A device employing an ion chamber has been developed for the sensitive, reliable, and very rapid detection of local ovetheatingin a gas-cooled electric machine before extensive damage can result. The device detects the presence of pyrolysis products generated by the thermal decomposition of organic materials such as insulations and polymers. The pyrolysis products include particles of the size of condensation nuclei (about 0.001 to 0.1 microns), and smaller, which have such a pronounced effect on the ion current in the ion chamber that a definite signal is obtained from a hot spot as small as one square inch in area in a large hydrogen-cooled generator. A considerable amount of test and operating experience has been acquired.