As we have seen, conductivity arises from mobile charge carriers. In metals, these mobile charge carriers are electrons; in an ionized gas, they are electrons and positively charged ions; in an electrolyte, these can be both positive and negative ions.
An important quantity is the mobility µ defined as the magnitude of the drift velocity per unit electric field:

The SI unit of mobility is m²/Vs and is 10⁴ of the mobility in practical units (cm²/Vs). Mobility is positive.

Hence,

where τ is the average collision time for electrons.