Maintenance and dissemination of the magnetic field unit at PTB

The different methods which are used at PTB to disseminate the unit of flux density are presented and compared with respect to their uncertainties. A DC flux density range from 10 /spl mu/T to 2 T is covered with the lowest relative uncertainty of 10/sup -6/ at fields of 1 mT to 2 mT. In case of AC fields, field coils are used as standards, the frequency response of which is checked by means of a simple but very reliable inductive measuring technique. A relative uncertainty of 3/spl middot/10/sup -3/ or even less is typically achieved for frequencies below 30 kHz.