Aluminum and magnets

[bea]

In my eyes some more things are suprizing:

aluminum is a really good conductor, its specific resistivity is not much larger than that of copper.
Despite of that the effect of cooling (=lowering of the resistivity) is very pronounced.

referring to pickups: the lamination of the core of Bisonics and Darkstars should have a major effect on the internal damping of the pickup resonance (but nevertheless a minor effect on its inductivity).

[David Schwab]

referring to pickups: the lamination of the core of Bisonics and Darkstars should have a major effect on the internal damping of the pickup resonance (but nevertheless a minor effect on its inductivity).

My question about the Bisonics is; are those laminations insulated from each other? I'm pretty sure they are not insulated. So my thought is they used laminations because it was easier to stamp out the parts, using thinner steel, than it would have been to drill out the keeper. Lots of keepers are stamped, but the ones in the Bisonics are a lot thicker.

I have seen many cheap Asian pickups made with multiple stamped laminations, which were clearly not done to eliminate eddy currents, but were just done to make the parts easier to fabricate. One example are cheap Mudbucker copies, which use multiple thinner keepers stacked up, as compared to the original Gibson pickups which had a machined part.

[bea]

My question about the Bisonics is; are those laminations insulated from each other? I'm pretty sure they are not insulated. So my thought is they used laminations because it was easier to stamp out the parts, using thinner steel, than it would have been to drill out the keeper.

Possibly. Apparently no one has analyzed the Bisonics (and similar) in that detail. But i remember from the Darkstar discussion that Fred Hammond was using thinner laminations in order to reduce eddy currents.

Anyway - even without laminations there is some (residual) resistance between those layers which should affect the eddy currents. I do notice this in my two Epi sidewinders - one with a full bar, the other one with those keeper bars. The one with the full bar sounds a bit different. But that's a secondary effect at best.

[überfuzz]

Hi!

If you're handy with a computer you can simulate these things quite well. Try some software in the lines of Comsol or similar. This would be is you don't like to get your hands dirty.

[Joe Gwinn]

My question about the Bisonics is; are those laminations insulated from each other? I'm pretty sure they are not insulated. So my thought is they used laminations because it was easier to stamp out the parts, using thinner steel, than it would have been to drill out the keeper. Lots of keepers are stamped, but the ones in the Bisonics are a lot thicker.

I have seen many cheap Asian pickups made with multiple stamped laminations, which were clearly not done to eliminate eddy currents, but were just done to make the parts easier to fabricate. One example are cheap Mudbucker copies, which use multiple thinner keepers stacked up, as compared to the original Gibson pickups which had a machined part.

Given the very low voltage per turn, even if there is no intentional insulation, the laminations may nonetheless be adequately insulated from one another.

For a numerical example, if a 5000-turn singlecoil pickup is generating five volts (only happens during attack transients), that's one millivolt per turn. This same millivolt is imposed on the core and an eddy-current loop, so if the core is a three-sheet laminated blade, that's one quarter of a millivolt per insulated gap traversed by the loop. The oxide on the laminations can stop such a low voltage, and if the laminations were thinly varnished before assembly, there will be zero eddy currents getting from one lamination to another.

Turning this around, one would have to weld the edges of the laminated stack together to ensure full eddy current effects.

[Mike Sulzer]

That is interesting about the oxide, even relevant to the roll of aluminum foil. Does aluminum oxide have a similar effect? Just for fun I made a four turn coil, put it flat on the table and set a 1 inch by 1 inch neo cylinder down on the table in the coil, many times, measuring about one mv, never more than two. So the voltages in that case are not very large either.

Given the very low voltage per turn, even if there is no intentional insulation, the laminations may nonetheless be adequately insulated from one another.

For a numerical example, if a 5000-turn singlecoil pickup is generating five volts (only happens during attack transients), that's one millivolt per turn. This same millivolt is imposed on the core and an eddy-current loop, so if the core is a three-sheet laminated blade, that's one quarter of a millivolt per insulated gap traversed by the loop. The oxide on the laminations can stop such a low voltage, and if the laminations were thinly varnished before assembly, there will be zero eddy currents getting from one lamination to another.

Turning this around, one would have to weld the edges of the laminated stack together to ensure full eddy current effects.

[bea]

Aluminum oxidizes almost immediately; so it is more or less always covered by a thin and very hard layer of its oxide (except in environments where oxygen is absent). The stuff is very hard (AFAIK second hardest behind diamond) and used in high quality sanding paper as well as a jewel (corundum / sapphire). Aluminum oxide is a bad conductor and therefore should have a similar effect as iron oxide on eddy currents in layered conductors.

[Mike Sulzer]

But the tube of aluminum foil can act as "shorted turn" low resistance conductor, implying that there is no effective insulation between layers, even with a low voltage. Is there perhaps something special about the oxide on silicon steel that would prevent this in a laminated pickup core? Maybe it is thicker, while the layer on Al foil is so thin that it does not insulate?

Aluminum oxidizes almost immediately; so it is more or less always covered by a thin and very hard layer of its oxide (except in environments where oxygen is absent). The stuff is very hard (AFAIK second hardest behind diamond) and used in high quality sanding paper as well as a jewel (corundum / sapphire). Aluminum oxide is a bad conductor and therefore should have a similar effect as iron oxide on eddy currents in layered conductors.

[bea]

In order to judge we need to compare a solid tube and a tube made of foil.
I do consider neither of the two kinds of oxide layer good insulators (in both cases you will not measure a noticeable resistance with a standard ohm meter!).

BTW: the surfaces of the contacting materials should be important as well, both its area as well as the surface quality: the surface of a typical iron sheet should be *much* rougher than the surface of the aluminum foil which is actually quite smooth as long as the foil is machine wound on its roll. Therefore the residual resistance between two typical steel sheets - especially with the fins of the holes in the darkstar cores - should be larger than in case of the aluminum.

[Joe Gwinn]

That is interesting about the oxide, even relevant to the roll of aluminum foil. Does aluminum oxide have a similar effect?

It does. But the layer on new aluminum foil is quite thin and easily broken, so there is some random inter-layer connection happening.

For such low voltages, even a layer of rust is sufficient. For example, the original telephone hybrid transformers were wound as solenoids one a core made of parallel lengths of black iron wire. The wire was varnished, mainly so mechanical motion wouldn't rub holes in the oxide layer.

[ScottA]

Aluminum oxidizes almost immediately; so it is more or less always covered by a thin and very hard layer of its oxide (except in environments where oxygen is absent). The stuff is very hard (AFAIK second hardest behind diamond) and used in high quality sanding paper as well as a jewel (corundum / sapphire). Aluminum oxide is a bad conductor and therefore should have a similar effect as iron oxide on eddy currents in layered conductors.

This is why you need a special cutting oil to machine aluminum. It oxidizes so vigorously that it will ignite regular cutting oil when fresh material is exposed.

Also true of other noble metals. We used to sputter coat an alloy called NiCoCrAlY. One time while cleaning out the chamber and exposing big peels of this stuff that had never seen air, a wastebasket full of methanol soaked kimwipes basically exploded in one of my colleague's faces.

[Mike Sulzer]

I suspect that more than that is going on since the connections are good enough to support magnet braking.

It does. But the layer on new aluminum foil is quite thin and easily broken, so there is some random inter-layer connection happening.