Permeability:

Some substances cause the magnetic lines of flux to get closer together than they are in the air. Some of the materials can make the lines of flux to become farther apart than they are in the air.

The first kind of material is ferromagnetic, and is of primary significance in magnetism. The ferromagnetic substances are the ones which can be magnetized. Iron and nickel are the examples of it. Various alloys are even more ferromagnetic than pure nickel or pure iron.

The other kind of material is known as diamagnetic. Wax, bismuth, dry wood, and silver are substances which decrease the magnetic flux density actually. No diamagnetic material decreases the strength of a magnetic field by anywhere near the factor which the ferromagnetic substances can increase it.

Permeability can be measured on a scale relative to a vacuum, or free space. Free space is given permeability 1. If you have a coil of wire having an air core, and a current is forced through the wire, then flux in the coil core is at a certain density, about the same as it would be in a vacuum. Thus, the permeability of pure air is about equal to 1. If you place iron core in coil, the flux density increases by the factor of about 60 to thousand times. Thus, the permeability of iron can range from 60 to as much as 8,000.

If you use certain permalloys as the core material in the construction of electromagnets, you can increase the flux density, and hence the local strength of the field, by as much as 1,000,000 times. Such substances therefore have permeability as great as 1,000,000.

If you feel compelled for some reason to make an electromagnet which is as weak as possible, you could make use of dry wood or wax for the core material. But generally, diamagnetic substances are used to keep the magnetic objects away, while minimizing interaction between them.

Diamagnetic metals have some useful property that they conduct electric current very well, but magnetic current poorly. They can be used for electrostatic shielding, which is a means of allowing magnetic fields to pass through while blocking the electric fields.