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http://www.ivorcatt.co.uk/x32a7.pdf
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IEE Science, Education and Technology IEE The history of electrical engineering 26th Weekend Meeting 10-12 July 1998 University of East Anglia DIGEST OF PAPERS PRESENTED
Organised by Professional Group D7 (History of technology) HEE/26 --------------------------------------------------------------------------------------------------------------------------------------------------------- LIST OF CONTENTS Introduction (Dr Colin Hempstead and Mr Johannes Hock) 1 Inventiveness and the thought processes of the engineer: Mr Jack Bridge 2 A difficulty in electromagnetic theory Dr Arnold Lynch and Mr Ivor Catt 3 4 12 P2/1 A DIFFICULTY IN ELECTROMAGNETIC THEORY by Arnold Lynch and Ivor Catt
We seem to have two different systems of electrical theory almost but not quite independent of each other. The difficulty has existed for more than a hundred years but appeared unimportant until the last twenty years or so. …. …. P2/2 Now we describe a problem which combines the two types of theory and shows the difficulty mentioned in the title of this paper. It arose about twenty years ago when fast-operating silicon chips were connected to one another. We idealise the problem slightly. Imagine a coaxial transmission line terminated by a matched load at the far end; and for simplicity let it be evacuated, and of very low resistance. Apply a step voltage to its input; a wave travels along it with the velocity of waves in free space. So after a time a current begins to flow in the terminating load; that is, electrons start to move through it. The problem is - where did they come from? Not from the input, because electrons have finite mass and so they cannot travel at the velocity of waves in free space. (Remember that we are considering a step voltage, not an alternating one.) One of us sent the problem to various people who might have been expected to provide an answer, and the responses were mainly of two kinds (ref. 1): (1) that the wave causes radial movements in the line as it passes over them, and that electrons displaced in this way at the far end make up the current; or (2) that electrons move along the line, with velocity less than the wave, but push other electrons on in front of them, keeping pace with the wave. This problem was mentioned in the Institution's Wheatstone Lecture last December. The lecturer said that electrons in a metal travel only slowly but that they can transmit a fast electromagnetic wave by "nudging" their neighbours ("nudging" was his word for it). Our comments on this are: each atom in a metal contributes a few free electrons, so there are rather more electrons than atoms and therefore they are spaced from each other by a little less than the spacing of the atoms - say about a tenth of a nanometre. The size of an electron is not known, but it is presumably much smaller than an atomic nucleus, which is about a millionth of a nanometre. That is, the electrons are spaced apart by more than 100,000 times their diameter. So they cannot deliver a nudge without moving, and they cannot move instantaneously because of their mass. . . REFERENCES
Dr. Arnold Lynch is an Honorary Research Fellow in the Dept. of Electronic Engineering, University College, London; correspondence can be addressed to him at 8 Heath Drive, Potters Bar, Herts, EN6 1EH |