OPTICAL PT

PURPOSE:To make the temperature characteristic of an apparatus as a whole small by making a voltage divider and an optical PT as a whole small and lightweight. CONSTITUTION:An optical PT is provided with a voltage divider 16 which divides a voltage to be measured and with a photosensor part 15 which applies the output voltage of the voltage divider 16 to an electromechanical crystal and which converts the output voltage into a light intensity corresponding to the output voltage, and it measures the magnitude of the voltage, to be measured, on the basis of the light intensity. The voltage divider 16 is constituted of a voltage divider body 16' as a gas capacitor composed of a changing part 17 to which the voltage to be measured is applied, of a single space electrode 18 surrounding the charging part 17 and of an insulating medium and of a solid capacitor 20 which is connected to the space electrode 18 and to the optical sensor part 15. The temperature-capacitance change rate characteristic of the solid capacitor 20 is offset by the temperature-output change rate characteristic, of the photosensor part 15, which is obtained by regulating the optical path length of the electrochemical crystal.