Method and apparatus for electromagnetically measuring parameters of electrically conductive high temperature materials

An apparatus for electromagnetic quantity measurement of at least one of the resistivity, temperature, dimension and spatial location of an electrically conductive material at high temperature, comprising:

• a transmitter coil for generating an electromagnetic alternating field in the electrically conductive material, an a.c. source for supplying the transmitter coil with alternating current at a frequency such that the electromagnetic alternating field penetrates to a depth .delta. in the electrically conductive material;
• a receiver coil located a distance D from the electrically conductive material wherein the ratio (.delta./D) lies between 0.1 and 3, for detecting secondary electromagnetic alternating fields dependent upon the measured parameter and generated by induced currents in the electrically conductive material;
• and signal processing means connected to the receiver coil for detecting and processing alternating voltage signals induced therein by said secondary electromagnetic alternating field and for distinguishing changes in the measured parameter from changes in other parameters, said signal processing means including means for selectively detecting the magnitude of alternating voltage signal components induced in the receiver coil which are phase-shifted relative to the alternating current supplied to the transmitter coil, said selective detecting means including a synchronous demodulator coupled to the receiver coil and to the a.c. source via a phase shifting circuit for substantially eliminating the effects of ambient surroundings on the electromagnetic field generated by the transmitter coil.