Superconductive device for measuring small magnetic fields - has quantum interference device and sample-and-hold circuit to stabilise field working range between Josephson elements

摘要

The appts. is for measurement of small magnetic fields, magnetic field gradients, electric currents and voltages is based on a D.c. superconducting quantum interference device (SQUID). A typical arrangement includes an a.c. source (1) which feeds a sawtooth waveform current to a SQUID. A voltage proportional to this current is fed to the input of a sample-and-hold circuit. When the maximal superconducting current is reached, a voltage change of the order of several millivolts is fed without delay to a differentiating circuit which suppliejs a voltage pulse to the sample-and-hold circuit. During this short time interval, the circuit stores a voltage (U1) proportional to the maximal current. By means of a voltage-current converter, a regulating current is generated and fed to the magnetic field to stabilise the working point. The magnetic field under control (F) is between two Josephson elements (J1, J2).