in order to generate a circularly polarized magnetic field. In

22
(a) Figure 12. Effect of 3rcl harmonic on sine wave: (a) sine wave (top) and sine wave combined with 6% 3rd

harmonic, in phase (bottom);

(b) sine wave and sine wave

6% 3rd

out of phase;

and sine wave

6% 3rd

out of phase.

Using the probes described in

the text, the waveforms with

6% 3rd harmonic would

correspond to what would be

observed if there was 2% 3rd

harmonic in the field.

combined with

harmonic, 180'

(c) sine wave combined with

harmonic, 90°

(b)

I

(c) 23
(a)

1 /^ /^x r\ A /^\ 1/ \' ^ / A VJ

'\j

A A A

/ A A V

(b)

Figure 13. Effect of 4th

harmonic on sine wave: (a) sine wave (top) and sine wave combined with 4% 4th harmonic,

in phase (bottom); (b) sine wave and sine wave combined

with 4% 4th harmonic, 180° out

of phase; (c) sine wave and

sine wave combined with 4% 4th

harmonic, 90° out of phase.

Using the probes described in

the text, the waveforms with

4% 4th harmonic would

correspond to what would be

observed if there was 1% 4th

harmonic in the field.

(c) 24
Figure 14. Effect of 5th

harmonic on sine wave: (a) sine wave (top) and sine wave combined with 3% 5th harmonic,

in phase (bottom); (b) sine wave

and sine wave combined with 3%

5th harmonic, 180° out of phase,

(c) sine wave and sine wave combined with 3% 5th harmonic,

90° out of phase. Using the

probes described in the text,

the waveforms with 3% 5th

harmonic would correspond to

what would be observed if there

was 0.6% 5th harmonic in the

field.

(b)

1

/ \ / A /

\ / \ /

/^,

/' \ / \ /

^ V

A -A /

Ax '

/

(c)

25
addition, the vertical magnetic field component is required to be in phase with

the vertical electric field in the animal study experiments supported by the

State of New York [43].

The phase relation of the magnetic field components can be determined by

simultaneously observing the waveform of the currents to the two sets of coils

producing the magnetic fields or by observing the magnetic field waveforms with

magnetic field probes of the type described in Section 4.2 The current

waveforms can be monitored by using a dual beam oscilloscope to observe the

voltages across small resistances (e.g., ~1 ohm) placed in series with leads

going to each set of coils. Similarly the emf's generated by the coil -type

probes, aligned with the magnetic field directions, can be observed with a dual

beam oscilloscope, provided that the field is great enough for the number of

turns of wire in the probe. The latter measurement approach is the preferred

approach because of its sensitivity to perturbations of the magnetic field in

the exposure system (see below). It should be noted that the phase of the emf's

will be shifted by -90" with respect to the actual magnetic field components

because, from Faraday's Law, the emf is proportional to the negative derivative

of the magnetic flux [44].

The ground-referenced flat probe described in Section 4.1 can be used with an oscilloscope to observe the waveform of the electric field. However, the

current induced in the probe is +90° out of phase with respect to the electric

field and the electronic circuit (.an inverting amplifier) introduces an additional 180° phase shift. Thus, just as in the case of the emf's produced by

the magnetic field probe, the signal from the disk probe-amplifier combination

will be -90° out of phase with the field that produced it. In the event that

both plates of the parallel plate system are energized with a center-tapped

transformer (Section 3.2), the ground referenced probe can be located on a nearby ground plane.

The suggested use of an oscilloscope for th6 phase measurements instead of a commercially available phase meter can be justified if precise measurements of

the phase relations between the various field components are not considered

necessary. The uncertainties associated with the use of an oscilloscope, a readily available laboratory instrument, are less than about ±2° as can be seen

in figure 15. Figure 15(a) and 15(b) show portions of two 60-Hz waveforms that

are out of phase by 90°. A departure from the quadrature relation by 4.5°

(i.e., one graticule division out of 40) is readily observable in figure 15(c).

By amplifying one of the waveforms as shown in figures 15(b) and 15(c), greater

measurement precision can be obtained. However, care should be exercised when

amplifying one of the waveforms because of the possibility of introducing

nonlinearities in the expanded waveform when overdriving the oscilloscope

amplifier. This technique assumes that the waveforms are free of distortion and

that the dual beams of the oscilloscope are triggered simultaneously. If

greater precision is required commercially available phase meters can be used in

place of the oscilloscope.

When the phase relations between the magnetic and electric field components are being characterized using the techniques described above, the magnetic field

measurements should be performed in the region of the exposure system where the

test animals will be located. Typically, this would be close to the bottom

26
(a)

Figure 15, Determining phase

relation between waveforms with

oscilloscope,

(a) Waveforms 90° out of phase,

(b) waveforms 90° out of phase,

(c) disDlacement produced by

4,5 shift in phase is

readily observed.

(b)

(c)

27
plate of the parallel plate system. Eddy currents in the conducting plate

(Section 3.7) can result in perturbations of the phase and magnitude of the

magnetic field which might not be apparent if the measurements are performed,

say, midway between the plates.

5. CORONA

Energizing parallel plates with the high voltages necessary to generate electric

fields that are many kV/m introduces the possibility of generating corona. One

accepted definition of corona [45] is that it is "A luminous discharge due to

ionization of the air surrounding a conductor caused by a voltage gradient

exceeding a certain critical value." In air the critical voltage gradient or electric field near the surface of a conductor is about 3 x 10^ V/m. Because

it is the electric field and not the voltage that determines the onset [46] of

corona, voltages of a few kilovolts can produce corona if applied to conductors

with inadequate insulation, sharp edges or points. Audible noise, light,

chemical products such as ozone, and radio-frequency radiation are all possible

by-products of corona. The current practice is to eliminate or minimize these

by-products because of their possible biological impact during field exposure

studies.

In general, the effects of corona can be reduced by increasing the radius of

curvature of energized and nearby grounded conductors. For example, the edges

of parallel plates can be fitted with metal tubing [19,22] and high voltage

leads can be made of metal tubing. Maximizing the distance between energized

and grounded conductors is another step that can be taken to reduce corona. In

this regard the distance between the parallel plates and magnetic field coils,

which are near ground potential, should be kept as large as practically

possible.

Qualitative methods to detect the presence of corona include listening for

audible noise, time exposure photography in a darkened room, and the use of an am radio near the exposure system as a detector of radio frequency radiation.

Some approximate correlations have been reported between audible noise and light

and the amount of charge in corona discharges under certain conditions [47,48].

Instrumentation and measurement techniques do exist for more precise

characterizations of the charge in corona. However, the measurement procedures

are beyond the scope of this report and the interested reader is referred to

references 47 and 48 for additional information. It is noted, as in the case of

many other physical parameters associated with field exposure studies, that

there is no consensus as to what is an acceptable level of electrical discharge

activity.

6. ACKNOWLEDGMENTS

The author is pleased to acknowledge the assistance of Barbara Frey and

Betty Meiselman during the preparation of the manuscript, and useful

conversations with other coworkers. The support of the Department of Energy is

also gratefully acknowledged.

28
7. REFERENCES AND NOTES

1. A. 0. Bulawka, W. G. Wisecup, L. A. Rosen, W. E. Feero, and F. M. Dietrich,

U. S. Department of Energy 60-Hz Electric Fields Bioeffects Research, IEEE

Trans. Power App. Syst., Vol. PAS-101, pp. 4432-4440, 1982.

2. R. Sander, J. Brinkmann, and B. Kuhne, Laboratory Studies on Animals and

Human Beings Exposed to 50 Hz Electric and Magnetic Fields, CIGRE

(International Conference on Large High Voltage Electric Systems) Paper

36-01, September, 1982.

3. J. Tuhackova and V. Cenkova, The effects of electric fields on the organism

of persons in the proximity of power transmission equipment at a voltage of

400 kV or higher, electra. Vol. 83, pp. 111-117, 1982.

4. J. P. Reilly, Human Reactions to ELF Electric and Magnetic Fields - An

Annotated Bibliography of Current Literature, Johns Hopkins University

Applied Physics Laboratory Report JHU PPSE T-27, July 1983.

5. Biological Effects of Extremely Low Frequency Electromagnetic Fields,

(Proceedings of the Eighteenth Annual Hanford Life Sciences Symposium at

Richland, Washington, 1978), Editors R. D. Phillips, M. T. Gillis, W. T.

Kaune, and D. D. Mahlum, available from NTIS, Springfield, Virginia 22161 as CONF-781016.

6. J. E. Bridges, Environmental Considerations Concerning the Biological

Effects of Power Frequency (50 or 60 Hz) Electric Fields, IEEE Trans. Power

App. Syst., Vol. PAS-97, pp. 19-32, 1978.

7. G. E. Atoian, Are There Biological and Psychological Effects Due To Extra

High Voltage Installations?, IEEE Trans. Power App. Syst., Vol. PAS-97,

pp. 8-18, 1978.

8. Experiments which will search for possible effects in biological systems

exposed simultaneously to 60-Hz electric and magnetic fields have begun in

the United States and Canada. These studies are sponsored by the State of

New York Department of Health.

9. W. T. Kaune, C. H. Allen, J. L. Beamer, J. R. Decker, M. F. Gillis,

R. D. Phillips, and R. L. Richardson, Biological Effects of 60-Hz Electric

Fields on Miniature Swine: Exposure Facility, IEEE Trans. Power App. Syst.,

Vol. PAS-99, pp. 120-128, 1980.

10. C. F. Ehret, 6. A. Sacher, A. Langsdorf, and R. N. Lewis, Exposure and

Data-Collection Facilities for Circadian Studies of Electric Field Effects

upon Behavior, Thermoregulation, and Metabolism in Small Rodents, in

Reference 5.

11. R. D. Phillips, L. B. Anderson, and W. T. Kaune, Biological Effects of High

Strength Electric Fields on Small Laboratory Animals, Report to the

Department of Energy, 1979, available from NTIS, Springfield, Virginia 22161

as Report DOE/TIC-10084.

29
12. See also the references cited in References 1, 5, and 6.

13. IEEE Committee Report, Measurement of Electric and Magnetic Fields From

Alternating Current Power Lines, IEEE Trans. Power App. Syst., Vol. PAS-97,

pp. 1104-1114, 1978.

14. D. W. Deno, "Transmission Line Fields," IEEE Trans. Power App. Syst.,

Vol. PAS-95, pp. 1600-1611, 1976.

15. R. B. Adler, L. J. Chu, and R. M. Fano, Electromagnetic Energy , Transmission and Radiation (John Wiley, New York, 1960) p. 15ff.

16. Test animals have also been positioned between parallel plates during

exposure to the field (Ref. 10). The surface electric field distribution

and current induced in the animal will differ significantly when compared

with animals in contact with one of the plates.

17. P. M. Morse and H. Feshbach, Methods of Theoretical Physics, Part II , (McGraw Hill, New York, 1953) p. 1245. ~

18. C. H. Shih, J. Diplacido, and B. J. Ware, Analysis of Parallel Plate

Simulation of the Transmission Line Electric Field as Related to Biological

Effects Laboratory Studies, IEEE Trans. Power App. Syst., Vol. PAS-96,

pp. 962-968, 1977.

19. D. T. Poznaniak, J. G. Johnson, G. W. McKee, and H. B. Graves, The Design of

Exposure Systems for Studying the Biological Effects of 60-Hz Electric

Fields on Small Plants and Animals, in Reference 5.

20. W. T. Kaune, Interactive Effects in 60-Hz Electric-Field Exposure Systems,

Bioelectromagnetics, Vol. 2, pp. 33-50, 1981.

21. F. R. Kotter and M. Misakian, AC Transmission Line Field Measurements, NBS

report prepared for U. S. Department of Energy, Report No. HCP/T-6010/E1

(1977). Available from NTIS, Springfield, Virginia 22161.

22. W. T. Kaune, A Prototype System for Exposing Small Laboratory Animals to

60-Hz Vertical Electric Fields: Electrical Measurements, in Reference 5.

23. It should be noted that the enclosure shown in figure 4 was placed

alongside others in the same parallel plate system to effectively raise

the level of the bottom plate to that of the screen floors in the

enclosures. Use of a single enclosure would result in some field

enhancement at and near the surface of the screen floor.

24. C. F. Ehret, private corrmunication. Daily cleaning of the animal enclosures

is necessary for long-term exposure studies (J. J. Russell and C. F. Ehret,

The Effect of Electric Field Exposure on Rat Circadian and Infradian

Rhythms, Abstracts, 4th Annual Biolectromagnetics Society Meeting, Los

Angeles, 1982, p. 101).

25. J. A. Creim, R. H. Lovely, W. T. Kaune, and R. D. Phillips, Attempts to

Produce Taste Aversion Learning in Rats Exposed to 60-Hz Electric Fields,

Bioelectromagnetics, in press.

30
I

26. See E. Weber, Electromagnetic Theory (Dover, New York, 1965), p. 131ff for

outline of derivation.

27. S. Ramo and J. R. Whinnery, Fields and Waves in Modern Radio

(John Wiley and Sons, New York, 1944), p. 141ff.

28. J. A. Stratton, Electromagnetic Theory (McGraw-Hill, New York, 1941),

pp. 262-263.

29. H. Nazooka, Magnetic Field of Circular Current, Phil. Mag., Vol. 41,

p. 377, 1921.

30. R. J. Caola, Jr., D. W. Deno, V.S.W. Dymek, Measurements of Electric and

Magnetic Fields In and Around Homes Near a 500 kV Transmission Line,

Paper 83WM 164-1 presented at the IEEE Power Engineering Society Meeting,

Jan. 30-Feb. 4, 1983.

31. D. R. Corson and P. Lorrain, Introduction to Electromagnetic Fields and

Waves (W. H. Freeman, San Francisco 1962) p. 285ff.

32. M. A. Plonus, Applied Electromagnetics (McGraw-Hill, New York 1978)

pp. 398-400.

33. H. D. Cohen and C. Graham, Human Exposure Facility for Double-Blind Testing

of 60-Hz Electric and Magnetic Fields. Abstracts, 5th Annual

Biolectromagnetics Society Meeting, Boulder, Colorado, 1983, p. 81.

34. For a brief introduction to the phenomena of ferromagnetism, paramagnetism,

and diamagnetism, see for example D. Halliday and R. Resnick, Physics , Parts I and II (J. Wiley, New York 1966) Chapt. 37.

35. It is noted that the disk probe can be fabricated from other materials and

that it may be necessary to introduce a small amount of capacitance in the

feedback circuit of the operational amplifier circuit to avoid

oscillations. To minimize attenuation of signals due to power frequency

harmonics which may be present in the electric field (see Section 4.4) the

capacitance should be kept as small as possible.

36. S. Stern, V. G. Laties, C. V. Stancampiano, C. Cox, and J. 0. de Lorge,

Behavioral Detection of 60-Hz Electric Fields by Rats, Bioelectromagnetics,

Vol. 4, pp. 215-247, 1983.

37. R. J. Spiegel, D. R. Kerns, E. H. Cooper, and E. L. Bronaugh, A Small

Accurate, Optically Isolated Electric Field Probe, IEEE Publ. No. A79 507-5,

1979.

38. Y. Hamasaki, H. Gotoh, M. Katoh, and S. Takeuchi, OPSEF: An Optical Sensor

for Measurement of High Electric Field Intensity, Electronics Letters,

Vol. 16, pp. 406-407, 1980.

39. M. Misakian, F. R. Kotter, and R. L. Kahler, Miniature ELF electric field

probe. Rev. Sci. Instrum. Vol. 49, pp. 933-935, 1978.

40. IEEE Recommended Practices for Measurement of Electric and Magnetic Fields

from AC Power Lines, IEEE Std 644-1979 (IEEE, Inc., New York 1979) p. 13.

31
41. The frequencies considered here are restricted to the power frequency

harmonics. Characterization of radio frequency fields are beyond the scope

of this report.

42. W. T. Kaune and M. L. Gil lis. General Properties of the Interaction Between

Animals and ELF Electric Fields, Bioelectromagnetics, Vol. 2, pp. 1-11,

1981.

43. For in vitro experiments, the horizontal magnetic field must be in phase

with the vertical electric field in air.

44. See Ref. 34, Chapter 35.

45. IEEE Standard Dictionary of Electrical and Electronic Terms, IEEE Std.

100-1977 (IEEE, Inc. New York, 1977).

46. The "onset" of corona must be described in terms of the sensitivity of the

instrumentation used for the measurements. For example, it is possible,

under controlled conditions, to measure the amount of charge in an avalanche

associated with a corona discharge as a function of voltage applied to the

conductor. It is normal to define the "onset voltage" of the corona as the

voltage at which discharges containing the minimum amount of charge that can

be detected, say 1 pC, as the onset voltage. Thus, there will be electrical

discharges at lower voltages and with smaller amounts of charge per

avalanche. See R. J. Van Brunt and M. Misakian, Mechanisms for Inception of

DC and 60-Hz AC Corona in SFg, IEEE Trans. Electrical Insulation, Vol.

EI-17, pp. 106-120, 1982 for discussion of corona inception in a point-plane

electrode system.

47. R. Bartnikas and E. J. McMahon, eds.. Engineering Dielectrics Volume 1,

Corona Measurements and Interpretation , AsTM Special Technical Publication

669, American Society for Testing and Materials, 1979. ^

48. F. H. Kreuger, Discharge Detection in High Voltage Equipment , (American

Elsevier Publishing Co., New York, 1964).

'

32
8. APPENDIX

Theory for measurement of the electric field strength at points away from

conducting surfaces and the description of a miniature electric field probe

suitable for field strength measurements between parallel plates are presented

here. While the miniature probe is less convenient to use compared to probes

with fiber-optic links [37,38], its size, sensitivity, and relatively low cost

of fabrication may justify its consideration if measurements of the field

strength between parallel plates are of interest. The operational amplifiers in

the detector (Fig. 4, Appendix) can be replaced by more modern components which

require less power and the power supply can be replaced by two 9-volt batteries.

33
Miniature ELF eiectric field probe

Martin Misakian, F. Ralph Kotter, and Richard L. Kahler

Electrosystems Division, National Bureau of Standards, Washington, D.C. 20234

(Received 3 October 1977; in final fonn, 10 March 1978) A miniature ac electric field probe having direct electrical connections with its batteryoperated electronics is described. Because its small size introduces little field perturbation,

fields generated by relatively small electrode structures in laboratory environments can readily be characterized.

i.JTRODUCTION

A miniature electric field (E field) probe which is well removed from its battery-operated electronics with

wires has recently been developed as part of a program

to evair ' , field measurement procedures and instru- mentaiion used under high-voltage ac transmission

lines. Because its small size introduces little field perturbation, the probe has been successfully employed to obtain 60-Hz f-field "maps" between labora- tory parallel-plate structures used for field meter cali- brations. It has also been used in a similar fashion to measure fields between small parallel plates used for biological experiments. Since calculation of these fields

is complicated by the presence of nearby ground planes

(e.g., walls, floor) and, in the case of biological studies,

animal enclosures, food, and water, the probe measure- ments provide an alternative and perhaps more ac- curate approach for characterizing the E field in a laboratory environment.

A search of the literature indicates that this is per- haps the first ac £-fieId probe, operable above the ground plane, having direct electrical connections be- tween the field sensing elements and electronics. Considerably more complex devices have been de- veloped which employ glass fiber optics to couple the probe with its signal processing circuits.'

I. THEORY

The theory of operation is readily developed by considering a conducting sphere of radius a and electric potential V sin(w/ -f </>) in a quasistatic uniform field E = Eq sinwr as shown in Fig. 1(a). The perturbed

electric field is given in spherical coordinates by^

E = -ar{[K sincu/ - V sin(w/ -I- (J))]ar-^ + Eo[l + 2{a/ry] cos0 sinw/}

-I- afl£o[l - ia/rf] sin0 sinwr, (1) where K sinw/ is the space potential at z = prior to the introduction of the sphere. At the sphere's surface

(r = a), the magnitude of E is

Eg = [K sino)/ - V sin(a>r + c^)]a~' + iEo sinojf cos 6. (2) and the surface charge density a is given by €oEg, where eo is the electric permittivity of space.

The total induced charge on the top hemisphere is found from the surface integral

Q

= aa^ sin0 dOdcf)

= Cj[K sinwl - V s'micot + <t>)] + Ca^o sinw/. (3) Here C, and C^ are constants which depend on conductor

geometry. The induced current is just

'""

d, = (jjCi[K cosw/ - V cos((Dt + (/))]

-I- wCafo coswr. (4) Less symmetric conductor geometries (e.g., a rec- tangular box) will lead to equations of the same form

for Q and /q. If the sphere's potential is set equal to the ambient

space potential, K sinw/, it can be seen from Eqs. (3) and (4) that measurement of the induced charge or cur- rent permits the determination of £o to within a known

constant. This constant can be found for other surface geometries by calibration. It should also be noted that the potential of the now electrically "floating" sphere

will be the same as the space potential of the equatorial

plane, = rr/l. The above matching of electric potentials is achieved

for large commercial 60-Hz f-field meters (used under

high-voltage transmission lines) by housing the battery

operated electronics and analog meter between the sensing electrodes ("hemispheres") and supporting the device in the E field with a long dielectric rod.^ The

q sin ait E. sin (lit

(a)

Fig. 1. Geometries for calculating induced charge and current for uniform (a) and nonuniform (b) quasistatic fields.

933 Rev. Set. Instrum. 49(7), Jul. 197B 933

34
Fig. 2. Schematic view of electric field probe. The wires are con- nected to the aluminum sensing elements with conducting adhesive and the elements are joined with epoxy. A thin dielectric sheet separates the hemispheres. The lead wires are fully shielded with

a 9.5-cm-long syringe needle handle and metal foil tape.

miniature £-field probe described in the next section

also "floats"" electrically but the potential is set using a potentiometer/

While the above theory has considered a uniform

field, the usefulness of the probe for measuring highly

non-uniform fields is also expected. The quasistatic

field near a point charge, q sinw/, is used to demonstrate this. An electrically isolated spherical £-field probe is now considered aligned along a field line at a distance d from q as shown in Fig. 1(b). The induced

current in the top hemisphere can be shown to equal^

/„ = (xiC^E coswt

1T211 / a ^ 24\7 (5) where

E = (\/A7Teo)q/d^

Comparison of Eqs. (4) and (5) shows that the induced

current for the nonuniform field case is the same as

that produced by a uniform field of amplitude Eq = q/47reod^ if the terms in {a/d) can be neglected. Thus

the induced current at a point in the highly nonuniform

field produced by a point charge is nearly the same

as that produced by a uniform field of equal magnitude

if d is sufficiently large. For example, if a/d = 0.1, the difference in induced current or charge (i.e., £-field measurement) produced by a uniform and highly non- uniform field is less than 1%. In this case, the field changes in magnitude over the dimensions of the sphere

by |A£/£| = Ma/d) = 0.4 (40%)!

The discussion in this section assumes that the charges

producing the field are not perturbed by the presence of

the conducting sphere.

II. APPARATUS AND METHOD

A schematic view of the miniature field probe is given in Fig. 2. The field sensing surfaces are electrically isolated portions of an aluminum sphere 1.2 cm

in diameter. Small diameter (—0.1 mm) magnet wires provide the connections between a shielded extension

cable and the sensing surfaces through a syringe needle shield which also serves as a handle for positioning the probe (Fig. 2 inset).

The probe and its associated battery operated elec- tronics are shown greatly simplified in Fig. 3. A 60-Hz

calibration £ field is generated with parallel plates and

the induced charge on the hemispherical sensing plates

is measured with a differential amplifier that is housed

in an electrically biased chassis box. This bias is adjusted

to be the space potential in the eouatorial plane near

the sphere using the space potential probe-ammeter®-potentiome^2r combination. A voltage null indicates when the bias is appropriately set. No

serious difficulty was encountered in achieving volt- age nulls when the lead wire from the potentiometer was approximately 2 m long. T!.e condition 4= [Eq. (4)] can develop, however, when very long connecting wires are used because of stray capacitance.

The phase shift must be eliminated with appropriate

circuitry (e.g., an RC network) to achieve a null. The handle of the field probe should be aligned with an equipotential surface. In uniform field, the equatorial

plane of the probe, shown in the inset of Fig. 3, will then coincide with this surface. The space potential

probe consists of a shielded (aluminum foil) subminiature coaxial cable -1.5 mm in diameter with ap- proximately 2 mm of insulated but unshielded center conductor at its sensing end. The potential probe is brought near the field probe along the equatorial plane.

The field probe shown in the inset of Fig. 3 has quartersphere sensing elements which permit measurement of two orthogonal field components without reorienting the probe. A switching arrangement indicated in Fig. 4 connects different pairs of elements for each measurement.

Figure 4 provides a more detailed view of the field probe electronics. Because of the small current flow be- tween the -I- input and chassis (less than ~I0 nA at 10 kV/m), the + input will effectively be at the chassis

bias potential which is set with the potentiometer. Thus

both hemispherical surfaces as well as the shielding will be set to the appropriate potential when a null is

POTEHTlOMETeH

Fig. 3. Simplified schematic view of field probe and associated

electronics. A space potential probe is used in conjunction with a

potentiometer to set the field probe potential and shielding to the value in the equatorial plane. The sphere is electrically "floating"

at the appropriate potential when the voltmeter is nulled.

934 Rev. Scl. Instrum., Vol. 49, No. 7, July 1978 35

Electric field probe 934
Fig. 4. Field probe amplifier and pvower supply. The induced current

is integrated to obtain a measure of the induced charge or £ field when the chassis potential is appropriately set. The AD 40J operational amplifier is employed because of its high input impedance and

low bias currents.'" The transfer function of the first stage is inde- pendent of frequency for 60 Hz and power frequency harmonics. A

switching arrangement ties together different pairs of sensing elements

to permit measurement of two orthogonal components of the E field.

achieved.' The space potential probe is removed during

field measurements.

Capacitive feedback has been used in the first-stage

amplifier in order to integrate the input current [Eq. (4)] and develop a frequency-independent output voltage

Vq' proportional to induced charge [Eq. (3)]. A second

stage of amplification follows and the rms value of the output is read with a battery-operated (rectified) average-sensing digital voltmeter (DVM). Because of the average sensing rms feature of the DVM, measure- ment of charge to determine E rather than current will lead to smaller errors if there are harmonic components in the E field.' This occurs because the charge

induced by a given £-field harmonic field component is not weighted by its harmonic number; harmonic components of current are so exaggerated however.

The probe is calibrated and tested in a uniform field produced with parallel plates 1 x 1 x 0.5-m separation

and energized using a center tapped current limited transformer. The expected constant vertical E field is observed along the vertical axis. Vertical ground planes

brought to within 0.25 m of the plate edges do not measurably affect this profile. Significant field perturbation

will occur, however, if the bottom plate of the apparatus

is grounded. Perturbation of the surface charge density

when the probe is within 5 cm of a plate surface

leads to a measurement error of less than 1%. The

sensitivity of the electronics shown in Fig. 4 is adequate

for measuring fields as low as 2 kV/m.

The theoretical expectation that highly nonuniform

fields can be measured with little error when the probe

is aligned with the field direction was not directly tested with the miniature probe. However, an investi- gation using large commercial field meters (—19 cm

vertical dimension) has confirmed within experimental

uncertainty (=e5%) that field meters calibrated to measure uniform fields can successfully be used in fields which change by —30% in magnitude over the probe

dimensions.*

ACKNOWLEDGMENT

We are pleased to acknowledge the assistance of

J, C. Palla during preparation of the manuscript.

This work was supported by the U.S. Department of

Energy under Contract No. EA-77-A-01-6010-A0-18-1.

' L. Wilhelmy, Elektrotech. Z. A 94, 441 (1973); A. R. Valentino,

Proc. IEEE International Symposium on Electromagnetic

Compatibility (July 1972), p. 265 (unpublished), available as riT Research Institute Report No. AD 742128 from NTIS,

Springfield, VA 22161. Since the time the present manuscript was submitted, we have learned that a probe similar to the NBS miniature probe has been developed at Ohio State University [see S. A. Sebo, report to Electric Power Research

Institute, EL-632, Vol. 1, Res. Proj. 753, pp. 5-13. January

1978 (unpublished)].

' P. Moon and D. E. Spencer, Field Theory for Engineers (Van

Nostrand, Princeton, NJ, 1%1), p. 225^

^ A forerunner of these electrically isolated field meters was a device referenced to ground ( V = 0) and used for a number of years to measure electric fields above the ground plane [C. J. Miller, IEEE Trans. Power App. Syst. PAS-86, 493 (1967)]. Under these conditions the first term in Eq. (4) does not vanish and erroneous measurements are obtained.

* The idea of electrically adjusting the probe potential to match the space potential for accurate field measurements is not new. It has been used for many years to measure the potential gradient of the earth's atmosphere. See, for example, M. Smiddy and

J. A. Chalmers, J. Atmosph. Terr. Phys. 12, 206 (1958) and

S. Gathman, Rev. Sci. Instrum. 39, 43 (1968).

* This result is given without derivation in the CIGRE article "Electrical Field Measurement in the Vicinity of HV Equipment

and Assessment of Its Bio-Physiological Perturbing Effects," by C. Mihaileanu et al. (CIGRE Paper 36-08, 1976, Paris, France).

It can be derived by considering an uncharged conducting sphere

in the field of a point charge and using the method of images.

* Model l(K)-10 electric field meter accessory. Electric Field Measurements Company, West Stockbridge, MA. The

identification of commercial equipment and their sources is made

to adequately describe the experimental results. In no case does this identification imply recommendation by the National Bureau of Standards, nor does it imply that the device is the

best available.

' In a uniform E field, the unperturbed space potential, <^„, at 2 = (Fig. 1) will be equal to the potential value in the equatorial plane after an electrically isolated sphere is introduced

into the field. In the present case, the sphere is initially at some

arbitrary potential <^ + </>„ and the equatorial plane near the sphere

will not be at the appropriate potential <^o- As the voltage

difference between the sphere and space potential probe is reduced, however, the potential surfaces near the sphere approach the geometry which is characteristic for an electrically isolated sphere.

* M. R. Negrete, IEEE WESCON Tech. Papers 9, pt. 6 (1%5).

' F. R. Kotter and M. Misakian, NBS report prepared for U.S.

Etepartment of Energy, Report No. HCP/T-6010/E1 (1977) (unpublished). Available from NTIS, Springfield, VA 22161.

"* The disclaimer in Ref. 6 again applies.

935 R«v. Scl. Instrum., Vol. 49, No. 7, July 1976 Electric fleid probe 935

36
NBS-n4A (REV. 2.8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS TN 1191

2. Performing Organ. Report NoJ 3. Publication Date

April 1984

4. TITLE AND SUBTITLE

Electrical Parameters in 60-Hz Biological Exposure Systems and Their Measurement;

A Primer

5. AUTHOR(S)

Martin Misakian

6. PERFORMING ORGANIZATION (/f joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

t. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, Stole, ZIP)

Division of Electrical Energy Systems

Department of Energy

ICOO Independence Avenue, SW

Washington, DC 20585

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual sunnmary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This report presents material which is intended to provide assistance in the

measurement of a number of electrical parameters that are of importance during

bioeffects research involving 60-Hz electric and magnetic fields, The

parameters that are considered are the electric field strength E, the magnetic

induction or flux density B, field uniformity, harmonic content, phase relations

between field components, and corona. Descriptions of the fields and methods

for their laboratory generation are surveyed, The text is purposely elementary

with references provided to aid the interested reader in obtaining a fuller

understanding of many of the topics. It is shown that using relatively simple

instrumentation, it is possible to characterize reasonably well the electric

and magnetic fields used in animal exposure studies.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

biological effects; corona; electric field; ELF; magnetic field; power frequency;

60-Hz; transmission lines

13. AVAILABILITY

[X~l Unlimited

I I For Official Distribution. Do Not Release to NTIS

S~] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C

20402.

\2Z\ Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

43

15. Price

USCOMM-DC 6043-P80

NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research of the National Bureau of Standards reports NBS research and develop- ment in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry,

engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement

methodology and the basic technology underlying standardization. Also included from time to time are survey articles on topics

closely related to the Bureau's technical and scientiTic programs. As a special service to subscribers each issue contains complete

citations to all recent Bureau publications in both NBS and non- NBS media. Issued six times a year. Annual subscription: domestic

$18; foreign $22.50. Single copy, $5.50 domestic; $6.90 foreign.

NONPERIODICALS

Monographs—Major contributions to the technical literature on

various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with in- terested industries, professional organizations, and regulatory

bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special publications

appropriate to this grouping such as wall charts, pocket cards, and

bibliographies. Applied Matbematics Series—Mathematical tables, manuals, and

studies of special interest to physicists, engineers, chemists,

biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NBS under

the authority of the National Standard Data Act (Public Law

90-396).

NOTE: The principal publication outlet for the foregoing data is the Journal of Physical and Chemical Reference Data (JPCRD)

published quarterly for NBS by the American Chemical Society

(ACS) and the American Institute of Physics (AlP). Subscriptions,

reprints, and supplements available from ACS, 1 155 Sixteenth St., NW. Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research results,

test methods, and performance criteria related to the structural and

environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on N BS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's technological marketplace. Order ihe above NBS publications from: Superintendent of Documents, Government Printing Office. Washington, DC 20402. Order the following NBS publications—FlPS and NBSIRs—from

the National Technical Information Service, Springfield, VA 22161

.

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended.

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717(38 FR 12315, dated May II, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Service , Springfield, VA 22161,

in paper copy or microfiche form.
U.S. Department of Commerce

National Bureau of Standards

Washington, D.C. 20234

Official Business

Penalty for Private Use $300