I think some clarification may be needed here on the theoretical concepts and I disagree with this idea. (I am a mechanical engineer, not an electrical one, so correct me if I'm wrong)
As much as I respect Bill Lawrence, I think he is misled on this particular one...
To the three people who might actually read this monster post, this is for you!
Eddy currents alter sound and output of a pickup and play an important role in pickup design.
Maybe, probably not...
Eddy currents are induced in metals in the vicinity of an AC magnetic field, creating a secondary magnetic field which opposes the inducing magnetic field of the coil.
Let me explain further... Eddy currents arise as a result of Lenz's Law, which states that an induced electromotive force always gives rise to a current, whose magnetic field opposes the original change in magnetic flux. In other words, a change in magnetic field passing through a metal, causes current to flow in the metal in a direction such that it creates its own magnetic field that is opposite in direction, and therefore opposes, the original magnetic field that created the current flow in the first place. The magnitude of the induced emf follows Faraday's Law of induction, which states that the magnitude of the induced emf is proportional to the rate of change of the magnetic flux.
Now, let's look at the strength calculation for eddy currents:
where:
k - 1 for thin sheets, 2 for thin wires
P - power dissipation (W/kg)
Bp - peak flux density (T)
d - thickness of the sheet or diameter of the wire (m)
f - frequency (Hz)
ρ - resistivity (Ωm)
D - density (kg/m^3)
The dimensions, conductivity and permeability of the metal, along with the frequency of the current in the coil, determine the magnitude and phase relation of the eddy currents.
Dimensions? Yes.
Conductivity? AKA reciprocal resistivity, yes.
Permeability? Nope (see above equation).
Frequency of the current in the coil? What coil? In the pickup? I am pretty sure the eddy currents will come about as a result of the magnets in the pickups and not as a result of the very small magnetic fields generated by the little current flowing in the pickup coils.
Phase relation? Do care to explain what that means in this application.
#A. The Tele bridge mount.
#B. The covers and baseplates of the classic humbucker.
#C. Any metal plates below the coils.
#D Metal pick guards for Strats.
#E. In general, all metal parts in the vicinity of the pickups.
Actually, he could have just said point #E and left the rest out.
He hasn't really spoken about what these eddy currents actually do!?! Let's find out by looking at an example of what eddy currents are used for and where they are a nuisance.
Induction heating is the process of using an electromagnet to heat or melt materials. The material to be heated must be conductive. A very powerful magnetic field is generated, but by using AC instead of DC. The AC results in a continuously changing magnetic field instead of a static one. This, in turn, causes very powerful eddy currents to flow in the material. Like using a wire of inappropriate gauge to carry a large current, the material will heat up and eventually melt.
Those of you familiar with transformer design will be aware that textbooks provide many example calculations based on what is called an ideal transformer. This ideal transformer is a true 1:1 power converter that has no losses - power in equals power out. In reality however, we know this not to be the case. If you have ever felt a running transformer you will notice it gets warm. Transformers lose efficiency as a result of heat generation. Now, why do they heat up? The answer is eddy currents. Changes in magnetic flux cause eddy currents to flow through the transformer core. Since the ferrite core material does not have a zero resistivity (only super conductors manage that) it will heat up. That is why transformer cores are made from laminated thin plates instead of a single solid core – the thin plates reduce the occurrence of eddy currents, making the transformer more efficient.
You should start to see that eddy currents can be used to transfer energy and are also responsible for losses. Sometimes these losses can be used to advantage. One such case is the eddy current brake.
An electromagnet (or permanent magnet in some cases) is brought near a rotating disk, much like a car's disk brake. As the disk rotates through the magnetic field, eddy currents are generated that flow in a direction to generate a new magnetic field that opposes the original field. Like pointing two magnets towards each other with north poles facing, an opposing force is experienced. The opposing force acts on the brake disk, causing it to slow down. A key issue here, is that these brakes are only effective when the disk is rotating at high speeds. At slow speeds the eddy currents are too small to create a large enough field to oppose the electromagnets.
There you have your final clue – eddy currents basically act like braking mechanisms in one way or another. They slow things down and something has to give.
So these mystical eddy currents affect the guitar somehow? This means that the effect the eddy currents have on the pickups would require that the eddy currents be large enough to cause their own magnetic field of sufficient magnitude in order to affect the behaviour of the pickups themselves.
Now, where could the eddy currents come from? The pickup magnets will move under the influence of the vibrating guitar strings. Moving magnets means a moving magnetic field, which means there will be a change in magnetic flux. We have the first requirement, a change in magnetic field. Now we need a conductor. What about the pickup coils? Well it is true that an induced emf will be present in the conducting coils, this is what the guitar amplifier lets us hear. However, eddy currents require an isolated conductor, and not a closed circuit like the guitar's pickups that are part of the amplifying circuit. So no eddy currents in the pickup coils, unless, a coil is shorted out and then forms a loop on its own, as is mentioned by Bill.
The only place eddy currents can form is in the metal parts of the guitar that are close enough to the moving pickup magnets. Now, suppose this actually happens – eddy currents flow in a metal component near the pickups. According to Bill, these eddy currents somehow affect the pickups. What he is insinuating here is that the eddy currents create a magnetic field of their own, that is so large that it opposes the magnetic field of the pickup magnets thereby changing the way they behave.
This brings me to the point that I have an issue with – I simply cannot believe that the magnetic field generated by the eddy currents is strong enough to have any noticeable effect. I believe that what is actually happening here is that conductive metal parts near the pickups are actually attracting the magnets (in a way damping them) and also shaping the magnetic field pattern. There is something called mu metal. It has extremely high permeability. It is used as a shielding for magnetic fields. However, it is not really a shielding - you cannot shield against a magnetic field, only attempt to redirect it. The higher the permeability of the shield, the more of the field is redirected and only a small percentage leaks through. See the following pictures:
Notice how the picture at the bottom left shows each colour at a different radius when compared to the original field in the picture top left? Not only is the mu metal "shielding" the field around the top and sides, it is also shaping the field at the bottom where there is no mu metal.
I think Bill is observing this effect on a smaller scale - the metal components are shaping the magnetic field around the pickups and damping the movement of the magnets in the pickups. That explains why he seems to think that permeability has something to do with this whole concept. Any takers on this viewpoint?
Disclaimer: I have no experience with making guitar pickups and it is 2am so I probably made lots of mistakes...
