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.