The different components of the cell voltage inside and outside of the electrolysis cell are described. The bath voltage contributes the greatest part to the cell voltage. Relationships for its ohmic and electrochemical parts are given namely: the reversible decomposition voltage, the ohmic bath voltage, the bubble voltage drop and the overvoltages. The bath voltage depends on the alumina concentration in the electrolytic bath. This relationship is the base of the automatic control of the alumina concentration by computer process control. When the alumina concentration is low the electrolysis reactions change and the anode effect occurs. The mechanism of the anode effect and its quenching is described.