On 02/17/2010 12:14 AM, Harry Veeder wrote: > > I did some googling on magnetic saturation... (some parts of the post > have been removed)
No prob, I snip all over the place, myself. ... [sal] >> The distinction you're drawing between a "cloak" and a "shield" is >> interesting but I think it hides the fundamental similarities >> between this design and all magnetic shield perpmos. > > My terminology is meant to illuminate the difference between the orbo > and all other magnetically shielded perpetual mobiles. OK, I can see that. [sal] >> They *all* share the very important trait of drawing no power (to >> operate the motor) while the shield is stationary; only the >> effects during shield motion really matter. > > Since there is relative motion between the putative shield and the > rotor, you can't say it requires no power once it is "in place". I don't follow this. There's always relative motion between some magnet and the 'shield'; in fact, that's the point of the shield: it lets you move some external magnet without resistance from the one behind the "shield". As to requiring no power, as far as I know, if you used superconducting wires for the coil and you were clever, you could energize the coil (somehow) and leave it running while the magnets move away with no additional power. So, in principle, the Steorn "cloak" also requires no power once it's in place. IOW all losses during magnet motion are indeed Joule heating, just as Sean et al claim; the "shield" isn't actually using any power. ... [sal] >> Consequently, a stronger external magnet will necessarily require >> a stronger internal field to overwhelm it. > > > Check this link to p. 360 from the book _Introduction to Magnetic > Materials_ By B. D. Cullity, C. D. Graham > > http://tiny.cc/FQicR > > If I read it correctly, it says that the applied magnetic field > required to rotate the saturated field Ms could, depending on the > material, be _infinitely small_! Sure, makes sense. > Therefore you do not need to > overwhelm the field of the permanent magnets, and with the right > materials, the field generated by the coils can be made as small as > you want. No, this doesn't follow. Remember, the fields themselves superpose linearly. It's only the behavior of the magnetic material in the presence of the applied field which is nonlinear. We can think of it like this: The field induced in the core is parallel to the applied field, and increases linearly with the applied field until it reaches 'saturation'. Real materials are more complex, but this will do for a thought picture. So, when the field strength is high enough to produce saturation, the core material is producing a fixed strength, rather strong, magnetic field. The direction of the field is determined by the external field (which is saturating the core). This is just like a compass needle, as described in your reference. The point is that the field of the (saturated) core material is *parallel* to the *applied* field. That applied field is the LINEAR superposition of the coil's field and the external magnet's field. For the field of the core to be entirely contained within the torus (which is must be, or must nearly be, for Naudin's dropping box experiment to work), the field of the core must be in the form of concentric rings around the axis of the core. But, since the core's field must be *parallel* to the applied field, that means the applied field -- which is the sum of the magnet's field and the coil's field -- must also be in the form of concentric circles around the torus axis. For that to be (nearly) the case, the coil's field, within the torus, must be vastly stronger than the magnet's field. Otherwise the field lines of the core would stick out of the torus and the core would not be fully shielded. To see this more clearly (but less precisely), we can use a reductio ad absurdum: Reduce the coil current to a microamp, and use just a few turns in the toroidal coil. Now, let's use an outrageously strong external magnet -- a nice superconducting magnet with a field of a few Tesla will do, I think. Now, do you think that *any* choice of core material will result in the core being shielded against the external magnet in that case? I don't. In fact the effect of the miniscule coil current will be nearly impossible to detect no matter how you go about it. And that illustrates the point that a stronger external field requires a higher (minimum) current in the coil to fully shield against it. ...
