I would like to approach the subject of "electronic shielding", as it's known by most musicians. Many guitar companies now deliver their instruments with shielded cavities, and this does in fact reduce the amount of interference captured by the circuit of an electric guitar, or bass. What electronic shielding doesn't do is reduce the amount of interference, or noise, captured by the pickups exposed poles pieces.

An valid argument can be made than pickups with metal covers are shielded, and I agree with that. The conductive cover, baseplate and cavity shielding do for a Faraday cage, therefore cancelling any interference. However, a shielded pickups cavity will have no effect on the amount of noise generated by uncovered, or plastic covered pickups, since no cage is formed.

The reason shielding works boils down to physics, as usual. Faraday cages are used in a number of applications throughout the world, and have been since 1836, when English scientist Michael Faraday invented them. They're used in power plants, guitars, entertainment show, guitars, university exam rooms and are the reason you won't be harmed when your car gets struck by lightning.

Here is how they work:

A Faraday cage is best understood as an approximation to an ideal hollow conductor. Externally or internally applied electromagnetic fields produce forces on the charge carriers (usually electrons) within the conductor; the charges are redistributed accordingly (that is, electric currents are generated). Once the charges have rearranged so as to cancel the applied field inside, the currents stop.

If a charge is placed inside an ungrounded Faraday cage, the internal face of the cage becomes charged (in the same manner described for an external charge) to prevent the existence of a field inside the body of the cage. However, this charging of the inner face re-distributes the charges in the body of the cage. This charges the outer face of the cage with a charge equal in sign and magnitude to the one placed inside the cage. Since the internal charge and the inner face cancel each other out, the spread of charges on the outer face is not affected by the position of the internal charge inside the cage. So for all intents and purposes, the cage generates the same DC electric field that it would generate if it were simply affected by the charge placed inside. The same is not true for electromagnetic waves.

If the cage is grounded, the excess charges will go to the ground instead of the outer face, so the inner face and the inner charge will cancel each other out and the rest of the cage will retain a neutral charge.

Effectiveness of shielding of a static electric field depends upon the geometry of the conductive material. In the case of a nonlinear varying electric field, and hence an accompanying varying magnetic field, the faster the variations are (i.e., the higher the frequencies), the better the material resists penetration, but on the other hand, the better it passes through a mesh of given size. In this case the shielding also depends on the electrical conductivity of the conductive materials used in the cages, as well as their thicknesses.