From ChrisAtUpw at aol dot com
Date Wed, 5 Feb 2003 213312 EST
Subject Re Preliminary new magnets diam check Notes
I have done a summary of some of Meredith's recent experiments and I have added some comments which may help in designs.
In a message dated 09/01/03, firstname.lastname@example.org writes:
The big magnets are from the E-Bay Gaussboys seller, as, item "Block 18"). They are literally metric in size, which is 50mm x 18mm x 6mm; or, 1.97" x .710" x .235". They are rated at 38.
They cost $7 each via E-Bay auction site, but $7.50 each per the Gaussboys regular web site. It "seems" to be a "regular" stock item at least for the time being. Not really a cheap item...but it seems to be ~ 10-15% greater in the amount of gauss field it generates; than say a R36 magnet which in reality probably slightly raises the amount of added aluminum dampening, optical flag,
or weight it could carry a little.
The steel base is a Steel Works item, and seems to be flatter overall than other welding steel material I've seen or tried before. The size is 4" width x 12" long x 1/4" thick. The item was obtained at a Denver area, Lowes, building supply store, but I think its around in other Ace, misc., stores that stock the Steel Works items. Price is ~ $7.50. However, I did visually compare the flatness with afew others, to settle on the selected purchase. Obviously the steel plate, has potential where with drilling, tapping of holes could lead to a complete adjustable setscrew
base seismo platform. In the picture above, is also a iron spacer of 1/8" square
keystock. A relatively common item in the U.S. They cost ~ $1 to $1.50 each. The big magnets all have the same pole upward (and opposite pole downward). The magnets all seem to come together on the steel base and keystock rather close together nicely, without any significant repelling away; which is probably related to the rougher steel base surface. The full diameter 1/8" spectrographic rod in the picture (4.35" L) does readily levitate in the "H" type (single) channel designed by David Lamb. It levitates about 5/8 of the diameter, above, the top
surface of the magnets. A quick check of a "half moon" (0.050" thick), showed it all, at and above the top of the magnet.
They could work as a "H" bridge cross piece part however. The magnets laid across the top, are to create a stable levitation; and/or control the magnetic field. They were literally a test "throw on", and it did flip the magnetic field from where the spec rod went from a "inverted pendulum", to a "handing pendulum" (S-G like).....WITHOUT having to create a filed curvature on the center area of the 1/8" iron keystock. As shown, the period was roughly 2-3 seconds, when the steel
base was leveled. I've not tried spec rod length adjustments with this layout yet, for longer period trials. The spec rod length as shown in the setup acted "like" a vertical mass hanging down on a regular spring, as it oscillated between the ends for quite some time; but of course it was in a horizontal plane. One could move the top magnets
a little bit inward (limited) to shorten the period. I've tried a similar setup before like this, but it didn't work near as well, as with this specific combination of magnets.
Here, the centering of the spec rod is dependent on the smaller magnets proximity quite literally. One can only speculate that the spec rods response to disturbances might be somewhat "looser", and not so amplitude dependent to a degree; as its without any filed curvature on top the iron spacer keystock. I think one could vary the number of the bigger magnets to about any desire for length. Although the magnets are separate "entities", I didn't really see very much noteable interaction in from what could be described as a simple induced
In a message dated 12/01/03, email@example.com writes:
In regard to the previous email, I note a much greater span of period, than the 3 seconds I first reported. That particular spec rods length was 4.345". With a full diameter 1/8" spec rod of 3.61" length and weighing 1 gram, it had a max induced
oscillation period of 13 seconds, while much smaller non-induced visually viewed offsets ranged from 6-10s. All this simply means is that other lengths may need to be checked for their respective responses. The 2 smaller Amazing magnets were kept at the end of the Gaussboys series lined up magnets. One could call these
"gap spanning magnets". I really hesitate to think that a long period is a "give", as there is likely a certain amount of drift possible in this.
The "tilt" sensitivity is quite prominent with this, as their is no top surface circular filed iron gap keystock here.
In a message dated 09/01/03, firstname.lastname@example.org writes:
The two smaller magnets are from the E-Bay seller "Amazing Magnets". They are 1" x 1/2" x 1/8" in thickness. They are rated at 40. They cost $1.30 each (in a quanity of 8, for $10.30); although the prices falls in quanity purchases. The magnet also "seems" to be a "regular" stock item for the time being. As for
its use, outside this setup; I've not tried it by itself yet; but the good R40 rating does merit other trials sometime.
In a message dated 25/01/03, email@example.com writes:
I suspect that the field is so intense that the 1/16" iron separators are saturating and hence there is little attraction between the separator and the baseplate. It may be that the field is uniform over the small section, whereas is isn't over the 1/8" spacers with the 0.2" magnets.
I used a big hunk of what I think is more of a soft iron grade. Then the small 1/16" key stock separators work. Am sure you are correct with the saturation aspect; via the base plate moreso it seems.
I also used the same keystock on the outside bottom edges of the magnet series.... that greatly helped the magnets from separating so much (~ 1/8") along the series chains of magnets. I didn't see or note any change in levitation height from adding keystock to the outside bottom edges of the magnets. This added keystock to the outside bottom edges maybe relivant and good for all similar
magnet setups of this nature.
The good news is that the gap field is very good....i.e, the pencil lead would now levitate with about 95% of its diameter above the top of the gap....very nice. For pencil leads and/or spec rod, this is the best ratio/percentage I've seen. This height equates to the lift/weight aspect. This is without adding on any sheet iron atop the mags; its possible that may not go smoothly either....but I'll check that out too, in time. The leveled maximum period with this is unknown....as I just used a full length Faber-Castell 1.4mm x 60.8mm lead...it was 3s. The period may get longer with a shorter lead.
The gap of the magnets is only ~ .0625", while the pencil lead is about .053" in diameter. Very small, and likely somewhat harder to work with than say the 1/8" spec rods of course.
The obvious "bad news", is that the base plate needs to be a certain grade and/or thickness, to even work at all. Stuff like bracket thin iron/steel won't work.
So......for a pencil lead setup.....there maybe some potential with this size of magnet after all. For the moment, I don't have other iron/steel around that is of sufficient size and thickness, to go forward with trying to make the pencil lead setup into a "H" variety.
Suspect one may still need to force the magnets together, and glue the outside keystock down to force the best series closure of the magnets.
I have had some thoughts on the operation of the diamagnetic seismometers which may be of interest in trying to develop new approaches. Consider two NdBFe rectangular magnets lying side by side on a thick soft iron backing plate with a lower height square soft iron rod in between - the 'normal' setup that we use. If the two magnets both have N poles on the top surface, the soft iron rod in between will form an intense S pole. The saturation magnetisation of the NdBFe magnets is about 12 kilo gauss - 1.2 Tesla. The saturation magnetisation of soft iron is about 21.5 kilo gauss - 2.15 Tesla - so the field near the central S pole will be approaching this value. If the magnets were infinite in length, uniform in magnetisation and were accurately level, the graphite rod would have an infinite period and be 'stable' at any axial place in the field. Real magnets only approach this performance after much work / effort.
Magnets do show variations in field along their length, particularly near the ends and this 'lumpiness' will limit the period which can be obtained. It may also give two or more axial positions where a rod is stable, depending on it's length in relation to the separation of the 'field lumps' at the ends of the magnets. The decreased field near the ends allows rods which have near the same overall length to slide off the end of the lift region. There is nothing to prevent this.
This effect is very obvious if you use a diamagnetic lead 1/2 to 3/4 the length of a magnet in a setup with reversed field junction, with gold rod cylindrical magnets. Centrally placed, it floats evenly and level. Allow it to move and the end nearest the junction dips and moves just past the junction and the other end near the centre of the magnet is strongly elevated. The same effect may be observed, but is much weaker, when the gold rods are oriented with similar adjacent poles at the junctions, rather than reversed. While reversed pole junctions are able to provide excellent damping, magnet junctions are more generally 'bad news' when you try to obtain long periods. If you need the damping, consider 2 large or 4 smaller magnets with a reversing join at the centre of the array.
The usual constructions involve placing two or three lines of 0.2" to 0.25" thick magnets on an soft iron backing plate with an 1/8" square section soft iron rod between the lines of magnets, in contact with the baseplate. Since all the magnets are magnetised through the top and bottom faces and are used with all top faces of the same polarity, they tend to repel, sometimes quite strongly. One 1/8" soft iron rod between each line is usually enough to form a pole of the other polarity and to allow the magnets to be pushed in contact. For very high energy magnets it may be necessary to add additional soft iron strips between the outsides of the line of magnets and the baseplate. It is preferable to use the smallest square section which allows full magnet contact, since this provides a second intense magnet pole and can reduce the field in the main channel.
The end field adjustment magnets repel the main array very strongly. Again, square section strip may be used on either side of the adjustment magnet to provide adequate attraction to the baseplate. These strips should terminate clear of the main magnets, leaving a short U shaped adjusting pole close to the end of the main magnets.
Now what sort of graphite rod movement is involved? Consider a system with a 10 sec period. Assuming that the motion is simple harmonic, the characteristics of motion will be similar to a pendulum bob. A period of 10 sec corresponds to a pendulum of about 25 metres. If the rod can move axially +/-5 mm and it's overall length is 60 mm, one end lifts by 60/2 x sinD, where D=5e-3/25 = 2e-4. This is about 6 microns. The average field squared over the rod needs to have a gradient of less than 1 in 833...! Comparing this to the several % field change seen when passing a Hall probe over the length of a magnet, highlights one main problem. The field needs to be far more even over the expected movement range than the magnets can normally provide.
The compensation to give a stable central lift region for the rod can be done in several ways. With this construction, long periods imply a high sensitivity to tilt.
The length of the rod can be trimmed to finish a bit less than half way along the final magnet. This assumes that the lift is greatest at the centre of a magnet, which is usually true. This can reduce the length of the rod that you can use considerably. Long rods may be preferable when considering lateral stability.
The central square iron polepiece can be slightly dished on the top face to give reduced lift in the middle and increased lift at either end. This is a time consuming task involving trial, measurement and error, but it is effective.
Additional magnets may be added either on top of the end magnets or just beyond the ends of the main magnets on the baseplate, to give an enhanced field over the end region. Adding physically higher / stronger magnets to the ends with the ability to slide them towards or away from main magnets can provide quite precise control of the end field and compensate for the normal reduction in lift near the end of the magnets. This can allow quite a lot of control of the period.
In a message dated 03/02/03, firstname.lastname@example.org writes:
As far as the end magnets present size, I could only get stabilization with two magnets stacked on each end, but, without, the sides of 1/8" iron (on the end
magnets only). The end magnets in this configuration seem to need a certain size magnet with a higher rating to normally be successful it seems. Stacking the magnets raises the pair gauss alittle higher than the gauss of the magnets in the rows. The period seemed to be ~ 5-6 seconds, with a (0.030") thick plastic spacer. There was 10 magnets in this configuration. (It was 3 sec with the original above arrangement)
The top surfaces of the main magnets may be fitted with soft iron plates. If you use galvanised or tinned iron, it reduces any rust problems. Marine varnish can also be quite effective. The capping strips may be the same width as the magnets, or slightly less, but they should not overlap the central pole (central gap). This enhances the field over the central pole and evens out longitudinal variations in the field, which is most useful. Useful thicknesses may range from 20 to about 60 thou. While straight capping sections have normally been used, there is no reason why the edges should not be dished laterally to control the field enhancement or why separate slightly angled strips could not be used for either end. It may be advantageous to chamfer the edge of thicker sections to about 45 degrees, using a file. There is a very intense field at the edge of the capping plates and this offers the possibility to stabilise the rod by repelling it sideways more at the ends in a pincer action.
The combination of capping plates to even out the fields along the magnets and additional end field magnets can provide the control precision required to give 10 second or longer periods.
However, you also need to choose the backing plate with some care. A rough guide as to the thickness T of backing plates for a rectangular magnet of dimensions A by B, is that 2.T.(A+B) > AxB. The backing plate needs to be quite accurately flat to minimise the magnetic resistance of the inevitable tiny gap. This is more important for large magnets. Ground high carbon flatstock is available, but it has an appreciable magnetic remanence. Mild steel is satisfactory magnetically. The surface of mild steel plate may need to be milled flat. With considerable effort it may be hand lapped flat. Bright cold rolled steel strip is probably the most satisfactory commercial backing material. It may be checked for flatness with a steel ruler to about 1 thou. Bright cold rolled mild steel strip is available 6.5 mm thick (1/4") from 40 mm wide (1.5") up and 8 mm thick (5/16") from 90 mm wide (3.5") up. There are some suppliers who specialise in small quantities / cut lengths. It may be an advantage to use a backing plate which is only a little bit wider than the main magnets, so long as they can still be pushed together longitudinally and are then stable in position.
Mechanical sticking effects may be observed with graphite rod assemblies. The very strong magnets allow tiny whiskers of magnetic material to accumulate and be held ~perpendicular to the surface of the magnet. You can sometimes spot them using a small 'pen' laser, but they may otherwise be near invisible. The surface of the magnet can be 'cleaned' with gaffer tape or similar, which has a rubber based adhesive and collects debris quite effectively. You press strips of it onto the surface and then peel it off. It can be quite difficult to clean a magnet thoroughly. It is a good idea to check for cleanliness before final assembly.
One added message from Meredith.