Plasma (physics) potential in the space between charged particles, independent of how it can be measured, is called the "plasma potential", or the "space potential". If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to what is termed a Debye sheath. The good electrical conductivity of plasmas makes their electric fields very small. This 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 (formula_6), but on the scale of the Debye length
Positive and negative sets union of a sequence of positive or negative sets is also positive or negative; more formally, if ("A") is a sequence of positive sets, then is also a positive set; the same is true if the word "positive" is replaced by "negative". A set which is both positive and negative is a μ-null set, for if "E" is a measurable subset of a positive and negative set "A", then both μ("E") ≥ 0 and μ("E") ≤ 0 must hold, and therefore, μ("E") = 0. The Hahn decomposition theorem states that for every measurable space ("X",Σ) with a signed measure μ,