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The German Lecture
Decades ago, Tolly Holt flew me to
Germany to give lectures at a conference at the university in Bonn. In one of
my three lectures, I wrote up Faraday’s Law of induction on the board; v =
-d(phi)/dt. I then said that, without any discussion, the text books followed
this with the formula (Integral)Eds = -d(phi)/dt. I said that this step was
illegal. Although I was only seconds into the
start of my lecture, I was immediately interrupted by professors and
lecturers present, who were appalled. A lengthy discussion followed. I said that if a loop of wire with a single
break was put around the perimeter of the surface under investigation,
a voltage drop could be measured. However, how would one measure the
voltage drop between two points some way away from each other along
the loop? It became clear that the layout of the two probe wires leading
to the measuring instrument became hopelessly involved in the measurement. The same sort of uncertainty arises if
we take Faraday’s Law of Induction, or the Biot-Savart law (or Ampere’s Rule)
and try to apply it to a TEM wave travelling at the speed of light. It is
necessary to think clearly about the physics involved, and not just hope the
maths will sort itself out on its own. Ivor
Catt 18june02 (Tolly = Dr. Anatol Holt) |
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