"greywolf42" wrote in message
...

George Dishman wrote in message
...

"greywolf42" wrote in message
...

And yes, they do have 'some kind of reality'. Sprinkle iron filings
on a
sheet of paper, then bring a strong magnet around. Always the same
pattern
at a given distance and orientation.

At any point in space the field has a strength and direction.
Suppose you mark where the lines lie on the paper with a pen
then remove the filings and carefully measure the field strength

a) on a line

b) between lines

Would the strength in (a) be greater than that in (b)?

Place a single filing on the paper and glue it down so it doesn't
fly off to the magnet. It creates an easier path for the field to
flow along in its immediate vicinity than passing through the air
so it distorts the field nearby and causes an increase of field
strength near the tips.

Well, yes. You've added another 'source' into the field, due to magnetic
induction (also noticed by Faraday and derived by Maxwell). However, the
'power' of that source is a function of the first magnet, and it's
distance.

I don't disagree with any of that. You have added
useful detail to my explanation of the mechanism.

It is a small effect but if a second filing
is placed nearby, it will be drawn to the first by the increased
local strength again shortening the path length and reinforcing
the effect.

If you let all the filings move equally, you won't get the above
distortion.

It gets more complex when considering 'all' but
just two illustrates the mechanism as I understand
it.

If you just 'drop' that first filing in place, it will always go to one of
the same lines that were traced out prior, or afterwards.

There I disagree. If you drop a single filing it
will always attach itself to the magnet if it is
free enough to move. Obviously friction usually
stops it.

There is nothing special about any location around the
magnet, the line will form in the direction of the field from
wherever you choose to fix the first filing.

That's because you've 'cheated' and arbitrarily 'glued' the 'first'
someplace other than a normal line-of-force. This creates a second
(interfering) source in the combined magnetic fields.

If you were correct, the filings would remain stationary when you move the
magnet along, underneath the paper. Yet still they move. (Try it.)

Once you have a chain of filings from one pole to
the other, attempting to move the magnet without
moving the filings would create a gap and all the
force is concentrated there. That is enough to
overcome the friction.

Going back to two filings with the first glued,
moving the magnet will not usually cause the
second to move but to rotate to remain aligned
with the field while still remaining in contact
with the first. (Try it ;-)

More scientifically, suppose we have this
arrangement:

N
|
| f --
|
S

--x--

A single filing is held at 'f' and the force
plotted against the distance 'x' from the magnet
as the filing is moved along a line perpendicular
to the axis. If you were right then there should
be a cyclical variation since the force should
have maxima when the filing is on a line and
minima in-between. This is not the case.

George