The possibility of the SQUID sensitivity improving to the magnetic field is demon-strated experimentally. This can be done providing magnetic coupling between the SQUID and the ferromagnetic circuit. This is important to increasing the low frequency sensitivity of HT_c SQUIDs which is otherwise low in the (1-10 Hz) range. Besides, the ferromagnetic circuit permits SQUID measurement of the magnetic field of distant "warm" (T approx approx 300K) sources as well as one-SQUID records of magnetic fluxes from several ferromagnetic circuits.
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