A method and apparatus for measuring the change in temperature of a material being heated. In one embodiment, the method includes introducing particles having magnetic susceptibility into the material heating the material and particles measuring, using a circuit having a resonant frequency, the change in the resonant frequency of the circuit as the temperature of the particles changes and correlating the change in resonant frequency to a change in temperature. In one embodiment, the apparatus includes a collection of magnetic particles in contact with the material undergoing heating a resonance circuit including a tank circuit including a coil and capacitance and having a resonant frequency an alternating current variable frequency source capable of tracking changes in the resonant frequency of the tank circuit to maintain the frequency of the alternating current at the resonant frequency of the tank circuit and a processor in communication with the alternating current source.
