The ozone sensor used within the ozonesonde is an iodine-iodide redox electrochemical concentration cell. It is made of 2 bright platinum electrodes immersed in potassium iodide solutions of different concentrations, contained in separate cathode and anode chambers, fabricated from polytetrafluoroethylene (Teflon TFE resin). The chambers are linked together with an ion bridge that serves as an ion pathway and retards mixing of the cathode and anode electrolytes, thereby preserving their concentrations. Driving emf for the cell is derived from a difference of potassium iodide concentrations present in the 2 half cells.
A chemical reaction starts as soon as ozone (in air) flows into the cathode solution. The reaction is an iodide-iodine redox reaction. You can measure the current when the switch S is closed. R is the load resistance of the circuit.