Today's automobiles monitor combustion efficiency using a galvanic oxygen sensor in the exhaust manifold. This sensor measures the oxygen pressure of the exhaust gas. The potential between two electrodes depends solely on the ratio of the partial pressures of oxygen at each electrode, separated by an oxygen ion conductor; zirconia . The chemical reactions (electron transfer) at each electrode are the same but in reverse of one another; at one electrode the reduced form of the chemical particle is being oxidised (releasing electrons) and at the other electrode the oxidised form is being reduced (accepting electrons). The voltage output of the sensor is sent in a feedback loop to control the air/fuel mixture for optimised combustion.