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Warren, "In practice, what length must a tube be, to work."
All mass has energy present already at the Isotope level. The power you get from making all the atoms vibrate up as one field will increase with the larger mass of a larger tube, a longer tube, etc.
In an antenna, which is a load, we use an inverse calculation of the length to calculate resonant frequency.
In mass resonance we use unity. The higher the water in a dam the higher the tension on the penstock by direct proportion. This is a hard thing to teach to an electrical engineer they want to view the length as degenerative or a load, rather then the source of the power.
The answer to your question, as long as you need to "feel it," larger will be easier to feel at first.
Since it will not move a meter in this form, that is your only means of detection that it is real.
For a circular field coupling of the energy you need to have a pretty good circle tube. Seams are not a big concern if the main loop is fairly accurate. There are other fields you can place into the meat of the thickness of the tube and with that method, out of round is less important, Out of round will work, as an area resonance but it will be hard to set up as a diameter measurement which will not be consistent. It is however the area inside the shape that you are vibrating up with this particular experiment and any area that matches will vibrate up even a square one.
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The SS iron atoms magnetic qualities are contained by the diamagnetic qualities of the chromium, near as I can tell, the two fields cancel out. 316 and 304 will not stick to a magnet, but a magnet will stick to my calipers, and they work fine for vibration manipulations even on Joe Cells. Joe cells are conditioning water and a magnetic field will take control of the Isotope level and change the tilt spin angles of some of the water so the field coherence will not be uniform on the entire water mass as one coherent field bubble.
Other kinds of fields however are stronger, and a magnet will have no effect on them, because this force is 137 times stronger then magnetic fields can become at it's maximum levels of field coherence.
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When you drop a magnet through Aluminum or Copper tubes, the resistance to motion is not from an opposing magnetic field, generated from "eddy currents." It is an inertial reaction from Isotope spin alignment of the fields having to turn in the copper out of alignment with the longer copper tube.
If you notice at first the distance of the gap in this field can be way too large for a magnetic responses, an interaction that drop off by the distance cubed law. Further if you do magnetic field strength testing with a hall effect sensor, as the magnet passes in the air versus the copper tube, the flux of the magnet is not bent in the lease and power level does not change of its field as if that field were being opposed, it is as if the copper was not even there. Copper does not disturb magnetic lines of flux at all, or bend them around like iron does.
With motion copper will repel both poles of the magnet equally, That is not how a magnetic field works.
Push any two magnets together in opposition and release them and measure the distance they move apart.
Now hold a magnet, up near a spinning copper cylinder, release it and see how far it flies. The inertial momentum is transferred from the motion of the copper, but in two opposing magnets inertial momentum is invisible.
http://www.resonantfractals.org/Magnetism/SpinningCylinder.htm http://www.resonantfractals.org/Magnetism/SpinningCylinder.htm
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I would not be worried about metallurgy, for this kind of experiment, it will work with any metal that has consistent structure all the way down it and parallel sides, not tapered.
SS is only better because you can feel it stronger with less mass and the vibration will be more pleasant.
It will work with glass, but the effect might be painful.
Dave L