Audio Post Production for Television and Film Third

Edition An introduction to technology and techniques

Hilary Wyatt - PDF Download (2025)

https://ebookultra.com/download/audio-post-production-for-
television-and-film-third-edition-an-introduction-to-technology-
and-techniques-hilary-wyatt/

Visit ebookultra.com today to download the complete set of

ebooks or textbooks
We have selected some products that you may be interested in
Click the link to download now or visit ebookultra.com
for more options!.

Acoustics and Audio Technology Third Edition Kleiner

https://ebookultra.com/download/acoustics-and-audio-technology-third-
edition-kleiner/

Forensic Science An Introduction to Scientific and

Investigative Techniques Third Edition Bell

https://ebookultra.com/download/forensic-science-an-introduction-to-
scientific-and-investigative-techniques-third-edition-bell/

Lighting technology a guide for television film and

theatre 2nd Edition Brian Fitt

https://ebookultra.com/download/lighting-technology-a-guide-for-
television-film-and-theatre-2nd-edition-brian-fitt/

Looking at Movies An Introduction to Film Third Edition

Richard M. Barsam

https://ebookultra.com/download/looking-at-movies-an-introduction-to-
film-third-edition-richard-m-barsam/
Audio anecdotes tools tips and techniques for digital
audio Vol 2 Barzel

https://ebookultra.com/download/audio-anecdotes-tools-tips-and-
techniques-for-digital-audio-vol-2-barzel/

Desktop Audio Technology Digital audio and MIDI principles

Music Technology 1st Edition Francis Rumsey

https://ebookultra.com/download/desktop-audio-technology-digital-
audio-and-midi-principles-music-technology-1st-edition-francis-rumsey/

An Introduction to Science and Technology Studies 2nd

Edition Sergio Sismondo

https://ebookultra.com/download/an-introduction-to-science-and-
technology-studies-2nd-edition-sergio-sismondo/

Syngas Production Methods Post Treatment and Economics

Production Methods Post Treatment and Economics 1st
Edition Adorjan Kurucz
https://ebookultra.com/download/syngas-production-methods-post-
treatment-and-economics-production-methods-post-treatment-and-
economics-1st-edition-adorjan-kurucz/

Mixing Recording and Producing Techniques of the Pros

Insights on Recording Audio for Music Video Film and Games
Book 2nd Edition Rick Clark
https://ebookultra.com/download/mixing-recording-and-producing-
techniques-of-the-pros-insights-on-recording-audio-for-music-video-
film-and-games-book-2nd-edition-rick-clark/
Audio Post Production for Television and Film Third
Edition An introduction to technology and techniques
Hilary Wyatt Digital Instant Download
Author(s): Hilary Wyatt, Tim Amyes
ISBN(s): 9781138459779, 1138459771
Edition: 3
File Details: PDF, 6.40 MB
Year: 2004
Language: english
Hila_Fm.qxd 9/11/04 3:02 PM Page i

Audio Post Production for

Television and Film
Hila_Fm.qxd 9/11/04 3:02 PM Page ii
Hila_Fm.qxd 9/11/04 3:02 PM Page iii

Audio Post Production for

Television and Film
An introduction to technology and
techniques

Third edition

Hilary Wyatt and Tim Amyes

AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD

PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO
Focal Press is an imprint of Elsevier
Hila_Fm.qxd 9/11/04 3:02 PM Page iv

Focal Press
An imprint of Elsevier
Linacre House, Jordan Hill, Oxford OX2 8DP
30 Corporate Drive, Burlington MA 01803

First published as the Technique of Audio Post-production in Video and Film 1990
Paperback edition 1993
Reprinted 1994, 1995 (twice), 1997, 1998
Second edition 1998
Reprinted 2000
Third edition 2005

Copyright © 2005, Hilary Wyatt and Tim Amyes. All rights reserved

The right of Hilary Wyatt and Tim Amyes to be identified as the authors of
this work has been asserted in accordance with the Copyright, Designs and
Patents Act 1988

No part of this publication may be reproduced in any material form (including

photocopying or storing in any medium by electronic means and whether
or not transiently or incidentally to some other use of this publication) without
the written permission of the copyright holder except in accordance with the
provisions of the Copyright, Designs and Patents Act 1988 or under the terms of
a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road,
London, England W1T 4LP. Applications for the copyright holder’s written
permission to reproduce any part of this publication should be addressed
to the publisher

Permissions may be sought directly from Elsevier’s Science and Technology Rights
Department in Oxford, UK: phone: (⫹44) (0) 1865 843830; fax: (⫹44) (0) 1865 853333;
e-mail: permissions@elsevier.co.uk. You may also complete your request on-line via the
Elsevier homepage (www.elsevier.com), by selecting ‘Customer Support’
and then ‘Obtaining Permissions’

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

Library of Congress Cataloguing in Publication Data

A catalogue record for this book is available from the Library of Congress

ISBN 0 240 51947 7

For information on all Focal Press publications visit our website at:
www.focalpress.com

Typeset by Newgen Imaging Systems (P) Ltd., Chennai, India

Printed and bound in Great Britain
Hila_Fm.qxd 9/11/04 3:02 PM Page v

Contents
Acknowledgements xi
About the authors xiii
Introduction to the third edition xv

PART 1 AUDIO BASICS 1

Chapter 1 The evolution of audio post production 3
An overview 3
A little history: the development of technology and techniques 5
Where we are now: post production today 15

Chapter 2 Digital recording and processing 18

The digital audio process 20
Sampling rate and pulse code modulation 21
Quantizing level 21
Storing digital audio data 22
Compression 23
Buffers 23
Interconnecting between digital audio systems 24

Chapter 3 Synchronizing and controlling audio post production equipment 27

SMPTE/EBU timecode 28
Timecode and speed 31
Identification and labelling 32
Longitudinal timecode (LTC) 33
Vertical interval timecode (VITC) 35
Burnt-in timecode 37
MIDI timecode (MTC) 37
Controlling equipment through synchronization 39
Synchronization modes 39
The control of tape/film transports 42

Chapter 4 Audio transfers and file formats 43

Compression 43
Linear transfers 44
Hila_Fm.qxd 9/11/04 3:02 PM Page vi

vi Contents

File transfers 47
File conversion software 53
Network systems 56

Chapter 5 Video, film and pictures 58

Film 58
Telecine 59
Video 59
Video compression 62
Film recording 65
Audio on video recorders 65
Viewing pictures in audio post production 66
Viewing images 66
Comparing film and video 67

Chapter 6 Film in audio post production 68

Film release 69
Conforming film 70
Film timecode 71
Sound on sprocketed film 73
Photographic film recording 73
Recording analogue optical soundtracks 74
Digital optical soundtracks 75

PART 2 THE POST PRODUCTION PROCESS 77

Chapter 7 Post production workflows 79

Chapter 8 Recording audio for post production 83

Aims 83
Types of microphone 83
Mono and stereo recording 85
Microphone position 86
Using multiple microphones 89
Production mixing 90
Studio and field recorders 91
Identing and logging takes 99
Studio-based recording 99
Field/location recording 101

Chapter 9 Editing picture and sound 107

An overview 107
Non-linear editing 108
System configuration 109
Hila_Fm.qxd 9/11/04 3:02 PM Page vii

Contents vii

Video resolution 109

The editing process 111
Logging the rushes 115
Digitizing sound and picture 116
Syncing sound and picture 116
Editing audio in the timeline 118
Audio tools 119
Outputting the audio edit 122
Spotting the soundtrack 126
Handing over to the sound editors 127

Chapter 10 The digital audio workstation 128

An overview 128
Digital audio editing 128
System configuration 129
Hard drives 131
Drive configurations 132
Working with picture 135
System requirements and interconnectivity 136
Audio editing tools 140
Mixing tools 141
Backing up 145
Setting up a tracklaying workspace 146
Choosing the right workstation for the job 147

Chapter 11 Preparing for the mix: editing production sound 150

Aims 150
The conform 151
Checking sync 152
Starting the dialogue edit 152
Boom or personal mic? 154
Handling twin/multiple-track material 155
Handling M/S recordings 155
Techniques for improving audio edits 155
Dialogue editing software 157
ADR spotting 157
ADR cue sheets 159
ADR spotting software 160
Attending the ADR session 160
Editing ADR 160
ADR fitting software 161
Splitting the dialogues for the mix 162
Crowd spotting 163
Attending the crowd session 164
Editing the crowd 165
Hila_Fm.qxd 9/11/04 3:02 PM Page viii

viii Contents

Chapter 12 Preparing for the mix: sound effects editing 166

Aims 166
Types of sound effect 167
Planning the tracklay 168
Sourcing sound effects 169
Starting the edit 172
Tracklaying for the surrounds 173
Tracklaying for the subs 174
Sound effects editing techniques 175
Sound effects plug-ins 177
Samplers and synthesizers 178
Presenting the tracks for the mix 178

Chapter 13 Post sync recording 180

Recording foley 180
Recording ADR 182
Crowd recording 184
Voice-over recording 185
Voice tracks for animation 186
ISDN (Integrated Switched Digital Network) 186

Chapter 14 Preparing for the mix: music 188

Aims 188
Types of music 189
Music and copyright 190
Planning the music 192
Sourcing music 194

Chapter 15 Monitoring and the environment 204

Monitoring loudspeakers 205
Stereo and multichannel sound 206
Acoustics and reverberation 208
Background noise 208
Workstation rooms 209
The importance of listening levels 209
Visual monitoring of recording levels 212

Chapter 16 Mixing and processing equipment 216

The mixing console 216
Types of mixing console 219
Inputs 220
Control of dynamics 225
Computerization of mixing operations 230
Hila_Fm.qxd 9/11/04 3:02 PM Page ix

Contents ix

Chapter 17 The mix 233

Operation of the controller 234
Console automation 234
The virtual mix 235
Cue sheets 236
Mixing using DAWs 236
Mixing in surround 239
Compatible mixes for television 243
Layback 244
Music and Effects mixes 245
Delivery requirements 246

Chapter 18 The transmission and reproduction of audio post

production material 247
The cinema chain 248
Digital television 249
Television transmission 249
Television chain – transmission 250
Metadata 252
Video on the web 252
Domestic video formats 253

Glossary 255

Index 277
Hila_Fm.qxd 9/11/04 3:02 PM Page x
Hila_Fm.qxd 9/11/04 3:02 PM Page xi

Acknowledgements
Thank you to Dennis Weinreich, Richard Conway and all my good friends at Videosonics for their
enthusiasm and support during the writing of this book, and their willingness to share their knowledge
and experience with me. I’d especially like to thank Jeremy Price, Simon Gershon, Dave Turner,
Michele Woods, Andrew Tyndale, Andrew Stirk, Barnaby Smyth, Howard Bargroff, Dan Johnson and
Smudger.

I’d also like to thank the many friends and colleagues within the industry who have been so generous
with their time and advice – Simon Bishop, Richard Manton, Tim Alban, Alex Mackie, Kate Higham,
Heidi Freeman, Sam Southwick, Thomas Drescher, Roger Knight, Jon Matthews, Ed Bulman,
Anthony Faust and Jim Guthrie.

Thanks to Peter Hoskins at Videosonics, Clare MacLeod at Avid, Liz Cox at Digidesign, and Mike
Reddick at AMS Neve for their help with many of the illustrations.

Hilary Wyatt

In writing this book many friends and colleagues in the industry have contributed. Among these are
Andy Boyle, Ian Finlayson, Cy Jack, Gillian Munro, Mike Varley, Alistair Biggar, Tim Mitchell,
Len Southam and many others.

The extract from the sound notes by Alfred Hitchcock for his film The Man Who Knew Too Much are
by kind permission of the Alfred Hitchcock Trust.

Tim Amyes
Hila_Fm.qxd 9/11/04 3:02 PM Page xii
Hila_Fm.qxd 9/11/04 3:02 PM Page xiii

About the authors

Hilary Wyatt is a freelance Dialogue Supervisor and Sound Effects Editor. She began her career in
1987, creating sound effects and editing music for a number of long-running British ‘cult’ animation
series.

Since then, Hilary has worked as a Sound Effects Editor on a wide range of productions, including
commercials, documentary, drama and feature films. In 1999 she supervised the dialogues on the British
gangster film Sexy Beast, and has since worked as Dialogue Supervisor on a number of British and
American features. Recent credits include Jojo In the Stars (animation), Absolute Power and
Dr Zhivago (TV), Bright Young Things, Dear Frankie, Something Borrowed and White Noise (features).

Tim Amyes has many years experience in post production, covering the whole production chain. As
well as being a former sound supervisor at Scottish Television, Tim has worked at two other compa-
nies in the British ITV network, both as a sound recordist and dubbing mixer. He has been involved in
industry training from the start, serving as one of the original members of Skillset, which was set up
to provide training standards (NVQs) for the UK television and film industries. He has also served on
both national industry and union training committees, and advised on the recent Scottish Screen/BFI
publication An Introduction to Film Language.

Currently, Tim lectures in audio, produces specialist corporate videos, and writes, having sold docu-
mentary scripts to both the BBC and ITV. A keen film enthusiast, he is a past member of the Scottish
Film Archive’s advisory committee.
Hila_Fm.qxd 9/11/04 3:02 PM Page xiv
Hila_Fm.qxd 9/11/04 3:02 PM Page xv

Introduction to the
third edition
The focus of this book is audio post production, one of the last stages in the creative process. By the
time it takes place, many crucial decisions will have been made regarding the sound – sometimes in
consultation with the sound post production team, sometimes not! It is important for those working in
audio post production to have a working knowledge of what happens on location, or in the studio, and
during the picture edit, as all these stages will have a bearing on their own work, both technically and
creatively. The third edition has therefore been completely rewritten and restructured to provide a step-
by-step guide to the professional techniques that are used to shape a soundtrack through the produc-
tion process.

This edition is split into two parts. Part 1 deals with the technical nuts and bolts of audio post produc-
tion – how audio is recorded, how sound and picture are synchronized together, how audio is trans-
ferred between systems, and how film and video technology works. You may find it useful to refer back
to these chapters when reading the second part of the book, which follows the path of production sound
from its original recording right through to the final mix and transmission. Part 2 is structured to
follow a typical post production workflow. It examines the equipment used at each stage, how it is
used, and it includes many of the practical techniques and shortcuts that are used by experienced
editors and mixers.

This book uses the generic terms ‘non-linear picture editor’ (abbreviated to NLE) and ‘digital audio
workstation’ (abbreviated to DAW) to describe systems in general use at the current time. On some
occasions we have been more specific, and have mentioned actual manufacturers where we felt it
important. However, we have only named equipment we feel is in standard use, and which is likely to
be around for many years to come. The reader should, however, bear in mind that some aspects of
audio technology are changing at a very rapid rate.

Hilary Wyatt
Hila_Fm.qxd 9/11/04 3:02 PM Page xvi
Hila_ch01.qxd 9/11/04 12:26 PM Page 1

Part 1

Audio Basics
Hila_ch01.qxd 9/11/04 12:26 PM Page 2
Hila_ch01.qxd 9/11/04 12:26 PM Page 3

1 The evolution of
audio post
production
Hilary Wyatt

An overview
The term audio post production refers to that part of the production process which deals with the
tracklaying, mixing and mastering of a soundtrack. Whilst the complexity of the finished soundtrack
will vary, depending on the type of production, the aims of the audio post production process are:

● To enhance the storyline or narrative flow by establishing mood, time, location or period through
the use of dialogue, music and sound effects.
● To add pace, excitement and impact using the full dynamic range available within the viewing
medium.
● To complete the illusion of reality and perspective through the use of sound effects and the recre-
ation of natural acoustics in the mix, using equalization and artificial reverbs.
● To complete the illusion of unreality and fantasy through the use of sound design and effects
processing.
● To complete the illusion of continuity through scenes which have been shot discontinuously.
● To create an illusion of spatial depth and width by placing sound elements across the stereo/
surround sound field.
● To fix any problems with the location sound by editing, or replacing dialogue in post production,
and by using processors in the mix to maximize clarity and reduce unwanted noise.
● To deliver the final soundtrack made to the appropriate broadcast/film specifications and mastered
onto the correct format.
Other documents randomly have
different content
way that a supply of the radium emanation can be obtained at
intervals from radium.
Hofmann and Strauss have observed a peculiar action of the cathode
rays on the active lead sulphate separated by them. They state that
the activity diminishes with time, but is recovered by exposure of the
lead for a short time to the action of cathode rays. No such action is
shown by the active lead sulphide. This effect is due most probably
to the action of the cathode rays in causing a strong
phosphorescence of the lead sulphate and has nothing to do with
the radio-activity proper of the substance.

23. Is thorium a radio-active element? The similarity of the

chemical properties of actinium and thorium has led to the
suggestion at different times that the activity of thorium is not due
to thorium itself, but to the presence of a slight trace of actinium. In
view of the difference in the rate of decay of the emanations of
thorium and actinium, this position is not tenable. If the activity of
thorium were due to actinium, the two emanations, as well as the
other products obtained from these substances, should have
identical rates of decay. Since there is not the slightest evidence that
the rate of decay of activity of the various products can be altered
by chemical or physical agencies, we may conclude with confidence
that whatever radio-active substance is responsible for the activity of
thorium, it certainly is not actinium. This difference in the rate of
decay of the active products is of far more weight in deciding the
question whether two bodies contain the same radio-active
constituent than differences in chemical behaviour, for it is quite
probable that the active material in each case may exist only in
minute quantity in the matter under examination, and, under such
conditions, a direct chemical examination in the first place is of little
value.
Recent work of Hofmann and Zerban and of Baskerville, however,
certainly tends to show that the element thorium is itself non-radio-
active, and that the radio-activity observed in ordinary thorium
compounds is due to the admixture with it of an unknown radio-
active element. Hofmann and Zerban[42] made a systematic
examination of the radio-activity of thorium obtained from different
mineral sources. They found generally that thorium, obtained from
minerals containing a large percentage of uranium, were more active
than those obtained from minerals nearly free from uranium. This
indicates that the radio-activity observed in thorium may possibly be
due to a transformation product of uranium which is closely allied
chemically to thorium and is always separated with it. A small
quantity of thorium obtained from the mineral gadolinite was found
by Hofmann to be almost inactive, whether tested by the electric or
by the photographic method. Later Baskerville and Zerban[43] found
that thorium obtained from a Brazilian mineral was practically devoid
of activity.
In this connection the recent work of Baskerville on the complexity
of ordinary thorium is of interest. By special chemical methods, he
succeeded in separating two new and distinct substances from
thorium, which he has named carolinium and berzelium. Both of
these substances are strongly radio-active, and it thus seems
probable that the active constituent observed in ordinary thorium
may be due to one of these elements.
If, as we have suggested, thorium itself is not active, it is certainly a
matter of surprise that ordinary commercial thorium and the purest
chemical preparations show about the same activity. Such a result
indicates that the methods of purification have not removed any of
the radio-active constituent originally present.
Whatever the radio-active constituent in thorium may ultimately
prove to be, it is undoubtedly not radium nor actinium nor any of the
known radio-active substances.
In later chapters, the radio-activity of thorium will, for simplicity, be
discussed on the assumption that thorium is itself a radio-active
element. The analysis of the changes which occur will thus not refer
to thorium itself but to the primary radio-active substance usually
found associated with it. The conclusions to be drawn from an
examination of the radio-active processes are for the most part
independent of whether thorium is itself radio-active or whether the
radio-activity is due to an unknown element. If thorium is not radio-
active itself, it is not possible to draw any conclusions upon the
question of the duration of the primary radio-activity associated with
it. Such a deduction cannot be made until the quantity of the radio-
active element present in thorium has been definitely determined.

24. If elements heavier than uranium exist, it is probable that they

will be radio-active. The extreme delicacy of radio-activity as a
means of chemical analysis would enable such elements to be
recognized even if present in infinitesimal quantities. It is probable
that considerably more than the three or four radio-elements at
present recognized exist in minute quantity, and that the number at
present known will be augmented in the future. In the first stage of
the search, a purely chemical examination is of little value, for it is
not probable that the new element should exist in sufficient quantity
to be detected by chemical or spectroscopic analysis. The main
criteria of importance are the existence or absence of distinctive
radiations or emanations, and the permanence of the radio-activity.
The discovery of a radio-active emanation with a rate of decay
different from those already known would afford strong evidence
that a new radio-active body was present. The presence of either
thorium or radium in matter can very readily be detected by
observing the rate of decay of the emanations given out by them.
When once the existence of a new radio-element has been inferred
by an examination of its radio-active properties, chemical methods of
separation can be devised, the radiating or emanating property
being used as a guide in qualitative and quantitative analysis.
 CHAPTER II.
IONIZATION THEORY OF GASES.

25. Ionization of gases by radiation. The most important

property possessed by the radiations from radio-active bodies is their
power of discharging bodies whether positively or negatively
electrified. As this property has been made the basis of a method for
an accurate quantitative analysis and comparison of the radiations,
the variation of the rate of discharge under different conditions and
the processes underlying it will be considered in some detail.
In order to explain the similar discharging power of Röntgen rays, the
theory[44] has been put forward that the rays produce positively and
negatively charged carriers throughout the volume of the gas
surrounding the charged body, and that the rate of production is
proportional to the intensity of the radiation. These carriers, or ions[45]
as they have been termed, move with a uniform velocity through the
gas under a constant electric field, and their velocity varies directly as
the strength of the field.

Fig. 1.
Suppose we have a gas between two metal plates A and B (Fig. 1)
exposed to the radiation, and that the plates are kept at a constant
difference of potential. A definite number of ions will be produced per
second by the radiation, and the number produced will depend in
general upon the nature and pressure of the gas. In the electric field
the positive ions travel towards the negative plate, and the negative
ions towards the positive, and consequently a current will pass
through the gas. Some of the ions will also recombine, the rate of
recombination being proportional to the square of the number
present. For a given intensity of radiation, the current passing
through the gas will increase at first with the potential difference
between the plates, but it will reach a limit when all the ions are
removed by the electric field before any recombination occurs.
This theory accounts also for all the characteristic properties of gases
made conducting by the rays from active substances, though there
are certain differences observed between the conductivity
phenomena produced by active substances and by X rays. These
differences are for the most part the result of unequal absorption of
the two types of rays. Unlike Röntgen rays, a large proportion of the
radiation from active bodies consists of rays which are absorbed in
their passage through a few centimetres of air. The ionization of the
gas is thus not uniform, but falls off rapidly with increase of distance
from the active substance.

26. Variation of the current with voltage. Suppose that a layer

of radio-active matter is spread uniformly on the lower of two
horizontal plates A and B (Fig. 1). The lower plate A is connected
with one pole of a battery of cells the other pole of which is
connected with earth. The plate B is connected with one pair of
quadrants of an electrometer, the other pair being connected with
earth.
The current[46] between the plates, determined by the rate of
movement of the electrometer needle, is observed at first to increase
rapidly with the voltage, then more slowly, finally reaching a value
which increases very slightly with a large increase in the voltage.
This, as we have indicated, is simply explained on the ionization
theory.
The radiation produces ions at a constant rate, and, before the
electric field is applied, the number per unit volume increases until
the rate of production of fresh ions is exactly balanced by the
recombination of the ions already produced. On application of a small
electric field, the positive ions travel to the negative electrode and the
negative to the positive.
Since the velocity of the ions between the plates is directly
proportional to the strength of the electric field, in a weak field the
ions take so long to travel between the electrodes that most of them
recombine on the way.
The current observed is consequently small. With increase of the
voltage there is an increase of speed of the ions and a smaller
number recombine. The current consequently increases, and will
reach a maximum value when the electric field is sufficiently strong to
remove all the ions before appreciable recombination has occurred.
The value of the current will then remain constant even though the
voltage is largely increased.
This maximum current will be called the “saturation” current, and the
value of the potential difference required to give this maximum
current, the “saturation P.D.”[47]
The general shape of the current-voltage curve is shown in Fig. 2,
where the ordinates represent current and the abscissae volts.
 Fig. 2.

Although the variation of the current with voltage depends only on

the velocity of the ions and their rate of recombination, the full
mathematical analysis is intricate, and the equations, expressing the
relation between current and voltage, are only integrable for the case
of uniform ionization. The question is complicated by the inequality in
the velocity of the ions and by the disturbance of the potential
gradient between the plates by the movement of the ions. J. J.
Thomson[48] has worked out the case for uniform production of ions
between two parallel plates, and has found that the relation between
the current i and the potential difference V applied is expressed by

Ai2 + Bi = V

where A and B are constants for a definite intensity of radiation and a

definite distance between the plates.
 Fig. 3.

In certain cases of unsymmetrical ionization, which arise in the study

of the radiations from active bodies, the relation between current and
voltage is very different from that expressed by the above equation.
Some of these cases will be considered in section 47.

27. The general shape of the current-voltage curves for gases

exposed to the radiations from active bodies is shown in Fig. 3.
This curve was obtained for ·45 grams of impure radium chloride, of
activity 1000 times that of uranium, spread over an area of 33 sq.
cms. on the lower of two large parallel plates, 4·5 cms. apart. The
maximum value of the current observed, which is taken as 100, was
1·2 × 10-8 amperes, the current for low voltages was nearly
proportional to the voltage, and about 600 volts between the plates
was required to ensure approximate saturation.
In dealing with slightly active bodies like uranium or thorium,
approximate saturation is obtained for much lower voltages. Tables I.
and II. show the results for the current between two parallel plates
distant 0·5 cms. and 2·5 cms. apart respectively, when one plate was
covered with a thin uniform layer of uranium oxide.
 Table I.

0·5 cms. apart

Volts Current
·125 18
·25 36
·5 55
1 67
2 72
4 79
8 85
16 88
100 94
335 100

Table II.

2·5 cms. apart

Volts Current
·5 7·3
1 14
2 27
4 47
8 64
16 73
37·5 81
112 90
375 97
800 100
The results are shown graphically in Fig. 4.
 Fig. 4.

From the above tables it is seen that the current at first increases
nearly in proportion to the voltage. There is no evidence of complete
saturation, although the current increases very slowly for large
increases of voltage. For example, in Table I. a change of voltage
from ·125 to ·25 volts increases the current from 18 to 36% of the
maximum, while a change of voltage from 100 to 335 volts increases
the current only 6%. The variation of the current per volt (assumed
uniform between the range of voltages considered) is thus about
5000 times greater for the former change.
Taking into consideration the early part of the curves, the current
does not reach a practical maximum as soon as would be expected
on the simple ionization theory. It seems probable that the slow
increase with the large voltages is due either to an action of the
electric field on the rate of production of ions, or to the difficulty of
removing the ions produced near the surface of the uranium before
recombination. It is possible that the presence of a strong electric
field may assist in the separation of ions which otherwise would not
initially escape from the sphere of one another’s attraction. From the
data obtained by Townsend for the conditions of production of fresh
ions at low pressures by the movement of ions through the gas, it
seems that the increase of current cannot be ascribed to an action of
the moving ions in the further ionization of the gas.

28. The equation expressing the relation between the current and
the voltage is very complicated even in the case of a uniform rate of
production of ions between the plates. An approximate theory, which
is of utility in interpreting the experimental results, can however be
simply deduced if the disturbance of the potential gradient is
disregarded, and the ionization assumed uniform between the plates.
Suppose that the ions are produced at a constant rate q per cubic
centimetre per second in the gas between parallel plates distant l
cms. from each other. When no electric field is applied, the number N
present per c.c., when there is equilibrium between the rates of
production and recombination, is given by

q = αN2,

where α is a constant.
If a small potential difference V is applied, which gives only a small
fraction of the maximum current, and consequently has not much
effect on the value of N, the current i per sq. cm. of the plate, is
given by

NeuV
i = -----
l

where u is the sum of the velocity of the ions for unit potential
gradient, and e is the charge carried by an ion.

uV
-----
 l

is the velocity of the ions in the electric field of strength

V
----
l

The number of ions produced per second in a prism of length l and

unit area of cross-section is ql. The maximum or saturation current I
per sq. cm. of the plate is obtained when all of these ions are
removed to the electrodes before any recombination has occurred.
Thus

I = q . l . e,

and

This equation expresses the fact previously noted that, for small
voltages, the current i is proportional to V.
Let

i/I = ρ,

then
Now the greater the value of V required to obtain a given value of ρ
(supposed small compared with unity), the greater the potential
required to produce saturation.
It thus follows from the equation that:
(1) For a given intensity of radiation, the saturation P.D. increases with
the distance between the plates. In the equation, for small values of
ρ, V varies as l2. This is found to be the case for uniform ionization,
but it only holds approximately for non-uniform ionization.
(2) For a given distance between the plates, the saturation P.D. is
greater, the greater the intensity of ionization between the plates.
This is found to be the case for the ionization produced by radio-
active substances. With a very active substance like radium, the
ionization produced is so intense that very large voltages are required
to produce approximate saturation. On the other hand, only a fraction
of a volt per cm. is necessary to produce saturation in a gas where
the ionization is very slight, for example, in the case of the natural
ionization observed in a closed vessel, where no radio-active
substances are present.
For a given intensity of radiation, the saturation P.D. decreases rapidly
with the lowering of the pressure of the gas. This is due to two
causes operating in the same direction, viz. a decrease in the
intensity of the ionization and an increase in the velocity of the ions.
The ionization varies directly as the pressure, while the velocity varies
inversely as the pressure. This will obviously have the effect of
causing more rapid saturation, since the rate of recombination is
slower and the time taken for the ions to travel between the
electrodes is less.
The saturation curves observed for the gases hydrogen and carbon
dioxide[49] are very similar in shape to those obtained for air. For a
given intensity of radiation, saturation is more readily obtained in
hydrogen than in air, since the ionization is less than in air while the
velocity of the ions is greater. Carbon dioxide on the other hand
requires a greater P.D. to produce saturation than does air, since the
ionization is more intense and the velocity of the ions less than in air.
29. Townsend[50] has shown that, for low pressures, the variation of
the current with the voltage is very different from that observed at
atmospheric pressure. If the increase of current with the voltage is
determined for gases, exposed to Röntgen rays, at a pressure of
about 1 mm. of mercury, it is found that for small voltages the
ordinary saturation curve is obtained; but when the voltage applied
increases beyond a certain value, depending on the pressure and
nature of the gas and the distance between the electrodes, the
current commences to increase slowly at first but very rapidly as the
voltage is raised to the sparking value. The general shape of the
current curve is shown in Fig. 5.

Fig. 5.

The portion OAB of the curve corresponds to the ordinary saturation

curve. At the point B the current commences to increase. This
increase of current has been shown to be due to the action of the
negative ions at low pressures in producing fresh ions by collision
with the molecules in their path. The increase of current is not
observed in air at a pressure above 30 mms. until the P.D. is increased
nearly to the value required to produce a spark. This production of
ions by collision is considered in more detail in section 41.

30. Rate of recombination of the ions. A gas ionized by the

radiation preserves its conducting power for some time after it is
removed from the presence of the active body. A current of air blown
over an active body will thus discharge an electrified body some
distance away. The duration of this after conductivity can be
examined very conveniently in an apparatus similar to that shown in
Fig. 6.

Fig. 6.

A dry current of air or any other gas is passed at a constant rate

through a long metal tube TL. After passing through a quantity of
cotton-wool to remove dust particles, the current of air passes over a
vessel T containing a radio-active body such as uranium, which does
not give off a radio-active emanation. By means of insulated
electrodes A and B, charged to a suitable potential, the current
between the tube and one of these electrodes can be tested at
various points along the tube.
A gauze screen, placed over the cross-section of the tube at D, serves
to prevent any direct action of the electric field in abstracting ions
from the neighbourhood of T.
If the electric field is sufficiently strong, all the ions travel in to the
electrodes at A, and no current is observed at the electrode B. If the
current is observed successively at different distances along the tube,
all the electrodes except the one under consideration being
connected to earth, it is found that the current diminishes with the
distance from the active body. If the tube is of fairly wide bore, the
loss of the ions due to diffusion is small, and the decrease in
conductivity of the gas is due to recombination of the ions alone.
On the ionization theory, the number dn of ions per unit volume
which recombine in the time dt is proportional to the square of the
number present. Thus

dn
--- = αn²,
dt

where α is a constant.
Integrating this equation,

1 1
--- – --- = αt,
n N

if N is the initial number of ions, and n the number after a time t.

The experimental results obtained[51] have been shown to agree very
well with this equation.
In an experiment similar to that illustrated in Fig. 6, using uranium
oxide as a source of ionization, it was found that half the number of
ions present in the gas recombined in 2·4 seconds, and that at the
end of 8 seconds one-fourth of the ions were still uncombined.
Since the rate of recombination is proportional to the square of the
number present, the time taken for half of the ions present in the gas
to recombine decreases very rapidly with the intensity of the
ionization. If radium is used, the ionization is so intense that the rate
of recombination is extremely rapid. It is on account of this rapidity of
recombination that large voltages are necessary to produce saturation
in the gases exposed to very active preparations of radium.
The value of α, which may be termed the coefficient of
recombination, has been determined in absolute measure by
Townsend[52], McClung[53] and Langevin[54] by different experimental
methods but with very concordant results. Suppose, for example,
with the apparatus of Fig. 6, the time T, taken for half the ions to
recombine after passing by the electrode A, has been determined
experimentally. Then

1
---- = αT,
N

where N is the number of ions per c.c. present at A. If the saturation

current i is determined at the electrode A, i = NVe, where e is the
charge on an ion and V is the volume of uniformly ionized gas carried
by the electrode A per second. Then

Ve
α = ---- .
iT

The following table shows the value of α obtained for different gases.

Value of α.

Gas Townsend McClung Langevin

Air 3420 × e 3384 × e 3200 × e

Carbon 3500 × e 3492 × e 3400 × e
Dioxide
Hydrogen 3020 × e
The latest determination of the value of e (see section 36) is 3·4 ×
10-10 E.S. units; thus α = 1·1 × 10-6.
Using this value, it can readily be shown from the equation of
recombination that, if 106 ions are present per c.c., half of them
recombine in about 0·9 sec. and 99% in 90 secs.
McClung (loc. cit.) showed that the value of α was approximately
independent of the pressure between ·125 and three atmospheres. In
later observations, Langevin has found that the value of α decreases
rapidly when the pressure is lowered below the limits used by
McClung.

31. In experiments on recombination it is essential that the gas

should be free from dust or other suspended particles. In dusty air,
the rate of recombination is much more rapid than in dust-free air, as
the ions diffuse rapidly to the comparatively large dust particles
distributed throughout the gas. The effect of the suspension of small
particles in a conducting gas is very well illustrated by an experiment
of Owens[55]. If tobacco smoke is blown between two parallel plates
as in Fig. 1, the current at once diminishes to a small fraction of its
former value, although a P.D. is applied sufficient to produce
saturation under ordinary conditions. A much larger voltage is then
necessary to produce saturation. If the smoke particles are removed
by a stream of air, the current returns at once to its original value.

32. Mobility of the ions. Determinations of the mobility of the ions,

i.e. the velocity of the ions under a potential gradient of 1 volt per
cm., have been made by Rutherford[56], Zeleny[57], and Langevin[58] for
gases exposed to Röntgen rays. Although widely different methods
have been employed, the results have been very concordant, and
fully support the view that the ions move with a velocity proportional
to the strength of the field. On the application of an electric field, the
ions almost instantly attain the velocity corresponding to the field and
then move with a uniform speed.
Zeleny[59] first drew attention to the fact that the positive and
negative ions had different velocities. The velocity of the negative ion
is always greater than that of the positive, and varies with the
amount of water vapour present in the gas.
The results, previously discussed, of the variation of the current with
voltage and of the rate of recombination of the ions do not of
themselves imply that the ions produced in gases by the radiations
from active bodies are of the same size as those produced by
Röntgen rays under similar conditions. They merely show that the
conductivity under various conditions can be satisfactorily explained
by the view that charged ions are produced throughout the volume of
the gas. The same general relations would be observed if the ions
differed considerably in size and velocity from those produced by
Röntgen rays. The most satisfactory method of determining whether
the ions are identical in the two cases is to determine the velocity of
the ions under similar conditions.
In order to compare the velocity of the ions[60], the writer has used an
apparatus similar to that shown in Fig. 6 on p. 40.
The ions were carried with a rapid constant stream of air past the
charged electrode A, and the conductivity of the gas tested
immediately afterwards at an electrode B, which was placed close to
A. The insulated electrodes A and B were fixed centrally in the metal
tube L, which was connected with earth.
For convenience of calculation, it is assumed that the electric field
between the cylinders is the same as if the cylinders were infinitely
long.
Let a and b be the radii of the electrode A, and of the tube L
respectively, and let V = potential of A.
The electromotive intensity X (without regard to sign) at a distance r
from the centre of the tube is given by

Let u1 and u2 be the velocities of the positive and negative ions for a
potential gradient of 1 volt per cm. If the velocity is proportional to
the electric force at any point, the distance dr traversed by the
negative ion in the time dt is given by
dr = Xu2 dt,
or

Let r2 be the greatest distance measured from the axis of the tube
from which the negative ion can just reach the electrode A in the
time t taken for the air to pass along the electrode.
Then

If ρ2 be the ratio of the number of the negative ions that reach the
electrode A to the total number passing by, then

Therefore

Equation 1.
Welcome to our website – the ideal destination for book lovers and
knowledge seekers. With a mission to inspire endlessly, we offer a
vast collection of books, ranging from classic literary works to
specialized publications, self-development books, and children's
literature. Each book is a new journey of discovery, expanding
knowledge and enriching the soul of the reade

Our website is not just a platform for buying books, but a bridge
connecting readers to the timeless values of culture and wisdom. With
an elegant, user-friendly interface and an intelligent search system,
we are committed to providing a quick and convenient shopping
experience. Additionally, our special promotions and home delivery
services ensure that you save time and fully enjoy the joy of reading.

Let us accompany you on the journey of exploring knowledge and

personal growth!

ebookultra.com