[Copyright notice]

Published by the Institution of Electrical Engineers, London and New York, ISBN 0-85296208-8. The cover was designed by GWA Design Consultants. This publication is copyright under the Berne Convention and the International Copyright Convention All rights reserved Apart from any copying under the UK Copyright Act 1956, part 1 section 7, whereby a single copy of an article may be supplied, under certain conditions, for the purposes of research or private study, by a library of a class prescribed by the UK Board of Trade Regulations (Statutory Instruments, 1957 No 868), no part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means without the prior permission of the copyright owners Permission is, however, not required to copy abstracts of papers or articles on condition that a full reference to the source is shown. Multiple copying of the contents of the publication without permission is always illegal. (c) 1979 The Institution of Electrical Engineers.