In the electrolysis of a salt solution, the quantity of electricity carried by the cations in one direction is proportional to their speed u, and the quantity carried by the anions in the opposite direction is proportional to their speed v. Hence the total current carried is proportional to u – v. The fraction of the total current carried by the cation is [equation]; this fraction is called the transference or transport numbers of the cation. Similarly the transport number of the anion is [equation]. The sum of the anion and cation transport numbers is always equal to unity. There are three methods in use for the determination of transport numbers.