IEC 61340-4-7:2025

IEC 61340-4-7 ®
Edition 3.0 2025-08
INTERNATIONAL
STANDARD
COMMENTED VERSION
Electrostatics -
Part 4-7: Standard test methods for specific applications - Ionization
ICS 17.200.99; 29.020 ISBN 978-2-8327-0669-5
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CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Test fixture and instrumentation. 8
5 Specific requirements for equipment categories . 10
5.1 Specific requirements for all ionization equipment . 10
5.2 Room ionization . 11
5.3 Laminar flow hood ionization . 14
5.4 Worksurface ionization . 16
5.5 Compressed gas ionizers – Guns and nozzles . 18
Annex A (informative) Theoretical background and additional information on the
standard test method for the performance of ionizers . 20
A.1 Introductory remarks . 20
A.2 Air ions . 20
A.3 Mobility and ion current . 20
A.4 Neutralization current . 21
A.5 Neutralization rate . 21
A.6 Ion depletion and field suppression . 21
A.7 Charged plate monitor and charge neutralization . 22
A.8 Relationship between charged plate monitor decay time and actual object . 22
A.9 Offset voltage . 22
A.10 Preparation of test area . 23
A.11 Ion transport in airflow . 23
A.12 Obstruction of airflow around the charged plate monitor . 23
A.13 Effect of "air blanket" . 24
A.14 Sources of measurement error . 24
A.14.1 Typical decay time variability . 24
A.14.2 Plate isolation . 24
A.14.3 Charging voltage . 24
A.14.4 Materials near the plate . 24
A.14.5 Other field-producing devices in test area . 24
A.14.6 Effect of offset voltage on decay time . 25
A.15 Importance of ionization equipment maintenance . 25
Annex B (normative) Method of measuring the capacitance of an isolated conductive
plate . 26
B.1 Method . 26
B.2 Equipment . 26
B.3 Procedure . 26
B.4 Example . 27
B.5 Sources of error . 27
B.5.1 Measuring equipment . 27
B.5.2 Poor plate isolation . 28
B.5.3 Objects in the environment . 28
B.5.4 Stray capacitance . 29
Annex C (informative) Safety considerations . 30
C.1 General . 30
C.2 Electrical . 30
C.3 Ozone . 30
C.4 Radioactive . 30
C.5 X-ray . 30
C.6 Installation . 30
Bibliography . 31
List of comments. 32

Figure 1 – Charged plate monitor components for non-contacting plate measurement . 9
Figure 2 – Charged plate monitor components for contacting plate measurement . 9
Figure 3 – Conductive plate detail of the non-contacting CPM. 10
Figure 4 – Conductive plate detail of the voltage follower CPM . 10
Figure 5 – Test locations for room ionization – AC bars, grids and DC bar systems . 12
Figure 6 – Test locations for room ionization – Single polarity emitter systems . 13
Figure 7 – Test locations for room ionization – Two DC-line systems . 13
Figure 8 – Test locations for room ionization – Pulsed DC emitter systems . 13
Figure 9 – Test locations for vertical laminar flow hood – Top view . 14
Figure 10 – Test locations for vertical laminar flow hood – Side view . 15
Figure 11 – Test locations for horizontal laminar flow hood – Top view . 15
Figure 12 – Test locations for horizontal laminar flow hood – Side view . 16
Figure 13 – Test locations for benchtop ionizer – Top view . 17
Figure 14 – Test locations for benchtop ionizer – Side view . 17
Figure 15 – Test locations for overhead ionizer – Top view . 18
Figure 16 – Test locations for overhead ionizer – Side view . 18
Figure 17 – Test locations for compressed gas ionizer (gun or nozzle) – Side view . 19

Table 1 – Test set-ups and test locations/ and points (TP) . 11
Table B.1 – Example measurement data . 27

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Electrostatics -
Part 4-7: Standard test methods for specific applications - Ionization

FOREWORD
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shall not be held responsible for identifying any or all such patent rights.
This commented version (CMV) of the official standard IEC 61340-4-7:2025 edition 3.0 allows
the user to identify the changes made to the previous IEC 61340-4-7:2017
edition 2.0. Furthermore, comments from IEC TC 101 experts are provided to explain the
reasons of the most relevant changes, or to clarify any part of the content.
A vertical bar appears in the margin wherever a change has been made. Additions are in green
text, deletions are in strikethrough red text. Experts' comments are identified by a blue-
background number. Mouse over a number to display a pop-up note with the comment.
This publication contains the CMV and the official standard. The full list of comments is available
at the end of the CMV.
IEC 61340-4-7 has been prepared by IEC technical committee 101: Electrostatics. It is an
International Standard.
This third edition cancels and replaces the second edition published in 2017. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) in Figure 5, a NOTE 3 was added to clarify that for AC bars and grids, a single emitter
alternating between +/− polarity is used;
b) in Annex B, the relative error for measurement equipment was updated to include the
consideration for the resolution of the voltmeter.
The text of this International Standard is based on the following documents:
Draft Report on voting
101/739/FDIS 101/744/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 61340 series, published under the general title Electrostatics, can
be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
INTRODUCTION
Grounding is the primary method used to limit static charge when protecting electrostatic
discharge sensitive items in the work environment. However, grounding methods are not
effective in removing static charges from the surfaces of non-conductive (insulative) or isolated
(ungrounded) 1 conductive materials. Air ionization techniques, by means of ionizer systems,
can be utilized to reduce this charge.
The preferred way of evaluating the ability of an ionizer to neutralize a static charge is to directly
measure the rate of charge decay. Charges to be neutralized may can be located on insulators
as well as on isolated conductors. It is difficult to charge an insulator reliably and repeatably.
Charge neutralization is more easily evaluated by measuring the rate of decay of the voltage of
an isolated conductive plate. The measurement of this decay should not interfere with or change
the nature of the actual decay. Four practical methods of air ionization are addressed in this
document:
a) radioactive emission;
b) high-voltage corona from AC electric fields;
c) high-voltage corona from DC electric fields;
d) soft X-ray emission.
This part of IEC 61340 provides test methods and procedures that can be used when evaluating
ionization equipment. The objective of the test methods is to generate meaningful, reproducible
data. The test methods are not meant to be a recommendation for any particular ionizer
configuration. The wide variety of ionizers, and the environments within which they are used,
will often require test methods different from those described in this document. Users of this
document should be prepared to adapt the test methods as required to produce meaningful data
in their own application of ionizers.
Similarly, the test conditions chosen in this document do not represent a recommendation for
acceptable ionizer performance. There is a wide range of item sensitivities to static charge.
There is also a wide range of environmental conditions affecting the operation of ionizers.
Performance specifications should be agreed upon between the user and manufacturer of the
ionizer in each application. Users of this document should be prepared to establish reasonable
performance requirements for their own application of ionizers.
Annex B provides a method for measuring capacitance of the isolated conductive plate.
1 Scope
This part of IEC 61340 provides test methods and procedures for evaluating and selecting air
ionization equipment and systems (ionizers).
This document establishes measurement techniques, under specified conditions, to determine
offset voltage (ion balance) and decay (charge neutralization) time for ionizers.
This document does not include measurements of electromagnetic interference (EMI), or the
use of ionizers in connection with ordnance, flammables, explosive items or electrically initiated
explosive devices.
As contained in this document, the test methods and test conditions can be used by
manufacturers of ionizers to provide performance data describing their products. Users of
ionizers are urged to modify the test methods and test conditions for their specific application
in order to qualify ionizers for use, or to make periodic verifications of ionizer performance. The
user will decide the extent of the data required for each application.
CAUTION: Procedures and equipment described in this document can expose personnel to hazardous electrical and
non-electrical conditions. Users of this document are responsible for selecting equipment that complies with
applicable laws, regulatory codes and both external and internal policy. Users are cautioned that this document
cannot replace or supersede any requirements for personnel safety.
2 Normative references
There are no normative references in this document.
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 61010-1, Safety requirements for electrical equipment for measurement, control, and
laboratory use - Part 1: General requirements 2
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1
air conductivity
ability of air to conduct (pass) an electric current under the influence of an electric field
3.2
air ions
molecular clusters of about ten molecules (water, impurities, etc.) bound by polarization forces
to a singly charged oxygen or nitrogen molecule
3.3
charge decay
decrease and/or neutralization or both of a net electrostatic charge
3.4
charged plate monitor
CPM
instrument using a charged metal plate of a defined capacitance and geometry which is
discharged in order to measure charge dissipation/ and neutralization properties of products or
materials
Note 1 to entry: This note applies to the French language only.
3.5
compressed gas ionizer
ionization device that can be used to neutralize charged surfaces and/or remove surface
particles or both with pressurized gas
Note 1 to entry: This type of ionizer may can be used to ionize the gas within production equipment.
3.6
corona
production of positive or negative ions by a very localized high electric field
Note 1 to entry: The field is normally established by applying a high voltage to a conductor in the shape of a sharp
point or wire.
3.7
decay rate 3
decrease of charge or voltage per unit time
3.7
decay time
time necessary for a voltage (due to an electrostatic charge) to decay from an initial value to
some chosen final value
3.8
emitter
conducting sharp object, usually a needle or wire, which will cause a corona discharge when
kept at a high potential
3.9
horizontal laminar flow
non-turbulent airflow in a horizontal direction
3.10
ionizer
device designed to generate positive and/or negative or both air ions
3.11
isolated conductor
non-grounded conductor
conductor that has sufficiently high resistance to ground that significant charge dissipation is
prevented within the timescale of interest 4
3.12
laminar flow hood ionization
device or system that provides local area ionization coverage in vertical or horizontal laminar
flow hoods or benches
3.14
non-contacting voltage measurement 5
measurement technique using an electrostatic fieldmeter or voltmeter to monitor the voltage
induced on an isolated conductive plate where there is no direct connection from the
measurement sensor to the isolated conductive plate
3.13
offset voltage
ion balance
observed voltage on the isolated conductive plate of a charged plate monitor (CPM) that has
been placed in an ionized environment
3.14
peak offset voltage
for pulsed ionizers, maximum value of the offset voltage for each polarity, as the ionizer cycles
between positive and negative ion outputs
3.15
room ionization
ionization system that provides large area coverage with air ions
3.16
worksurface ionization
ionization device or system used to control static charges at on a worksurface
Note 1 to entry: This type Worksurface ionizers include benchtop ionizers, overhead worksurface ionizers and
laminar flow hood ionizers.
3.17
vertical laminar flow
non-turbulent airflow in a vertical direction
3.20
contacting voltage measurement 6
measurement technique using high input impedance circuitry used to monitor the voltage induced on
an isolated conductive plate where there is a direct connection from the circuitry to the conductive
plate
4 Test fixture and instrumentation
WARNING – Procedures and equipment described in this document can expose personnel to
hazardous electrical and non-electrical conditions. Users of this document are responsible for
selecting equipment that complies with applicable laws, regulatory codes and both external and
internal policy. Users are cautioned that this document cannot replace or supersede any
requirements for personnel safety. See Annex C for safety considerations. 7
The instrument described in this document to make performance measurements on air
ionization equipment is the charged plate monitor (CPM); refer to Figure 1 and Figure 2. The
conductive plate shall be (15,0 ± 0,1) cm × (15,0 ± 0,1) cm and the total capacitance of the test
circuit, with plate, while the instrument is in its normal operating mode, shall be 20 pF ± 2 pF
as measured per Annex B. See Figure 3 and Figure 4. The instrument described in this
document may can also be used for compliance verification of air ionizers.
For the isolated conductive plate design shown in Figure 3, there shall be no objects, grounded
or otherwise, closer than dimension "A" of the conductive plate, except the supporting insulators
or plate voltage contacts, as shown in Figure 3 (refer to Annex B). For the conductive plate
assembly shown in Figure 4, there shall be no objects, grounded or otherwise, within 2,54
2,5 cm of the plate assembly in any direction, other than a support structure (e.g. a tripod)
located below the ground plate of the assembly.
The conductive plate, when charged to the desired test voltage, shall not decay by more than
10 % of the test voltage within 5 min, in the absence of ionization.
The voltage on the conductive plate shall be monitored in such a way that the system conforms
to 4.1, 4.2 and 4.3. The response time of the monitoring device shall be sufficient to accurately
measure changing plate voltages with a response time sufficient to accurately measure
changing plate voltages. Changing plate voltages are a function of the ionization technology
type being monitored. 8
For safety reasons (see Clause C.1), the voltage source used to charge the conductive plate
should be current limited. The voltage source used to charge the conductive plate shall meet
the requirements of IEC 61010-1. 9

NOTE See Figure 3 for a detailed drawing of the non-contacting CPM.
Figure 1 – Charged plate monitor components for non-contacting plate measurement

NOTE See Figure 4 for a detailed drawing of the contacting CPM.
Figure 2 – Charged plate monitor components for contacting plate measurement

(15,0 ± 0,1) cm × (15,0 ± 0,1) cm
Conductive plate
Dimension A
Insulator
Ground plate ≥ 15
Ground
cm
IEC
Figure 3 – Conductive plate detail of the non-contacting CPM

Figure 4 – Conductive plate detail of the voltage follower CPM
5 Specific requirements for equipment categories
5.1 Specific requirements for all ionization equipment
See Annex A for information regarding theoretical background and additional information on the
standard test method for the performance of ionizers. 10
For the types of ionization equipment listed in 5.2, 5.3, 5.4 and 5.5, the following specific
requirements apply:
a) Decay time test – The conductive plate of the test fixture shall be charged to an initial test
voltage and allowed to decay to 10 % of the initial test voltage. The time required shall be
monitored and recorded for both polarities of initial charge. This time is referred to as the
decay time (refer to 4.1 and Figure 1). The conductive plate of the test fixture shall be
charged to an initial charging voltage, which shall be higher than the initial test voltage. The
time required for the voltage to decay from the initial test voltage to 10 % of that value shall
be recorded (refer to Clause 4 and Figure 1). Measurements shall be made with both
polarities of charging voltage. 11
b) Offset voltage test – The conductive plate shall be momentarily grounded to remove any
residual charges and to verify the zero setting of the voltage monitoring device. The plate
is then monitored within the ionized environment, per the procedure described for each
equipment category. The resulting observed voltage is referred to as the offset voltage.
c) Locations – The decay time and offset voltage shall be measured and recorded 12 for each
test location/ and point (TP) described in the test location figures (see Table 1).
d) Same conditions – Decay time and offset voltage shall be measured under the same
conditions without any equipment adjustments. If ionizers from different categories are to
be compared, the same test voltages shall be used for all tests. 13
e) Peak offset voltage – In the case of pulsed ionizers, offset voltage shall be measured and
reported recorded in peak values using the test equipment described in Clause 4.
f) Other parameters – Application specific parameters such as humidity, temperature, air
speed, etc., for example shall be recorded.
Table 1 – Test set-ups and test locations/ and points (TP)
Charged plate
Offset voltage
initial voltage
Figure Number of test
Equipment category measurement time
(both polarities)
references locations
interval
V
Room ionization
Bars, grids, AC Figure 5 2 (1 to 5) min 1 000
Bars, pulsed and DC Figure 5 2 (1 to 5) min 1 000
Single polarity emitter Figure 6 3 (1 to 5) min 1 000
DualTwo DC line Figure 7 3 (1 to 5) min 1 000
Pulsed DC emitter Figure 8 2 (1 to 5) min 1 000
Laminar flow hood
Figure 9 and
Vertical 8 (1 to 5) min 1 000
Figure 10
Figure 11 and
Horizontal 6 (1 to 5) min 1 000
Figure 12
Worksurface ionization
Benchtop Figure 13 and
12 (1 to 5) min 1 000
Figure 14
Overhead Figure 15 and
12 (1 to 5) min 1 000
Figure 16
Compressed gas ionization
Guns and nozzles Figure 17 1 10 s to 1 min 1 000

5.2 Room ionization
The area around the charged plate monitor should be cleared for a horizontal distance of 1,5 m
in all directions. The ionization system should be operated for a minimum of 30 min to stabilize
conditions in the test area.
To avoid affecting the test, the test technician should be grounded and stand outside the 1,5 m
cleared area.
Decay time from a 1 000 V initial test voltage to a 100 V final test voltage shall be measured
for both positive (+) and negative (−) polarities.
The air speed at the test location shall be recorded.
Measurements should be taken with the charged plate monitor at a distance of 1,5 m from the
ionizer under test. Since installed ionizer heights can vary, a consistent measurement height
should be selected for the evaluation of different systems. This height and the ionizer mounting
height shall be recorded in the test results.
The minimum number of test locations is determined by the type of system (see Table 1 and
refer to Figure 5 through Figure 8).
Decay time as described in 5.1 a), shall be measured at each test location.
Offset voltage as described in 5.1 b) and e) shall be determined at each test location. Offset
voltage shall be measured after a period of at least 1 min to allow the reading to stabilize (5 min
maximum).
NOTE 1 Example For AC grids (shaded areas, less than 100 % coverage) and pulsed or steady-state DC bars. For
AC bars or grids, the shaded areas represent areas where the coverage is less than 100 %.
NOTE 2 TP1 is directly under grid or bar while TP2 is centred between grids or bars.
NOTE 3 For AC bars and grids, a single emitter alternating between +/− polarity is used.
Figure 5 – Test locations for room ionization – AC bars, grids and DC bar systems
Three measurement locations required.
Figure 6 – Test locations for room ionization – Single polarity emitter systems

Three measurement locations required.
Figure 7 – Test locations for room ionization – Dual Two DC-line systems

Two measurement locations required.
Figure 8 – Test locations for room ionization – Pulsed DC emitter systems
5.3 Laminar flow hood ionization
The test should be performed on a surface that does not contain obstructions to airflow. Unless
otherwise specified, the test surface should be static dissipative or conductive and properly
grounded.
To avoid affecting the test, the test technician should be properly grounded.
Decay time from a 1 000 V initial test voltage to a 100 V final test voltage shall be measured
for both positive (+) and negative (−) polarities.
The air speed at test location TP4, as shown in Figure 9 and Figure 11, should shall be
recorded.
For a vertical laminar flow hood, the test set-up is shown in Figure 9 and Figure 10. Data shall
be taken at test positions TP1 through TP8 as shown in Figure 9.
For a horizontal laminar flow hood, the test set-up is shown in Figures 9 Figure 11 and 10
Figure 12. Data shall be taken at test positions TP1 through TP6 as shown in Figure 11.
Decay time as described in 5.1 a), shall be measured at each test location.
Offset voltage as described in 5.1 b) and e) shall be determined at each test location. Offset
voltage shall be measured after a period of at least 1 min, or as necessary to allow the reading
to stabilize (5 min maximum).
Dimensions in centimetres
Eight measurement locations required, all dimensions nominal.
Figure 9 – Test locations for vertical laminar flow hood – Top view
Dimensions in centimetres
All dimensions nominal.
Figure 10 – Test locations for vertical laminar flow hood – Side view
Dimensions in centimetres
Six measurement locations required, all dimensions nominal.
Figure 11 – Test locations for horizontal laminar flow hood – Top view
Dimensions in centimetres
All dimensions nominal.
Figure 12 – Test locations for horizontal laminar flow hood – Side view
5.4 Worksurface ionization
The test should be performed on a surface that does not contain obstructions to airflow. Unless
otherwise specified, the test surface should be static dissipative or conductive and properly
grounded.
To avoid affecting the test, the test technician should be properly grounded.
Decay time from a 1 000 V initial test voltage to a 100 V final test voltage shall be measured
for both positive (+) and negative (−) polarities.
The unit shall be measured with the heater on and off, if so equipped. The unit shall be tested
with any filters in place if so equipped. Measurements shall be made at both minimum and
maximum airflows for units with variable airflow. The air speed shall be measured and included
in the test results. End users should test ionizers with the same configuration of operating
heaters and filters that they intend to use.
For benchtop units, the ionizer shall be placed as shown in Figure 13 and Figure 14. Airflow
shall be directed at test location TP2 and measured at test locations TP2 and TP5. The charged
plate monitor shall face the ionizer. Measurements with the charged plate monitor shall be made
at test locations TP1 through TP12 as shown in Figure 13.
For overhead units, the ionizer shall be placed as shown in Figure 15 and Figure 16. Airflow
shall be measured at test locations TP5 and TP8. Measurements with the charged plate monitor
shall be made at test locations TP1 through TP12 as shown in Figure 15.
Decay time as described in 5.1 a), shall be measured at each test location.
Offset voltage as described in 5.1 b) and e) shall be determined at each test location. Offset
voltage shall be measured after a period of at least 1 min, or as necessary to allow the reading
to stabilize (5 min maximum).
Dimensions in centimetres
Twelve measurement locations required, all dimensions nominal.
Figure 13 – Test locations for benchtop ionizer – Top view
Dimensions in centimetres
All dimensions nominal.
Figure 14 – Test locations for benchtop ionizer – Side view
Dimensions in centimetres
Twelve measurement locations required, all dimensions nominal.
Figure 15 – Test locations for overhead ionizer – Top view
Dimensions in centimetres
All dimensions nominal.
Figure 16 – Test locations for overhead ionizer – Side view
5.5 Compressed gas ionizers – Guns and nozzles
The test should be performed on a surface that does not contain obstructions to airflow. Unless
otherwise specified, the test surface should be static dissipative or conductive and properly
grounded.
To avoid affecting the test, the test technician should be properly grounded.
Decay time from a 1 000 V initial test voltage to a 100 V final test voltage shall be measured
for both positive (+) and negative (−) polarities.
Unless otherwise specified, the input pressure shall be 200 kPa. End users should test
compressed gas ionizers in the same configuration of input pressure and distance that they
intend to use.
The tests shall be performed using the test set-up shown in Figure 17.
Decay time as described in 5.1 a), shall be measured at the test location.
Offset voltage as described in 5.1 b) and e) shall be determined at the test location. Offset
voltage shall be measured after a period of at least 10 s or as necessary to allow the reading
to stabilize (1 min maximum).
Dimensions in centimetres
One measurement location required, all dimensions nominal.
Figure 17 – Test locations for compressed gas ionizer (gun or nozzle) – Side view

Annex A
(informative)
Theoretical background and additional information on the standard test
method for the performance of ionizers
A.1 Introductory remarks
The field from static charges, located on insulators or isolated conductors, can be neutralized
by oppositely charged air ions depositing on the charged bodies.
A.2 Air ions
Air ions are molecular clusters consisting of about ten molecules (often water) around a (singly)
charged oxygen or nitrogen molecule. Normally, relatively few ions are present in the air.
Typically, the number is less than 1 000 per cm . These "natural" ions are usually formed by
radiation from radioactive materials in the air, in the ground or in building materials.
For neutralization purposes, much higher ion concentrations are needed. Although radioactivity
can also be employed in such situations, the most common ion production method is by collision
between neutral molecules and electrons accelerated in an electric field with field strengths
exceeding 3 MV/m (at atmospheric pressure). This is generally referred to as high voltage
corona ionization.
A.3 Mobility and ion current
If an ion is exposed to an electric field E, it will move with an average drift speed (v) proportional
to E, i.e.
v= kE
(A.1)
where k is the mobility of the ion.
–4 2 –1 –1
Ordinary air ions have mobilities in the range of 1 to 2 × 10 m V s (square metre per volt-
second).
If the air has a concentration n of positive ions with the mobility k, and charge e, an electric field
E will cause an electric current to flow in the direction of E with the density j.
j enkE λE
(A.2)
The constant λ (enk) is called the positive conductivity of the air (or more precisely, the polar
conductivity due to the positive ions). Negative ions will move in the opposite direction of the
field, but Formula (A.2) can still be used to calculate the current density from negative ions,
when e is taken as the numerical value of the ion charge. The current density from negative
ions will thus also be in the direction of the field.
==
A.4 Neutralization current
If a body completely surrounded by ionized air is given a charge q, an electric field is established
around the body and charges will flow towards it and away from it. The field will vary from point
to point, but is always proportional to the charge q. The current towards the body is carried by
the ions of polarity opposite to that of q, and is known as the neutralization current. The
neutralization current is proportional to the charge q and to the relevant opposite conductivity
of the surrounding air.
A.5 Neutralization rate
If the conductivity does not change, the relative rate of charge neutralization is constant and
the charge will decay exponentially with a time constant τ equal to the permittivity of the air e
o
divided by the conductivity λ.
τλ= e /
o
(A.3)
It should be noted that it is the conductivity, not the ion concentration itself that determines the
neutralizing ability of the air. If the particle concentration of the air is increased, by smoke for
example, the average mobility of the ions, and thus the conductivity, can decrease by a factor
of ten or more. The number of charged particles per unit volume of air, i.e. the ion concentration,
can still be more or less constant.
A.6 Ion depletion and field suppression
The conditions for fulfilling Formula (A.3) are almost never met.
It was assumed that the conductivity was not affected by the neutralization process. In the case
of room ionization, for example, the field from the charge to be neutralized can partially deplete
the air of ions. This lowers the air conductivity, making the neutralization slower than predicted
by Formula (A.3). The degree of ion depletion will increase with the field strength from the
charged body. The rate of neutralization
...