wrote:
> Hi Meredith,
>
> Your setup reminds me of an experiment of mine I performed over two
> years ago. Like you, I used 3 Nd rectangular magnets and a pyrographite
> slab. I added 2 cube magnets near the ends to get a stable period. I put a
> 0.004" thick aluminum damping vane on top of the pyrographite. A small
> lens was mounted in the damping vane as part of a planned optical pickoff.
> The magnetic field for damping was furnished by four cube magnets on top of
> the structure.
>
> I never made the optical pickoff to complete the sensor, because tilt
> tests disclosed that the motion of the graphite was somewhat "sticky". I
> think small variations in the levitating magnetic fields are responsible. I
> tried thin iron shims in order to homogenize the field, but then the field
> was too weak to support levitation. It is my belief that iron pole pieces
> are necessary for the successful implementation of a diamagnetic
> seismometer/tiltmeter.
>
> Paramagnetism of the aluminum vane was a minor problem also.
>
> Cheers,
>
> Bob
>
>
>
>
Hi Bob,
Thanks for your email note. It is real good to "hear" from someone with similar experiences.
All your experiences do have a relevance; and many I've also seen in the past with this type of setup
and other quite different variations.
The "sticky" terminology in particular is that yes; it is normally a very real problem. It "seems", to be
most visually prevalent when only a single piece of pyrolytic graphite is used. By "sticky" I mean it
seems to sometimes be anchored in one or more spots and even with some light induced tilt where
one would surely expect a movement response; they occasionally do not get any. Of course, I'am
not referring to debris (small hair, etc.) that may actually be the cause sometimes. With PG, its
possible to impart small magnetic particles thereon which will react in the magnetic fields of course.
Actually the ONLY reason, I'am trying again is that with two separated but interconnected pieces of
PG, I know I'am seeing quite a improvement in overall response. I can't really presently describe the
"why"; except perhaps the joint sum of the two pieces are kind of like a differential diamagnetic
response....or.....if this sounds right; it is kind of like a electronic resistance bridge were the voltage
differential is seen in total, rather than in one partial sum segment resistor. Yes, I'am not a qualified
scientist. I've also looked at totally switching out the PG, with other different pieces, and the results
look the same; so its obviously not just specific to acouple PG pieces. Also, the PG pieces have
been on and off the magnets numerious time for various test trials; so the results are real. The PG
pieces are also fairly close to each other in dimensions; which might be necessary (?). The PG
pieces I have are not precision machined so they vary in flatness and in other dimensions by around
~ 0.010".
Another feature I like is the variable period with the variable PG spacing. How I'am going to do that
with flimsy aluminum is yet to be attempted. One may have to settle for whatever period they select
before any "glueing" down of the aluminum. Even being able to do so, is unique.
I've even put a large optical magnifying lens next to the model; to check for minute lockups, and
of course whether it physically oscillates around a reference "zero" mark I put on the magnet. I can't
say I've definitely seen any sticking yet on this model; but if it happens, I will.
One good indication that it works better is that I do see increased wider span tilt oscillation motion
effects with the 2 PG pieces than I've ever seen with one piece of PG. It seems to be ~ 3X over
that of just using 1 piece of PG.
I don't yet really know the result of adding on steel/iron atop the magnets; as I've not yet done so.
I think I'll try 1/2" width X 1/8" thick iron initially. The iron is commercially common size stuff, but
it does have dual rounded outer width edges; which might not be totally ideal. The size is common;
but its different, as it was used as a desk drawer guide and was coated with a thin layer of copper
and then chrome plated. Its probably (?) bright steel and not the black structural variety.
I've seen thin ~ 1/16" thick iron/steel work very well on kind of a "U" channel type setup...where the
(then) spectrographite rod levitated lower down on the deliberately lower middle channel magnet.
That was Chris's recommendation and it did dramatically improve its levitation height.
Yes; the aluminum paramagnetism has been seen in the past also; and its going to show up here
also. The aluminum could be responsible for some of the sticking, with their various impurities.
I'am using K&S Engineering aluminum; which seems to be purer stuff. With the severe weight
limitations; its hard to consider using other metal/s. I suppose I can hang up a piece of the stuff
on a long thread, and bring a magnet near it; to just see a rough indication of its general paramagnetism.
I am also presently using 2 plastic spacers inbetween the magnets; which are 0.035" thick X ~0.475"
in width. The spacer is a idea from John Lahr; and it does help alittle for some slightly increased
levitation. I may try to find some thin plastic or non-ferrious metal that is 5/8" in width and somewhere
around 0.020 to 0.0315" thick, that could aid in setting up the iron on the magnet tops. The iron will
be magnetically normally forced against the joints for alignment. If I don't glue and leave it as is; then
if it doesn't work too well; I can easily remove the iron and go on from there.
Of course; I don't know yet know how it will all turn out; but....its all fun stuff to just see what happens.
Take care, Meredith
On Dec 6, 2007 7:39 PM, <
Bobhelenmcclure@.......> wrote:
Hi Meredith,
Your setup reminds me of an experiment of mine I performed over two years ago. Like you, I used 3 Nd rectangular magnets and a pyrographite slab. I added 2 cube magnets near the ends to get a stable period. I put a
0.004" thick aluminum damping vane on top of the pyrographite. A small lens was mounted in the damping vane as part of a planned optical pickoff. The magnetic field for damping was furnished by four cube magnets on top of the structure.
I never made the optical pickoff to complete the sensor, because tilt tests disclosed that the motion of the graphite was somewhat "sticky". I think small variations in the levitating magnetic fields are responsible. I tried thin iron shims in order to homogenize the field, but then the field was too weak to support levitation. It is my belief that iron pole pieces are necessary for the successful implementation of a diamagnetic seismometer/tiltmeter.
Paramagnetism of the aluminum vane was a minor problem also.
Cheers,
Bob