| http://www.w3.org/ns/prov#value | - magnetic alternating fields having a frequency of 50 Hz and above, said means for shielding consisting of an inner shell of a low-retentivity material having a relative permeability greater than 104, and an outer shell of aluminum; electronic means for evaluating signals from said gradiometers, said electronic means including amplifier means for amplifying signals from analog outputs of said DC-SQ
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