An electrical apparatus includes a spiral electrode and an interface circuit. The spiral electrode is disposed on a distal end of a probe for insertion into a body of a patient. The interface circuit is configured to (a) transfer a radiofrequency (RF) ablation signal to the electrode for ablating tissue in the body, (b) output a voltage that develops across the electrode in response to an external magnetic field, for measuring a position of the distal end in the body, and (c) transfer electrical current through the electrode for measuring a resistivity that is indicative of tissue temperature in a vicinity of the electrode.