PERMEABILITY

Permeability, symbolized by the lowercase Greek mu (μ), is measured on a scale relative to a vacuum, or free space. A perfect vacuum is assigned, by convention, a permeability figure of exactly 1. If current is forced through a wire loop or coil in air, then the flux density in and around the coil is about the same as it would be in a vacuum. Therefore, the permeability of pure air is about equal to 1. If you place an iron core in the coil, the flux density increases by a factor ranging from a few dozen to several thousand times, depending on the purity of the iron. The permeability of iron can be as low as about 60 (impure) to as high as about 8,000 (highly refined).
If you use special metallic alloys called permalloys as the core material in electromagnets, you can increase the flux density, and therefore the local strength of the field, by as much as 1 million (10⁶) times. Such substances thus have permeability as great as 10⁶.
If, for some reason, you feel compelled to make an electromagnet that is as weak as possible, you can use dry wood or wax for the core material. Usually, however, diamagnetic substances are used to keep magnetic objects apart while minimizing the interaction between them.