FLEMING DOES NOT RULE OK!
These could be what Physics World
calls Lateral Thoughts, because I originally wrote this horizontally with my leg in plaster. Here follow some snapshots of my journey through Physics, which is not a straightforward one like that of Marco Polo, but a meandering one like that of the 14th century Moroccan traveller Ibn Battuta
. But this is no random selection, but a selected album entitled “Electromagnetism.”
The prologue to this tale finds me as a pre-teenager in the mid 1950’s, poring over my Pictorial Encyclopaedia
. I eagerly drink in the graphical information along with the textual, and today I still recall the story of our Earth being pulled out of our Sun, along with the other planets, by a passing star. My chief custard pie, though, is reserved for the illustrator who depicted the arteries and ventricles of the heart, on both sides, as blue, and similarly the veins and auricles in red.
One thing the encyclopaedia did get right, though, was the rectification of alternating current. There on the page was a diagram and explanation of the thermionic diode. Enter Sir John Ambrose Fleming
(1849 – 1945), at this stage a hero of our story. Having already designed Marconi’s 1901 Poldhu transmitter, in 1905 he presented the diode at the Royal Society. An intensely practical and inventive man, but as we shall see later a little bit weak on theory, he likened its operation to a one-way valve, and so introduced a terminological discontinuity between us Brits and our transatlantic cousins which persists to this day.
Forward to the end of the 50’s, and studying O-level physics. I have by now taken apart a television chassis, and my head is full of triodes, pentodes, etc, all with electrons rushing to the anode. But at school I have to do calculations with current coming out of the positive terminal of a cell. I am confused. Mathematically, I’ve been fluent in negative numbers for years after being taught elementary algebra most thoroughly by our Welsh Rugby master. But apart from temperature, I have no real physical conception of them. I’m in good company here, though, since neither did the great René Descartes. In mechanics it was, rather, Nicolas Malebranche
(1638 – 1715) who sorted this out, replacing Descartes’ motus
by momentum as we know it today.
Into the the 60’s, and A-level physics. I have already been snared in the brambles of electromagnetic and electrostatic systems of units (you can view them from a safe distance here
– Oh the liberation of SI units!) and now I am confronted with Fleming’s Left-Hand and Right-Hand Rules. As I am taught them, for a generator the current is indicated by the seC
ond finger, which confuses me even further since as a piano player I count my middle finger as number 3.
Now my A-level results are in, and listed in this order: Chemistry B
; Mathematics A
; Physics D
. My Physics teacher looks, and says BAD
! On to university to study Chemistry, but with Physics as a first-year subsidiary subject, I am confronted with a textbook with pages full of upside down triangles, which I now know as Del (in-joke for Brits: Only Fools, Horses, and Physicists!
Small wonder that I am now hurtling away from Physics like comet Hale-Bopp out of the Solar System. But physics departments do sometimes need chemists, and a few years later in the early 70’s, I find myself in a reasonably stable orbit in the Reading Department, albeit rather far from the centre of things, like a Kuiper Belt Object.
For about thirty years I persist in this orbit, far enough away to avoid being volatilized but still receiving enough gentle insolation to allow thoughts to reorganize. Slowly, slowly, all that undigested mechanics and thermodynamics begin to make sense. I even develop a form of Fleming’s rules, for the right hand only, that is more consistent, and can convey Ye Olde Anglo-American (Heaviside-Gibbs) Vector Multiplication without them even realizing it’s maths.
| ||Motor ||Generator|
|Thumb ||What you put in ||Current ||Movement|
|Index finger ||What’s there ||Field ||Field|
|Middle finger ||What you get out ||Motion ||Current|
Fleming’s two rules are still vexing students. Here is part of an exchange on MadSci.org
, from 2001:
Student: Please help! We were told a bit about these rules and that one works with a dynamo and one with a motor, but I don't even know the difference between those two. And who was Fleming?
.... and at the end of the reply ....
I checked what my fingers represented in the CRC Handbook of Chemistry and Physics to make sure I was right (or left?). Good luck.
Vexing instructors too. But when this bee in my bonnet buzzes too loudly, I get the following responses:
“That’s what the government tell us to put in the curriculum”. But who tells the government what to put?
“You can’t go generalizing to the less bright students”. True, but I am writing from something of a worm’s-eye view.
“I get the LHR across by explaining how ridiculous it is”. That shows, O instructor, how talented you are, but there are few who can match your ability.
“Don’t think that what suits you suits everyone else”. But has anyone really done any research here as to what does work for young people?
Fleming’s nemesis came when he published an essay in Nature, 1930, in which he described sidebands in AM radio as “a kind of mathematical fiction [that] does not correspond to any reality in nature”. Their acoustic equivalent had been observed in 1875, and described mathematically in 1894 by Lord Rayleigh. Fleming himself should have known better, having studied under the direction of James Clerk Maxwell, and refutations of his essay came in from all directions.