An exhaust gas oxygen sensor comprises a zirconia disc 1 having a catalytic electrode 5 on one face thereof and a non-catalytic electrode 7 on the other face and is adapted for mounting in an internal combustion engine exhaust duct 17 with both electrodes exposed to the exhaust gases. With this arrangement, the relatively small change in the partial pressure of the oxygen at the non-catalytic electrode 7 between a rich and lean mixture as compared with the relatively large change produced at the catalytic electrode 5 by the catalytic action provides a suitable reference for operation of the cell. The disc 1 is mounted on edge in a ceramic base 3 with the plane of the disc parallel to the exhaust gas flow to minimize abrasion by particle bombardment. Electrical leads 13, 15 connected to annular conductors 9, 11 in electrical contact with the electrodes 5, 7 on the respective faces of the zirconia disc 1 extend through the ceramic base 3 to form a two prong connector. IMAGE
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