Defined for ionic species B by where is the charge number of the ionic species B, is the Faraday constant, and is the electric mobility of species B. In most current practice is taken as unity, i.e. ionic conductivity is taken as that of species such as Na⁺, Ca²⁺/2, La³⁺/3 etc. To avoid ambiguity the species considered should be clearly stated, e.g. as .