Measuring device having a squid magnetometer with a modulator for measuring magnetic fields of extremely low frequency

An apparatus for measuring magnetic fields which change only at an extremely low frequency contains a SQUID magnetometer which comprises a superconducting flux transformer for inductively coupling the measuring signal into a d-c SQUID sensor. With this apparatus, also quasi-static magnetic fields with frequencies far below 1 Hz should be detectable. A device for modulating the measuring signal to be coupled into the SQUID sensor is associated at least with the flux transformer, the modulation frequency of which is in a frequency range characteristic for low-noise operation of the SQUID. The modulation frequencies are generally above 1 Hz and optionally even above 1 kHz.