Device for Producing Very Low Magnetic Fields

Abstract

A volume containing a reduced magnetic field may be obtained inside a lead foil sack by opening the folded sack at 4.2°K when the lead is superconducting. A multi‐stage magnetic flux ``vacuum pump'' based upon this idea was used to reduce earth's field to below 10⁻⁶ Oe in a volume of several hundred cubic centimeters. The superconducting sack shields this volume against the magnetic field fluctuations of the laboratory environment. The estimated noise amplitude in the low field space is 10⁻⁸ Oe. A glass Dewar inserted into the sack permits working in the low field space at controlled temperatures near room temperature. The system has had extensive use for the study of the long term stability of flux‐gate magnetometer sensors.