"What is vector potential ?"

Roger has answered:

......, mathematically it is just a mysterious function, first recognised

circa 1850,   >.which, when it curls, produces the

.magnetic field.  Hence the later association of it with the term Magnetic
.vector potential.  It was later found that one could derive all of
.Maxwel',s equations from this single equation, so it began to seem that
.there was something  more to it than just a pretty name......

.............The next breakthrough came circa 1960 when
.Abranov and Bohm wrote a letter to Nature prophesising, on very good
.grounds,  that in electron interference experiments, where a beam of
.electrons is fired at a double slit, it should be possible to change the
.phase of the interference pattern on the screen behind the slits by
.introducing a small source of vector potential, derived of its magnetic
.field component,  eg by using a very long thin solenoid or, in later work

a

.tiny toroidal coil, half way between the slits.  This was later
.demonstrated many times experimentally and it was clearly shown that only
.the vector potential was responsible for the phenomenon, so some people,
.mainly physicists at that time, began to have a real respect for it.

Without knowing more details of the experiment I can't comment. However, the
effect of magnetic fields on electron beams is well known - the fact that
you can view this screen illustrates the effect of this phenomena.

The engineers were much slower at recognising its importance in radio
signal emission and radiation ......

And with good reason. In our search for the Holy Grail of efficient
electrically small antennas we do consider many strange devices - some
invoking Maxwells equations to prove viability. However, the ability to
radiate or receive efficiently, particularly on LF, sorts out the wheat from
the chaff.

but in the last decade it has received considerable
attention in the radio research laboratories in the USA, especially those
concerned with the defence industry.

Good for obtaining funding for 'Son of Star Wars'.
Anyone in the UK, other than Roger, engaged on this line of enquiry?

  As an example of the fundamental nature of the vector potential, in the
last few years  there have been a number of papers in the American journals
from research labs and defence consultants concerning contra-wound toroidal
antennas..............
American literature in particular an early paper by Carron, N.J: American
Journal of Physics,1995, 63, pp 99-103, Carron was unaware of my
experimental work at the time.

Carron's patent 0043591 for his toroid antenna runs to 67 pages! and has
been in the public domain for some time.(date of filing 13/7/1981)
It quotes the relationship of vector and scalar potential to electric and
magnetic fields and the permeability of free space but does not use these
units in describing the action of his toroid antennas.

    All of this may seem to be a bit too theoretical because one cannot
actually see the vector potential,  one only observes the effects of its
presence, but, after all, the same applies to magnetism.  One never
observes a magnetic field, - only the effects that it produces.

One can measure magnetic and electrostatic fields (static or dynamic).
See PA0SE's instrument in the LF Handbook, page 83, for measuring
electromagnetic field strength.

Now, has anyone come up with an instrument for measuring Vector Potential?

A passage from Basic Electrotechnics, by B. L. Goodlet states:
"If the vector potential at a point due to current or flux is known, it is
possible to calculate the induced electric field at that point if the
current or flux changes".
This implies that if Vector Potential cannot be measured then it must be a
mathematical go-between to simplify calculations.

While in the rarefied atmosphere of Electromagnetic Theory it appears that
we  have Electric Vector Potential, Magnetic Vector Potential, and Electric
Scalar potential. There may be a Magnetic Scalar Potential but I haven't
seen one yet.

Until someone can convince me otherwise I feel that these are just
mathematical entities. Whether they will provide the key to a better LF
antenna than the one devised by Marconi remains to be seen..

Regards,
Peter, G3LDO

<[email protected]>