Just a word of practical advice for folks thinking of trying Halbach arrays: Don't get the common 1/4" N42 cubes, and expect ANY glue known to man to allow you to glue them together in a classic Halbach, it is very difficult to do. Recently i tried this for a conventional alternative energy project i am part of here.
The closest you will likely be able to glue them, even with channel walls on either side as reinforcement to provide a much greater area for glue, and to keep them from turning... is about 1/8" apart. And even this small distance appears to matter to the overall effect. Trying to get these suckers to sit right next to each other without a fastener is virtually impossible, they are too strong for that (even clamping them overnight didn't help). I even tried that aluminum foil adhesive tape used in heating/cooling ducting as a little shield barrier in between, but it did not help much either because the field extends way beyond the cube.
So it is better to get either much weaker mags, or ones that are drilled for fasteners (or carefully and SLOWLY drill them all). And half of them must be drilled "N to S", and the other half drilled parallel to the N-S poles.
The closest you will likely be able to glue them, even with channel walls on either side as reinforcement to provide a much greater area for glue, and to keep them from turning... is about 1/8" apart. And even this small distance appears to matter to the overall effect. Trying to get these suckers to sit right next to each other without a fastener is virtually impossible, they are too strong for that (even clamping them overnight didn't help). I even tried that aluminum foil adhesive tape used in heating/cooling ducting as a little shield barrier in between, but it did not help much either because the field extends way beyond the cube.
So it is better to get either much weaker mags, or ones that are drilled for fasteners (or carefully and SLOWLY drill them all). And half of them must be drilled "N to S", and the other half drilled parallel to the N-S poles.
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