Electrical resistivity and conductivity results in the important concept of "quasineutrality", which says the density of negative charges is approximately equal to the density of positive charges over large volumes of the plasma ("n" = <Z>"n"), but on the scale of the Debye length there can be charge imbalance. In the special case that "double layers" are formed, the charge separation can extend some tens of Debye lengths. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the Boltzmann relation: Differentiating this
Positive and negative sets there is a partition of "X" into a positive and a negative set; such a partition ("P","N") is unique up to μ-null sets, and is called a "Hahn decomposition" of the signed measure μ. Given a Hahn decomposition ("P","N") of "X", it is easy to show that "A" ⊆ "X" is a positive set if and only if "A" differs from a subset of "P" by a μ-null set; equivalently, if "A"−"P" is μ-null. The same is true for negative sets, if "N" is used instead of "P". Positive and negative sets In measure theory, given a measurable space ("X",Σ)