Abstract: A platinum oxygen-pressure measuring electrode is connected with a reference electrode. An indicating device for indicating the oxygen pressure is connected with both of the electrodes, and an arrangement is interposed between the electrodes and an object to be measured and has a surface which is to be placed against the object. The arrangement offers to the diffusion of oxygen from the surface to the measuring electrode a resistance which is greater than the resistance which is offered to such diffusion of the oxygen through the object to be measured.