General EditorJohn Lackie

CHAMBERSAn imprint of Chambers Harrap Publishers Ltd7 Hopetoun CrescentEdinburghEH7 4AY

This edition first published by Chambers Harrap Publishers Ltd 2007

r Chambers Harrap Publishers Ltd 2007

Previous edition published in 1999. Previously published in 1995 as LarousseDictionary of Science and Technology. First published as Chamberss TechnicalDictionary in 1940 by W&R Chambers Ltd (revised 1958, 1971, 1974 and 1984).

All rights reserved. No part of this publication may be reproduced, stored in aretrieval system, or transmitted by any means, electronic, mechanical, photocopyingor otherwise, without the prior permission of the publisher.

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

ISBN-13: 978 0550 100719

Text design by Ken WilsonTypeset by Macmillan India LtdPrinted in Italy by LegoPrint S.p.A

Preface

This edition of the Chambers Dictionary of Science and Technology is based substantially upon theedition produced in 1999 which, for the first time, incorporated panel entries giving additionaldetail on a range of topics. The dictionary has its origin in 1940, when it was called ChamberssTechnical Dictionary, although it always had a substantial basic science content. It became theDictionary of Science and Technology in 1971, changed in 1984 to the Chambers Science andTechnology Dictionary, then transformed again in 1995 into the Larousse Dictionary of Science andTechnology (with illustrations for the first time) before reverting to the present title in 1999.

The first question I asked when I had the privilege of taking over as General Editor fromProfessor Peter Walker, the architect of major changes in recent editions, was about the role ofa dictionary in these days of search engines and the Internet. It is still a valid question, but Ithink the answer is probably that a dictionary should provide crisp, authoritative definitionsthat are useful to a diverse audience of non-specialists, especially those who are not connectedto the Web or who do not have the time or inclination to dredge for a definition among sitesthat may or may not be reliable. This puts the onus on the Editor and the specialist con-tributors to ensure the accuracy of the content, which I hope we have managed. That said, itwould be remarkable if a few mistakes had not crept through; I hope they are rare and trivial,and apologize in advance if we have perpetuated or propagated errors. Given the complexity ofmodern science and technology and the diversity of the technical language that is employed, itis difficult for a small number of people, no matter how catholic or eclectic in their interests, tobe expert in everything. The most challenging situation is when terminology has evolved andsubtly changed: only the aficionado may realize that the old definition is no longer accurateand that current usage is different and the users are always right if the language is evolving, asit will in our constantly developing technological world. But if you are reading the olderliterature, the older meaning may well be correct, so we have deleted very few entries, evenwhen there is a strong suspicion that the term (or even the technology) is obsolete.

What is new

The obvious areas where neologisms are proliferating are molecular bioscience and informa-tion technology, and these have received the most attention. Biology is emerging from a periodof schismatic diversity, and the pervasive influence of molecular biology has forced a commonlexicon on most bioscientists. It therefore seemed sensible to merge botany, zoology, biologyand immunology (each of which had separate subject fields in the last edition); where, after all,were genetics, cell biology, virology? A new Bioscience subject field was therefore formed it isinclusive rather than exclusive, and has gained several hundred new headwords. Indeed, muchof the bioscience terminology is equally familiar to the modern medical practitioner but wehave retained Medicine, splitting off Pharmacology as a home for the many drugs that wesuspect are often looked up. Environmental Sciences has become the category for most ecologyand meteorology terms, as well as terms relating to waste management and pollution.Psychology has emerged as a new category, incorporating some of the entries formerly labelledBehaviour (others have gone into Bioscience), but with many new additions it is, after all,one of the most popular subjects at both school and university level. Agriculture, one of theoldest of technologies, has gained a place, and some of the details in Forestry entries have beenpruned to accommodate the agricultural ones. Modern telecommunications are so dependentupon digital information processing that the former Telecommunications and Computer

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Sadly, although I was able to discuss the preparation of this edition with Peter, he died in early 2006. The presentformat of the dictionary owes much to his efforts over several earlier editions.

Science fields have been merged into Information and Communications Technology, with itsinevitable three-letter acronym, ICT. This area too has gained a substantial number of newheadwords.

It should be noted that these subject fields are only a rough indication of how an entry shouldbe classified. They are important in differentiating between very different meanings (biosci-entists and electrical engineers use cell in rather different ways, for example), but relativelyunimportant in fields that communicate extensively and have boundaries that are fuzzy (mostbioscientists use pharmacological tools, often to research medically important diseases, some-times even diseases that are partly the province of psychology).

There are a few new panels, notably on world mortality statistics and on the human brain, andsome changes have been made to the appendices. Otherwise, the major change is the growth ofthe dictionary by the addition of some 2,000 new headwords.

Arrangement

Entries are presented in alphabetical order, apart from those that are numbers, which arepresented in numerical order in a section of their own at the beginning of the dictionary.Single-letter entries appear at the beginning of each letter section. Greek letters are placedunder the anglicized equivalent (omega under O, for example). Numbers preceding chemicalnames are generally ignored, so that 1,1,1-trichloroethane is under T, below 3:4:40-trichloro-carbanilide and above trichloroethanoic acid. Abbreviations are placed in their alphabeticalposition, but the definition is usually given under the full version. Where some ambiguitymight arise, a cross-reference is given. Spellings are generally British English, but where con-fusion could arise the US version is given in the earlier occurrences (thus, under heme-, thereader is directed to haem-). An exception arises with the names of some drugs which haveBritish Approved Names (BANs) generally used as the headword, but these BANs have beenharmonized with the recommended international non-proprietary names (rINNs) and aresometimes therefore Americanized (thus sulfasalazine, even though sulphur is the generalspelling used elsewhere). Even so, some things may well not be where the reader expects, andan alternative location should be tried.

An evolving dictionary

In deciding upon new headwords for this edition we have relied partly upon new wordsidentified by Chambers Wordtrack, the programme used to identify new usages for otherChambers titles, notably The Chambers Dictionary. In addition, specialist contributors, in-cluding myself, have added terms that we felt were missing.

A more vexed question is when to omit headwords. Inevitably some terms become obsoleteand are occupying space that would be better devoted to new words; but if the reader is seekinga definition of a term used in an older text, where will they find it, except perhaps in an olderedition of this dictionary? In this edition we have been conservative and deleted few entries;pressure on space, and a wish that the dictionary remains a single manageable volume, mayforce the issue during future revisions.

Italic and bold

Italic is used for:(1) alternative forms of, or alternative names for, the headword. Synonyms are generally

given at the end of the definition;(2) terms derived from the headword, often after adj or pl;(3) variables in mathematical formulae;

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(4) generic and specific names in binomial classification of organisms;(5) after abbrev for, symbol for, etc, where the meaning is defined within the entry or is

obvious and there is no cross-referenced entry.

Bold is used for:(1) cross-references, either after see, cf , abbrev for, symbol for, etc, or in the body of

an entry. Cross-referencing indicates that there is a headword elsewhere which givesadditional information (although the absence of a cross-reference does not necessarilymean that there is not such an entry);

(2) vector notation in formulae, etc.

Trade names

Trade and proprietary names are identified by an initial capital, the prefix TN or suffix s, orby a statement.

Illustrations

With a few exceptions, these were drawn by Professor Peter Walker.

Acknowledgements

This edition of the dictionary relies substantially upon the content of the earlier editionsmentioned above, and the efforts of earlier contributors must be acknowledged.

For this edition, Ephraim Borowski scrutinized the Mathematics entries, Alan Cooper theChemistry ones; Brian Grout wrote the new Agriculture entries and Heather Parton checkedthe Food Science entries. Richard Smithson advised on and Julie MacLoughlin checked thenew Psychology section, and added various entries. Peter Tiplady checked and added to theMedicine section, and wrote a new panel; Nicholas Graham merged the former Telecommu-nications and Computing sections and added extensively to the new ICT section, as well aswriting a new panel. Adrian Webster added a few new Astronomy entries and Mike Weighellchecked the Chemical Engineering entries. The Bioscience, Pharmacology and EnvironmentalScience areas were handled by me, with help from Katie Brooks of Chambers, especially on theImmunology entries. I am also grateful to Neville Hankins, who copy-edited with a keen eye.But in the end, if there are errors, they are mine; I hope they are few and I apologize inadvance.

John Lackie

v

Contributors

General EditorJohn Lackie

ContributorsEphraim Borowski

Katie BrooksAlan Cooper

Nicholas GrahamBrian Grout

Julie MacLoughlinHeather Parton

Richard SmithsonPeter TipladyAdrian WebsterMike Weighell

Editorial AssistanceKatie BrooksLianne Vella

CopyeditorNeville Hankins

Publishing ManagerCamilla Rockwood

Production and Prepress ControllerKaren Stuart

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Contents

Preface iii

Panels viii

Subject Categories x

Abbreviations xi

The Dictionary 11322

Appendices 1323

SI prefixes 1325

SI conversion factors 1325

Conversion factors between SI, CGS and FPS units 1328

Physical constants, standard values and equivalents in SI units 1329

Units of measurement 1330

SI derived units 1330

Subatomic particles 1331

Electromagnetic spectrum 1332

Radio and radar frequencies used in aircraft navigation 1333

Letter designations of the frequency bands 1334

Graphical symbols used in electronics 1335

Greek alphabet 1336

Mathematical symbols 1336

Planets 1337

Planetary satellites 1337

The constellations 1338

Time 1339

Geological time 1340

Earthquake severity measurement scales 1345

The periodic table 1346

Taxonomy of living organisms 1351

Amino acids 1356

Chronology of discoveries and inventions 1358

Nobel Prize winners 19502006 1362

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Panels

Active transport 16

Aerodynamics 22

Angiosperms (flowering plants) 46

Astronomical telescope 74

Atmospheric boundary layer 77

Atmospheric pollution 78

Atomic structure 79

Auto-immunity 84

Bacteria 95

Binding energy of the nucleus 120

Biological engineering polymers 122

Black hole 128

Bonding 138

Human brain 148

Bridges and materials 154

C4 photosynthetic pathway 168

Calvin cycle, photosynthetic carbon reduction cycle 173

Cell cycle 196

Cell wall 199

Cement and concrete 200

Ceramics processing 204

Chromosome 220

Climatic change 230

Close packing of atoms 234

Communications satellite 250

Cosmic rays 281

Cosmology 282

Creep and deformation 290

Critical mass 292

Crystal lattice 299

DNA and the genetic code 358

Drilling rig 372

Earth 384

Earthquake 386

Elastomers 396

Electroceramic processing 399

Electron microscope 404

Enzyme 420

Fatigue 450

Ferromagnetics and ferrimagnetics 457

Fibre assemblies 459

Food irradiation 485

Galaxy 508

Gels 518

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Genetic manipulation 520

Glasses and glass-making 528

Grand unified theories 539

Hardness measurements 562

HertzsprungRussell diagram 575

High-performance polymers 580

Immune response 612

Impact tests 614

Internet 638

Intrinsic and extrinsic silicon 642

Laser 681

Lifetime study of the nuclear bomb survivors 696

Liquid crystal displays 706

Mitochondrion 774

Mortality 787

Non-ionizing fields and radiation 822

Nuclear reactors 828

Numerical weather forecast 831

Oklo natural fission reactor 838

Paper and paper-making 866

Plate tectonics 916

Polymers 930

Polymer synthesis 932

Polyvinyl chloride 935

Printed, hybrid and integrated circuits 953

Protein 961

Pulsar 968

Quantum theory 978

Quasar 982

RAD/DSDM Methodology 988

Radiation 990

Radiometric dating 995

Redshiftdistance relation 1010

Risk assessment 1035

Rocket 1038

Rubber toughening 1046

Rusting 1048

Semiconductor fabrication 1076

Silicon, silica, silicates 1095

Space 1117

Space station 1118

Steels 1143

Steroid hormones 1147

Stratosphere and mesosphere 1155

Sun as a star 1167

Superconductors 1170

Thermosets 1206

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Trace element analysis 1224

Transmissible spongiform encephalopathy 1231

Troposphere 1242

Twinned crystals 1250

Tyre technology 1252

Uranium isotope enrichment 1262

Vertebrate evolution 1277

Vitamins 1285

Wood - structure and properties 1306

Subject Categories

(Abbreviations are shown in brackets)

x

Acoustics (Acous)Aeronautics (Aero)Agriculture (Agri)Architecture (Arch)Astronomy (Astron)Automobiles (Autos)Bioscience (BioSci)Building (Build)Chemical Engineering (ChemEng)Chemistry (Chem)Civil Engineering (CivEng)Crystallography (Crystal)Electrical Engineering (ElecEng)Electronics (Electronics)Engineering (Eng)Environmental Sciences (EnvSci)Food Science (FoodSci)Forestry (For)General (Genrl)Geology (Geol)Glass (Glass)Image Technology (ImageTech)

Information and CommunicationsTechnology (ICT)

Mathematical Sciences (MathSci)Medicine (Med)Mineral Extraction (MinExt)Mineralogy (Min)Nuclear Engineering (NucEng)Paper (Paper)Pharmacology (Pharmacol)Physics (Phys)Plastics (Plastics)Powder Technology (PowderTech)Printing (Print)Psychology (Psych)Radar (Radar)Radiology (Radiol)Ships (Ships)Space (Space)Surveying (Surv)Textiles (Textiles)Veterinary Science (Vet)

Abbreviations

(Showing main abbreviations used in the dictionary; others will be found as headwords.)

abbrev abbreviationac alternating currentAD Anno Dominiadj adjectiveam before noonapprox approximatelyat no atomic numberBC Before Christbp boiling pointBP before present (with 1950 as

present)C Celsius (centigrade)/Centralc. circacc cubic centimetre(s)cf compareCGS centimetregram(me)secondcm centimetre(s)colloq colloquial(ly)conc concentratedcu cubicdc direct currentdim diminutiveE east(ern)eg for exampleesp especiallyetc and so onF Fahrenheitfig. figurefl oz fluid ounce(s)Fr Frenchft foot/feetg acceleration due to gravityg gram(s)gall gallon(s)gen genitiveGer GermanGk Greekh hoursie that isin inch(es)K Kelvinkb (kbp) kilobase (kilobase pair)kg kilogram(s)km kilometre(s)l litre(s)

lb poundsLt (Latin)m metre(s)mg milligram(s)min minute(s)MKS(A) metrekilogram(me)second

(ampere)ml millilitre(s)mm millimetre(s)mp melting pointMt Mountmya million years agon nounN north(ern)NB note well (nota bene)no. numbernos numbersoz ounce(s)p (pp) page (pages)% per centpl pluralpm after noonpt pint(s)ram relative atomic massrel. d. relative densityrms root-mean-squares second(s)S South(ern)SI Syste`me International

(dUnites)sing singularsp (spp) species (sing/pl)spec specificsq squarestp standard temperature and

pressureTN trade (proprietary) nameTV televisionUK United KingdomUS United States (of America)v verbW west(ern)yd yard(s)yr year(s)

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1, 2, 3y38 rule (MathSci) See three-eighths rule.2- (Genrl) See two-2,4,5,-T (BioSci) Abbrev for 2,4,5-trichlorophenoxyethanoicacid, a selective herbicide that is widely used but withincreasing public concern.

2E (Min) Apparent optic axial angle as measured in air.2V (Min) The optic axial angle when measured in themineral.

4:2:2 (ImageTech) A standard prescribing the ratio offrequencies used in digital component video systemsfor sampling the luminance signal and two colourdifference signals 135 MHz for luminance and675 MHz for each of the colour difference signals.Extended 4:2:2 prescribes 18 MHz and 9 MHz respectively,providing greater resolution for 625-line wide-screen and1250-line high definition. There is also a 4:1:1 standard. Seefsc.

4GL (ICT) Abbrev for fourth-generation language.4-methyl-aminophenol (Chem) See entry under M.NH2C6H4OH; a solid crystalline compound, soluble inacids and alkalis and readily oxidized in air, used as aphotographic developer and widely marketed under theTN Rodinal.

5-HT (Med, BioSci) Abbrev for 5-hydroxytryptamine. Seeserotonin.

7S antibody (BioSci) Immunoglobulin with sedimentationcoefficient of about 7S. Term often used as synonym forIgG. See 19s antibody.

8 mm time code (ImageTech) See time code.10 base 2 (ICT) A transmission standard for local areanetworks under the ieee 802.3 standard. This systemuses thin coaxial cable and is capable of basebandtransmission at 10 Mbps and a maximum cable length of185 m between repeaters. Also thin Ethernet, thin-net.

10 base 5 (ICT) A transmission standard for local areanetworks under the ieee 802.3 standard. This systemuses a thick coaxial cable and is capable of basebandtransmission at 10 Mbps and a maximum cable length of500 m between repeaters. Also standard Ethernet, thickEthernet.

10 base T (ICT) A transmission standard for local areanetworks under the ieee 802.3 standard. This systemuses unshielded twisted pair cable and is capable ofbaseband transmission at 10 Mbps and a maximum cablelength of 100 m between repeaters.

12mo (Print) See duodecimo. Also duodecimo.16 mm (ImageTech) A narrow gauge of motion picture film,widely used for non-theatrical production and presenta-tion.

16mo (Print) See sextodecimo.

16:9 switching (ImageTech) The facility for a TV or VCR tosense and switch automatically to wide-screen display orrecording.

18-electron rule (Chem) The structures of most stableorganometallic compounds of the transition elements,eg ferrocene, can be rationalized by showing that 18 valenceelectrons can be associated with each metal atom. Seeelectron octet.

18mo (Print) See octodecimo.19S antibody (BioSci) Immunoglobulin with sedimentationcoefficient of about 19S. Term often used as synonym forIgM. See 7s antibody.

21 cm line (Astron, Phys) A line in the radio spectrum ofneutral hydrogen at 21105 cm. It is caused by thespontaneous reversal of direction of spin of the electronin the magnetic field of the hydrogen nucleus, but it may bedetected only in the vast hydrogen clouds of the Galaxy.

24mo (Print) The 24th part of a sheet, or a sheet folded fourtimes to make 24 leaves of 48 pages.

24-hour rhythm (Psych) See circadian rhythm.30 Doradus (Astron) See tarantula nebula.32 bit (ICT) A processor or computer bus, handling dataconsisting of 32 binary digits.

32mo (Print) The 32nd part of a sheet, or a sheet folded fivetimes to make 32 leaves or 64 pages. More than four foldsare not in fact practicable.

35 mm (ImageTech) The standard gauge of motion picturefilm used for professional production and presentation.Also a popular photographic film format. See leica.

61 Cygni (Astron) A faint visual binary star system in theconstellation Cygnus. One of the components is itself abinary system. One of the nearest stellar objects to the Sun,it was the first to have its trigonometric parallax measured.Distance 342 pc.

64mo (Print) The 64th part of a sheet or a sheet folded sixtimes to make 64 leaves or 128 pages. More than four foldsare not practicable and the sheet would be cut into fourparts, each being folded four times.

70 mm (ImageTech) The widest gauge of motion picturefilm.

80 Ursae Majoris (Astron) See alcor.100 Hz scanning (ImageTech) A means of reducing TVflicker by storing every field and showing it twice atdouble rate.

802.11 (ICT) A family of specifications developed by theIEEE for wireless LAN technology. The 802.11 standardspecifies an over-the-air interface between a wireless clientand a base station or between two wireless clients. See wi-fi.

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AA (BioSci) Symbol for adenine.A (Eng) See ae.A (Phys) Symbol for ampere.A (Phys) Symbol for angstrom.A (Chem) Symbol for: (1) relative atomic mass (atomicweight); (2) helmholtz free energy.

A (Genrl) Symbol for area.A (Phys) Symbol for: (1) absolute temperature; (2)magnetic vector potential.

[A] (Phys) A strong absorption band in the deep red of thesolar spectrum (wavelength 762128 nm) caused by oxygenin the Earths atmosphere. The first of the fraunhoferlines.

a (Genrl) Symbol for atto-.a (Phys) Symbol for: (1) acceleration; (2) amplitude;(3) linear absorption coefficient.

a- (Genrl) Prefix signifying on. Also shortened form of ab-,ad-, an-, ap-.

a- (Chem) Abbrev for: (1) asymmetrically substituted; (2)ana-.

a (Genrl) See under alpha.a (Chem) Symbol for: (1) substitution on the carbon atom ofa chain next to the functional group; (2) substitution on acarbon atom next to one common to two condensedaromatic nuclei; (3) substitution on the carbon atom nextto the hetero-atom in a hetero-cyclic compound; (4) astereoisomer of a sugar.

a (Phys) Symbol for: (1) absorption coefficient; (2)acceleration; (3) angular acceleration; (4)attenuation coefficient; (5) fine-structure con-stant; (6) helium nucleus.

[a]Dt (Chem) Symbol for the specific optical rotation of a

substance at t1C, measured for the D lines of the sodiumspectrum.

aa (Geol) A term of Hawaiian origin for lava flows with arough, jagged surface.

Aalenian (Geol) The oldest stage of the Middle Jurassic. Seemesozoic.

AAMI (Med) Abbrev for age-associated memory impairment.A-amplifier (Acous) An amplifier associated with, orimmediately following, a high-quality microphone, as inbroadcasting studios. NB Not the same as class-aamplifier.

A & AEE (Aero) Abbrev for Aeroplane & ArmamentExperimental Establishment, at Boscombe Down, UK.

A and R display (Radar) See r-display.Ab (BioSci) Abbrev for antibody.ab (Build) Abbrev for as before in eg bills of quantities.ab- (ElecEng) Prefix to name of unit indicating derivation inthe CGS system, eg abampere.

ABA (BioSci) Abbrev for abscisic acid.abactinal (BioSci) Pertaining to that part of the surface of anechinoderm that lacks tube feet.

abacus (Arch) The uppermost part of a column capital orpilaster, on which the architrave rests.

abacus (MathSci) A bead frame. Used as an arithmeticcalculating aid.

abambulacral (BioSci) Pertaining to any of the five groovesalong which the tube feet of echinoderms are located.

abampere (Phys) Unit of electric current in the CGS electro-magnetic system of units. One abampere equals 10 A.

abamurus (Arch) A supporting wall or buttress, built to addstrength to another wall.

abandonment (MinExt) Voluntary surrender of legal rightsor title to a mining claim.

abandonware (ICT) Computing slang for software that isno longer distributed by its original publisher.

abapical (BioSci) Pertaining to, or situated at, the lowerpole: remote from the apex.

abatjour (Arch) An opening to admit light and generally todeflect it downwards; a skylight.

abattoir (Agri) A facility where animals are slaughtered andprepared for sale as meat.

abaxial (BioSci) Remote from the axis. Used to describe eg aleaf or petal that faced away from the axis during earlydevelopment (and usually, therefore, the under-surface ofan expanded leaf). Cf adaxial.

abaxial (ImageTech, Phys) Said of rays of light which do notcoincide with the optical axis of a lens system.

Abbe refractometer (Chem) An instrument for measuringdirectly the refractive index of liquids, minerals andgemstones.

ABC (ElecEng) Abbrev for aerial bunched conductors.ABC (ImageTech) Abbrev for automatic beam control.ABC proteins (BioSci) Membrane proteins involved in activetransport or regulation of ion channel function and havingan ATP binding cassette. A well-known example is the cysticfibrosis transmembrane conductance regulator (CFTR).

abdominal air sac (BioSci) Posterior part of the lung in birds.abdominal cavity (Med) See peritoneal cavity.abdominal gills (BioSci) In the aquatic larvae of manyinsects, paired segmental leaf-like or filamentous expan-sions of the abdominal cuticle for respiration.

abdominal limbs (BioSci) Segmented abdominal appen-dages in most Crustacea which are used for swimming,setting up currents of water for feeding and/or respiration,or for carrying eggs and young. In Diplopoda, segmentedambulatory appendages on the abdomen.

abdominal regions (Med) Nine regions into which thehuman abdomen is divided by two horizontal and twovertical imaginary planes, ie right and left hypochondriac,right and left lumbar, right and left iliac, epigastric,umbilical and hypogastric.

abdominoplasty (Med) A plastic surgery procedure for thestomach. Also tummy tuck.

abducens (BioSci) In vertebrates, the sixth cranial nerve,purely motor in function, supplying the rectus externusmuscle of the eye.

abduction (Med) The action of pulling a limb or part awayfrom the median axis.

abductor (BioSci) Any muscle that draws a limb or partaway from the median axis by contraction, eg the abductorpollicis, which moves the thumb outwards.

Abeggs rule (Chem) Empirical rule that the solubility ofsalts of alkali metals with strong acids decreases fromlithium to caesium, ie with increase of relative atomicmass, and those with weak acids follow the opposite order.Sodium chloride is an exception to this rule, being lesssoluble than potassium chloride.

Abeggs rule of eight (Chem) Rule that the sum of themaximum positive and negative valencies of an element iseight, eg sulphur in SF6 and H2S.

Abel flash-point apparatus (MinExt) A petroleum-testingapparatus for determining the flash point.

Abelian group (MathSci) A group in which the groupoperation is commutative. It is important in the study ofrings and vector spaces.

abelite (Chem) Explosive, composed mainly of ammoniumnitrate and trinitrotoluene.

aberrant (BioSci) Having characteristics not strictly inaccordance with the type.

aberration (Astron) An apparent change of position ofa heavenly body, due to the speed of light having afinite ratio to the relative velocity of the source and theobserver.

aberration (Phys) In an image-forming system, eg an opticalor electronic lens, failure to produce a true image, eg apoint object as a point image. Geometrical aberrationsinclude spherical aberration, coma, astigmatism, curvatureof the field and distortion. See chromatic aberration.

abhesive (Eng) A substance which prevents two materialssticking together, eg teflon on frying pans.

abiogenesis (BioSci) The development of living organismsfrom non-living matter; either the spontaneous generationof yeasts, bacteria, etc, believed before Pasteur, or thegradual process postulated for the early Precambrian inmodern theories of the origin of life. Also spontaneousgeneration.

abiotic (BioSci) Pertaining to non-living things.ABL (EnvSci) Abbrev for atmospheric boundary layer(panel).

ablation (Geol) (1) Any one of the processes by which snowand ice are lost from a glacier, mainly by melting andevaporation (sublimation). (2) Removal of surface layers ofa meteorite and tektites during flight.

ablation (Med) Removal of body tissue by surgery.ablative polymer (Eng) A material which degradescontrollably in an aggressive environment, esp on re-entryspacecraft. Extreme temperatures are reached on a heatshield, so it is protected with an ablation shield made of egsilicone polymer. The same principle is used in intumes-cent paints for fire resistance.

Abney law (Phys) Rule stating that if a spectral colour isdesaturated by the addition of white light, and if itswavelength is less than 570 nm, its hue then moves towardsthe red end of the spectrum, while if the wavelength ismore than 570 nm its hue moves towards the blue.

Abney level (Surv) Hand-held instrument in which anglesof steep sights are measured while simultaneously viewinga spirit-level bubble.

Abney mounting (Phys) A form of mounting for a concavediffraction grating, in which the eyepiece (or photographicplate holder) is fixed at the centre of curvature of thegrating and the slit can move around the circumference ofthe Rowland circle, to bring different orders of thespectrum into view.

abnormal glow discharge (Electronics) A discharge carry-ing current in excess of that which is required to cover thecathode completely with visible radiation.

abnormal reflection (ICT) Reflection from the ionosphereof a radio wave whose frequency is greater than thecritical frequency.

ABO blood group substances (BioSci) Large glycopeptideswith oligosaccharide side chains bearing ABO antigenicdeterminants identical to those of the erythrocytes of thesame individual, present in mucous secretions of personswho possess the secretor gene.

ABO blood group system (BioSci) The most important ofthe antigens of human red blood cells for bloodtransfusion serology. Humans belong to one of fourgroups: A, B, AB and O. The red cells of each group carryrespectively the A antigen, the B antigen, both A and Bantigens, or neither. Natural antibodies are present againstthe blood group antigen that is absent from the redcells. Thus persons of group A have anti-B, of group Bhave anti-A, of group O have anti-A and anti-B, and group

AB have neither. Before blood transfusion the bloodmust be cross-matched to ensure that red cells of onegroup are not given to a person possessing antibodiesagainst them.

abomasitis (Vet) Inflammation of the abomasum.abomasum (BioSci) In ruminant mammals, the fourth ortrue stomach. Also reed.

A-bomb (Phys) See atomic bomb.aboral (BioSci) Opposite to, leading away from, or distantfrom, the mouth.

abort (ICT) The unplanned failure of a program or a pieceof software.

abort (Space) To terminate a vehicles flight either by failureor by deliberate action to prevent dangerous consequences;if manned, a predetermined sequence of events is followedto ensure the safety of the crew.

abortifacient (Med) A term describing anything whichcauses artificial abortion; a drug which does this.

abortion (Med) (1) Expulsion of the fetus from the uterusduring the first three months of pregnancy. Abortion maybe spontaneous or induced. (2) Termination of thedevelopment of an organ.

abortive infection (BioSci) Viral infection of a cell in whichthe virus fails to replicate fully, or produces defectiveprogeny; the effect can still be cytopathogenic.

abortive transformation (BioSci) Temporary transfor-mation of a cell by a virus that does not becomeintegrated into the host DNA.

abradant (Eng) A substance, usually in powdered form,used for grinding. See abrasive.

abrade (Eng) To scratch or tear away two surfaces in contactby relative motion.

Abrams law (CivEng) Rule that the ratio of water tocement for chemical action to impart strength to concreteis 035:1.

abranchiate (BioSci) Lacking gills.abrasion (Eng) Surface damage due to an abrasive or torubbing contact.

abrasion (Geol) Mechanical wearing away of rocks byrubbing during movement.

abrasion (Med) A rubbed-away area of the surface coveringof the body, ie of skin or of mucous membrane.

abrasion hardness (MinExt) Resistance to abrasive wear,under specified conditions, of metal or mineral.

abrasive (Eng) A hard substance, usually in powdered form,used for the removal of material by scratching andgrinding, eg silicon carbide powder (carborundum).

abrasive blast cleaning (Build) A method for preparingsteel for painting whereby abrasive particles, eg copper slag,are projected under air pressure through a nozzle. Veryeffective in removing rust and mill scale, leaving an anchorpattern (a pattern of minute projections) on the substrateaffording good paint adhesion.

abrasive papers (Build) Special papers coated in gritused for flatting down. Supplied in a range of grits fromvery fine to coarse as either dry or waterproof abrasivepapers.

abrasive wear (Eng) Mechanism of wear due to thepresence in one or both surfaces of hard particles (egcarbide in steels), or to hard particles trapped betweenthem.

abreaction (Psych) In psychoanalytic theory, an intenseemotional outburst to a previously repressed experience;the therapeutic effect is known as catharsis.

A/B roll editing (ImageTech) Video editing using twosource players (A and B) enabling dubbing from both.Necessary if scenes are to be superimposed. See editcontroller, edit recorder, edit suite.

A/B roll printing (ImageTech) A method of film printingwith alternate scenes assembled in two rolls, each havingblack spacing equivalent in length to the omitted scene;double printing from the two allows the inclusion of fadeand dissolve effects and avoids visible splice marksbetween scenes in 16 mm printing.

Abel flash-point apparatus 4 A/B roll printing

ABS (Plastics) Abbrev for acrylonitrile-butadiene-styrene. Seecopolymer.

ABS brake (Autos) See antilock brake.abscess (Med) A localized collection of pus in infectedtissue, usually confined within a capsule.

abscisic acid (BioSci) A sesquiterpenoid plant growthsubstance (C15H20O4) with a variety of reported effects,eg inhibiting growth, causing stomatal closure andpromoting senescence, abscission and dormancy. Alsodormin. Abbrev ABA.

abscissa (MathSci) For rectilineal axes of co-ordinates, thedistance of a point from one axis measured in a directionparallel to another axis, usually horizontal. The signconvention is that measurements to the right from theaxis of ordinates are positive, measurements to the leftnegative. Pl abscissae. Cf cartesian co-ordinates.

abscission (BioSci) The organized shedding of parts of aplant by means of an abscission layer.

abscission layer (BioSci) In the abscission zone, a layerof cells the disjunction or breakdown of which causesabscission. Also separation layer.

abscission zone (BioSci) Zone at the base of leaf, petal,fruit, etc, that contains the abscission layer andprotective layer.

absolute (MathSci) (1) See modulus. (2) A conic (aquadric in three dimensions) formed by the assemblage ofthe points at infinity on a line (in general two points). Itsform determines the metrical properties of the geometricalsystem being operated. Thus in Euclidean geometry, theabsolute is the degenerate conic comprising the line atinfinity taken twice, while in non-Euclidean geometry, theabsolute is either a real conic (hyperbolic geometry) or animaginary conic (elliptic geometry).

absolute address (ICT) Code designation of a specificmemory location as determined by the hardware. Cfrelative address.

absolute age (Geol) The geological age of a fossil, mineral,rock or event, generally given in years. Preferred synonymradiometric age, isotopic age. See panel on radiometricdating.

absolute alcohol (Chem) Water-free ethanol; rel.d. 0793(1551C); bp 7841C; obtained from rectified spirit byadding benzene and refractionating. Very hygroscopic.

absolute ampere (Phys) The standard MKS unit of electriccurrent; replaced the international ampere in 1948. Seeampere.

absolute block system (CivEng) See block system.absolute ceiling (Aero) The height at which the rate ofclimb of an aircraft, in standard atmosphere, would bezero; the maximum height attainable under standardconditions.

absolute coefficient (MathSci) A coefficient with anabsolute value, ie a multiplier which is numerical ratherthan symbolic.

absolute configuration (Chem) The arrangement ofgroups about an asymmetric atom, esp a tetrahedrallybonded atom with four different substituents. See chir-ality, cahningoldprelog system.

absolute convergence (MathSci) The property of aninfinite series when the sum of absolute values converges;ieP

ar is absolutely convergent if the seriesPjarj is

convergent.absolute electrometer (Phys) A high-grade attracted-diskelectrometer in which an absolute measurement ofpotential can be made by weighing the attraction betweentwo charged disks against gravity.

absolute filter (Phys) A filter which removes mostparticulate matter from gases.

absolute humidity (EnvSci) See vapour concentration.absolute instrument (Phys) An instrument which measuresa quantity directly in absolute units, without the necessityfor previous calibration.

absolute lethal concentration (BioSci) The lowest con-centration of a substance in an environmental medium

which kills all the test organisms or species under definedconditions. Abbrev LC100.

absolute magnitude (Astron) See magnitude.absolute permeability (ElecEng) See permeability.absolute potential (Chem) The theoretical true potentialdifference between an electrode and a solution of its ions,measured against a hypothetical reference electrode, havingan absolute potential of zero, with reference to the samesolution.

absolute pressure (Phys) Pressure measured with respect tozero pressure, in units of force per unit area.

absolute reaction rate (Chem) The reaction rate deter-mined from statistical thermodynamics; uses the assump-tion of the theory of absolute reaction rates that the rate ofa chemical reaction is governed by the rate of crossing anenergy barrier or of forming an activated complex. Seearrhenius theory of dissociation.

absolute-rest precipitation tank (Build) A tank used forbatch treatment of sewage, as opposed to one taking acontinuous flow. After two- or three-hour settlement, thetop water is drawn off from above and the precipitatedsludge from below.

absolute temperature (Phys) Temperature measured withrespect to absolute zero, ie the zero of the kelvinthermodynamic scale of temperature, a scalewhich cannot take negative values. See kelvin, rankinescale.

absolute threshold (Psych) The minimal intensity of aphysical stimulus required to produce a response.

absolute units (Phys) Units derived directly from thefundamental units of a system and not based on arbitrarynumerical definitions. The differences between absoluteand international units were small; both are now super-seded by the definitions of si units.

absolute value (MathSci) See modulus.absolute wavemeter (ICT) A wavemeter in which thefrequency of the injected radio-frequency signal is bycalculation of physical properties (circuit elements ordimensions) of a resonant circuit line or cavity.

absolute weight (Phys) The weight (or mass) of a body in avacuum.

absolute zero (Phys) The least possible temperature for allsubstances. At this temperature the molecules of anysubstance possess no heat energy. A figure of 273151C isgenerally accepted as the value of absolute zero. Seekelvin.

absorbance (Chem) The logarithm of the ratio of theintensity of light incident on a sample to that transmittedby it. It is usually directly proportional to the concentra-tion of the absorbing substance in a solution. See beerslaw, transmittance.

absorbance (Paper, Textiles) The capacity of materialssuch as textile fibres and paper to absorb liquids. Seeregain.

absorbed dose (Radiol) Quantity of energy imparted byionizing radiation to a unit mass of biological tissue. Unitis the gray (Gy). See panel on radiation.

absorbency test (Paper, Textiles) Any test method formeasuring the capacity of materials such as textile fibresand paper to absorb liquids or fluids. Results are usuallyexpressed as the gain in weight of the test piece, thecapillary rise in a test strip in given time, or the timerequired to reach a predetermined capillary rise.

absorber (Phys) Any material which converts energy ofradiation or particles into another form, generally heat.Energy transmitted is not absorbed. Scattered energy isoften classed with absorbed energy. See absorptioncoefficient.

absorber rod (NucEng) See control rod.absorbing material (Phys) Any medium used for absorbingenergy from radiation of any type.

absorbing well (CivEng) A shaft sunk through animpermeable stratum to allow water to drain through toa permeable one.

ABS 5 absorbing well

absorptance (Phys) A measure of the ability of a body toabsorb radiation; the ratio of the radiant flux absorbed bythe body to that incident on the body. Formerlyabsorptivity, absorptive power.

absorptiometer (Chem) Apparatus for determining thesolubilities of gases in liquids or the absorption of light.

absorption (BioSci) (1) The uptake of a drug (see adme),or other compounds, eg nutrients from the intestinaltract, into the body. (2) The use of reagents to removeunwanted substances, eg antibodies or antigens, from amixture.

absorption band (Phys) A dark gap in the continuousspectrum of white light transmitted by a substance whichexhibits selective absorption. See fig. at absorptionspectrum.

absorption capacitor (ElecEng) A capacitor connectedacross a spark gap to damp the discharge.

absorption coefficient (Chem) The volume of gas,measured at stp, dissolved by unit volume of a liquidunder normal pressure (ie one atmosphere).

absorption coefficient (Phys) (1) At a discontinuity(surface absorption coefficient): (a) the fraction of theenergy which is absorbed; or (b) the reduction ofamplitude, for a beam of radiation or other wave systemincident on a discontinuity in the medium through whichit is propagated, or in the path along which it istransmitted. (2) In a medium (linear absorption coefficient),the natural logarithm of the ratio of incident and emergentenergy or amplitude for a beam of radiation passingthrough unit thickness of a medium. (The mass absorptioncoefficient is defined in the same way but for a thickness ofthe medium corresponding to unit mass per unit area.) NBTrue absorption coefficients exclude scattering losses, totalabsorption coefficients include them. See atomic absorp-tion coefficient.

absorption discontinuity (Phys) See absorption edge.absorption dynamometer (Eng) A dynamometer whichabsorbs and dissipates the power which it measures; eg theordinary rope brake and the Froude hydraulic brake. Cftransmission dynamometer.

absorption edge (Phys) The wavelength at which there isan abrupt discontinuity in the intensity of an absorptionspectrum for electromagnetic waves, giving the appearanceof a sharp edge in its photograph. This transition is due toone particular energy-dissipating process becomingpossible or impossible at the limiting wavelength. InX-ray spectra of the chemical elements the K-absorptionedge for each element occurs at a wavelength slightly lessthan that for the K-emission spectrum. Also absorptiondiscontinuity.

absorption hygrometer (EnvSci) An instrument by whichthe quantity of water vapour in air may be measured. Aknown volume of air is drawn through tubes containing adrying agent such as phosphorus pentoxide; the increase inweight of the tubes gives the weight of water vapour in theknown volume of air.

absorption inductor (ElecEng) See interphase trans-former.

absorption lines (Phys) Dark lines in a continuousspectrum caused by absorption by a gaseous element.The positions (ie the wavelengths) of the dark absorptionlines are identical to those of the bright lines given by thesame element in emission.

absorption nebula (Astron) See dark nebula.absorption plant (MinExt) Plant where oils are removedfrom natural gas by absorption in suitable oil.

absorption refrigerator (Eng) A plant in which ammoniais continuously evaporated from an aqueous solutionunder pressure, condensed, allowed to evaporate (soabsorbing heat) and then reabsorbed.

absorption spectrum (Phys) The system of absorptionbands or lines seen when a selectively absorbing substanceis placed between a source of white light and a spectro-scope. See kirchhoffs law. Fig. $

absorption spectrum Lines on a continuum.

absorption tubes (Chem) Tubes filled with solid absorbentfor the absorption of moisture (eg silica gel) and gases (egcharcoal).

absorption wavemeter (ElecEng) A wavemeter whichdepends on resonance absorption in a tuned circuit,constructed with very stable inductance and capacitance.

absorptive power (Phys) See absorptance.

absorptivity (Phys) See absorptance.abstraction (ICT) A principle of object-oriented design, inwhich common data or functionality are removed fromindividual objects into a self-contained layer, making themconsistently available from a single source.

abstraction (Psych) The mental process of arriving at anabstract idea or concept from specific examples.

AB toxin (BioSci) A multi-subunit toxin with two majorcomponents: an active (A) portion and a portion that isinvolved in binding (B) to the target cell. In well-knownexamples (cholera, diphtheria, and pertussis toxins), the Asubunit has ADP-ribosylating activity.

abundance (BioSci) See frequency, relative abun-dance.

abundance (Phys) For a naturally occurring element, theproportion or percentage of one isotope to the total. Alsoabundance ratio.

abundant number (MathSci) A natural number for whichthe sum of the proper factors is greater than the numberitself; eg 18 is abundant since 1 + 2 + 3 + 6 + 9 > 18. Cfdeficient number, perfect number.

abura (For) Tropical WAfrican hardwood (Mitragyna) witha fine, even texture.

abutment (CivEng) A structure provided to withstandthrust, eg end supports of an arch or bridge. See knapsackabutment.

abutment load (MinExt) In stoping or other deep-levelexcavation, weight transferred to the adjacent solid rock byunsupported roof.

abutting joint (Build) A timber joint whose plane is at rightangles to the fibres, the fibres of both joining pieces beingin the same straight line.

abyssal (EnvSci) A term describing the ocean floorenvironment between c.4000 and 6000 m. Also bathysmal.Cf bathyal, littoral.

abyssal deposits (Geol) Pelagic marine sediments, accu-mulating in depths of more than 2000 m including, withincreasing depth, calcareous oozes, siliceous oozes and redclay (500 m).

abyssal plain (Geol) A flat region of the deep ocean floorwith a slope of less than 1:1000.

abyssopelagic (EnvSci) Relating to the open waters of theabyssal zone.

abzyme (BioSci) A catalytic antibody, one that has enzymicactivity.

Ac (Chem) Symbol for actinium.Ac (Eng) The transformation temperature on heating of thephase changes of iron or steel, subscripts indicating thedesignated change, eg Ac1 is the eutectoid (7231C) and Ac3the ferrite/austenite phase boundary.

Ac (EnvSci) Abbrev for altocumulus.

AC-3 (ImageTech) TN for the digital audio coding used in35 mm motion picture film to provide six-channelsurround sound. It uses data blocks recorded opticallybetween the perforations, leaving room for a conven-tional soundtrack. It is also suitable for multi-channel TVaudio, as well as video software and home cinema.

ac (ElecEng) Symbol for alternating current.

absorptance 6 ac

ac- (Chem) Abbrev indicating substitution in the alicyclicring.

acacia (For) See myrtle.acacia gum (Chem) See gum arabic.acalculia (Psych) Inability to make simple mathematicalcalculations. Also dyscalculia.

acamprosate (Pharmacol) A drug thought to be beneficialin maintaining abstinence in alcohol-dependent patients.

Acanthamoeba (Med) A protozoon which can survive intapwater and may cause keratitis if contact lenses arewashed in tapwater.

acanthite (Min) An ore of silver, Ag2S, crystallizing in themonoclinic system. Cf argentite.

acantho- (Genrl) Prefix from Gk akantha, spine, thorn.Acanthocephala (BioSci) A phylum of elongate worms witha rounded body and a protrusible proboscis, furnishedwith recurved hooks. There is no mouth or alimentarycanal. The young stages are parasitic in various Arthro-poda, the adults in fish and aquatic birds and mammals.Thorny-headed worms.

acanthoma (Med) A tumour of epidermal cells.acanthosis nigricans (Med) A rare disease characterized bypigmentation and warty growths on the skin, oftenassociated with cancer of the stomach or uterus.

acariasis (Vet) Contagious skin disease caused by mites(acari).

Acarina (BioSci) An order of small Arachnida, with globular,undivided body. The immature stages (hexapod larvae)have six legs. The order is a large worldwide group,occupying all types of habitat, and of great economicimportance. Many are ectoparasitic. Mites and ticks.

acarophily (BioSci) A symbiotic association between plantsand mites. Also acarophytism.

acatamathesia (Psych) The inability to comprehend data(objects, language, etc) presented to the senses.

acaulescent (BioSci) Having a short stem.acauline (BioSci) Stemless or nearly so. Also acaulose.ac balancer (ElecEng) An arrangement of transformers orreactors used to equalize the voltages between the wires of amultiple-wire system. Also static balancer.

ac bias (Electronics) A high-frequency signal applied to amagnetic tape recording head along with the signal to berecorded. This stabilizes magnetic saturation and improvesfrequency response, at the same time reducing noise anddistortion. The bias signal frequency has to be many timesthe highest recording frequency.

AC-boundary layer (Acous, Aero) See stokes layer.accelerated ageing test (ElecEng) A stability test for cablesusing twice normal working voltage. It is claimed this givesquick results that correlate with service records.

accelerated fatigue test (Eng) Test which applies a cyclicloading schedule, which can be of varying frequency and/or amplitude, to a machine or component simulating itsloading in service, but at a higher rate, to determine its safefatigue life before it is reached in service. See panel onfatigue.

accelerated freeze-drying (FoodSci) freeze-drying inwhich the final sublimation under vacuum is accelerated byheat. Abbrev AFD.

accelerate-stop distance (Aero) The total distance, underspecified conditions, in which an aircraft can be brought torest after accelerating to critical speed for an enginefailure at take-off.

accelerating chain (Electronics) The section of an electronbeam tube or system, eg cathode-ray tube or electronmicroscope, in which electrons are accelerated by voltageson accelerating electrodes. Also used in particle accel-erators.

accelerating electrode (Electronics) An electrode in athermionic valve or cathode-ray tube maintained at a highpositive potential with respect to the electron source. Itaccelerates electrons in their flight to the anode but doesnot collect a high proportion of them.

accelerating machine (Electronics) See accelerator.

accelerating potential (Electronics) The potential appliedto an electrode to accelerate electrons from a cathode.

acceleration (Phys) The rate of change of velocity, expressedin metres (or feet) per second squared. It is a vectorquantity and has both magnitude and direction.

acceleration due to gravity (Phys) Acceleration withwhich a body would fall freely under the action of gravityin a vacuum. This varies according to the distance from theEarths centre, but the internationally adopted value is9806 65 m s2 or 321740 ft s2. Abbrev g. See helmertsformula.

acceleration error (Aero) The error in an airborne magneticcompass due to manoeuvring; caused by the verticalcomponent of the Earths magnetic field when the centre ofgravity of the magnetic element is displaced from normal.

acceleration stress (Space) The influence of acceleration(or deceleration) on certain physiological parameters of thehuman body. Humans can withstand transverse accelera-tions better than longitudinal ones, which have a profoundeffect on the cardiovascular system. The degree of tolerancealso depends on the magnitude and duration of theacceleration.

acceleration tolerance (Space) The maximum accelerationforce that an astronaut can withstand before blacking outor otherwise losing control.

accelerator (Aero) A device, similar to a catapult, butgenerally mounted below deck level, for assisting theacceleration of aircraft flying off aircraft carriers. Landversions have been tried experimentally.

accelerator (Autos) A pedal connected to the carburettorthrottle valve of a motor vehicle, or to the fuel injectioncontrol.

accelerator (BioSci) Any muscle or nerve that increases rateof action.

accelerator (Build) A hardener or catalyst mixed withsynthetic resins in two pack materials to speed up thehardening rate.

accelerator (Chem) (1) A substance which increases thespeed of a chemical reaction. See catalysis. (2) Asubstance which increases the efficient action of anenzyme. (3) Any substance effecting acceleration of thevulcanization process of rubber. The principal types arealdehyde derivatives of Schiff s bases: butyraldehyde-butylidene-aniline, di-orthotolyl-guanidine, diphenyl-guanidine, benzthiazyl disulphide, tetramethyl-thiurandisulphide and zinc dimethyl-dithiocarbamate.

accelerator (CivEng) Any substance mixed with cementconcrete for the purpose of hastening hardening.

accelerator (Electronics) Machine used to accelerate chargedparticles to very high energies. See betatron, cyclo-tron, linear accelerator, synchrocyclotron,synchrotron.

accelerator (ICT) A special circuit board that is placedwithin a computer to speed up some aspect of its operation.

accelerator (ImageTech) A chemical used to increase therate of development; eg sodium carbonate or borax.

accelerator pump (Autos) A small cylinder and pistonfitted to some types of carburettor, and connected to thethrottle so as to provide a momentarily enriched mixturewhen the engine is accelerated.

accelerometer (Acous, Electronics) A transducer used toprovide a signal proportional to the rate of acceleration ofa vibrating or other body, usually employing the piezo-electric principle. See pick-up head.

accelerometer (Aero) An instrument, carried in aircraft,guided missiles and spacecraft, for measuring accelerationin a specific direction. Main types are indicating,maximum-reading, recording (graphical) and counting(digital, totalling all accelerations above a set value). Seeimpact accelerometer, vertical-gust recorder.

acceptable daily intake (FoodSci) Estimated daily intake inmg kg1 of a substance (eg additive, residue) regarded ashaving no obvious harmful effect to humans during theirlifespan. Abbrev ADI.

ac- 7 acceptable daily intake

acceptance angle (Electronics) The solid angle withinwhich all incident light reaches the photocathode of aphototube.

acceptance test (ICT) The phase of the testing cycle inwhich an entire system is tested by a group of users,generally customers, in order to establish its acceptabilityfor purchase or implementation.

acceptor (Chem) (1) The reactant in an induced reactionwhose rate of reaction with a third substance is increasedby the presence of the inductor. (2) The atom whichaccepts electrons in a co-ordinate bond.

acceptor (Electronics) Impurity atoms introduced in smallquantities into a crystalline semiconductor and having alower valency than the semiconductor, from which theyattract electrons. In this way holes are produced, whicheffectively become positive charge carriers; the pheno-menon is known as p-type conductivity. See donor,impurity.

acceptor level (Electronics) See energy levels.access charge (ICT) A charge for access to a computer ortelecommunications network.

access eye (Build) A screwed plug provided in soil,waste and drain pipes at bends and junctions, to clear astoppage.

accessorius (BioSci) (1) A muscle that supplements theaction of another muscle. Also accessory. (2) In vertebrates,the eleventh cranial nerve or spinal accessory.

accessory bud (BioSci) A bud additional to a normalaxillary bud.

accessory cell (BioSci) (1) Cell other than a lymphocytethat takes part in an immune reaction, eg in antigenpresentation and/or by modulating the function of thelymphocyte. Usually a macrophage or dendritic cell. (2) Inplants, a subsidiary cell in a stomatal complex.

accessory chromosome (BioSci) See sex determination.accessory gearbox (Aero) A gearbox, driven remotely froman aero-engine, on which aircraft accessories, eg hydraulicpump and electrical generator, are mounted.

accessory glands (BioSci) Glands of varied structure andfunction in connection with genitalia, esp of Arthropoda.

accessory hearts (BioSci) See accessory pulsatoryorgans.

accessory minerals (Geol) Minerals which occur in small,often minute, amounts in igneous rocks; their presence orabsence makes no difference to classification and nomen-clature.

accessory pigments (BioSci) Pigments found in chloro-plasts and blue-green algae that transfer their absorbedenergy to chlorophyll a during photosynthesis. Theyinclude chlorophylls b, c and d, the carotenoids and thephycobilins.

accessory plates (Min) Quartz wedge, gypsum plate andmica plate. Used with petrological microscope to helpdetermine the optical character of a mineral as an aid in itsexamination.

accessory pulsatory organs (BioSci) In some insects andmolluscs, sac-like contractile organs, pulsating indepen-dent hearts; variously situated on the course of thecirculatory system. Also accessory hearts.

accessory shoe (ImageTech) Mounting forming part of thecamera body onto which separate units such as flash gunsand rangefinders may be fitted.

access time (ICT) The time interval between the instant atwhich data are called from memory and the instant atwhich the data can be used. It can vary from microsecondswith fast store to minutes with magnetic tape.

access to store (ICT) Entry or extraction of data from amemory location. The method and speed of access dependson the type of memory. See backing store, fast store,random access memory, serial access memory.

Accipitriformes (BioSci) An order comprising the diurnalbirds of prey and raptors. It Includes eagles, harriers,vultures and buzzards. See also falconiformes, whichare sometimes considered a separate order.

ac circuit (ElecEng) A circuit which passes alternatingcurrent as opposed to direct current, eg it may have acapacitor in series, which blocks direct current.

acclimatization (BioSci) Adaptation to environmentalstress; reversible physiological adjustment in an organismwhen moved to a new environment, usually taking days orweeks. Also acclimation. See hardening.

accommodation (Med) The natural alteration of theeffective focal length of the eye in order to see objectsdistinctly at varying distances. The range of vision for ahuman eye is from about 250 mm to infinity. Power ofaccommodation usually diminishes with advancing age.

accommodation (Psych) The creation of new cognitiveschemas when objects, experiences or other information donot fit with existing schemas.

accommodation rig (MinExt) Offshore rig with sleeping,supply and recreational facilities.

ac commutator motor (ElecEng) An ac motor whichembodies a commutator as an essential part of itsconstruction. See ac series motor, compensatedinduction motor, repulsion motor, schragemotor.

accordion (Print) US for concertina fold.accoucheur (Med) A physician who practises midwifery; anobstetrician.

accretion (Astron) The process in which a celestial body,particularly an evolved star in a binary system, is enlargedby the accumulation of extraneous matter falling in undergravity.

accretion (BioSci) External addition of new matter: growthby such addition.

accretion (Geol) The process of enlargement of a continentby the tectonic coalescence of exotic crustal fragments.

accretion disc (Astron) The disc of material at the edge of ablack hole, which has been attracted from a neighbouringstar and which emits X-rays as its inner edge disappearsinto the gravitational field of the hole.

accumulated temperature (EnvSci) The integrated pro-duct of the excess of air temperature above a thresholdvalue and the period in days during which such excess ismaintained.

accumulation point (MathSci) One of a set of points, suchthat every neighbourhood of it includes at least one pointof the set. Also limit point.

accumulator (ICT) Special storage register associated withthe arithmetic logic unit, used for holding the resultsof a computation or data transfer.

accumulator (ElecEng) Voltaic cell which can be chargedand discharged. On charge, when an electric current ispassed through it into the positive and out of the negativeterminals (according to the conventional direction of flowof current), electrical energy is converted into chemicalenergy. The process is reversed on discharge, the chemicalenergy, less losses in both potential and current, beingconverted into useful electrical energy. Accumulatorstherefore form a useful portable supply of electric power,but have the disadvantages of being heavy and of being atbest 70% efficient. More often known as battery; alsoreversible cell, secondary cell, storage battery.

accumulator (Eng) Bottle or other reinforced reservoir forstoring pressurized gas or fluid during moulding. Its usehelps conserve energy during injection moulding cycle.

accumulator box (ElecEng) A vessel usually made of plasticwhich contains the plates and electrolyte of an accumulator.

accumulator grid (ElecEng) The lead grid which forms oneof the plates of a leadacid accumulator having pastedplates.

accumulator traction (ElecEng) See battery traction.accumulator vehicle (ElecEng) See battery traction.ACE inhibitor (Pharmacol) Abbrev for an angiotensin-converting enzyme inhibitor.

acellular (BioSci) Not partitioned into cells. Sometimes usedfor unicellular but also for multinucleate or coenocytic. Seecoenocyte.

acceptance angle 8 acellular

acenaphthenequinone (Chem) Chemical, crystallizing inyellow needles, sparingly soluble in water. Forms the basisof scarlet and red vat dyes of the Ciba type.

acentric (BioSci) Having no centromere; applied tochromosomes and chromosome segments.

acentrous (BioSci) Having a persistent notochord with novertebral centra, as in the Cyclostomata.

acephalous (BioSci) Showing no appreciable degree ofcephalization: lacking a head region, as in Pelecypoda.

acervulus (BioSci) A dense cushion-like mass of conidio-phores and conidia formed by some fungi. Adj acervulate.

acet-, aceto- (Genrl) Prefixes from Lt acetum, vinegar.acetabulum (BioSci) (1) In Platyhelminthes, Hirudinea andCephalopoda, a circular muscular sucker. (2) In insects, athoracic aperture for insertion of a leg. (3) In vertebrates, afacet or socket of the pelvic girdle with which the pelvic finor head of the femur is articulated. (4) In ruminantmammals, one of the cotyledons of the placenta.

acetal (Chem) (1) 1,1-diethoxy ethane. CH3CH(OC2H5)2. Bp1041C. A colourless flammable liquid used as a solvent. (2)A term applied to any compound of the type RCH(OR0)2,where R and R0 are organic radicals and R may be hydrogen.

acetaldehyde (Chem) Ethanal. CH3CHO. A colourless,ethereal, pungent liquid, bp 211C, mp 1211C, rel.d. 08,oxidation product of ethanol. An intermediate forproduction of ethanoic acid; also an important rawmaterial for the synthesis of organic compounds.

acetaldehyde

C CO

H

H

H

H

H

acetal resin (Chem) See polyoxymethylene.acetals (Chem) The dehydration products of aldehydes, withan excess of alcohols present. Also 1,1-dialkoxyalkanes.

acetamide (Chem) Ethanamide. CH3CONH2. A primaryamide of ethanoic acid, crystalline, soluble in water andalcohol, mp 821C, bp 2221C.

acetate fibres (Textiles) Artificial fibres; continuous fila-ments and staple fibres manufactured from celluloseacetate produced from cotton linters or wood pulp.Between 74 and 92% of the hydroxyl groups in the originalcellulose are acetylated. For triacetate fibres the cellulose ismore highly acetylated.

acetate film (ImageTech) Film with its photographicemulsion coated on a base of cellulose triacetate, of lowflammability. Also non-flam film, safety film.

acetates (Chem) Ethanoates. Salts of acetic (ethanoic) acid;eg sodium acetate. Also esters of acetic acid.

acetic acid (Chem) Ethanoic acid. CH3COOH. Synthesizedfrom acetylene (ethyne), also obtained by the destructivedistillation of wood and by the oxidation of ethanol; mp1661C, bp 1181C, rel.d. (201C) 10497. Main ingredient ofvinegar and used as a food preservative. See preservationindex.

acetic anhydride (Chem) Ethanoic anhydride. (CH3CO)2O.The anhydride of acetic acid. Colourless liquid, bp 1371C.Used industrially for preparation of cellulose acetate andacetylsalicylic acid. Valuable laboratory acetylating agent.

acetic fermentation (Chem) The fermentation of diluteethanol solutions by oxidation in presence of bacteria, espBacterium aceti. Acetic acid is formed.

acetin (Chem) Monoacetin, glyceryl monoacetate.CH3COOC3H5(OH)2. Bp 1301C, rel.d. 122, a colourlesshygroscopic liquid; used as an intermediate for explosives,a solvent for basic dyestuffs and a tanning agent.

aceto- (Genrl) See acet-.acetogens (BioSci) Autotrophic bacteria that live in certainbasalts. They use hydrogen gas for energy and derivecarbon from inorganic carbon dioxide. They excrete simpleorganic compounds that other bacteria consume.

acetone (Chem) CH3COCH3. Bp 561C, of ethereal odour, avery important solvent and basis for organic synthesis.Acetone is the simplest saturated ketone and is a usefulsolvent for acetylene. It is found in the blood and urine ofpatients with uncontrolled diabetes mellitus or in starva-tion. Also propanone.

acetone cyanhydrin (Chem) 2-hydroxy 2-methyl propano-nitrile. (CH3)2C(OH)CN. Addition product of acetone andhydrogen cyanide.

acetone resin (Chem) A synthetic resin formed by thereaction of acetone with another compound, such asphenol or formaldehyde.

acetonitrile (Chem) CH3CN. A polar organic solvent.acetonuria (Med) See ketonuria.acetophenone (Chem) Phenyl methyl ketone. C6H5COCH3.Mp 201C, bp 2021C, rel.d. 103, large colourless crystals orliquid, soluble in most organic solvents, insoluble in water;used for organic synthesis and in perfumery.

acetoxyl group (Chem) The group CH3COO.acetylation (Chem) Ethanoylation. Reaction which has theeffect of introducing an acetyl radical (CH3CO) into anorganic molecule.

acetylators (BioSci) Individuals who can acetylate ormetabolize substances in the liver such as many commondrugs. A single gene mutation determines whether anindividual is a fast or slow acetylator.

acetylcelluloses (Chem) See cellulose acetates.acetyl chloride (Chem) Ethanoyl chloride. CH3COCl. Mp1121C, bp 511C, rel.d. 1105. Colourless liquid, ofpungent odour, used for synthesis, in particular forintroducing the acetyl group into other compounds.

acetylcholine (BioSci) An important neurotransmitter,particularly at the neuromuscular junction but alsoinvolved in nervenerve transmission (at chemicalsynapses) in the brain.

acetylcholine esterase (BioSci) An enzyme, found in thesynaptic clefts of cholinergic synapses, that breaks downthe neurotransmitter acetylcholine into acetate and cho-line, thus limiting the size and duration of the postsynapticpotential. Many nerve gases and insecticides are potentacetylcholine esterase inhibitors, and thus prolong thetimecourse of postsynaptic potentials.

acetylcholine esterase inhibitors (Pharmacol) A classof drugs that inhibit the breakdown of acetylcholine andthus increase availability as a neurotransmitter in thecentral nervous system. This appears to be beneficialin treatment of Alzheimers disease. Examples aredonezepil, rivastigmine. Up to one-half of patients treatedwith these drugs show a slower rate of decline incognition.

acetyl CoA (BioSci) The acetylated form of coenzyme A;important in intermediary metabolism, particularly in thetca cycle.

acetylene (Chem) Ethyne. HCCH. A colourless, poisonousgas, owing its disagreeable odour to impurities; soluble inethanol, in acetone (25 times its volume at stp) and inwater. Bp 841C, rel.d. 091. Prepared by the action ofwater on calcium carbide and catalytically from naphtha.Used for welding, torches, illuminating (historically), aceticacid synthesis and for manufacturing derivatives.

acetyl group (Chem) Ethanoyl group. CH3CO. Theradical of acetic acid.

acetylide (Chem) Ethynide. Carbide formed by bubblingacetylene through a solution of a metallic salt, eg cuprousacetylide, Cu2C2. Violently explosive compounds.

acetylsalicylic acid (Chem) C6H4(OCOCH3) COOH. Mpapproximately 1281C. Used in medical and veterinarypractice as an analgesic, antipyretic and antirheumatic. Theacid or its salts are the active components of aspirin.

aceval (Aero) Abbrev for air combat evaluation.ACF diagram (Geol) A triangular diagram used to representthe chemical composition of metamorphic rocks. Thethree corners of the diagram are Al2O3, CaO andFeO+MgO.

acenaphthenequinone 9 ACF diagram

ac generator (ElecEng) An electromagnetic generator forproducing alternating emf and delivering ac to an outsidecircuit. See alternator, induction generator.

achaenocarp (BioSci) See achene.achalasia (Med) Failure to relax.achalasia of the cardia (Med) Failure to relax on the partof the sphincter round the opening of the oesophagus intothe stomach.

achene (BioSci) A dry indehiscent, one-seeded fruit, formedfrom a single carpel and with the seed distinct from thefruit wall. Also achaenocarp, akene.

Achernar (Astron) A conspicuous blue-white star in theconstellation Eridanus, the ninth brightest star in the sky.Distance 35 pc. Also Alpha Eridani.

Achilles tendon (BioSci) In mammals, the united tendon ofthe soleus and gastrocnemius muscles.

achlorhydria (Med) Absence of hydrochloric acid fromgastric juice.

acholuric jaundice (Med) See spherocytosis.achondrite (Geol) A type of stony meteorite which comparesclosely with some basic igneous rocks such as eucrite.

achondroplasia (Med) Dwarfism characterized by shortnessof arms and legs, with a normal body and head. Adjachondroplastic.

achroglobin (BioSci) A colourless respiratory pigmentoccurring in some Mollusca and some Urochorda.

achroite (Min) See tourmaline.achromatic lens (Phys) A lens designed to minimizechromatic aberration. The simplest form consists oftwo component lenses, one convergent, the other diver-gent, made of glasses having different dispersive powers,the ratio of their focal lengths being equal to the ratio ofthe dispersive powers.

achromatic prism (Phys) An optical prism with a minimumof dispersion but a maximum of deviation.

achromatic sensation (Phys) A visual perception of grey.Represented by the equal energy point on a chromaticitydiagram.

achromatic stimulus (Phys) Stimulus which produces anachromatic sensation.

achromatopsia (BioSci) Absence of colour vision, the normin many species but rare in humans; due to an absence ofcones in the retina.

achylia gastrica (Med) Complete absence of pepsin andhydrochloric acid from the gastric secretion.

acicle (BioSci) A stiff bristle, or slender prickle; sometimeswith a glandular tip.

aciclovir (Pharmacol) An antiviral drug used to treat herpesvirus infections such as chickenpox, shingles, cold soresand herpes. It inhibits DNA polymerase, but does noteliminate the virus. It is effective only if given at the veryoutset of the infection. Formerly acyclovir.

acicular (BioSci, Min) Needle-shaped; used to describe theneedle-like habit of crystals.

acid (Chem) Normally, a substance which: (1) dissolves inwater with the formation of hydrogen ions; (2) dissolvesmetals with the liberation of hydrogen gas; or (3) reactswith a base to form a salt. More generally, a substancewhich tends to lose a proton (bronstedlowry theory)or to accept an electron pair (see lewis acids and bases).

acid (Geol) See acid rock.acid amides (Chem) A group of compounds derived froman acid by the introduction of the amino group in place ofthe hydroxyl radical of the carboxyl group.

acid anhydride (Chem) Compound generating a hydroxylicacid on addition of (or derived from the acid by removalof) one or more molecules of water, eg sulphur (VI) oxide:SO3 +H2O-H2SO4.

acid azides (Chem) The acyl derivatives of hydrazoic acid,obtainable from acid hydrazides by treatment withnitrous acid. They are very unstable.

acid brittleness (Eng) The brittleness developed in steel in apickling bath, through evolution of hydrogen. Cf hydro-gen embrittlement.

acid chlorides (Chem) Compounds derived from acids bythe replacement of the hydroxyl group by chlorine.

acid cure (MinExt) In extraction of uranium from its ores,lowering of gangue carbonates by puddling with sulphuricacid before leach treatment.

acid deposition (EnvSci) Acid compounds emitted into theatmosphere which then return to the surface either in theform in which they were discharged or as new compoundsformed by reaction in the atmosphere. Includes drydeposition, usually of sulphur and nitrogen oxides nearthe source, and wet deposition, which follows whenacids are washed from the atmosphere by precipitation.This includes acid rain and occult deposition.

acid drift (MinExt) The process by which ores, pulps andproducts become acidic through pick-up of atmosphericoxygen through standing.

acid dyes (ImageTech) Dyes which have their colourassociated with the negative ion or radical.

acid egg (ChemEng) A pump for sulphuric acid, of simpleand durable construction, with few moving parts. The acidis run into a pressure vessel, usually egg-shaped, fromwhich it can be forcibly expelled by compressed air.

acid esters (Chem) Compounds derived from acids inwhich part of the replaceable hydrogen has been exchangedfor an alkyl radical.

acid fixer (ImageTech) Fixing solution (hypo) with theaddition of an acid (sodium bisulphite or potassiummetabisulphite) to prevent staining.

acid growth hypothesis (BioSci) The hypothesis thatauxin-stimulated plant cell elongation results fromincreased proton extrusion (see proton-translocatingatpase) with an increased wall extensibility as a result ofits lower pH.

acid hydrazides (Chem) Hydrazine derivatives into whichan acyl group has been introduced.

acid hydrolases (BioSci) Hydrolytic enzymes that operatebest in acid conditions. Usually refers to the phosphatases,glycosidases, nucleases and lipases that are found in thelysosome and that are responsible for digesting phagocy-tosed material.

acidimetry (Chem) The determination of acids by titrationwith a standard solution of alkali. See titration,volumetric analysis.

acidity (Chem) (1) The extent to which a solution is acid,normally expressed as its ph value. Cf alkalinity. (2) Theconcentration of any species in a solution which istitratable by a strong base.

acidity regulator (FoodSci) A substance used to increase,reduce or stabilize the acidity of a product or anintermediate product. To retain pH within specified range,a buffer salt (usually the sodium or calcium salt of a fattyacid) may be added.

acidizing (MinExt) Improving the flow of oil from alimestone formation by pumping acid into it.

acid mine water (MinExt) Water containing sulphuric acidas a result of the breakdown of the sulphide minerals inrocks. Acid mine water causes corrosion of miningequipment, and may contaminate water supplies intowhich it drains.

acidophiles (BioSci) Bacteria able to grow at very low pH(o1). Their enzymes function under these extremeconditions and can be used in biotechnological processesinvolving an environment that would inhibit normalenzymes. Cf thermophiles.

acidophilic (BioSci) (1) Cells or tissues that are easilystained with acid dyes. (2) Micro-organisms or plants thatflourish in an acidic environment.

acidosis (Med) A condition in which the hydrogen ionconcentration of blood and body tissues is increased fromthe normal range of 3643 nmol l1. Respiratory acidosis iscaused by retention of carbon dioxide by the lungs;metabolic acidosis by retention of non-volatile acids(renal failure, diabetic ketosis) or loss of base (severediarrhoea).

ac generator 10 acidosis

acid process (Eng) A steel-making process in which thefurnace is lined with a siliceous refractory, and for whichiron low in phosphorus is required, as this element is notremoved. See basic process.

acid process (Paper) Any pulp digestion process utilizing anacid reagent, eg a bisulphite liquor with some free sulphurdioxide.

acid radical (Chem) A molecule of an acid minus replace-able hydrogen.

acid rain (EnvSci) A form of wet deposition in which acidmolecules or particles in the atmosphere are returned to thesurface having been washed out by rain or snow (acid snow)as it falls. The unnatural acidity (pH 355) is caused mainlyby the oxides of sulphur and nitrogen from the burning ofcoal and oil. See panel on atmospheric pollution.

acid refractory (Eng) See silica.acid resist foils (Eng) Blocking foils for use in etchingmetal. The foil is stamped onto paper and the excess foilblocked onto the metal rule or other object which is thenexposed to an acidic etching fluid such as ferric chloride.

acid rock (Geol) An igneous rock with >63% quartz.acid salts (Chem) Salts formed by replacement of part of thereplaceable hydrogen of the acid.

acid slag (Eng) Furnace slag in which silica and aluminaexceed lime and magnesia.

acid smut (EnvSci) See acid soot.acid soil complex (BioSci) Combination of aluminiumand/or manganese toxicity with calcium deficiency thataffects a relatively calcicole (or non-calcifuge) plantgrowing on an acid soil; preventable horticulturally byliming. Cf lime-induced chlorosis.

acid solution (Chem) An aqueous solution containing morehydroxonium ions than hydroxyl ions, ie with ph valueo7; one which turns blue litmus red.

acid soot (EnvSci) A pollutant, consisting of particles ofcarbon bound together by water containing sulphuric acid,formed as a by-product of the incomplete combustion ofcarbon-based fuel. Also acid smut.

acid steel (Eng) Steel made by an acid process.acid stop (ImageTech) Weak acid processing solution usedimmediately after the developer to halt its chemicalactivity and neutralize it before fixing. Also stop-bath.

acid value (Chem) The measure of the free acid content ofeg vegetable oils and resins, indicated by the number ofmilligrams of potassium hydroxide (KOH) required toneutralize 1 g of the substance.

aciniform (BioSci) Berry-shaped; eg in spiders, the acini-form glands producing silk and leading to the median andposterior spinnerets.

acinostele (BioSci) A protostele in which the xylem is star-shaped in cross-section with phloem between the arms as inthe roots of most seed plants and the stems of Lycopodium.

Ackermann steering (Autos) An arrangement whereby aline extended from the track-arms, when the wheels are setstraight ahead, should meet on the chassis centre-line at2/3 of the wheelbase from the front, allowing the inner stubaxle to move through a greater angle than the outer.

Acker process (Chem) A process for production of sodiumhydroxide. Molten sodium chloride is electrolysed, using amolten lead cathode, and the resulting leadsodium alloy isdecomposed by water, yielding pure lead and pure sodiumhydroxide. Obsolete.

acknowledgement signal (ICT) A signal transmitted alonga circuit from B to A when triggered by a signal from A to B.

A-class insulation (ElecEng) Insulating material which willwithstand temperatures up to 1051C.

ACM (ICT) Abbrev for Association for Computing Machinery.US professional association.

ac magnet (ElecEng) Electromagnet excited by alternatingcurrent having normally a laminated magnetic circuit. Seeshaded pole.

acme screw-thread (Eng) A thread having a profile angle of291 and a flat crest and root, used eg for lathe lead screwfor easy engagement by a split nut.

acmite (Min) A variety of aegirine; also used for theNaFe+3 Si2O6 end-member.

ac motor (ElecEng) An electric motor which operates from asingle or polyphase alternating current supply. Seecapacitor motor, induction motor, synchronousmotor.

acne (Med) Inflammation of a sebaceous gland. Pimples inadolescents are commonly due to infection with the acnebacillus.

acnode (MathSci) See double point.acoelomate (BioSci) A term for an animal without acoelom, an acoelomate animal.

acoelomate triploblastica (BioSci) Animals with threeembryonic cell layers but no coelom. They consist of thePlatyhelminthes, Nematoda and some minor phyla, ie allthe helminth phyla.

acoelomatous (BioSci) See acoelomate.acoelous (BioSci) Lacking a gut cavity.acontia (BioSci) In Anthozoa, free threads, loaded withnematocysts, arising from the mesenteries or the mesen-teric filaments, and capable of being discharged via themouth or via special pores.

acotyledonous (BioSci) The embryo of a vascular plant,having no cotyledons.

acoustic absorption (Acous) Transfer of energy intothermal energy when sound is incident at an interface.

acoustic absorption factor (Acous) The ratio of theacoustic energy absorbed by a surface to that which isincident on the surface. For an open window this can be100, for painted plaster 002. The value varies with thefrequency of the incident sounds, eg for 2 cm glass fibre itis 004 at 125 Hz, 080 at 4000 Hz. Also acoustic absorptioncoefficient.

acoustical mass (Acous) The quantity M, where oM is thepart of the acoustical reactance which corresponds to theinductance of an electrical reactance: o is the pulsatance,given by 2pf where f is the frequency in hertz. Alsoacoustical inertia.

acoustical stiffness (Acous) For an enclosure of volume V,the quantity given by S = rc2/V, where c is velocity ofpropagation of sound and r is density. It is assumed thatthe dimensions of the enclosure are small compared withthe sound wavelength and that the walls around thevolume do not deflect.

acoustic amplifier (Acous) An amplifier of mechanicalvibrations.

acoustic branch (Phys) A branch of the dispersion curve(frequency o against wavenumber q) for crystal latticevibrations for which o is proportional to q for small q. Fora crystal containing n atoms per unit cell, the dispersioncurve has 3n branches of which three are acoustic branches.The branches are characterized by different patterns ofmovement of the atoms. See optic branch.

acoustic centre (Acous) The effective source point of thespherically divergent wave system observed at distantpoints in the radiation field of an acoustic transducer.

acoustic compliance (Acous) The reciprocal of theacoustical stiffness.

acoustic construction (Build) Building construction whichaims at the control of transmission of sound, or ofmechanical vibration giving rise to sound, particularlyunwanted noises. The parts of the structure are separatedby air spaces or acoustic absorbing material and can bedecoupled by the interposing of springs. Also discontinuedconstruction.

acoustic coupler (ICT) A device that enables a digital signalto be transmitted over the telephone network using anordinary telephone handset.

acoustic delay line (ICT) A device, magnetostrictive orpiezoelectric, eg a quartz bar or plate of suitable geometry,which reflects an injected sound pulse many times withinthe body.

acoustic distortion (Acous) Distortion in sound-reprodu-cing systems.

acid process 11 acoustic distortion

acoustic emission (Eng) Non-destructive-testing method ofinvestigating deformation and failure processes in materialsby the signals generated when the elastic waves released bythem are detected at the materials surfaces.

acoustic feedback (ICT) Instability or oscillation in asecond reproduction system caused by the microphone orpick-up receiving vibrations from the loudspeaker.

acoustic filter (Acous) Filter which uses tubes andresonating boxes in shunt and series as reactance elements,providing frequency cut-offs in acoustic wave transmission,as in an electric wave filter.

acoustic grating (Acous) A diffraction grating for produc-tion of directive sound. Spacings are much larger than inoptical gratings owing to the longer wavelength of soundwaves. Both transmission and reflection gratings are used.

acoustic impedance (Acous) The complex ratio of soundpressure on surface to sound flux through surface, havingimaginary (reactance) and real (resistance) components,respectively. Unit is the acoustic ohm.

acoustic interferometer (Acous) Instrument in whichmeasurements are made by study of interference patternset up by two sound or ultrasonic waves generated at thesame source.

acoustic lens (Acous) A system of slats or discs to spread orconverge sound waves.

acoustic microscope (Acous) Microscope based on acousticwaves (longitudinal compressions and rarefactions ofdensity) at microwave frequencies: the interaction of anacoustic wave with a material is sensitive to its elasticproperties. Images can be created by modulating a displaywith the intensity received by a detector/specimen systemscanned synchronously (ultrasonic imaging). Couplingbetween electrical signals and acoustic vibrations exploitsthe piezoelectric effect.

acoustic model (Acous) A scale model of a room (egconcert hall) or structure which is used to measurequalities important for architectural acoustics and noisecontrol, eg sound distribution. The scale is typicallybetween 1:10 and 1:20. In order to adjust the wavelength,the frequency has to be increased by a factor 1020.

acoustic ohm (Acous) Unit of acoustic resistance, reactanceand impedance, equal to 105 Pa s m3.

acousticolateral system (BioSci) In vertebrates, afferentnerve fibres related to the neuromast organs and to the ear;receptors in aquatic forms of relatively slow vibrations.

acoustic perspective (Acous) The quality of depth andlocalization inherent in a pair of ears, which is destroyed ina single channel for sound reproduction. It is transferablewith two microphones and two telephone ear receiverswith matched channels, and more adequately realized withthree microphones and three radiating receivers with threematched channels.

acoustic plaster (Build) Rough or flocculent plaster whichhas good acoustic absorbing properties and which can beused for covering walls. Added to the mix is finealuminium, which evolves gas on contact with water andso aerates the mass. These tiny holes lower the acousticimpedance and so reduce the reflection of incident soundwaves.

acoustic pressure (Acous) See sound pressure.acoustic radiator (Acous) A device to generate and radiatesound. The most common radiators are: (1) vibratingela