Magnetic field detector for thermonuclear reactor

Two ferromagnetic metal thin film resistor patterns having substantially the same shape and substantially the same resistance variation coefficient relative to temperature are disposed perpendicular to each other. One of them is for the measurement and disposed in the direction substantially aligned with the external magnet fields as an object of the measurement. The other of them is used as a reference, and the ferromagnetic metal thin film resistor patterns are connected as two resistors of a bridge circuit for the measurement, and a non-magnetic metal having satisfactory heat conductivity is connected on the back of an insulator to be a base. In addition, there is disposed a temperature controlling power source means for controlling voltage applied so that the resistance value of the metal resistor patterns is made constant. Then, the effect due to the change of the resistance by the temperature of the sensor can be offset, and the influence of temperature by nuclear heat generation can be offset by designing it such that the temperature is always constant by using an insulation member having good heat conductivity. (I.S.)